Stefano Mammola
email: stefano.mammola@unito.it@tel

Elena Piano

Florian Malard

Philippe Vernon

Marco Isaia
email: marco.isaia@unito.it@tel

Dr Enrico Rezende

Extending Janzen's hypothesis to temperate regions: A test using subterranean ecosystems

Keywords: Climate Variability Hypothesis, Janzen's hypothesis, Range size, Thermal tolerance, Climate change, Rapoport's rule, Subterranean biology, Western Italian Alps

HAL is

INTRODUCTION

Understanding the role of climate in determining species geographical distribution is a pivotal research topic in biogeography, macroecology and climate change biology [START_REF] Deutsch | Impacts of climate warming on terrestrial ectotherms across latitude[END_REF][START_REF] Mittelbach | Evolution and the latitudinal diversity gradient: speciation, extinction and biogeography[END_REF][START_REF] Muñoz | Janzen's hypothesis meets the Bogert effect: connecting climate variation, thermoregulatory behavior, and rates of physiological evolution[END_REF][START_REF] Shah | Climate variability predicts thermal limits of aquatic insects across elevation and latitude[END_REF][START_REF] Sunday | Global analysis of thermal tolerance and latitude in ectotherms[END_REF]). Janzen's mountain passes hypothesis [START_REF] Janzen | Why mountain passes are higher in the tropics[END_REF] is a core contribution to understand how climatic variability influences a species thermal tolerance and, through this, its dispersal and elevational range. This hypothesis was originally developed to explain biodiversity patterns in tropical mountains. Janzen began by assuming that the degree to which a topographic barrier limited dispersal depends on the temperature gradient across that barrier. Then, he assumed that a greater seasonal stability of temperature, like in tropical mountains, would select for species with narrow thermal tolerance over time. By linking together these two assumptions, he predicted that tropical species would have greater difficulty than temperate species to disperse across mountain passes, because they would be more likely to encounter unsuitable climates. Consequently, Janzen concluded that mountain passes were figuratively "higher" in the tropics.

Accepted Article

. [START_REF] Ghalambor | Are mountain passes higher in the tropics? Janzen's hypothesis revisited[END_REF][START_REF] Sheldon | Fifty years of mountain passes: a perspective on Dan Janzen's classic article[END_REF]). Yet, the challenge of comparing thermal tolerance, range size and dispersal ability of geographically-distant animal communities occurring in tropical and temperate mountain systems emphasizes the difficulty of rigorously testing this framework. Indeed, a comprehensive test of Janzen's hypothesis-i.e. a research tackling simultaneously all of its empirical assumptions-was proposed only after five decades from its original formulation [START_REF] Polato | Narrow thermal tolerance and low dispersal drive higher speciation in tropical mountains[END_REF][START_REF] Smith | Janzen's mountain passes hypothesis is comprehensively tested in its fifth decade[END_REF]. By studying invertebrate assemblages in tropical versus temperate mountain rivers, [START_REF] Polato | Narrow thermal tolerance and low dispersal drive higher speciation in tropical mountains[END_REF] recently demonstrated that the mountain passes hypothesis might hold true for riverine communities. Building upon this milestone result, it should be theoretically possible to generalize Janzen's hypothesis more broadly, by testing whether it can be used to explain biodiversity patterns in non-tropical ecosystems with reduced climatic seasonality.

Caves represent unique experimental arenas for testing the assumptions of Janzen's hypothesis. Deep terrestrial subterranean habitats-deep caves and the maze of fissures and voids inaccessible to humans-are among the terrestrial habitats with the greatest climatic stability [START_REF] Badino | Underground meteorology. What's the weather underground[END_REF]. In temperate caves, there are steep gradients in thermal seasonality from the outer (annual temperature variations comparable to the outside; [START_REF] Pipan | Temperature variation and the presence of troglobionts in terrestrial shallow subterranean habitats[END_REF] to the inner sectors (annual variations in the order of few tenths of degrees; [START_REF] Badino | Underground meteorology. What's the weather underground[END_REF][START_REF] Cigna | Modern trend(s) in cave monitoring[END_REF]. This provides a unique opportunity to test for relations among habitat specialization, thermal tolerance and dispersal capacity in animal communities at a very local, rather than a continental, scale. Indeed, comparing geographically-distant communities such as temperate and tropical communities bears the risk of confounding the influence of short-and long-term climate variability on specialization and dispersal [START_REF] Isaia | The fate of clades in a world of recurrent climatic change: Milankovitch oscillations and evolution[END_REF][START_REF] Stevens | The latitudinal gradients in geographical range: how so many species co-exist in the tropics[END_REF]). This confounding factor has long been debated when it comes to explaining the mechanisms causing an increase in species range size at higher latitudes [START_REF] Morueta-Holme | Habitat area and climate stability determine geographical variation in plant species range sizes[END_REF][START_REF] Rohde | Rapoport's rule is a local phenomenon and cannot explain latitudinal gradients in species diversity[END_REF][START_REF] Veter | Is Rapoport's rule a recent phenomenon? A deep time perspective on potential causal mechanisms[END_REF], i.e. the Rapoport's rule as described by [START_REF] Stevens | The latitudinal gradients in geographical range: how so many species co-exist in the tropics[END_REF].

Accepted Article

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Here, we provide the first explicit test of Janzen's hypothesis in subterranean habitats, by measuring the thermal tolerance of the widest sample of congeneric subterranean organisms to date, and relating thermal tolerance to the degree of subterranean specialization and to the extent of the elevation range. We used spiders of the genus Troglohyphantes (Araneae: Linyphiidae) as experimental model organisms (Fig. 1).

Troglohyphantes spiders show different levels of subterranean habitat specialization, thus representing multiple replicates of the ecological transition along a gradient from shallow and thermally variable to deep and thermally stable subterranean habitats [START_REF] Isaia | The fate of clades in a world of recurrent climatic change: Milankovitch oscillations and evolution[END_REF]Mammola et al., 2018a). We tested eleven congeneric species distributed in the restricted biogeographic area of the Western Alps [START_REF] Isaia | The fate of clades in a world of recurrent climatic change: Milankovitch oscillations and evolution[END_REF]Mammola, Goodacre, & Isaia, 2018), hence evaluating differences in thermal tolerance and dispersal among species sharing a common biogeographic history but occurring in habitats of contrasted thermal seasonality.

We initially explored the upper thermal tolerance of these species via thermal test experiments in a climatic chamber. Subsequently, we estimated the subterranean specialization of each species using a quantitative approach based on morphological traits (Mammola et al., 2018a), and related this specialization to their thermal tolerance. We predicted that increasing levels of subterranean specialization would result in narrower ranges of thermal tolerance, because the more specialized subterranean species colonize the deeper, seasonally stable cave habitats. Then, we related thermal tolerance to the elevational range extent as well as to the variation in temperature across that range (hereafter 'realized thermal niche breadth'). We predicted that more specialized subterranean species would exhibit both narrower elevational ranges and smaller realized thermal niche breadths. Corroborating the two predictions would provide an extension of Janzen's hypothesis to habitats with reduced thermal seasonality other than to tropical mountains [START_REF] Polato | Narrow thermal tolerance and low dispersal drive higher speciation in tropical mountains[END_REF], by linking a subterranean species' morphological and thermal specialization to its dispersal potential across elevation gradients.

Accepted Article

. [START_REF] Fick | Worldclim 2: New 1-km spatial resolution climate surfaces for global land areas[END_REF].

Accepted Article

We collected live specimens of eleven species of Troglohyphantes from subterranean sites (caves, mines, subterranean military bunkers; hereinafter 'caves') across the Western Italian Alps (Table 1). With the exception of a very rare species inhabiting interstitial subterranean habitats of difficult access (T. giachinoi Isaia & Mammola), we sampled all known Troglohyphantes species occurring in this region. Samplings were conducted from early fall to late winter, which in our experience (Isaia & Mammola, personal observations) corresponds to the period where Troglohyphantes spiders are generally mostly available in .

MATERIALS & METHODS

Thermal overlap for deep and shallow cave habitats

To test whether thermal gradients are less overlapping for spiders inhabiting deep cave sectors, we used best available thermal records in the Western Italian Alps to compare the overlap in temperature at shallow and deep cave sectors. We used thermal data collected between 2012 and 2014. Temperature measurements were taken every 3 h through the whole sampling period, using Hygrochron TM dataloggers (accuracy: ±0.5°C) placed both at the cave entrance (1-10 m from the surface) and deeper inside the cave (>100 m or at the deepest point when caves were less than 100 m in length). Data were taken in multiple caves across our study region, covering an elevational range from 400 to over 2,000 m a.s.l.

(full details in [START_REF] Mammola | Climate change may drive cave spiders to extinction[END_REF]. Following [START_REF] Scheffers | Vertical (arboreality) and horizontal (dispersal) movement increase the resilience of vertebrates to climatic instability[END_REF], we plotted the range of thermal regimes and determined daily overlap values between the superficial and deep subterranean temperature regimes across caves opening at different elevations. We showed thermal overlap during only 2.5 months, because this was a timeframe for which all dataloggers were properly and simultaneously working in all caves (see details in [START_REF] Mammola | Climate change may drive cave spiders to extinction[END_REF]. The reported temperature patterns remain roughly the same for the whole period in which temperatures were recorded.

Sampling protocol

Accepted Article

. caves. Due to the reduced detectability and generally low density of individuals, for a few species it was necessary to sample multiple caves to obtain a sufficient number of specimens (from 11 to 36) for the analysis (Table 1). In each cave, we collected spiders by hand and placed them in individual Eppendorf Tubes® of 5ml. We stored all vials in a coolbag and transported it the same day to the laboratory at the University of Turin (Italy), where we conducted the experiments. In the laboratory, we placed specimens in a IPP 30 Peltier Memmert climatic chamber, each specimen in individual Petri dish, and acclimated them for one day at the temperature of the cave. Air humidity was kept at 100% during the acclimation and during the experiments, by placing two Petri dishes filled with water inside the climatic chamber. During acclimation, small insects collected on the cave walls during sampling were offered ad libitum to each specimen tested. Owing to the high resistance to starvation of subterranean spiders [START_REF] Isaia | The fate of clades in a world of recurrent climatic change: Milankovitch oscillations and evolution[END_REF], specimens were not fed throughout the experiments.

For each sampled cave, we derived the internal mean annual temperature from continuous temperature measurements taken between 2012 and 2013 [START_REF] Mammola | Climate change may drive cave spiders to extinction[END_REF], or unpublished temperature records taken via the same methodology (Grotte della Maissa). For two caves lacking field-collected temperature records (indicated with an asterisk in Table 1), we downloaded annual temperature series from the same period from the nearest thermo-hygro-pluviometric weather station. After correcting the data with the standard environmental lapse rate (0.57°C/100m; [START_REF] Rubel | The climate of the European Alps: Shift of very high resolution Köppen-Geiger climate zones 1800-2100[END_REF], we calculated the mean annual temperature and used this value as a direct proxy of the cave temperature [START_REF] Badino | Underground meteorology. What's the weather underground[END_REF][START_REF] Sánchez-Fernández | The deep subterranean environment as a potential model system in ecological, biogeographical and evolutionary research[END_REF].

We conducted experiments to determine the upper critical temperature with the climatic chamber, starting from the mean annual cave temperature, with an increasing slow temperature rate of 1°C/day. We evaluated the individual critical temperature (CT) as the total paralysis, that is the temperature at which the individual was immobile and no appendage movements were visible [START_REF] Le Lann | Thermal tolerance of sympatric hymenopteran parasitoid species: does it match seasonal activity?[END_REF]. We observed that after the

Accepted Article

. heating process, the spiders never recovered after being removed from the climatic chamber, a typical pattern when the temperature at which heat stupor occurs is close to the lethal temperature [START_REF] Wu | Exceptional thermal tolerance and water resistance in the mite Paratarsotomus macropalpis (Erythracaridae) challenge prevailing explanations of physiological limits[END_REF]. We expressed the CT 50 as the measure of temperature at which 50% individuals experienced total paralysis. The experiment continued until all the specimens had stopped moving (CT 100 ). For each species tested, we kept approximately 10% of the specimens as controls in stable climatic conditions, to exclude the confounding effect of causes of mortality other than temperature alteration (e.g., starvation, stress). No mortality was recorded in controls during the experiments.

After the experiments we sexed each specimen (male, female or juvenile), confirmed species identity under a stereomicroscope, and acquired morphological measures to quantify the degree of subterranean specialization.

Morphological measures

For estimating the degree of subterranean specialization of each species tested, we measured three morphological traits, namely leg elongation (sum of leg I articles divided by body size), eye regression (sum of eye diameter divided by length of the ocular area) and profile reduction (cephalothorax height divided by cephalothorax width). The reliability of these measures to represent specialization to deep subterranean habitats in Troglohyphantes species is provided in Mammola et al. (2018a). To standardize measurements and obtain comparable estimations, we considered only adult females in the estimation, thereby avoiding problems related to sexual size dimorphism. We acquired measures in millimeters (mm) from digital pictures made with a Leica EC3 digital camera installed on a Leica M80 stereoscopic microscope (up to 60x magnification), and with the Leica LAS EZ 3.0 software (Leica Microsystems, Switzerland).

Elevational range extent and realized thermal niche breadth

Accepted Article

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We assembled a database of occurrence localities of each Western alpine Troglohyphantes species, using recent literature geo-referenced records [START_REF] Isaia | The fate of clades in a world of recurrent climatic change: Milankovitch oscillations and evolution[END_REF][START_REF] Mammola | Alpine endemic spiders shed light on the origin and evolution of subterranean species[END_REF]Mammola et al., 2018a[START_REF] Mammola | Climate change may drive cave spiders to extinction[END_REF]) (Fig. 1). We overlaid elevation and mean annual temperature rasters at 30 arcsec resolution (1970-2000;[START_REF] Fick | Worldclim 2: New 1-km spatial resolution climate surfaces for global land areas[END_REF] to the occurrence records of each species. We calculated the elevational range as the difference between maximum and minimum elevations and the realized thermal niche breadth as the difference between maximum and minimum temperatures experienced by each species across its localities.

Statistical analyses

All statistical analyses were conducted using R software (R Core Team, 2017). We used the internal mean annual temperature of the caves as the baseline to calculate the delta with the species critical temperatures (∆T, a measure of thermal tolerance expressed as the difference between the CT and the mean temperature of the natural habitat of each specimen). We tested for differences in ∆Ts between sexes (males, females or juveniles) with separate ANOVA, via the formula (R notation):

∆T ~ sex * species For the four species collected in multiple caves (T. konradi, T. lucifer, T. lucifuga, T. vignai), we also used separate ANOVA to test for differences in ∆Ts across populations and taking into account the sex, via the formula:

∆T(species) ~ cave + sex
We constructed standard survival curves with binomial Generalized Linear Models (GLM), whereby we modeled the percentage survival rate of each species as a function of ∆T (response variable).

Accepted Article

The use of annual mean temperature to calculate ∆T may bias results for low specialized species because air temperature can vary substantially across seasons in .

We performed a cluster analysis for grouping Troglohyphantes species into homogeneous classes of subterranean specialization, using the morphological traits measured on adult females. Using standardized values of traits, we estimated clusters via Euclidean distances and the Ward2's hierarchical clustering method [START_REF] Murtagh | Ward's hierarchical agglomerative clustering method: which algorithms implement Ward's criterion?[END_REF], with the R package 'fastcluster' [START_REF] Müllner | fastcluster: fast hierarchical, agglomerative clustering routines for R and Python[END_REF]. We relied on the Gap statistic for determining the optimal number of clusters k [START_REF] Tibshirani | Estimating the number of clusters in a data set via the gap statistic[END_REF], allowing for a maximum k equal to the number of species tested and performing 500 bootstraps.

To verify if the shape of the survival curves varied in response to the degree of subterranean specialization of each species, we modeled the survival rate of all individuals tested as a function of ∆T in interaction with the class of subterranean specialization, with a binomial Generalized Linear Mixed Model (GLMM) The mixed part of the model was designed to account for data dependence, using species identity as random factor. We fitted the models in R package 'lme4' [START_REF] Bates | Fitting linear mixed-effects models using lme4[END_REF], with the formula: Survival rate ~ ∆T * Class of subterranean specialization + random factor (Species)

We tested the relationships between ∆T and both the elevational range and the realized thermal niche breadth of each species with Phylogenetic Generalized Least Squares (PGLS) models, hence accounting for phylogenetic non-independence among species. We introduced the latest Troglohyphantes phylogeny (Mammola et al., 2018a) into the PGLSs via a Pagel correlation structure, with the R package 'nlme' [START_REF] Pinheiro | nlme: Linear and Nonlinear Mixed Effects Models[END_REF]. Prior to this analysis, ∆T was log-transformed to account for one outlying observation (T. lucifer). We tested for differences in elevation range extent and realized thermal niche breadth between species belonging to different classes of subterranean specialization with ANOVA.

Accepted Article

. superficial cave sectors (Fig. 2). Therefore, we explored whether the results of our analysis would change if we computed ∆T using the cave temperature values at the time when the specimens were collected in the caves, rather than the mean annual cave temperaturte (Appendix S1 in Supporting Information).

Accepted Article

For all species tested, ∆T did not significantly varied depending on the sex of the specimens (ANOVA: F 18,152 = 0.31, p= 0.99). In the case of three species (T. lucifuga, T. vignai, T. konradi) for which we tested two populations, no differences in ∆T between populations (ANOVA: T. konradi: F 2,21 = 0.81, p= 0.46; T. lucifuga: F 2,10 = 3.45, p= 0.09; T.

vignai: F 2,24 = 1.65, p= 0.21) were detected. In the case of T. lucifer, most individuals were collected in one out of the three caves (Grotta del Ghiaccio di Bosconero), preventing to fit a robust ANOVA due to a significant imbalance in the levels of the factor. Also, we found no .

RESULTS

Thermal overlap for deep and shallow cave habitats

Variation in ambient temperature was extremely low in deep sectors of caves, with virtually no detectable daily variations and seasonal variations in the order of one tenth of degree.

We observed no overlap in thermal regimes between deep caves habitats at different elevations (Fig. 2B). By contrast, thermal regimes were more variable in the superficial cave sectors and there was a substantial overlap between thermal regimes among these habitats at different elevations (Fig. 2A).

Experimental test on thermal tolerance

A total of 247 specimens belonging to 11 species were collected in 13 caves across the Western Italian Alps. The species T. bolognai was excluded from the analysis, because high mortality of individuals occurred during the acclimation phase before the beginning of the test. All remaining species were successfully tested in the climatic chamber (Table 1).

Narrow thermal tolerance was observed for most Troglohyphantes (Fig. 3A), with the majority of species reaching their CT 50 at ∆T ranging from 1 to 4 °C (Table 1). T. bornensis and T. konradi proved to be the least tolerant species, reaching their CT 50 at ∆T of 1 °C, and its CT 100 at ∆T ranging from 3 and 5 °C, respectively. Conversely, T. lucifer was the most tolerant species, reaching its CT 50 at ∆T 19 °C and its CT 100 at 27 °C.

Accepted Article

. effects of sex and population on ∆T when we estimated ∆T using the cave temperature values at the time when the specimens were collected in the caves (Appendix S1).

Subterranean habitat specialization

Using morphological traits and a Ward2 hierarchical clustering method, we assigned each species to a distinct class of subterranean specialization. The Gap statistic revealed an optimal k of 3, corresponding to three discrete classes of species with similar levels of morphological subterranean specialization (Fig. 4). The first cluster included all individuals belonging to T. lucifer, T. lucifuga and T. pluto, namely species with minor morphological specialization to subterranean life (eyes normally developed and abdominal pattern present) and often found in the vicinity of the cave entrance or in shallow subterranean habitats ("Low" class of subterranean specialization in Fig. 4). The second cluster comprised species with a moderate degree of morphological specialization ("Intermediate" class in Fig. 4), and included most individuals of the species T. bornensis, T. iulianae, T. nigraerosae and T.

vignai. The third cluster comprised highly specialized species ("High" class in Fig. 4), namely T. konradi, T. lanai and T. pedemontanus, with pronounced eye and profile reduction, appendage elongation and absence of abdominal pigmentation. Clusters were mostly homogeneous, with the exception of six misplaced individuals highlighted in Fig. 4.

Relationship between thermal tolerance, specialization and elevational range

GLMM revealed a significant interaction between ∆T and the class of specialization, indicating that the shape of the survival curve varied according to the degree of subterranean habitat specialization (Fig. 3B; see also Fig. S1). We found a significant positive relationship between a species' elevational range and thermal tolerance (∆T), with increasing elevational range extents at increasing values of ∆T (Fig. 3C; PGLS: estimated β±SE= 930.06±184.66, p< 0.01) and no phylogenetic signal in the residuals (Pagel λ= -0.71). The extent of a species' elevational range decreased significantly with increasing specialization to deep subterranean life (Fig. 3D; ANOVA: F 2,8 = 5.47, p< 0.05). We recovered the same significant trend with respect to the realized thermal niche breadth, with increasing variation in temperature among localities of a species' range at increasing values of ∆T (Fig. 3E; PGLS: estimated β±SE= 4.02±0.92, p< 0.01; Pagel λ= -0.31). Furthermore, realized thermal niche breadth decreased significantly with increasing specialization to deep subterranean life (Fig. 3F; F 2,8 = 4.61, p< 0.05).

Accepted Article

. Our extension of Janzen's mountain passes hypothesis to temperate subterranean ecosystems is based on the integration of multiple lines of evidence and is grounded in organismal and habitat measures. Rather than inferring thermal tolerance breadth from species distributions (see [START_REF] Cardillo | The relationship between environmental niche breadth and geographic range size across plant species[END_REF], we quantified thermal traits via experimental tests on the widest simultaneous set of congeneric subterranean organisms to date (Fig. 3A). We also used annual temperature data series measured at the habitat level, that allowed us to characterize the steep gradients of thermal variability in caves (Fig.

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DISCUSSION

Extending Janzen's hypothesis to temperate subterranean ecosystems

Five decades after its publication, [START_REF] Janzen | Why mountain passes are higher in the tropics[END_REF] milestone contribution has been listed

among the 100 most influential papers in ecology [START_REF] Courchamp | 100 articles every ecologist should read[END_REF]. The mountain passes hypothesis is seminal not only in providing a mechanistic explanation for biodiversity patterns in tropical versus temperate mountain systems [START_REF] Ghalambor | Are mountain passes higher in the tropics? Janzen's hypothesis revisited[END_REF][START_REF] Sheldon | Fifty years of mountain passes: a perspective on Dan Janzen's classic article[END_REF], but also because it can potentially be transferred to a large variety of habitats displaying steep gradients of thermal seasonality, including for example soils [START_REF] Bahrndorff | The rapid cold hardening response of Collembola is influenced by thermal variability of the habitat[END_REF][START_REF] Raschmanová | Community composition and cold tolerance of soil Collembola in a collapse karst doline with strong microclimate inversion[END_REF], polar waters [START_REF] Peck | Extreme sensitivity of biological function to temperature in Antarctic marine species[END_REF][START_REF] Pörtner | Thermal limits and adaptation in marine Antarctic ectotherms: an integrative view[END_REF], and deep sea [START_REF] Rex | Globalscale latitudinal patterns of species diversity in the deep-sea benthos[END_REF]). Yet, the ecological complexity of most ecosystems and the challenge posed by the geographic scale at which this hypothesis can be tested, have largely prevented ecologists from comprehensively testing the underlying assumptions of Janzen's hypothesis and transferring it to non-tropical settings [START_REF] Gill | Cryptic species diversity reveals biogeographic support for the 'mountain passes are higher in the tropics' hypothesis[END_REF][START_REF] Mccain | Vertebrate range sizes indicate that mountains may be 'higher' in the tropics[END_REF][START_REF] Polato | Narrow thermal tolerance and low dispersal drive higher speciation in tropical mountains[END_REF][START_REF] Scheffers | Vertical (arboreality) and horizontal (dispersal) movement increase the resilience of vertebrates to climatic instability[END_REF]. Due to their ecological simplicity and environmental stability, subterranean habitats emerge as an ideal model system in which to explore eco-evolutionary questions in general [START_REF] Mammola | Finding answers in the dark: caves as models in ecology fifty years after Poulson and White[END_REF][START_REF] Sánchez-Fernández | The deep subterranean environment as a potential model system in ecological, biogeographical and evolutionary research[END_REF] and hypotheses pertaining to thermal niche breadth in particular [START_REF] Eme | Integrating phylogeography, physiology and habitat modelling to explore species range determinants[END_REF][START_REF] Mermillod-Blondin | Thermal tolerance breadths among groundwater crustaceans living in a thermally constant environment[END_REF][START_REF] Pallarés | Heat tolerance and acclimation capacity in unrelated subterranean arthropods living under common and stable thermal conditions[END_REF][START_REF] Raschmanová | Testing the climatic variability hypothesis in edaphic and subterranean Collembola (Hexapoda)[END_REF][START_REF] Rizzo | Lack of evolutionary adjustment to ambient temperature in highly specialized cave beetles[END_REF].

Accepted Article

PGLS analyses revealed a weak phylogenetic signal in our dataset, implying that two closely related Troglohyphantes species are not necessarily more physiologically similar than two Troglohyphantes species selected at random from the phylogenetic tree. This result .

2). An additional novelty of this study was to use a quantitative method supported by statistical inference to estimate the level of subterranean specialization of the different species (Fig. 4), and to incorporate this information in the evaluation of the thermal tolerance (Fig. 3B).

Thermal tolerance and subterranean specialization

By integrating morphological measures and physiological experiments, we were first able to demonstrate that an increased specialization to subterranean habitats was accompanied by a concomitant reduction in thermal tolerance. Overall, intermediate and highly subterranean specialized Troglohyphantes species reached CT 50 already between 1 and 4 °C above their natural ambient temperature, and CT 100 between 3 and 10 °C. Within the intermediate class of specialization, T. vignai was the only species to exhibit a greater thermal tolerance. In particular, the species behaved similarly to T. pluto, which clustered in the low specialized class (Fig. 4). In this regard, it is worth noting that populations of T. vignai display a high genetic structuring [START_REF] Mammola | Alpine endemic spiders shed light on the origin and evolution of subterranean species[END_REF], which casts some doubts about the current taxonomic status of this species and, consequently, about its thermal tolerance limits. The low specialized species T. lucifuga and T. lucifer, showed the widest thermal tolerances, reaching CT 50 at 7 and 19 °C above their natural ambient temperature, respectively, and CT 100 at 15 and 27 °C. In other words, species that are morphologically specialized for life in shallow subterranean habitats proved to be able to withstand a two-fold larger temperature increase than species morphologically specialized for life in deep subterranean habitats.

This result is consistent with the fact that the seasonal variability of shallow subterranean habitats is much greater than that of deep subterranean habitats [START_REF] Pipan | Temperature variation and the presence of troglobionts in terrestrial shallow subterranean habitats[END_REF].

Accepted Article

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probably owed much to our study's specificity which consisted in using a regional scale for testing Janzen's hypothesis. Using species from temperate and tropical regions, species within these two regions are more closely related to each other and they display similar physiological traits; hence the strong phylogenetic signal typically observed in a number of studies [START_REF] Gutiérrez-Pesquera | Testing the climate variability hypothesis in thermal tolerance limits of tropical and temperate tadpoles[END_REF]. Conversely, when using multiple congeneric species from the same region but specialized to different habitats-in our case, deep and shallow subterranean habitats-there is a greater chance that some species from the most climatically-variable habitat evolved species adapted to the more climatically-stable one.

This yields sister species exhibiting contrasted thermal tolerance, hence the weak phylogenetic signal observed in our study.

Subterranean specialization and elevational range extent

Having adapted to narrow thermal conditions (Fig. 2B), specialized subterranean organisms would have more difficulty in crossing topographic barriers than less specialized species, because they would be more likely to encounter temperatures outside their thermal tolerance [START_REF] Ghalambor | Are mountain passes higher in the tropics? Janzen's hypothesis revisited[END_REF]. In the alpine context in which Troglohyphantes species are found, subterranean dispersal over elevational gradients would be unlikely, in account for the significant variation in temperature and reduced overlap in thermal regime between deep cave habitats at different elevations (Fig. 2B; [START_REF] Rubel | The climate of the European Alps: Shift of very high resolution Köppen-Geiger climate zones 1800-2100[END_REF]. The small-range size of subterranean species has long been solely attributed to habitat fragmentation (e.g. [START_REF] Christman | The relationship between cave biodiversity and available habitat[END_REF][START_REF] Moldovan | Habitat fragmentation and its effects on groundwater populations[END_REF], without considering the potential influence of thermal specialization on dispersal propensity [START_REF] Eme | Integrating phylogeography, physiology and habitat modelling to explore species range determinants[END_REF]). Yet, this study is the first to establish a relationship between the extent of a subterranean species' range across the elevation gradient and its thermal tolerance. Habitat fragmentation and thermal specialization most likely act synergistically to constrain dispersal. Habitat

Accepted Article

Our extension of Janzen's hypothesis to subterranean habitat also provides a potential mechanism for explaining the high rate of local endemism and increased species packing that are typically observed in disparate subterranean systems (e.g. [START_REF] Hedin | High-stakes species delimitation in eyeless cave spiders (Cicurina, Dictynidae, Araneae) from central Texas[END_REF][START_REF] Njunjić | Comprehensive evolutionary analysis of the Anthroherpon radiation (Coleoptera, Leiodidae, Leptodirini)[END_REF][START_REF] Ribera | Ancient origin of a Western Mediterranean radiation of subterranean beetles[END_REF][START_REF] Wessel | Founder effects initiated rapid species radiation in Hawaiian cave planthoppers[END_REF]. It is acknowledged that strong topographic heterogeneity could both inflate speciation rate by increasing specialization and imposing barriers to dispersal, and decrease the extinction rate by promoting local survival during climate shifts [START_REF] Jetz | Geographic range size and determinants of avian species richness[END_REF][START_REF] Ohlemüller | The coincidence of climatic and species rarity: high risk to small-range species from climate change[END_REF][START_REF] Eme | Integrating phylogeography, physiology and habitat modelling to explore species range determinants[END_REF]. In the case of our model organisms, the specialization to a habitat with limited seasonal variability may have contributed to promote reproductive isolation, leading to high adaptive radiation in .

fragmentation restricts maladaptive gene flow between populations, thereby facilitating thermal specialization, which itself creates physiological barriers to dispersal.

A potentially important caveat of the present study is the use of range size as a surrogate for dispersal capacity [START_REF] Lester | The relationship between dispersal ability and geographic range size[END_REF]). Yet, there are several lines of evidence which suggest that dispersal within the genus Troglohyphantes indeed depends on the degree of subterranean specialization. In T. vignai, the only specialized species of Troglohyphantes for which a population genetic study was carried out, mitochondrial and nuclear genetic data at the population/species interface indicated very limited gene flow, with many populations being strongly isolated from each other [START_REF] Mammola | Alpine endemic spiders shed light on the origin and evolution of subterranean species[END_REF]. Furthermore, widely distributed species like T. lucifuga, but also other low subterranean specialized Troglohyphantes species distributed in central and eastern Alps, are typically found both in caves and more superficial habitats, including deep leaf litter strata, voids in rocky debris, and other moist and shaded retreats [START_REF] Isaia | The fate of clades in a world of recurrent climatic change: Milankovitch oscillations and evolution[END_REF]Mammola et al., 2018a[START_REF] Mammola | Climate change may drive cave spiders to extinction[END_REF]. This ecological plasticity would enhance dispersal across shallow habitats and would explain their larger distribution ranges [START_REF] Mammola | Alpine endemic spiders shed light on the origin and evolution of subterranean species[END_REF].

Conversely, specialized and short-ranged species of Troglohyphantes are almost exclusively collected in caves, empirically suggesting a reduced capacity to disperse through nonsubterranean habitats [START_REF] Isaia | The fate of clades in a world of recurrent climatic change: Milankovitch oscillations and evolution[END_REF].

Accepted Article

.

the genus and to a high proportion of small-range endemics, even co-occurring within limited geographic extents [START_REF] Isaia | The fate of clades in a world of recurrent climatic change: Milankovitch oscillations and evolution[END_REF]Mammola et al., 2018a). In the Western Alps as much as six closely related species are known to coexist in the district of Alpi Liguri, i.e. just about 3,000 km 2 (Fig. 1). Yet, the occurrence of non-overlapping thermal regimes among deep subterranean habitats at different elevations and the very narrow thermal tolerance of specialized subterranean species question the idea that sharp elevational climatic gradients may favor local survival through short distance dispersal during climate shifts.

Climatic controls on subterranean biodiversity patterns

Our findings contribute to reconcile contrasted views about the relative importance of two distinct scales of climate variability in shaping patterns of subterranean biodiversity [START_REF] Zagmajster | Geographic variation in range size and beta diversity of groundwater crustaceans: insights from habitats with low thermal seasonality[END_REF], a topic that has been recently discussed also in surface ecosystems [START_REF] Scheffers | Tropical mountain passes are out of reach -but not for arboreal species[END_REF][START_REF] Scheffers | Vertical (arboreality) and horizontal (dispersal) movement increase the resilience of vertebrates to climatic instability[END_REF]. Using correlative methods, [START_REF] Zagmajster | Geographic variation in range size and beta diversity of groundwater crustaceans: insights from habitats with low thermal seasonality[END_REF] and [START_REF] Eme | Do cryptic species matter in macroecology? Sequencing European groundwater crustaceans yields smaller ranges but does not challenge biodiversity determinants[END_REF] showed that long-term temperature variability accounted for a substantial variation in range size of groundwater crustaceans across

Europe. Yet, the lack of latitudinal variation in temperature seasonality in subterranean habitats does not necessarily imply that the observed pattern of increasing range at higher Troglohyphantes species occur in areas that were free of ice during the Pleistocene (Mammola et al., 2018b: p. 237, fig. 2), which is consistent with a scenario of restricted

Accepted Article

.

dispersal following the retreat of Pleistocene glaciers. On the other hand, the distribution range of low specialized species, especially T. lucifuga, stretches in areas that were entirely covered by the primary ice shield during the Pleistocene, which could be explained by the species having colonized the area in recent time, after the glaciers retreated [START_REF] Mammola | Climate change may drive cave spiders to extinction[END_REF].

Our study has also important implications for determining the vulnerability of subterranean species to anthropogenic climate change. Climate change is expected to affect deep subterranean habitats [START_REF] Mammola | Climate change going deep: the effects of global climatic alterations on cave ecosystems[END_REF][START_REF] Taylor | Ground water and climate change[END_REF], especially by increasing ambient temperature and reducing moisture content, although with a certain delay due to the thermal inertia of the rock [START_REF] Mammola | Climate change going deep: the effects of global climatic alterations on cave ecosystems[END_REF]. In this context, the likelihood of evolutionary responses will depend on the magnitude and rates of warming within subterranean habitats. To date, the few available studies attempting to predict the future response of subterranean species to altered climatic conditions were based on correlative species distribution models (SDM) projected onto future climate change scenarios [START_REF] Mammola | Applying species distribution models to caves and other subterranean habitats[END_REF]. However, such predictions may be unreliable if species display greater thermal tolerance than those reflected by their current geographic ranges [START_REF] Sánchez-Fernández | Thermal niche estimators and the capability of poor dispersal species to cope with climate change[END_REF]. Results of the present study suggest that the ecological specialization-dispersal tradeoff prevents specialized subterranean species from exploiting regions with uncertain climates, a proposal which is entirely consistent with previous SDM forecasts of habitat quality for Troglohyphantes spp. both in the Pleistocene and under future climate change scenarios [START_REF] Mammola | Climate change may drive cave spiders to extinction[END_REF]. By considering the results of this study in the light of these future habitat change projections, we can predict that most specialized subterranean species will unlikely be able to overcome climatic alterations by means of dispersal. However, we observed variability in the individual response to increasing temperature among the tested populations, with some individuals being able to withstand greater temperature increases in respect to the population average (Table 1). On a positive note, this variability suggests that more tolerant individuals may possibly undergo selection

Figure 1 .

 1 Figure 1. Distribution of Troglohyphantes spiders in the Western Italian Alps. For each species, localities and centroid of the distribution are reported. For visual presentation, the Minimum Convex Polygon (MCP) encompassing all localities of each species is drawn. Shades of gray in the background represent mean annual temperature, as derived from Bioclim 2 (Fick & Hijmans, 2017).

Figure 2 .

 2 Figure 2. Compared to shallow cave habitats (A), deep cave habitats show extremely reduced thermal seasonality and no overlap in thermal regimes across elevational gradient (B). Seasonal experimental temperature records shown in the graph were taken in winter (20 Nov 2012 -04 Feb 2013) across caves at different elevations in the Western Italian Alps, using HygrochronTM

Figure 3 .

 3 Figure 3. Thermal tolerance (∆T) of Western alpine Troglohyphantes species according to the class of subterranean specialization, and relations with elevational range extent and realized thermal niche breadth. A) Survival rate as a function of ∆T. Survival curves represent best fits to the data, according to GLMs. B) Predicted relationship between the survival rate of Troglohyphantes species and ∆T, in interaction with the class of morphological adaptation to the subterranean conditions, as derived from GLMM. C) Relationship between elevational range extent and ∆T, according to PGLS. D) Differences in the extent of elevational range among the three classes of morphological specialization to subterranean life. E) Relationship between the thermal breadth experienced by species across their elevational range and ∆T, according to PGLS. F) Differences in realized thermal breadth among classes of morphological specialization to subterranean life. Color coding in C-F refers to the legend in the inset B.

Figure 4 .

 4 Figure 4. Overall morphological similarity among the Troglohyphantes species considered in the analyses. Clustering is based on three morphological traits (bottom panel) and on Euclidean distances and Ward2's hierarchical clustering. Three distinct clusters correspond to the three classes of subterranean specialization: low (T. lucifuga, T. lucifer, T. pluto), intermediate (T. bornensis, T. nigraerosae, T. vignai), and high (T. konradi, T. lanai, T. pedemontanus) specialized species. Misplaced individuals are highlighted with the color of their putative cluster. Morphological drawings by Elena Pelizzoli.

  latitudes is primarily driven by climatic variability acting at much longer time-scales than the seasonal scale. If, as pointed out by the results of this study, the absence of thermal seasonality drives subterranean species towards narrow thermal specialization and low dispersal capacity, then these species are also less likely to survive major climate change and to recolonize vacant areas once climate becomes favorable again. The two temporal scales of climate variability might have acted in concert to shape the current distribution of Troglohyphantes species in the Western Alps. On the one hand, all specialized subterranean

Table 1 .

 1 Overview of the species of Troglohyphantes tested and summary statistics. Cave [cadastre code] (Coordinates)= sampling locality, cadastre code in square brackets ("Art." denotes artificial sites) and approximate geographic coordinates in WGS84 decimal degrees; CaveT= mean annual temperature of the sampling locality; Spec.= subterranean specialization according to the cluster analysis; n= number of specimens tested (controls are included in the count); CT 50/100 = temperature at which 50% or 100% individuals experienced total paralysis; ∆T 50/100 = temperature at which 50% or 100% individuals experienced total paralysis relative to the mean annual cave temperature at which the specimens were sampled.

		Cave
						CT			∆T
	Species	[cadastre code]	CaveT Spec.	n	mean±sd	CT50 CT100	mean±sd	∆T50 ∆T100
		(Coordinates)				(range)			(range)
		Grotte della Maissa
		[Pi 1214, 1215, 1219]	8.5
	T. konradi Brignoli	(44.26 N, 7.40 E)		High	32	10.69±1.76 (8-15)	11	15	1.92±1.23 (1-5)	1	5
		Sotterranei di Vernante [Art.]
			9*
		(44.25 N, 7.52 E)
		Grotta delle Arenarie
	T. lanai Isaia & Pantini	[Pi 2509]	9.2	High	14	10.02±0.75 (9.2-11.2)	11	12	1.82±0.75 (1-3)	2	3
		(45.71 N, 8.31 E)
		Grotta di Bossea
	T. pedemontanus Gozo	[Pi 108]	8.9	High	15	11.88±0.96 (10.5-13.5)	13	14	3.38±0.96 (2-5)	4	5
		(44.24 N, 7.84 E)
		Grotte di Pugnetto
	T. bornensis Isaia & Pantini	[Pi 1501-1504]	8.7	Interm. 11	9.44±0.73 (9-11)	9	11	0.94±0.73 (0.5-2.5)	1	3
		(45.27 N, 7.14 E)

* Internal mean annual temperature estimated from the closest weather station

  Conversely, the survival curve for low specialized Troglohyphantes declined more smoothly (Low: estimated β±SE 2.60±0.84).

The high and intermediate class curves were significantly different from the low one (GLMM: High: estimated β±SE= -1.43±0.19, p-value< 0.01; Intermediate: estimated β±SE= -0.68±0.07, p< 0.01), indicating a Accepted Article . steep decline in survival rate at increasing ∆T.

ACKNOWLEDGEMENTS

This work was funded by University of Turin and Compagnia di San Paolo (Grant Award: CSTO162355). FM was supported by grants from the French programs STYGOMICS (CNRS Défi Enviromics), CONVERGENOMICS (no. ANR-15-CE32-0005) and EUR H2O'Lyon (ANR-17-EURE-0018). A special thanks goes to Alexandra Jones for assistance in the laboratory and to Andrea John Dejanaz for carrying out morphological measures; to Filippo Milano, Nicolò Chiappetta and Raffaella Zerbetto for fieldwork assistance; Yael Lubin for proof-reading our English and useful comments; and two anonymous reviewers for their constructive comments.

Data accessibility statement: Data supporting this study are available in Figshare (doi:10.6084/m9.figshare.8058302) [the link will become available upon acceptance]

Author contributions statement:

SM, MI and EP conceived the idea and designed methodology; .

of key physiological traits in a climate change scenario, resulting in phenomena of local adaptation and in situ persistence.

Concerns over the fate of biodiversity in a changing climate have revived biodiversity research into Janzen's thoughts [START_REF] Sheldon | Fifty years of mountain passes: a perspective on Dan Janzen's classic article[END_REF]. By extending Janzen's hypothesis to climatically-stable habitats other than tropical mountains, we pave the way for integrating the ecological specialization/dispersal tradeoff into understanding the differential responses of species to climate instability across a broad range of ecosystems. Ultimately, our contribution shows that the "move or adapt" dilemma in response to climate change [START_REF] Nogués-Bravo | Cracking the code of biodiversity responses to past climate change[END_REF] may in part already be set by the specialization/dispersal tradeoff. Therefore, we encourage scientists to take into consideration physiological barriers to dispersal into their modelling fitting exercises, when predicting the species response to climate change.

Accepted Article

. All authors contributed critically to the drafts and gave final approval for publication.

Disclosure statement:

The authors report no conflicts of interest.

SUPPORTING INFORMATION

Additional supporting information may be found in the online version of this article. Appendix S1. Alternative results using seasonal cave temperature values to estimate ∆T.


Botein Florence Robert-Gangneux

Olgica Djurković-Djaković
email: olgicadj@imi.bg.ac.rs

Serological and molecular screening of umbilical cord blood for Toxoplasma gondii infection

Serological and molecular screening of umbilical cord blood for Toxoplasma gondii infection; a reply to Botein et al

Florence Robert-Gangneux, Olgica Djurković-Djaković

We have read the paper by Botein et al. [START_REF] Botein | Serological and molecular screening of umbilical cord blood for Toxoplasma gondii infection[END_REF] (https://doi.org/10.1111/tid.13117) on the screening of umbilical cord blood for Toxoplasma gondii to prevent transplantationtransmitted infections. Indeed, as explained by the authors, there could be a risk of disseminated infection in the recipient if the cord blood (CB) contains circulating Toxoplasma parasites, an occurrence which is possible only if the fetus has been congenitally infected. Congenital infection occurs when the mother acquires primary infection during pregnancy. Considering a 30% seroprevalence of Toxoplasma gondii infection in Egypt, [START_REF] Ibrahim | Molecular and serological prevalence of Toxoplasma gondii in pregnant women and sheep in Egypt[END_REF] and a risk for acquiring toxoplasmosis during pregnancy previously estimated to be about 1% in a population of roughly similar prevalence, [START_REF] Berger | Toxoplasmosis among pregnant women in France: risk factors and change of prevalence between 1995 and 2003[END_REF] along with an overall transmission rate to the fetus of 30%, one would expect to observe 0.21 seroconversions in the 100 women included in this study. Of note, the sensitivity of PCR on cord blood is not 100%, even in the absence of specific treatment of the mother, [START_REF] Sterkers | Diagnosis of congenital toxoplasmosis by polymerase chain reaction on neonatal peripheral blood[END_REF] thus the number of cases detected by PCR would be even lower. In contrast to this estimation drawn from well-established data, Botein et al. found a positive PCR in 11 CB samples suggesting a seroconversion rate of about 52% among women who were seronegative at the beginning of pregnancy. Although it can be argued that Egyptian women may have a greater risk of infection due to poorer knowledge regarding hygienic measures to prevent toxoplasmosis, this seroconversion rate is simply unrealistic.

Overall, these findings raise suspicion of possible contamination, as frequently observed with nested PCR methods, which are therefore in no way a reference molecular diagnostic method.

Accepted Article

. Another point is the interpretation of the CB serological results. The detection of anti-Toxoplasma IgG in CB reflects the presence of specific IgG in maternal serum, meaning that the mother is either immunized (chronically infected) or has seroconverted during pregnancy, so it is poorly informative regarding the risk of fetal infection. By contrast, the detection of specific IgM, which are not transferred through the placental barrier, indicates fetal production of specific IgM, and therefore fetal infection. The presence of anti-Toxoplasma IgM alone in CB is rare, as it would imply that the fetus has been infected simultaneously with the mother shortly before birth (no passive transfer of maternal IgG, as they would not yet be synthesized by the mother), and unrealistic to occur in 6%. Specific IgM detection is in fact one of the biological tools to diagnose congenital toxoplasmosis at birth. However, the sensitivity of IgM detection ranges from 45% to 85%. [START_REF] Robert-Gangneux | Epidemiology of and diagnostic strategies for toxoplasmosis[END_REF] Consequently, the absence of IgM in CB cannot exclude newborn infection, thus there is indeed a risk of having circulating parasites in IgM-negative CB from infected neonates. In this setting, maternal serological screening at delivery would be more appropriate to exclude CB potentially risky for recipients, i.e. CB from mothers who acquired toxoplasmosis during pregnancy and of whom 30% may have transmitted Toxoplasma parasites to their fetus. As mentioned by the authors, specific anti-Toxoplasma IgM can be detected months after infection. In our experience, more than 95% of women who seroconverted in early pregnancy still present with IgM at delivery (unpublished data). Provided that the sensitivity of ELISA is high, and the test is designed for diagnostic purposes, the approach to screen mothers, and to exclude those with Toxoplasma-specific IgM antibodies from CB donation, would be simple and more appropriate than a nested PCR.

  Olgica Djurkovic-Djakovic, MD/PhD National Reference Laboratory for Toxoplasmosis Centre of Excellence for Food-and Vector-borne Zoonoses Institute for Medical Research, University of Belgrade Dr. Subotica 4 P.O.Box 102 11129 Belgrade, Serbia To the Editor,

Authors Contribution statement

FRG and ODjDj both conceived the letter. FRG drafted the letter and ODjDj revised it, and they both approved the final version.


Alice Baudouin

Sylvain Gatti

Florence Levréro

Céline Genton

Romane H Cristescu

Vincent Billy

Peggy Motsch

Jean-Sébastien Pierre

Pascaline Le Gouar

Nelly Ménard
email: nelly.menard@univ-rennes1.fr

Disease avoidance, and breeding group age and size condition the dispersal patterns of western lowland gorilla females

Keywords: skin disease, social dispersal, female emigration and immigration, breeding male quality, social environment, polygynous mammal, gorillas

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Male-biased dispersal and philopatry of females are the most common patterns in social mammal species [START_REF] Greenwood | Mating systems, philopatry and dispersal in birds and mammals[END_REF]. However, in polygynous species structured in multifemale groups defended by one male [e.g. most social equids, several tropical bats, several primate species, (Clutton-Brock and Lukas 2012)], adult females can disperse several times throughout their lifetime [START_REF] Handley | Advances in our understanding of mammalian sex-biased dispersal[END_REF]. The social dispersal of a groupliving female involves a change of social partners and mates (Clutton-Brock and Lukas 2012).

Social dispersal, similarly to locational dispersal, plays a major role in individual fitness as well as in the genetics and dynamics of the overall population [START_REF] Greenwood | Mating systems, philopatry and dispersal in birds and mammals[END_REF][START_REF] Clobert | Dispersal[END_REF].

Proximal forces influencing social dispersal are intrinsically linked to the major risks and benefits of group living. The major proximal forces described as potential drivers of female social dispersal are inbreeding avoidance, reduction of predation risk, reduction of feeding and intra-sexual competition, selection of a higher-quality male (Sterck et al. 2005, Clutton-Brock and[START_REF] Clutton-Brock | The evolution of social philopatry and dispersal in female mammals[END_REF], and reduction of pathogen transmission through behavioral avoidance of diseased conspecifics [START_REF] Curtis | Infection-avoidance behaviour in humans and other animals[END_REF][START_REF] Sarabian | Hygienic tendencies correlate with low geohelminth infection in free-ranging macaques[END_REF][START_REF] Poirotte | Mandrills use olfaction to socially avoid parasitized conspecifics[END_REF]. Social dispersal implies first that females decide whether to remain in a social group or leave it, and then that they choose the target group into which to immigrate [START_REF] Isbell | Differential costs of locational and social dispersal and their consequences for female group-living primates[END_REF]VanVuren 1996, Debeffe et al. 2015). These decisions require gathering information on other social groups and suggest a social and fitness cost-benefit analysis [START_REF] Isbell | Differential costs of locational and social dispersal and their consequences for female group-living primates[END_REF]VanVuren 1996, Clobert et al. 2009). Studies on wildlife and humans show that individuals attempt to avoid disease transmission using external signs of disease on conspecifics such as behavioral changes [START_REF] Croft | Effect of gyrodactylid ectoparasites on host behaviour and social network structure in guppies Poecilia reticulata[END_REF], chemical cues (Behringer et al. 2006, Kavaliers andCholeris 2018), or visual sickness symptoms [START_REF] Axelsson | Identification of acutely sick people and facial cues of sickness[END_REF]. In group-living species, females

Accepted Article

. may choose smaller social groups despite increased predation risks [START_REF] Côté | Parasitism and group-size in social animals -A metanalysis[END_REF] and choose healthy mates (Altizer et al. 2003, Tybur and[START_REF] Tybur | Mate preferences and infectious disease: theoretical considerations and evidence in humans[END_REF] to limit the risk of inter-individual disease transmission. However, among the range of behaviors reducing the risk of disease transmission [for review see e.g. [START_REF] Hart | Behavioral adaptations to pathogens and parasites -5 strategies[END_REF])], the extent to which disease avoidance conditions the choice of social and/or mating partners and promotes dispersal has remained poorly investigated in social species [START_REF] Behringer | Avoidance of disease by social lobsters[END_REF][START_REF] Debeffe | Parasite abundance contributes to conditiondependent dispersal in a wild population of large herbivore[END_REF], Kavaliers and Choleris 2018).

Disease avoidance behavior may be difficult to identify because individual dispersal decisions likely result from compromises involving other (and potentially confounding) factors such as body condition and reproductive status, food or sexual competition, predation and infanticide risks, or availability of a high-quality male [START_REF] Clobert | Dispersal[END_REF][START_REF] Curtis | Infection-avoidance behaviour in humans and other animals[END_REF].

For instance, being in a good condition (e.g. high body fat, no injury or disease) allows an individual to disperse since it is better equipped to bear dispersal costs [START_REF] Debeffe | Condition-dependent natal dispersal in a large herbivore: heavier animals show a greater propensity to disperse and travel further[END_REF], whereas diseased or parasitized individuals may be too weak for this task [START_REF] Debeffe | Parasite abundance contributes to conditiondependent dispersal in a wild population of large herbivore[END_REF]. Alternatively, heavily diseased or parasitized individuals might be more prone to disperse in a 'leave-it' strategy as a last chance attempt to improve their situation [START_REF] Isbell | Differential costs of locational and social dispersal and their consequences for female group-living primates[END_REF]VanVuren 1996, Debeffe et al. 2014). Pregnancy, lactation, or the presence of unweaned thus dependent offspring can reduce the likelihood of female dispersal [START_REF] Sterck | Female dispersal, inbreeding avoidance and mate choice in Thomas langurs (Presbytis thomasi)[END_REF][START_REF] Robbins | Dispersal patterns of females in the genus Gorilla[END_REF][START_REF] Sicotte | The influence of male takeovers on female dispersal in Colobus vellerosus[END_REF]. Consequently, nulliparous adult females (i.e. that have not yet reproduced) may have more opportunities to disperse than parous females and may disperse several times before their first pregnancy [START_REF] Linklater | Social dispersal but with philopatry reveals incest avoidance in a polygynous ungulate[END_REF]Cameron 2009, Robbins et al. 2009). In addition, females may disperse from large groups to smaller ones (Clutton-Brock and Lukas 2012, [START_REF] Marjamaeki | Local density and group size interacts with age and sex to determine direction and rate of social dispersal in a polygynous mammal[END_REF], thereby limiting food or intra-sexual competition [START_REF] Stokes | Female dispersal and reproductive success in wild western lowland gorillas (Gorilla gorilla gorilla)[END_REF][START_REF] Sicotte | The influence of male takeovers on female dispersal in Colobus vellerosus[END_REF].

Accepted Article

. We examined the extent to which disease avoidance is involved in the dispersal decisions of western lowland gorilla adult females, after controlling the potential influence of other factors such as the reproductive status of females, intra-group competition, and adult male reproductive and protective value. We used a long-term dataset on the immigration to and emigration from known social groups of individually identified gorillas some bearing yawslike skin lesions. Yaws is a disease caused by a Treponema species described in humans [START_REF] Mitjà | Yaws[END_REF] and in non-human primates (Cousins 2007, Giacani and[START_REF] Giacani | The endemic treponematoses[END_REF].

The consequences of yaws can be debilitating, the worst cases lead to deformities and severe handicaps [START_REF] Levréro | Yaws disease in a wild gorilla population and its impact on the reproductive status of males[END_REF][START_REF] Mitjà | Yaws[END_REF]. Early stages of the disease are regarded as infectious and can be determined visually according to presence of red, ulcerative, puritic granulomatous papillomas (Engelkens et al. 1991[START_REF] Antal | The endemic treponematoses[END_REF]. This disease spreads mainly through skin contact between individuals (Engelkens et al. 1991, Giacani and[START_REF] Giacani | The endemic treponematoses[END_REF]. Lesions are apparent and mostly facial, and therefore may serve as visible cues to avoid diseased conspecifics.

Gorillas are polygynous; their breeding groups are composed of a dominant adult male called a silverback and several adult females with their offspring. Less than 60% of silverbacks lead breeding groups, while the others are unmated males [START_REF] Gatti | Population and group structure of western lowland gorillas (Gorilla gorilla gorilla) at Lokoué, Republic of Congo[END_REF]). The reproductive and protective abilities of the leader silverback are believed to decline at the end of his breeding-group tenure, which lasts up to 12 years [START_REF] Breuer | Variance in the male reproductive success of western gorillas: acquiring females is just the beginning[END_REF]. A previous study on one western lowland gorilla population showed that females with an unweaned infant avoided emigration, possibly to limit infanticide risks from silverbacks [START_REF] Stokes | Female dispersal and reproductive success in wild western lowland gorillas (Gorilla gorilla gorilla)[END_REF].

Moreover, after their natal dispersal, females experienced up to three secondary dispersal events in their life and preferred transferring into smaller groups [START_REF] Stokes | Female dispersal and reproductive success in wild western lowland gorillas (Gorilla gorilla gorilla)[END_REF]. We focused our study on secondary dispersal, excluding cases of natal emigration that are predominantly linked to incest avoidance.

Accepted Article

.

We tested three hypotheses of potential influences of disease avoidance in female dispersal at emigration and immigration steps of their dispersal process, encompassing relevant factors of the socio-sexual environment of females and their disease status. First, females should leave diseased silverbacks for silverbacks in good health. Second, females should emigrate from, and avoid immigrating into, breeding groups with numerous diseased individuals.

Finally, diseased females should disperse less than healthy ones. Simultaneously, we controlled for the possible influence of 1) the presence of an unweaned infant and the females' reproductive status; 2) intra-group competition, using group size as a proxy; 3) silverbacks' reproductive and protective abilities, using group composition, e.g. the number of offspring of several age-classes as a proxy of silverback age. The presence of only the infant age-class indeed indicates early breeding-group tenure by a young fully mature silverback that is assumed to provide better protection against infanticide and predation than an older male at the end of his tenure.

Materials and methods

Study sites and populations

We collected demographic data from two different gorilla populations [Lokoué,(00°54'23''N,15°10'33''E) and Romani (00°41'28"N; 14°53'31"E)] in the Odzala-Kokoua National Park (Republic of Congo). We monitored the Lokoué population from 2001 to 2003 and the Romani population over 11 years (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015). Each gorilla was identified and its sex and age-class were determined according to physical characteristics and behavior with the help of a database of photographs and videos (for details on study periods and observation methods see Appendix S1 : Table S1). We differentiated nulliparous females, who have never been observed with an infant and have non-elongated nipples and undeveloped breasts, from

Accepted Article

. parous females. We distinguished individuals that displayed "severe" skin lesions from the others according to the presence of red, ulcerative and/or granulomatous papillomas characteristic of yaws' infectious stages and irreversible lesions such as tissue necrosis and bone deformations (Engelkens et al. 1991, Giacani and[START_REF] Giacani | The endemic treponematoses[END_REF]. Among the 593 individuals identified, the overall prevalence was around 22% while 13.3% were categorized as "severely diseased" (i.e. showing severe skin lesions, for description of the disease and prevalence per population and year, see Appendix S2: Fig. S1, Table S1). We identified each of the 109 different gorilla units (59 breeding groups; 50 unmated units; for details on samples per population and year, see Appendix S3: Table S1) and we recorded its composition. According to the "ageing" process of breeding groups (BG) subsequently to their formation (Appendix S4), we classified them into four stages : young BG

Dispersal status of adult females

At each observation of breeding groups we recorded changes, if any, in adult females. We then categorized each adult female according to her dispersal status in a given year as follows: i) resident female: an adult female that has remained in the same breeding group; ii) emigrating female: an adult female that has left her breeding group; iii) immigrating female: a new adult female in a group (for information on gorilla social organization and dispersal patterns see Appendix S5). No female was observed as immigrant and emigrant the same year. We used the term "transfer event" when we knew both the original and target groups of the dispersing female. Females not observed again after emigrating were considered as having

Accepted Article

. emigrated out of the population. We could not differentiate them from those who died, but none of them showed evidence of bad body conditions. Females not known within the population before they immigrated were considered to have immigrated from a neighboring population.

Analyses of dispersal patterns

To test our hypotheses on emigration and immigration patterns, we built two respective datasets (DataS1). We assumed no significant difference in habitat quality among gorilla units because they live in the same dense Marantaceae forest and their home ranges overlap (see Appendix S1). Since we had no a priori hypotheses on differences between the two populations, data were pooled. The emigration dataset included the female-related response variable (i.e. each female was either an emigrant or a resident per year, named "female-year" observation, with a binomial distribution weighted by the total number of adult females within the group), and 9 explanatory variables (i.e. the characteristics of the adult female and the characteristics of her breeding group, see details in Fig. 1). We recorded 513 female-year observations. Resident females made up 469 female-years while 44 emigration events occurred (for details per population and year, see Appendix S3: Table S1). The immigration dataset included the gorilla unit-related response variable (i.e. either chosen or not by the immigrant female, named "unit-year" observation), and the 8 explanatory variables (i.e., the characteristics of the gorilla units, see details in Fig. 1). We analyzed immigration patterns with a demographic data set of 56 different breeding groups and 54 non-breeding units with unmated silverbacks (either solitary or living in non-breeding groups). Totals of 97 different silverbacks either solitary or breeding group leaders, were identified. We recorded 313 unityear observations, out of which 68 unit-years had female immigration (for details on samples per population and year, see Appendix S3: Table S1).

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. We used a generalized linear mixed model [GLMM, binomial family, logit link, glmer function, lme4 R-package, [START_REF] Bates | Fitting linear mixed-effects models using lme4[END_REF]] that considered population, group or female identity and year as random factors, to analyze each of our two datasets. Redundant explanatory variables were detected using canonical coefficients and were not considered in the same model during the model selection procedure (Appendix S6, Table S1). We used a model selection with an information-theoretic framework and Akaike Information Criterion adjusted for small sample sizes [AICc, [START_REF] Burnham | Model selection and multimodel inference: a practical information-theoretic approach[END_REF]]. No interaction was tested in the selection model procedure. Marginal and conditional R² of the selected models (ΔAICc < 2) were computed. We estimated standardized coefficients of the selected variables and their 95% confidence intervals using a conditional model averaging procedure. The significance of variables was determined when the 95% confidence interval did not include zero. Technical details on the statistical approach are described in Appendix S6: Tables S1-S4.

Concerning immigration patterns, our GLMM analysis was designed to reveal differences in female choice among unit characteristics. However, it did not account for differences in unit type availability. Therefore, to assess if the choice of unit made by immigrating females differed from a random choice, we used a Manly standardized selection ratio [widesIII function, adehabitatHS R package [START_REF] Manly | Resource selection by animals: statistical design and analysis for field studies[END_REF]

]: . . 1 i i n ij j j u W u    
Where W i is the selection ratio for the population (n animals) for the ith resource category, u i. is the number of type i resource units used by all animals, πij is the known proportion of resources available to animal j that are in category i, u .j is the resource units used by the jth animal.

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In our analysis, the individuals are identified and the use and availability of units are measured for each one. We tested the significance of the observed distribution of the probability under a null hypothesis of a random choice using the log-likelihood statistic [START_REF] Manly | Resource selection by animals: statistical design and analysis for field studies[END_REF] for each of the following resource categories: silverbacks' skin disease status, number of severely diseased individuals in breeding groups, gorilla unit category, breeding group size, the infant-to-adult female ratio. Finally, the differences in the characteristics of the two units involved in female transfers were tested with Wilcoxon paired and chi-squared tests.

Results

Factors influencing the female emigration probability

Three plausible models best explained the probability of female emigration (conditional R² ranging from 0.46 to 0.53, Appendix S6: Table S3). They indicate that the probability for a female to emigrate increased with the number of severely diseased individuals (0.33 [0.09 -0.58] 95%CI, 2 on the 10 best models Appendix S6, Fig. 2a), with the presence of severe skin lesions on the silverback (1.53 [0.42 -2.63] 95%CI, 4 on the 10 best models Appendix S6, Fig. 2b), and with the age of the breeding group, females leaving more senescent groups (1.62

[0.55 -2.69] 95%CI, 4 on the 10 best models Appendix S6, Fig. 2c) than young ones. By contrast, the probability for a female to emigrate decreased with the presence of an unweaned infant (-2.89 [-4.10 --1.67] 95%CI, 10 on the 10 best models Appendix S6, Fig. 2d, see Appendix S7 for complementary results). The skin disease status of females had nonsignificant impact on their probability to emigrate (-0.66 [-1.75 -0.43] 95%CI, 3 on the 10 best models Appendix S6).

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Factors influencing female immigration and target unit selection

We never observed a female immigrating with her infant. Among the 68 immigration events, 59 targeted breeding groups, nine females joined unmated silverbacks, mostly solitary males but one who was in a non-breeding group. The probabilities for a female to immigrate into a young group was ten times higher than the probability to join a solitary male (-3. Concerning the influence of young breeding group characteristics (the most selected group category) on female immigration, two plausible models were selected amongst all possible ones (conditional R² respectively of 0.82 and 0.83, Appendix S6: Table S4). They indicate that the probability for females to immigrate into a young breeding group decreased with the presence of severely diseased individuals (-2.44 [-5.69 --0.81] 95%CI, 3 on the 8 models Appendix S6, Fig. 3b), and with the size of the group (-0.43 [-0.72 --0.14] 95%CI, 2 on the 8 models Appendix S6, Fig. 3c).

Manly's index analysis [START_REF] Manly | Resource selection by animals: statistical design and analysis for field studies[END_REF] strengthened the importance of the factors selected with GLMM. Most notably, females significantly avoided groups with more than two individuals showing severe signs of skin disease (Fig. 4a). In addition, females significantly selected young breeding groups whereas they avoided mature and senescent groups and nonbreeding units; i.e. unmated silverbacks (Fig. 4b). They significantly selected groups composed of 2 to 4 individuals while they avoided groups composed of 9 and more individuals (Fig. 4c). Females preferred breeding groups without infants, which predominantly characterized young groups, to groups where all females had an unweaned infant (Fig. 4d).

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. This analysis confirmed that the silverback's skin disease had no significant effect on female choice.

Transfer characteristics

We recorded 16 transfer events concerning 15 females for which we knew both the original and target groups of the dispersing female. The number of severely infected individuals in the target groups was significantly lower than in the original groups (0.43 vs. 1.6, paired Wilcoxon's test, P = 0.02). This result remained relevant when correcting by group size (average ratio of infected individuals in original groups three times higher than in target groups ; 0.15 vs 0.05, paired Wilcoxon's test, P = 0.06). The original breeding groups of transferring females were on average larger than the size of their target groups (9.6 vs. 5.9, paired Wilcoxon's test, P = 0.0171). Most transfers (63%, N = 16) involved females emigrating from senescent or mature groups, and no female transferred into a target group older than their original group.

Discussion

We confirmed that the presence of an unweaned infant appears as a predominant constraint that limits females' secondary social dispersal in gorillas [START_REF] Sicotte | Female mate choice in mountain gorillas[END_REF][START_REF] Stokes | Female dispersal and reproductive success in wild western lowland gorillas (Gorilla gorilla gorilla)[END_REF].

Under normal conditions, delaying dispersal until their offspring is weaned is considered as a female strategy against infanticide risks [START_REF] Robbins | Impact of male infanticide on the social structure of mountain gorillas[END_REF]). On the contrary, the death of the leading silverback induces involuntary transfers of mother-infant pairs and increase the probability of infanticide [START_REF] Robbins | Impact of male infanticide on the social structure of mountain gorillas[END_REF]. However, in western lowland gorillas, infanticide is considered to be less common than in mountain gorillas (Yamagiwa et al. 2009, Robbins and[START_REF] Robbins | Dispersal patterns of females in the genus Gorilla[END_REF]: in at least two different populations, mother-infant pairs involuntarily transferred into new social groups where the infants survived although the leader silverbacks were likely not their fathers [START_REF] Stokes | Female dispersal and reproductive success in wild western lowland gorillas (Gorilla gorilla gorilla)[END_REF][START_REF] Genton | Recovery potential of a Western lowland gorilla population following a major Ebola outbreak: results from a ten year study[END_REF].

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Influence of severe skin lesions

We found that silverbacks' disease status influenced emigration females' decision but not their immigration decision. A previous study showed that unmated males (i.e. solitaries and silverbacks in non-breeding units) suffered more from skin diseases than mated males (i.e.

silverbacks in breeding groups) in the Lokoué population, suggesting that females avoided mating with diseased silverbacks [START_REF] Levréro | Yaws disease in a wild gorilla population and its impact on the reproductive status of males[END_REF]. Our results suggested that females leave infected silverbacks, leading silverback to lose their mating status. However, other factors such as silverback age and body size [START_REF] Caillaud | Influence of male morphology on male mating status and behavior during interunit encounters in western lowland gorillas[END_REF], previous interactions between the silverback and the female, or relatedness, are potentially involved in female choices. Further studies are needed to determine the role of these factors on females' mate choice in gorilla populations.

In gorillas, joining a new mated male means joining his breeding group. We found that the presence of skin disease in breeding groups determined the dispersal decision of females.

Females showed a greater propensity to emigrate from breeding groups that contained numerous severely diseased individuals. Similarly, immigrating females avoided groups with more than two diseased individuals. Several studies conducted in captive conditions showed that primates, including gorillas, could have a relative numerousness judgment; e.g. gorillas are able to discriminate between two food quantities, selecting the larger one [START_REF] Anderson | Relative numerousness judgment and summation in young and old Western lowland gorillas[END_REF]. Therefore, these abilities may also be expressed in the wild when it is crucial to avoid disease. Severely diseased individuals are expected to increase the risk of contamination by inter-individual transmission [START_REF] Giacani | The endemic treponematoses[END_REF]. Hence, by avoiding contact with diseased individuals and leaving their group, female gorillas decrease the risk of being contaminated, as observed in other species [START_REF] Kiesecker | Behavioral reduction of infection risk[END_REF][START_REF] Behringer | Avoidance of disease by social lobsters[END_REF][START_REF] Vanpé | Immune gene variability influences roe deer natal dispersal[END_REF]. Other behavioral social avoidance of contagious individuals by healthy ones has rarely been found in the wild [START_REF] Behringer | Avoidance of disease by social lobsters[END_REF][START_REF] Poirotte | Mandrills use olfaction to socially avoid parasitized conspecifics[END_REF]. Mandrills

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(Mandrillus sphinx) are able to socially exclude conspecifics infected by endoparasites by avoiding grooming with them [START_REF] Poirotte | Mandrills use olfaction to socially avoid parasitized conspecifics[END_REF]. Grooming increased with a decreasing endoparasite load, revealing that healthy individuals avoided the risk of infection. By contrast, no disease avoidance has been found in other social species [Mungos mungo, [START_REF] Fairbanks | No evidence for avoidance of visibly diseased conspecifics in the highly social banded mongoose (Mungos mungo)[END_REF]]. Interestingly, these authors suggested that in this highly social species that lives in stable, long-term groups where group members frequently interact, individuals could hardly avoid diseased conspecifics. Therefore, this underlined the potential influence of the social systems on individuals' strategies to avoid pathogen contamination. In gorillas, although breeding groups may be stable all along the silverback tenure [up to 12 years, [START_REF] Breuer | Variance in the male reproductive success of western gorillas: acquiring females is just the beginning[END_REF]], females without an unweaned infant may prevent disease infection by conspecifics during this tenure by deciding to emigrate and choosing the unit they immigrate into.

Chemical mechanisms can be involved in the detection of diseased conspecifics, with cues in the environment [START_REF] Kiesecker | Behavioral reduction of infection risk[END_REF][START_REF] Behringer | Avoidance of disease by social lobsters[END_REF] or in the feces [START_REF] Poirotte | Mandrills use olfaction to socially avoid parasitized conspecifics[END_REF]). In the case of skin disease in gorillas, we suppose that the intensity of skin lesions represented relevant visual cues of infection risks, similarly to humans who detect sick persons on the basis of visual symptoms [START_REF] Axelsson | Identification of acutely sick people and facial cues of sickness[END_REF]. In particular, in humans as in gorillas, the face concentrates many traits used in inter-individual communication and is predominantly scanned by conspecifics as compared to the whole body [START_REF] Kano | Face and eye scanning in gorillas (Gorilla gorilla), orangutans (Pongo abelii), and humans (Homo sapiens): unique eye-viewing patterns in humans among hominids[END_REF]). An abnormal aspect of the face can be used to appraise the health status of other individuals [START_REF] Regenbogen | Behavioral and neural correlates to multisensory detection of sick humans[END_REF]. We suppose that in gorillas, behavioral signals, comparable to disgust in humans, can function as a disease-avoidance mechanism that could be acquired throughout individual development and into adulthood [START_REF] Oaten | Disgust as a disease-avoidance mechanism[END_REF][START_REF] Curtis | Infection-avoidance behaviour in humans and other animals[END_REF]. The use of social information on infection status of conspecifics plays an important role in disease avoidance (Kavaliers and Choleris 2018). Mechanisms of social cognition encompass assessing conspecifics disease status and decision on interaction or not with them (Kavaliers

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. and Choleris 2018). Gorillas could acquire, from other group members, a behavioral aversion towards individuals severely infected by skin diseases. In addition, owing to their long maturing period in social groups, they probably learn signals of disease risks from known diseased conspecifics by associating physical cues of infection and deleterious consequences (such as difficulty in breathing and eating due to tissue necrosis and perforation of the palate and nasal septum, difficulty in moving due to irreversible bone lesions).

Contrary to our hypothesis, the skin disease status of the females themselves did not influence their propensity to disperse. Dispersal is generally considered to induce energy and time costs, increase mortality risks, and reduce the advantage of being in a familiar social unit or habitat [loss of social rank or selection of a suboptimal environment [START_REF] Bonte | Costs of dispersal[END_REF]].

Therefore, the individual body condition is a factor involved in the decision to disperse [START_REF] Debeffe | Condition-dependent natal dispersal in a large herbivore: heavier animals show a greater propensity to disperse and travel further[END_REF], although studies yielded contrasting results depending on the environmental context [START_REF] Linklater | Social dispersal but with philopatry reveals incest avoidance in a polygynous ungulate[END_REF][START_REF] Loe | No evidence of juvenile body mass affecting dispersal in male red deer[END_REF][START_REF] Galliard | Patterns and processes of dispersal behaviour in arvicoline rodents[END_REF].

The disease status of individuals is assumed to influence their body condition and thus their dispersal, but studies investigating the effects of disease on the dispersion probability remain scarce [START_REF] Brown | Ectoparasitism as a cause of natal dispersal in cliff swallows[END_REF][START_REF] Vanvuren | Ectoparasites, fitness, and social behaviour of yellow-bellied marmots[END_REF][START_REF] Debeffe | Parasite abundance contributes to conditiondependent dispersal in a wild population of large herbivore[END_REF]. Roe deer (Capreolus capreolus) highly parasitized by endoparasites delayed their natal dispersal to accumulate sufficient body condition to sustain locational dispersal [START_REF] Debeffe | Parasite abundance contributes to conditiondependent dispersal in a wild population of large herbivore[END_REF]. By contrast, yellow-bellied dispersing marmots (Marmota flaviventris) tended to host higher ectoparasite loads [START_REF] Vanvuren | Ectoparasites, fitness, and social behaviour of yellow-bellied marmots[END_REF], and similarly heavily parasitized cliff swallow (Hirundo pyrrhonota) nestlings dispersed more than slightly parasitized ones [START_REF] Brown | Ectoparasitism as a cause of natal dispersal in cliff swallows[END_REF]. In our study, individual gorillas weakened by skin disease supported the cost of social dispersal because the impact on their body condition was not deleterious enough to prevent their dispersal. Alternatively, as in other dispersing primates, gorillas may take advantage of interunit encounters to switch groups without adding the cost of a risky solitary period (Isbell

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. [START_REF] Vanvuren | Ectoparasites, fitness, and social behaviour of yellow-bellied marmots[END_REF]. In our studied populations, neither a potential influence of habitat quality nor an interaction with the level of infection in units on dispersal female decision could be evoked. This is because of i) the study sites are mostly covered by the dense Marantaceae forests providing ample staple foods (mainly Haumania sp.), and ii) the overlapping of all units' home ranges. The social environment may therefore be a more important driver of female dispersal decision than the ecological environment.

Females' preference for young mated males

Gorilla females showed a propensity to leave older breeding groups to preferentially immigrate into young breeding groups rather than join unmated silverbacks. These patterns underlie the formation and ageing processes of breeding groups [START_REF] Parnell | Group size and structure in western lowland gorillas (Gorilla gorilla gorilla) at Mbeli Bai, Republic of Congo[END_REF]. In most mammal species, young mature males are supposed to have reduced access to mates because of a reduced capacity to protect females (Clutton-Brock and McAuliffe 2009). For example, northern elephant seal (Mirounga angustirostris) females avoid mating with males that are unable to provide protection and call on older dominant males to get rid of the younger ones that harass them [START_REF] Mcmahon | Harem choice and breeding experience of female southern elephant seals influence offspring survival[END_REF]. Comparatively, in our study, unmated silverbacks were possibly of less reproductive and protective value because they were either too young to form their own breeding group or old post-reproductive silverbacks. Females preferentially transferred toward younger breeding groups, i.e. with fully mature males at the peak of their physical abilities. Moreover, among those young groups, females preferred the smallest ones, possibly limiting competition for access to resources or to male proximity. By contrast, they avoided the mature or senescent breeding groups led by older silverbacks as compared to young breeding groups. Indeed, according to the classical group' life history, a silverback may form his breeding group once he is around 15-18 years old (i.e. a young breeding group), then he is aging during his subsequent tenure that can last up to 12 years while his group is maturing then becoming senescent [START_REF] Breuer | Variance in the male reproductive success of western gorillas: acquiring females is just the beginning[END_REF][START_REF] Genton | Recovery potential of a Western lowland gorilla population following a major Ebola outbreak: results from a ten year study[END_REF]).

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Females have a marked preference for breeding groups without any infant, therefore in the early phase of male tenure (i.e. no female having yet given birth). This preference reflects the strong attractiveness of those males, which is thought to result in fast female recruitment once a first female has joined a solitary silverback.

Conclusion & Perspectives

Dispersal of gorilla females results from a multifactorial decision based on the characteristics of females, silverbacks, and social groups. Our study highlights that the social environment of adult females, and in particular the presence of diseased individuals, determines their decision to disperse. The mechanisms of females' appraisal of their social environment and of the disease status of their conspecifics remain to be investigated.

Dispersal of individuals in a disease context is often linked to an increase of disease transmission and spread [START_REF] Nunn | Emerging infectious diseases and animal social systems[END_REF]. If infected individuals decide to immigrate into groups with low prevalence of the disease, and lacking resistance from social conspecifics against immigrants, they might increase the transmission rate, to the benefit of the spread of the pathogen. However, uninfected individuals may limit the risk of infection by avoiding groups with a large number of infected individuals, which will preserve host population viability. Disease avoidance behaviors by hosts influence disease dynamics and spread by reducing the intensity and duration of disease outbreaks and viral prevalence [START_REF] Dolan | Host behavior alters spiny lobsterviral disease dynamics: a simulation study[END_REF]. Integrating disease avoidance behaviors in epidemiological models would allow us to better understand host-pathogen co-evolutive dynamics.

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  [one silverback + adult females with infants (≤ 4 years old)]; juvenile BG [one silverback + adult females and offspring up to juveniles (4-7.5 years old)], mature BG [one silverback + adult females and offspring up to sub-adults (7.5-10 years old for females, 7.5-11 years old for males)], senescent BG [one silverback, adult females and several cohorts of offspring from infants to blackbacks (11-14 years old)].

  60 [- 5.60 --1.62] 95%CI), or to immigrate into juvenile(-2.77 [-4.97 --0.58] 95%CI) or senescent groups(-4.60 [-8.83 --0.38] 95%CI), Fig.3a). No female chose mature groups. The silverbacks' skin disease status had no effect on female immigration (0.08 [-1.69 -1.95] 95%CI).

Figure legends Figure 1 .

 legends1 Figure legends

Figure 2 .

 2 Figure 2. Predicted probability for adult females to emigrate from a breeding group related to

Figure 3 .

 3 Figure 3. Predicted probability for adult females to immigrate into a gorilla unit related to a)

Figure 4 .

 4 Figure 4. Manly's selection indices testing the preferences of immigrating females for

Acknowledgments

We thank the team of the ECOFAC program (EU): C. Aveling, B. Djoni-Djimbi, P. Ngouembe, J-M. Froment, P. Marshall, P. Montuir, P. Mortier, P. Rickmounie, T. Smith, J. de Winter, and African Parks Networks: D. Zeller, L. Lamprecht, P. Darcis, and G. Le Flohic for

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