S Périquet

H Fritz

E Revilla

D Macdonald

A Loveridge

G Mtare

Périquet

Fritz

AJ Loveridge

& Valeix

Dynamic interactions between

Keywords: African lion, carnivore intraguild interactions, Crocuta crocuta, Panthera leo, spotted hyaena

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INTRODUCTION

Apex predators, through their effects on the abundance, distribution and behaviour of their prey and mesopredators, have a profound influence on the functioning of communities and ecosystems [START_REF] Ritchie | Predator interactions, mesopredator release and biodiversity conservation[END_REF][START_REF] Ripple | Status and Ecological Effects of the World's Largest Carnivores[END_REF]. In most ecosystems, several species of apex predators coexist. Understanding the functioning of their guild is key if we want to grasp the full impact of the changes in the abundance and distribution of apex predator populations that occur worldwide [START_REF] Ripple | Status and Ecological Effects of the World's Largest Carnivores[END_REF][START_REF] Chapron | Recovery of large carnivores in Europe's modern human-dominated landscapes[END_REF]. Intraguild interactions amongst predators are diverse. Negative interactions have been the most studied, with predators not only competing through depletion of common prey but also posing threats to each other through kleptoparasitism [START_REF] Höner | The response of spotted hyaenas to long-term changes in prey populations: functional response and interspecific kleptoparasitism[END_REF][START_REF] Trinkel | Competitive interactions between spotted hyenas and lions in the Etosha National Park, Namibia[END_REF] and direct interference during aggressive interactions with risk of injury [START_REF] Linnell | Interference interactions, co-existence and conservation of mammalian carnivores[END_REF] and intraguild predation [START_REF] Palomares | Interspecific killing among mammalian carnivores[END_REF]. These negative interactions can cause subordinate species to spatially and/or temporally avoid dominant ones [START_REF] Durant | Living with the enemy: avoidance of hyenas and lions by cheetahs in the Serengeti[END_REF][START_REF] Vanak | Moving to stay in place: behavioral mechanisms for coexistence of African large carnivores[END_REF], similarly to the ecology of fear in predator-prey interactions [START_REF] Courbin | Multi-trophic resource selection function enlightens the behavioural game between wolves and their prey[END_REF][START_REF] Palmer | A 'dynamic' landscape of fear: prey responses to spatiotemporal variations in predation risk across the lunar cycle[END_REF][START_REF] Kohl | Do prey select for vacant hunting domains to minimize a multi-predator threat?[END_REF]. However, this is not always the case, and evidence has accumulated on the absence of spatial and temporal avoidance between sympatric apex predators [START_REF] Cozzi | Fear of the dark or dinner by moonlight? Reduced temporal partitioning among Africa's large carnivores[END_REF][START_REF] Miller | Lions and leopards coexist without spatial, temporal or demographic effects of interspecific competition[END_REF]. This absence of spatio-temporal avoidance may be due to the fact that (i) avoidance mechanisms occur at very fine spatio-temporal scales [START_REF] Broekhuis | Risk avoidance in sympatric large carnivores: reactive or predictive?[END_REF][START_REF] Du Preez | Impact of risk on animal behaviour and habitat transition probabilities[END_REF][START_REF] Swanson | In the absence of a "landscape of fear": How lions, hyenas, and cheetahs coexist[END_REF]) and often go undetected, (ii) dominance is dynamic with the outcome of an interaction depending on the relative group size of each of the competing species [START_REF] Cooper | Optimal hunting group size: the need for lions to defend their kills against loss to spotted hyaenas[END_REF], and (iii) coexisting apex predators also benefit from positive interactions between them. Positive effects of interactions have been less studied and underestimated in community ecology [START_REF] Bruno | Inclusion of facilitation into ecological theory[END_REF]. They range from scavenging opportunities (feeding from a carcass, [START_REF] Wilson | Scavenging: how carnivores and carrion structure communities[END_REF] and kleptoparasitism opportunities (displacing a competitor from its kill, [START_REF] Krofel | The noble cats and the big bad scavengers: effects of dominant scavengers on solitary predators[END_REF] to indirect facilitation whereby prey, through anti-predator behavioural adjustments to minimize the risk of predation by one predator species, become more vulnerable to another predator species. For example, elk (Cervus elpahus) shift to habitats that have a more complex structure to reduce predation risk from wolves (Canis lupus), which significantly increases predation risk from pumas (Puma concolor, [START_REF] Atwood | Spatial partitioning of predation risk in a multiple predator-multiple prey system[END_REF].

A pair of sympatric apex predators is not characterized by one type of interaction only, and different, sometimes contrasting, interactions may occur. First, different interactions may operate at different scales. [START_REF] Sivy | Fatal Attraction? Intraguild Facilitation and Suppression among Predators[END_REF] revealed negative responses of several carnivore species to wolf abundance at the landscape scale (overall lower occupancy probability in presence of wolves), but positive responses of the same species to localized wolf activity (increased detection probability of coyote, Canis latrans, closer to wolf GPS clusters). Second, the environmental conditions and the life history traits of the species may also determine the relative costs and benefits of engaging in one type of interaction, which is illustrated by the seasonal dynamics of some foraging tactics, such as scavenging [START_REF] Pereira | Facultative predation and scavenging by mammalian carnivores: seasonal, regional and intra-guild comparisons[END_REF]. Many studies inferring negative or positive interactions between sympatric apex predators have been conducted at the large scales of static interactions, focusing on the relative local abundance and temporal niche of the predators (e.g. [START_REF] Schuette | Occupancy patterns and niche partitioning within a diverse carnivore community exposed to anthropogenic pressures[END_REF][START_REF] Swanson | Cheetahs and wild dogs show contrasting patterns of suppression by lions[END_REF][START_REF] Sivy | Fatal Attraction? Intraguild Facilitation and Suppression among Predators[END_REF][START_REF] Miller | Lions and leopards coexist without spatial, temporal or demographic effects of interspecific competition[END_REF][START_REF] Rasphone | Temporal partitioning by felids, dholes and their potential prey in northern Laos[END_REF]). These large-scale studies can only focus on predictive responses to potential interactions, while fine-scale studies are necessary to understand the reactive responses to actual encounters between apex predators. Little is known on the fine-scale dynamics of inter-specific interactions due to the challenges of simultaneously monitoring large carnivores, but some pioneering research has started (for prey-predator relationships see [START_REF] Courbin | Reactive responses of zebras to lion encounters shape their predator-prey space game at large scale[END_REF]; for apex predators intra-specific interactions see [START_REF] Jordan | Dynamics of direct inter-pack encounters in endangered African wild dogs[END_REF][START_REF] Broekhuis | Using GPS collars to investigate the frequency and behavioural outcomes of intraspecific interactions among carnivores: A case study of male cheetahs in the Maasai Mara, Kenya[END_REF]; for apex predator interspecific interactions see du [START_REF] Du Preez | Impact of risk on animal behaviour and habitat transition probabilities[END_REF]. The simultaneous GPS-collaring of sympatric apex predators can provide information on the direction and speed of a carnivore's movement with respect to the other carnivore involved in the interaction. GPS-tracking has therefore the potential to reveal which species is attracted to or avoid one another, and under which circumstances.

In African savannas, African lions (Panthera leo) and spotted hyaenas (Crocuta crocuta) are the two most common large predators and evidence has accumulated on the richness of their interactions. These were reviewed by Périquet et al. (2015a) and it was shown that the coexistence of spotted hyaenas (hyaenas hereafter) and lions is a complex balance between negative (exploitative competition for prey, kleptoparasitism, aggressive interactions and direct killing) and positive interactions (scavenging opportunities). Périquet et al. (2015a) highlighted the need for the study of the spatio-temporal dynamics of these interactions. Here, we used data from GPS-collars deployed simultaneously on lions and hyaenas to (i) identify the encounters between the two species based on the proximity of simultaneous GPS locations, (ii) assess which species provoked the encounter by approaching the other one, (iii) determine which species was dominant during the encounter and triggered the displacement of the other species, and (iv) identify when one species attempted to avoid the encounter by moving away from the other one. We further evaluated the environmental conditions in which encounters occurred.

First, waterholes act as prey hotspots in arid and semi-arid savannas [START_REF] Valeix | Behavioral adjustments of African herbivores to predation risk by lions: Spatiotemporal variations influence habitat use[END_REF][START_REF] Valeix | Temporal dynamics of dry-season water-hole use by large African herbivores in two years of contrasting rainfall in Hwange National Park, Zimbabwe[END_REF]), so we expected encounters to occur mainly close to waterholes. Second, African arid and semi-arid savannas are characterized by contrasting (wet and dry) seasons. Large herbivore body condition is highest in the wet season, when plant productivity peaks, and decreases as the dry season progresses [START_REF] Rn | Adaptive Herbivore Ecology[END_REF]. Hyaenas are cursorial hunters, i.e. chase down their prey over long distance [START_REF] Kruuk | The Spotted Hyena: A Study of Predation and Social Behavior[END_REF][START_REF] Mills | Kalahari hyaenas: Comparative behavioural ecology of two species[END_REF], and are therefore more likely to kill weak individual prey. Their hunting success is thus likely to be lower in the wet season when prey body condition is high. These seasonal variations in prey vulnerability to predation provided us with a good opportunity to assess the temporal dynamics of hyaena-lion interactions.

Lions are the main cause of mortality for adult hyaenas and are expected to be dominant at most encounters, especially when an adult male is present (reviewed in Périquet et al. 2015a).

Hence, we expected hyaenas to generally avoid encounters with lions and flee from an encounter when it occurs, except in the wet season when prey are scattered through the landscape and in good condition and therefore more difficult to kill and when hyaenas should actively search for kleptoparasitism/scavenging opportunities from lion kills.

MATERIAL AND METHODS

Study area

Hwange National Park (HNP) is located in north-western Zimbabwe (19°00'S, 26°30'E) and covers an area of approximately 14,600 km 2 . The study area (≈ 1,500 km 2 ) is located in the northern part of HNP, in the Main Camp area (Appendix S1: Fig. S1), and is characterized by Kalahari sandy soils. The vegetation is primarily bushed woodland and bushland savanna, interspersed with small patches of grassland [START_REF] Arraut | The 2013-2014 vegetation structure map of Hwange National Park, Zimbabwe, produced using free satellite images and software[END_REF]. HNP is semi-arid (mean annual rainfall=600 mm, coefficient of variation=25%) with a wet season from November to April and a dry season from May to October [START_REF] Chamaillé-Jammes | Spatial patterns of the NDVI-rainfall relationship at the seasonal and interannual time scales in an African savanna[END_REF]. Availability of surface water is primarily from rainwater collected in natural depressions. However, most of these natural depressions do not hold water during the dry season, when water is artificially pumped in about 50 waterholes spread throughout HNP (Appendix S1: Fig. S1). Hyaena density in the study area is 11.0 hyaenas/100 km 2 (Unpubl. data) and lion density is 4.3 lions/100 km 2 [START_REF] Loveridge | Conservation of large predator populations: Demographic and spatial responses of African lions to the intensity of trophy hunting[END_REF]. Wild dogs (Lycaon pictus), cheetahs (Acinonyx jubatus) and leopards (P. pardus) are also present in the study area but less abundant.

Predator GPS data

From July 2009 to October 2013, nine adult hyaenas (eight females and one male) belonging to four different clans were equipped with GPS collars (see Périquet 2014 for details). Hyaenas spent most of their time foraging alone (Périquet et al. 2015b) and their fission-fusion dynamics precluded from assessing group sizes during interactions. During the same period, 27 adult lions (20 males and 7 females) were also equipped with GPS collars (see Benhamou et al. 2014 for details). At least one male lion per coalition and one female lion per pride were collared. As lionesses from the same pride and males from the same coalition spend most of their time together in the study area [START_REF] Valeix | Behavioral adjustments of African herbivores to predation risk by lions: Spatiotemporal variations influence habitat use[END_REF], we assumed that individual locations generally represent those of their pride and coalition. Relevant animal care protocols were followed during capture and collaring of carnivores (Wildlife Drugs Sub-committee of the Drugs Control Council of Zimbabwe and Zimbabwe Veterinary Association, Wildlife Group, and licenses to acquire, possess and administer game capture drugs/dangerous drugs), and permissions were provided by the appropriate agencies (Research Council of Zimbabwe, Zimbabwe Parks and Wildlife Management Authority). All applicable institutional and/or national guidelines for the care and use of animals were followed. Collars were programmed to take hourly fixes during the night (between 6 pm and 6 am). Lion collars were not set up to record GPS locations at the same time as hyaena collars and the frequency of locations varied from one location every hour to one location every 2h at night (12 individuals with a collar set up to record a GPS location every 2h).

A prerequisite for two animals to encounter each other is that their space use overlaps. We preliminarily quantified the static interactions between hyaenas and lions using home range overlap metrics. Using a 2D measure of home range overlap, we found that on average 97.9 ± 0.5 % (range = 85.4-100%) of a given hyaena home range was overlapping with at least one lion home range (see Appendix S2 for details).

Proximity index and encounter definition

Proximal fixes were used to identify and define the encounters between hyaenas and lions. We defined proximal fixes as simultaneous hyaena and lion GPS locations within 200m of each other. This threshold is conservative compared to other studies (500m in [START_REF] Broekhuis | Using GPS collars to investigate the frequency and behavioural outcomes of intraspecific interactions among carnivores: A case study of male cheetahs in the Maasai Mara, Kenya[END_REF] or [START_REF] Courbin | Reactive responses of zebras to lion encounters shape their predator-prey space game at large scale[END_REF]800m in Jordan et al. 2017) as we wanted to be sure that both species were aware of each other's presence. For each hyaena-lion pair with overlapping home ranges, we computed the proximity index as the proportion of the number of proximal fixes over the total number of simultaneous fixes for the pair [START_REF] Long | A critical examination of indices of dynamic interaction for wildlife telemetry studies[END_REF]. This proximity index is a measure of how often hyaenas encounter lions in the landscape. In order to quantify if hyaena and lions encountered each other more often than expected by chance, we generated proximity indices by randomly creating new GPS locations for hyaenas but keeping the lion locations unchanged. These random locations were generated in a similar way as in a Step Selection Function framework [START_REF] Fortin | Wolves influence elk movements: behavior shapes a trophic cascade in Yellowstone National Park[END_REF]. Each hyaena used location was set as the starting point from which we created 100 steps (a step is the straight line movement between two successive GPS locations) by randomly drawing step length and turning angle from their respective empirical distributions obtained by pooling step data from all other hyaenas, as recommended by [START_REF] Fortin | Wolves influence elk movements: behavior shapes a trophic cascade in Yellowstone National Park[END_REF]. In case of a hyaena GPS location falling within 200 m of several lion GPS locations, we used the distance to the closest lion location.

Movement of the given hyaena-lion pair during each night containing at least one pair of proximal fixes was plotted and visually examined. We categorized whether an encounter occurred at a large prey carcass (> 120 kg) that could provide scavenging opportunities. We thus defined encounters occurring in the presence of a carcass when proximal fixes were located near a known carcass (found in the field during systematic hyaena GPS cluster searches or opportunistically) or near a large and clearly defined lion cluster (>5h within a 200 m radius). Previous work using GPS clusters to investigate potential lion kill sites used cluster of 2 GPS locations within 100m of each other (e.g. [START_REF] Tambling | Feasibility of using proximity tags to locate female lion Panthera leo kills[END_REF][START_REF] Tambling | Estimating carnivoran diets using a combination of carcass observations and scats from GPS clusters[END_REF][START_REF] Davidson | Seasonal Diet and Prey Preference of the African Lion in a Waterhole-Driven Semi-Arid Savanna[END_REF]), but since we did not investigate these clusters in the field and lions hardly ever rest for more than 5h in the same location at night [START_REF] Schaller | The Serengeti lion-A Study of Predator-Prey Relations[END_REF] we feel confident that clusters identified using our definition represent carcasses. When there was no clear GPS cluster near the proximal fixes, we assumed that the encounter occurred in the absence of a carcass. While we acknowledge that with this classification, small carcasses would be missed, these are unlikely to provide scavenging opportunities. In the presence of small and loose clusters (multiple GPS locations not tightly grouped together and scattered within a radius of ~100 m), we classified the encounter as undefined in terms of carcass presence. Depending on the situation, multiple pairs of proximal fixes could be assigned to the same encounter (for instance, proximal fixes occurring in the vicinity of a given carcass).

Encounter characteristics

Seasonality -Each encounter was attributed to the wet or dry season based on the date when it occurred. We used a Chi 2 test to test for the effects of the presence of a carcass on the seasonality of encounters.

Distance to water -Because waterholes play an important role in the spatial ecology of both lions [START_REF] Valeix | How key habitat features influence large terrestrial carnivore movements: waterholes and African lions in a semi-arid savanna of north-western Zimbabwe[END_REF]) and hyaenas [START_REF] Périquet | Sharing the top: How do Spotted hyaenas cope with Lions? Apex predator coexistence in a wooded savanna[END_REF] in HNP, we computed the distance between the first proximal fixes and the nearest pumped waterhole (distance to water hereafter) for each encounter. We tested the effect of distance to water on the probability of encounter using Generalized Linear Models (GLMs) with a binomial error structure with encounter (coded 0/1) as a the dependent variable and distance to water as an explanatory variable. We used separated models for encounter in the presence and absence of a carcass, irrespective of the season. For each model, 500 locations with no encounter were drawn randomly from the hyaena GPS dataset only including nights during which no encounter between hyaenas and lions was identified.

Duration -The duration of an encounter in the presence of a carcass was defined as the time elapsed between the first and the last proximal fixes still associated with the given carcass (≤200m), even if they were separated by several hours and interspersed with some nonproximal fixes (it is indeed common for carnivores to visit a carcass over several consecutive days, [START_REF] Cozzi | Effects of Trophy Hunting Leftovers on the Ranging Behaviour of Large Carnivores: A Case Study on Spotted Hyenas[END_REF]. We extracted the GPS data of the two species from the night of the first proximal fixes to the night of the last proximal fixes for subsequent analyses. The duration of an encounter in the absence of a carcass was calculated from the first to the last pair of consecutive proximal fixes. Since all encounters away from carcasses were restricted to one night, we extracted the GPS data of the two species for the night when the encounter occurred for subsequent analyses. Because of high heterogeneity in the duration between encounter in the presence and absence of a carcass, we used a Wilcoxon rank sum test to determine the effect of carcass presence on encounter duration.

Which species provoked the encounter?

For all encounters (presence and absence of a carcass), we computed the speed (in m/h) of movement during the hour preceding the start of the encounter for both species. Speed was only computed if the last GPS location was taken a maximum of 2 h before the start of the encounter.

For encounters in the presence of a carcass, we further performed a visual examination of the movement of the two species before the start of the encounter (GPS locations preceding the first pair of proximal fixes but belonging to the same night) to identify which species was the first at the carcass (≤ 200 m). It is noteworthy that the species arriving first at a carcass did not necessarily kill the prey. For encounters in the absence of a carcass, we also visually examined the movement of each species before the encounter and defined two situations: (1) one species was in the vicinity (≤ 200 m) of the encounter location while the other one moved towards the encounter location, in which case the species moving towards the encounter location was considered as attracted to the location of the other species, or (2) both species showed clear movement before the encounter. In this case, we calculated, for each species, an attraction score for each step of the 4 hours preceding the encounter.

The attraction score was based on the direction of the step compared to the simultaneous step made by the other species (see Fig. 1a for details). The attraction score for a step could take 5 values: +1 if the step of species A was directed toward the step of species B ± 10° (case of "strong attraction"); +0.5 if the step of species A was directed toward the step of species B ± 30° -but excluding cases of strong attraction -(case of "mild attraction"); -1 when the step of species A was directed opposite the step of species B (± 10°) (case of "strong avoidance"); -0.5 when the step of species A was directed opposite the step of species B ± 30° -but excluding cases of strong avoidance -(case of "mild avoidance"); and 0 in any other case ("independent movement"). Finally, we summed the scores of the steps of the 4 hours preceding the encounter to calculate an overall attraction score for each species (see Fig. 1b for an example). Based on these overall scores, we established if one species was clearly attracted to the location of the other species and therefore likely to provoke the encounter (positive overall score while the other species had a null or negative overall score) or avoiding the location of the other species (negative overall score while the other species had a null or positive overall score). Only GPS locations separated by a maximum of 2 h and only encounters before which at least two steps could be computed were taken into account. In addition, to be sure that hyaenas and lions could have detected each other's presence, we only considered steps for which both starting and ending points were located within 2 km of the other species.

Which species was dominant over the encounter?

For encounters in the presence of a carcass, if the species present at the carcass moved away (further than 200 m from the carcass) in the 2 hours following the arrival of the second species, the latter was considered dominant. However, if the species at the carcass was not displaced, and the arriving species spent less than 2h in the vicinity of the carcass (≤200 m) before moving away (even if it came back to the carcass at a later stage during the encounter), the former was considered as dominant. Staying at the carcass after the end of the encounter (if the first arriving species was not displaced) was not considered as an indication of dominance. Indeed, while a carcass can initially provide a lot of food (assuming it was killed or discovered in good condition), its profitability decreases with time. Thus, a species leaving the carcass might do so when it is not profitable enough to stay longer. For each encounter, we established whether at least one adult male lion was present as the presence of a male lion has often been reported as the determinant of the outcome of aggressive interactions between lions and hyaenas [START_REF] Cooper | Optimal hunting group size: the need for lions to defend their kills against loss to spotted hyaenas[END_REF][START_REF] Höner | The response of spotted hyaenas to long-term changes in prey populations: functional response and interspecific kleptoparasitism[END_REF]. At the time of the study, all male coalitions had at least one individual collared in the study area. We further computed the speed of movement during the hour following the end of the encounter for both species. Speed was only computed if the first GPS location was taken maximum 2h after the end of the encounter.

To test for differences in speed and distance to the carcass between the two species, we used linear mixed effect models including speed or distance to the carcass as the dependent variable, species as the explanatory variable, and carcass identity as a random intercept. Both speed and distance to carcass were log transformed to meet normality assumptions.

For encounters in the absence of a carcass, we used the attraction score approach described above for the 4h following the end of the encounter, to assess which species avoided the other one after the encounter. When there were insufficient data to characterize the movement of the two species for 4h after an encounter, we used speed during the hour immediately following the encounter to determine if one species was clearly moving away from the other one. In this case, we considered a ratio of hyaena speed to lion speed above 2 to be indicative of a behaviour of lion avoidance by hyaenas and of less than 0.5 to be indicative of a behaviour of hyaena avoidance by lions.

We tested for a significant difference in the speed at which lions and hyaenas moved before and after an encounter depending on the presence of a carcass using a linear model including the interaction between species and carcass presence (coded as 0/1) as explanatory variables and speed as dependent variable. Speed was log transformed to meet the model assumptions.

All data extraction and analyses were conducted using the R software (R Core Team 2020, version 3.5.2) with the packages adehabitaHR (version 0.4.15), adehabitatLT (version 0.3.23, Calenge 2006), lmer (version 3.0-1, Bates 2010). Values are provided as the mean ± standard error (SE) unless mentioned otherwise.

RESULTS

Proximity index and encounter definition

Proximity index -We found a total of 108 proximal fixes out of 170,120 simultaneous fixes in our dataset, which resulted in an average proximity index for a given hyaena of 0.0011± 0.0023 (range=0-0.0107, n=90). The average of the randomly generated proximity index was lower (0.0005±0.00001, range=0-0.0076, Fig. 2) suggesting that hyaenas and lions came into contact more often than expected by chance.

Encounter definition -Based on visual examination of hyaena and lion movements around proximal fixes, we defined 70 encounters between the two predators. Twenty-eight of them occurred in the presence of a carcass and 35 occurred in the absence of a carcass. For seven encounters, the presence or absence of a carcass could not be established with certainty and their characteristics are not discussed hereafter (see Appendix S3 for a complete list of encounters and their characteristics).

Encounter characteristics

Seasonality -More than half of the encounters in the presence of a carcass occurred during the wet season (57.1%, n=12; Fig. 3a,Appendix S3). Most encounters in the absence of a carcass occurred during the dry season (77.1%, n=27; Fig. 3a,Appendix S3). Encounters in the presence of a carcass were significantly more likely to occur during the wet season ( 2 =6.4, p=0.01).

Distance to water -In the presence of a carcass, we found no effect of the distance to water on the probability of encounter (p=0.64), suggesting that these were evenly distributed in the landscape. However, encounters in the absence of a carcass were more likely to occur as the distance to water decreased (z value=-0.36, p=0.0003), see distribution of the encounters depending on the distance to water in Fig. 3b).

Duration -Excluding the six encounters characterized by one pair of proximal fixes only (i.e. for which we could not estimate duration), encounters in the presence of a carcass lasted on average 40±9h ranging from 1 to 162h (nearly seven days, Fig. 3c, see also Appendix S4: Fig. S2 for examples of encounters). For encounters in the absence of a carcass, 80 % of the 35 encounters were characterized by only one pair of proximal fixes (Fig. 3c, see also Appendix S4: Fig. S4 for examples of encounters) and they were significantly shorter than encounters in the presence of a carcass (1.4±0.2h, range: 1-6h, W=2088, p=0.004).

Which species provoked the encounter?

Encounters in the presence of a carcass -Of the 28 encounters at a carcass, lions were the first at the carcass in 71% of the cases (n=20, Fig. 4). The few cases (n=7) when hyaenas arrived first corresponded to situations with only lionesses present. In the hour preceding the encounter, hyaenas moved significantly faster (1,011±230m/h, n=27) than lions (580±200 m/h, n=27, t=3.12, p=0.002, Appendix S4: Fig. S1).

Encounters in the absence of a carcass -A male lion was present in 34% (n=12) of the encounter in the absence of a carcass. When movement could be characterized before the encounter (n=26), hyaenas appeared to be attracted to lions (42% of the encounters) or to ignore them, but they never seemed to avoid them (Fig. 4, Appendix S3: Table S2). In contrast, lions showed movement characteristics of avoidance before the encounter in 11.5% of no effect of the presence of a carcass on hyaena movement speed after an encounter (t=0.11, p=0.90, Appendix S4: Fig. S1 andS3).

DISCUSSION

The analysis of simultaneous GPS locations of hyaenas and lions showed that these two apex predator species did not avoid encountering each other. Rather, they encountered each other more often than expected by chance. Encounters between lions and hyaenas may occur through either active mechanisms of attraction between the two predators or passive mechanisms whereby the two predators are attracted to the same locations in the landscape.

Wet season: time for intraguild facilitation

During the wet season, lions and hyaenas encountered each other mainly at carcasses and throughout the landscape (i.e. independently of water sources). As we expected, hyaenas arrived at carcasses after the lions, and travelled far and fast to reach the carcasses. Lions killing prey and feeding on a carcass are indeed likely to be heard (noises of high-speed chase, struggle, and prey distress call) and smelt (blood and stomach content odour) by hyaenas from quite a distance, and hyaena intraspecific communication may also play a role resulting in hyaena moving up to 5 km in the hour preceding its arrival to a carcass that had just been killed by lions. Further, lions were dominant over the access to carcasses, and hyaenas did not stay near the carcasses while the lions were present (Appendix S5). However, hyaenas came back regularly to the carcasses for up to one week after their first arrival. This suggests that hyaenas regularly checked whether lions were still at the carcass and fed on the carcass only after the lions had gone. Hyaenas are particularly well-equipped to make use of carcass parts usually disregarded by other species [START_REF] Kruuk | The Spotted Hyena: A Study of Predation and Social Behavior[END_REF], and are therefore able to benefit from whatever was left by lions. Our results showed that hyaenas hardly ever displaced lions from a carcass. This is consistent with result from an experiment conducted in South Africa showing that interactions occur at large carcasses over which lions maintain preferential access [START_REF] Amorós | Hyaenas and lions: how the largest African carnivores interact at carcasses[END_REF]. Previous studies showed that the likelihood of hyaenas wining an encounter with lions depends on the group sizes of the two species (with hyaenas likely to take over a kill when they outnumber lions by a factor of 4 -Cooper 1991) and the presence of an adult male lion [START_REF] Kruuk | The Spotted Hyena: A Study of Predation and Social Behavior[END_REF][START_REF] Cooper | Optimal hunting group size: the need for lions to defend their kills against loss to spotted hyaenas[END_REF][START_REF] Trinkel | Competitive interactions between spotted hyenas and lions in the Etosha National Park, Namibia[END_REF]. We do not know the number of hyaenas and lions involved in the encounters mentioned in this study. However, male lions were present during most of these interactions. As the population of adult male lions increased after a moratorium on trophy hunting around HNP in 2005-2008[START_REF] Loveridge | Conservation of large predator populations: Demographic and spatial responses of African lions to the intensity of trophy hunting[END_REF], male lions are now more likely to be present and thus hyaenas less likely to win their encounters with lions. In our study, even though we cannot infer if the prey was killed or found dead by lions, we suggest that lions producing/finding a carcass provide extra scavenging opportunities for hyaenas by making the given carcass more easily located.

Carcasses are widely used resources across ecosystems, which have been overlooked for terrestrial mammals, and their ecological importance, along with that of scavenging, has recently gained increased recognition in community ecology [START_REF] Wilson | Scavenging: how carnivores and carrion structure communities[END_REF][START_REF] Pereira | Facultative predation and scavenging by mammalian carnivores: seasonal, regional and intra-guild comparisons[END_REF]. For instance, wolverine (Gulo gulo) take advantage of prey killed by wolf (Canis lupus) and lynx (Lynx lynx, van Dijk et al. 2008) and access to these carcasses, especially during winter, has a stabilizing effect on wolverine populations as well as on the whole food web [START_REF] Wilson | Scavenging: how carnivores and carrion structure communities[END_REF]. Altogether, our results illustrate that intraguild facilitation (through scavenging opportunities) may be an important process to explain the coexistence between lions and hyaenas. As expected based on the hunting mode of hyaenas, our results further suggest that hyaena scavenging from lion kills mainly occurs in the wet season. Seasonal variations in the prevalence of scavenging have already been highlighted [START_REF] Pereira | Facultative predation and scavenging by mammalian carnivores: seasonal, regional and intra-guild comparisons[END_REF].

Dry season: time for interference competition?

We revealed a significant number of encounters between lions and hyaenas in the dry season.

They encountered each other mainly in the absence of a carcass. In the dry season, hyaenas are likely to be more efficient killers than during the wet season and more carcasses from herbivore natural mortality are likely to be available in the landscape, which will increase the chance of locating a carcass from which lions are absent. Altogether, this may explain the lower number of interactions at carcasses between hyaenas and lions in the dry season.

Interestingly, during the dry season, lions and hyaenas encountered each other mainly close to waterholes.

In arid and semi-arid savannas, large herbivores tend to aggregate around scarce water sources in the dry season [START_REF] Thrash | Dry season herbivore densities around drinking troughs in the Kruger National Park[END_REF][START_REF] Redfern | Surface-water constraints on herbivore foraging in the Kruger National Park, South Africa[END_REF]. This is the case in HNP [START_REF] Valeix | Behavioral adjustments of African herbivores to predation risk by lions: Spatiotemporal variations influence habitat use[END_REF][START_REF] Valeix | Temporal dynamics of dry-season water-hole use by large African herbivores in two years of contrasting rainfall in Hwange National Park, Zimbabwe[END_REF] where waterholes can be considered as prey hotspots during the dry season. Both predators are attracted to these areas [START_REF] Valeix | How key habitat features influence large terrestrial carnivore movements: waterholes and African lions in a semi-arid savanna of north-western Zimbabwe[END_REF][START_REF] Périquet | Sharing the top: How do Spotted hyaenas cope with Lions? Apex predator coexistence in a wooded savanna[END_REF], and are therefore more likely to encounter each other there. Our results suggest that management decisions to artificially provide water throughout the year may influence apex predator interactions by creating, maintaining and strengthening prey hotspots.

Encounters in the absence of a carcass were very short and usually limited to a single contact point with one or both species moving away immediately afterwards. This suggests that in the absence of food, predators avoid spending time near each other. Hyaenas did not avoid these encounters, and even appeared to provoke many of them. Surprisingly, lions seemed to avoid the encounter on a few occasions. Additionally, hyaenas did not appear to flee after an encounter, while lions quickly moved away on more than half of the encounters. Altogether, these results are quite counterintuitive and suggest that hyaenas were not scared of proximity to lions and were even attracted to them. These observations could result from hyaenas following lions to assess kleptoparatism and scavenging opportunities, as foxes have been observed to follow striped hyaenas (Hyena hyena, [START_REF] Macdonald | Observations on the behaviour and ecology of the striped hyena, Hyena hyena in Israel[END_REF]). Yet, we found that hyaenas hardly ever displace lions from carcasses and that very few encounters at carcasses occur in the dry season, making this scenario unlikely. The observed patterns could result from two scenarios: a passive interference scenario whereby the two predators actively select for areas rich in prey and lions leave a patch due to prey disturbance following hyaena arrival, or an active interference scenario whereby hyaenas actively seek to encounter lions near waterholes to displace them from these prime hunting grounds. In the two scenarios, our results would suggest an interference competition between the two species, with potential negative effects from hyaenas on lions, which would challenge the widely accepted view that lions are dominant over hyaenas. While many studies (including this one) found lions to be dominant over hyaenas in terms of food access [START_REF] Cooper | Optimal hunting group size: the need for lions to defend their kills against loss to spotted hyaenas[END_REF][START_REF] Höner | The response of spotted hyaenas to long-term changes in prey populations: functional response and interspecific kleptoparasitism[END_REF][START_REF] Trinkel | Competitive interactions between spotted hyenas and lions in the Etosha National Park, Namibia[END_REF], to our knowledge, this is the first time that interactions between the two species away from food resources are studied.

Limits and future directions

Our results show the great potential of GPS data loggers to provide extensive information on interactions between species. It is however important to note that the frequency of encounters was underestimated as the proximity index was calculated from GPS locations separated by one hour at least, and short encounters that occurred between consecutive 1h locations may have been undetected. Additionally, we could not quantify the encounters involving a collared animal and a non-collared one. Significant improvements in battery life should soon allow the use of this technology with high frequency of GPS fixes acquisition that would provide a much finer spatiotemporal scale to study interactions between apex predators. This would allow testing the scenarios sketched above about the underlying mechanisms of the interactions. We also encourage the development of methods to quantify attraction and avoidance from simultaneous movement data (as started by [START_REF] Jordan | Dynamics of direct inter-pack encounters in endangered African wild dogs[END_REF][START_REF] Chisholm | Parsimonious test of dynamic interaction[END_REF].

Additionally, only one individual (at best two) per predator group was radio-collared, and since hyaena groups are characterized by a highly dynamic fission-fusion [START_REF] Kruuk | The Spotted Hyena: A Study of Predation and Social Behavior[END_REF][START_REF] Smith | Social and ecological determinants of fissionfusion dynamics in the spotted hyaena[END_REF], we cannot say with certainty whether some hyaenas arrived before or stayed after the collared one. Hyaenas also behave differently depending on their social rank [START_REF] Smith | Social and ecological determinants of fissionfusion dynamics in the spotted hyaena[END_REF]) and they have marked personality which could influence the outcome of interactions with lions [START_REF] Watts | Responses of Spotted Hyenas to Lions Reflect Individual Differences in Behavior[END_REF].

Conclusion

While interference competition occurs between lions and hyaenas, as it is widely accepted between apex predators (e.g. [START_REF] Broekhuis | Risk avoidance in sympatric large carnivores: reactive or predictive?[END_REF][START_REF] Vanak | Moving to stay in place: behavioral mechanisms for coexistence of African large carnivores[END_REF][START_REF] Du Preez | Impact of risk on animal behaviour and habitat transition probabilities[END_REF], the extent of facilitation through scavenging opportunities should not be ignored [START_REF] Van Dijk | Diet shift of a facultative scavenger, the wolverine, following recolonization of wolves[END_REF][START_REF] Mattisson | Influence of intraguild interactions on resource use by wolverines and Eurasian lynx[END_REF][START_REF] Pereira | Facultative predation and scavenging by mammalian carnivores: seasonal, regional and intra-guild comparisons[END_REF][START_REF] Sivy | Fatal Attraction? Intraguild Facilitation and Suppression among Predators[END_REF]. Our results show that the balance between intraguild competition and facilitation is likely to be affected by environmental conditions and at least in semi-arid savanna ecosystems is most probably driven by water availability and seasonality.

FIGURE 1A 709

Figure 1 :

 1 FIGURES CAPTIONS

Figure 2 :

 2 Figure 2: Distribution of the 100 proximity indices randomly generated compared to the

Figure 3 :

 3 Figure 3: a) Monthly distribution of encounters between hyaenas and lions depending on the

Figure 4 :

 4 Figure 4: Creation and outcome of encounters occurring in the presence versus absence of a

ACKNOWLEDGEMENTS

The Zimbabwe Research Council and the Zimbabwe Parks and Wildlife Management Authority are kindly acknowledged for providing the opportunity to carry out this research. We deeply thank Nicholas Elliot, Jane Hunt and Brent Stapelkamp for the collection of lion GPS data. This study was supported by the HERD project (Hwange Environmental Research Development), funded by the ANR FEAR (ANR-08-BLAN-0022), the CNRS, the RP-PCP platform and by the Hwange Lion Project supported by grants from The Darwin Initiative for Biodiversity Grant 162/09/015, The Eppley Foundation, Disney Foundation, Marwell Preservation Trust, Regina B. Frankenburg Foundation, The Rufford Maurice Laing Foundation, Panthera Foundation, and the generosity of Joan and Riv Winant. SP salary was provided by the ENS of Lyon and UCBL Lyon1. ER was supported by the Spanish Ministry of Science and Innovation co-funded by FEDER (projects CGL2009-07301/BOS, CGL2012-252

35931/BOS and CGL2017-83045-R). This work was performed using the computing facilities of the CC LBBE/PRABI. This work benefited from ideas developed in the framework of the ANR project FUTURE-PRED (ANR-18-CE02-0005-01). We thank two anonymous reviewers for their helpful comments on a previous version of the manuscript.

encounters (Fig 4,Appendix S3: Table S2) and of attraction in 15% of encounters. In the hour preceding the encounter, hyaenas moved on averaged at 1,045±195 m/h (n=34), which was not significantly different from the lion speed of 766±136 m/h (n=31,t=0.47,p=0.64,Appendix S4: Fig S3).

Which species was dominant over the encounter?

Encounters in the presence of a carcass -We found that lions were dominant over access to the carcass in 18 out of the 20 encounters where dominance could be clearly established (Fig. 4, Appendix S3: Table S1, see Appendix S4: Fig. S2 for examples of encounters). Hyaenas were dominant only once. During the 22 encounters that were characterized by more than one pair of proximal fixes, hyaenas moved on average 739±47 m/h (range=1-5,600m/h, n=479 steps) which was significantly more than lions (115±13m/h, range =0-1900m/h, n=409 steps, t=15.98, p<0.0001). Hyaenas were also found significantly further away from the carcass than lions (hyaena: 1,614m±127m, range=4-15,400m, n=544 locations; lion: 250±2m, range=0-1,900m, n=483 locations; t =20.48, p<0.0001). In the hour following the end of the encounter, hyaenas moved at 527±129m/h (n=28), which was significantly more than lions (60±32m/h, n=27, t=5.49, p<0.0001, Appendix S4: Fig. S1).

Encounters in the absence of a carcass -When movement could be characterized after the encounter (n=33), hyaenas seemed to move away from lions in only 18% of the cases (Fig. 4, Appendix S3: Table S2), whereas lions moved away from hyaenas in 58% of the cases (Fig. 4, Appendix S3: Table S2). In the hour following the end of the encounter, hyaenas moved at 578±146m/h (n=33) and lions at 658±118m/h (n=30, Fig. S3) and the difference was not significant (t=1.07, p=0.28). After an encounter, we found a significant effect of the interaction between species and the presence of a carcass (F=14.48, p=0.002) with lions moving faster in the absence of a carcass compared to when a carcass was present (t=6.73, p<0.0001).We found