Dieter Lukas
email: dieter_lukas@eva.mpg.de

Elise Huchard

The evolution of infanticide by females in mammals

Keywords: social competition, sexual selection, phylogenetic comparison, sociality, kinship

In most mammalian species, females regularly interact with kin, and it may thus be difficult to understand the evolution of some aggressive and harmful competitive behaviour among females, such as infanticide. Here, we investigate the evolutionary determinants of infanticide by females by combining a quantitative analysis of the taxonomic distribution of infanticide with a qualitative synthesis of the circumstances of infanticidal attacks in published reports.

Our results show that female infanticide is widespread across mammals and varies in relation to social organization and life-history, being more frequent where females breed in groups and have intense bouts of high reproductive output. Specifically, female infanticide occurs where the proximity of conspecific offspring directly threatens the killer's reproductive success by limiting access to critical resources for her dependent progeny, including food, shelters, care or a social position. In contrast, infanticide is not immediately modulated by the degree of kinship among females, and females occasionally sacrifice closely related juveniles.

Our findings suggest that the potential direct fitness rewards of gaining access to reproductive resources have a stronger influence on the expression of female aggression than the indirect fitness costs of competing against kin.

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Introduction

Recent work has emphasized that competitive strategies of female mammals are often strikingly symmetrical to those observed in males, including displays and ornaments, fighting and weaponry, dominance hierarchies, and reproductive suppression [Clutton-Brock 2007[START_REF] Henry | [END_REF]Stockley & Bro-Jørgensen 2011;Clutton-Brock & Huchard 2013]. However, while interactions among conspecific male mammals are often contextual and temporally limited to competition over access to mating partners [Emlen & Oring 1977;[START_REF] Connor | Male dolphin alliances in Shark Bay: changing perspectives in a 30-year study[END_REF], interactions among females tend to occur across extended periods and multiple settings [START_REF] Gowaty | Sex roles, contests for the control of reproduction, and sexual selection. Sexual selection in primates: New and comparative perspectives[END_REF][START_REF] Th | Mammalian Societies[END_REF]. Females may thus compete over a diversity of resources -including food, resources necessary to breed (burrows, home-range), or offspring care [Clutton-Brock 2007]. In addition, since female mammals are typically philopatric, they may often compete with kin. It has therefore proven difficult to identify the determinants of overt female-female competition, in particular in societies that are structured around female kinship. The challenge has been to understand how the direct benefits of competition may be balanced with the indirect fitness costs of competing against kin, especially in the case of extremely harmful behaviour such as infanticide [Clutton-Brock et al. 2001;Young et al. 2006].

The killing of rivals' offspring represents one violent manifestation of intrasexual competition, and a significant source of juvenile mortality in some populations [START_REF] Palombit | Infanticide: male strategies and female counter-strategies[END_REF]], with adults of both sexes committing infanticide. It has been intensely studied in male mammals, where fifty years of field research have shown that it has evolved as a sexually selected strategy over access to mating partners [Hrdy 1974]. In cases where the presence of a dependent offspring prevents the mother from becoming pregnant again, committing an infanticide allows the killer to create extra reproductive opportunities. This strategy is particularly common in polygynous societies where one or a few alpha male(s) monopolize mating opportunities over short periods before losing dominance to others [ van Schaik & Janson 2000;[START_REF] Lukas | The evolution of infanticide by males in mammalian societies[END_REF]Palombit 2015]. Since in most instances infanticide is committed by males who recently joined the group, they are unlikely to kill any related offspring [START_REF] Hrdy | Infanticide among Animals: A Review, Classification, and Examination of the Implications for the Reproductive Strategies of Females[END_REF]]. In contrast, as for other forms of female competition, little is known about the determinants and consequences of infanticide by females other than the mother, although it may be more prevalent than infanticide by males, both within and across taxa [START_REF] Hrdy | Care and Exploitation of Nonhuman Primate Infants by Conspecifics Other Than the Mother[END_REF], Blumstein 2000;[START_REF] Digby | Infanticide by female mammals: implications for the evolution of social systems[END_REF]].

Unlike males, female killers do not benefit from extra mating opportunities [Agrell et al. 1998;[START_REF] Digby | Infanticide by female mammals: implications for the evolution of social systems[END_REF]], because male mammals generally do not invest into offspring care to the extent that it would prevent them from mating with other females [Kleiman & Malcom 1981]. If anything, killing a dependent juvenile may exacerbate female mating competition by speeding-up the resumption to fertility for the mother of the victim. Several plausible scenarios explaining the occurrence of infanticide by females have been proposed based on a synthesis of natural observations [reviewed by [START_REF] Digby | Infanticide by female mammals: implications for the evolution of social systems[END_REF]]. Symmetrically to the patterns observed for male infanticide, predation for nutritional gains (H1: 'exploitation' hypothesis) may not provide a general explanation for female infanticide as killers have relatively rarely been observed to consume victims partially or entirely (e.g. [Goodall 1986;Blumstein 2000]).

Instead, hypotheses regarding the adaptive benefits of female infanticide have focused on how killings might facilitate access to resources that are critical to successful reproduction (H2: 'resource competition' hypotheses) [START_REF] Digby | Infanticide by female mammals: implications for the evolution of social systems[END_REF]]. Female killers might be defending access to an exclusive territory or shelter, or attempting to expand their breeding space when they target victims outside their home-range (H2.1: 'breeding space' hypothesis) (as in blacktailed prairie dogs [Hoogland 1985] or Belding's ground squirrels [START_REF] Sherman | Reproductive competition and infanticide in Belding's ground squirrels and other animals[END_REF]). In species where females only associate temporally to breed, killers may defend access to their own milk, by discouraging attempts to suckle from unrelated juveniles (H2.2: 'milk competition hypothesis) (as in Northern elephant seals: [Le Boeuf et al. 1973]). In species who breed cooperatively, killers may defend access to extra offspring care by group mates other than the mother by altering the helper-to-pup ratio in their own group (H2.3: 'allocare' hypothesis) (as in meerkats [Clutton-Brock et al. 2001;Young et al. 2006], banded mongooses [Gilchrist 2006;Cant et al. 2014], or marmoset [Digby 1995]). Finally, in species who live in stable groups, killers may defend their offspring's future social status (in species with stable hierarchies) or group membership (in species with forcible evictions) by eliminating future rivals (H2.4: 'social status' hypothesis) (as in some Old World primates [START_REF] Hrdy | Care and Exploitation of Nonhuman Primate Infants by Conspecifics Other Than the Mother[END_REF][START_REF] Digby | Infanticide by female mammals: implications for the evolution of social systems[END_REF]). Finally, it is unclear whether females might refrain from committing infanticide when this is likely to lead to indirect fitness costs. If so, they might be less likely to commit infanticide in species where most interacting females are related, or they might be able to target unrelated offspring exclusively. Alternatively, the benefits of competition in any or all these circumstances might be high enough to outweigh any potential indirect fitness costs.

Here, we present an investigation of the distribution and circumstances of infanticide by female mammals, based on data from 289 species from across 14 different Orders collected from the primary literature. The combination of a quantitative synthesis of the taxonomic distribution of infanticide with a qualitative analysis of the circumstances of infanticidal attacks (including traits of the killer and victim) can contribute to reveal the ecological, lifehistory or social determinants of female reproductive competition across mammalian societies, and their relevance to the occurrence of female associations and interactions within and among matrilines. Our aim is to provide a starting point for the investigation of the likely causes and situations under which female infanticide occurs, bearing in mind that our analytical framework suffers from several caveats. First, infanticide is uncommon and difficult to observe, so that some species might wrongly be classified as 'non-infanticidal'. Second, the analyses rely on a rough categorization of the circumstances of infanticide and may oversimplify its determinants, which could be influenced by multiple factors in a given species.

We first summarize the social organisation and life-histories of species in which infanticide by females has been observed, in order to evaluate the conditions under which infanticide is most common (Table 1). In particular, we investigate whether philopatry and average relatedness among groups of interacting females reduce the occurrence of female infanticide.

Next, we perform specific phylogenetic analyses to test core predictions generated by each hypothesis and assess whether females have been observed to commit infanticide in species in which they are most likely to benefit from such killings. In addition, we investigate whether population-level information on the traits of killers and victims are compatible with predictions generated by each hypothesis. All our predictions and tests are summarized in Table 2. We first show that, across all species, the distribution and occurrence of infanticide by females is better explained by resource competition than by exploitation. We next test support for each for the four resource-competition hypotheses. We assess whether (i) instances where females kill offspring in neighbouring ranges ('breeding space hypothesis') are most likely explained by competition over breeding space; whether (ii) instances where females kill offspring born in the same breeding association by competition over milk ('milk competition hypothesis'); whether (iii) instances where females kill offspring in groups where usually only a single female reproduces by competition over offspring care ('allocare hypothesis'); and whether (iv) instances where females kill offspring born in groups with multiple breeders by competition over social status or group membership ('social status hypothesis').

Materials & Methods

Following [START_REF] Digby | Infanticide by female mammals: implications for the evolution of social systems[END_REF], we use a broad definition of infanticide as an act by one or more nonparents that makes a direct or significant contribution to the immediate or imminent death of conspecific young. This definition excludes instances where mothers kill their own offspring (which are considered to result from parent-offspring conflict [O 'Connor 1978] rather than from intrasexual competition) and includes cases where infants die as the result of the physical aggression (direct infanticide) as well as cases where the enforced neglect of an infant, such as kidnapping, ultimately causes death (indirect infanticide). Although the latter cases are often excluded from studies of infanticide due to their proximate form of 'overzealous' allomaternal care [START_REF] Hrdy | Care and Exploitation of Nonhuman Primate Infants by Conspecifics Other Than the Mother[END_REF]], their ultimate consequence -infant death -contributes to shape their evolution as infanticidal behaviour. We included infanticide records from both wild and captive populations for which the killer was unambiguously identified as an adult female. For each species, we recorded whether observations occurred in a captive setting or under natural conditions. Species for which no case of infanticide has ever been observed were included only if detailed observations on individual females and juveniles were available, either from repeated captive observations or from field studies occurring across at least three reproductive seasons, to minimize the risk of misclassifying them as "non-infanticidal". Given that infanticide is difficult to observe, we focused on studies that were performed in circumstances under which it could be expressed and detected (co-housing of multiple females; detailed observations or specific females from before birth until weaning) or on studies that followed the fate of one or more offspring cohort(s) and recorded the causes of juvenile mortality. Data were collected from searches through the scientific literature, starting with major reviews on the topic of female infanticide [START_REF] Wasser | Reproductive suppression among female mammals: implications for biomedicine and sexual selection theory[END_REF]Agrell et al. 1998;[START_REF] Ebensperger | Strategies and counterstrategies to infanticide in mammals[END_REF]Blumstein 2000;[START_REF] Digby | Infanticide by female mammals: implications for the evolution of social systems[END_REF]Ebensperger & Blumstein 2007;Stockley & Bro-Jorgensen 2011;Clutton-Brock & Huchard 2013] and performing backward and forward citation searches to identify relevant observations. While we might not have included all species in which the status of infanticide by females is documented (i.e., known to be either absent or present), we believe that this search strategy should not lead to systematic biases with regards to the tested hypotheses. We repeated all analyses excluding the 42 species for which we only found observations in captive settings (27 species with and 15 species without female infanticide), which did not change any of the results.

For the comparative analyses, we extracted data for each species on variables linked to the four different hypotheses (see Table 2). From published databases, we obtained information on: social organisation (classified as: solitary breeders [breeding females have exclusive ranges in which they do not tolerate any other breeding individual], pair breeders [home-ranges contain a single breeding female and a single breeding male but may contain additional nonbreeding individuals], associated breeders [females share the same space for breeding but associations are unstable and tend not to last beyond the breeding season], or social breeders [several breeding females share the same home-range across multiple breeding seasons]) [START_REF] Lukas | Climate and the distribution of cooperative breeding in mammals[END_REF]; female philopatry and dispersal (whether most breeding females have been born in their current locality/group or elsewhere) [START_REF] Lukas | Group structure, kinship, inbreeding risk and habitual female dispersal in plural-breeding mammals[END_REF]; carnivory (whether the diet of a species includes meat or not) [START_REF] Wilman | EltonTraits 1.0: Species-level foraging attributes of the world's birds and mammals[END_REF]; infanticide by males (whether males have been observed to kill conspecific young) [START_REF] Lukas | The evolution of infanticide by males in mammalian societies[END_REF]; environmental climatic harshness (a principal component, with high values indicating that rainfall is low and temperatures are cold and unpredictable across the known range of a species) [Botero et al. 2014]; maternal investment (mean body size of offspring at weaning multiplied by the mean number of offspring per year, divided by mean body mass of adult females) [START_REF] Sibly | How body mass and lifestyle affect juvenile biomass production in placental mammals[END_REF]; the use of burrows or nest holes for breeding (information was taken from the papers used to extract information on the absence or presence of infanticide by females); litter size (number of offspring per birth); offspring mass at birth (grams); weaning age (age at which offspring are independent, in days); inter-birth interval (time between consecutive births, in days) [START_REF] Jones | PanTHERIA: A Species-Level Database of Life History, Ecology, and Geography of Extant and Recently Extinct Mammals[END_REF]; energetic value of milk (MJ/ml based on the protein, sugar, and fat composition) [Langer 2008;Barton & Capellini 2011;Hinder & Milligan 2011]; offspring care by fathers and/or non-parental group members (whether offspring receive milk or food from or are being regularly carried by group members who are not the mother) [START_REF] Lukas | Climate and the distribution of cooperative breeding in mammals[END_REF]. In addition, we completed information obtained from these databases by collecting extra data from the primary literature on dominance hierarchies and mechanisms of rank acquisition in social groups (whether all adult females can be arranged in a dominance hierarchy and if so, whether an individual's rank is influenced by age and/or nepotism); and forcible evictions (whether females use aggression to exclude other females from their own social group). For each species in which females had been observed to kill conspecific young, we used the primary literature to record as much information as possible regarding the characteristics of the killer (age and reproductive state) and of the victim (age, sex, and relatedness to killer) to test specific predictions. The full dataset is provided in Supplementary Table 1 (comparative data) and Supplementary Table 2 (individual characteristics data), with all references for data specifically collected here in Supplementary File 1.

In addition to performing comparisons assessing contrasts in the presence or absence of infanticide across all species in our sample, we classified species into different types according to each of the four resource competition hypotheses (Table 2). This classification also aimed at controlling for a potential confounding effect of social organization, as our analysis might otherwise detect factors associated with the evolution of sociality if females are more likely to have been observed to commit infanticide in some social systems. For the breeding space hypothesis, we only included instances of infanticide in which females did not share a homerange with the mother of the victims; for the milk competition hypothesis, we restricted the sample to associated breeders; for the allocare hypothesis, we only included pair; and for the social competition hypothesis, we only looked at social.

For the comparative analyses, the phylogenetic relatedness between species was inferred from the updated mammalian supertree [START_REF] Rolland | Faster Speciation and Reduced Extinction in the Tropics Contribute to the Mammalian Latitudinal Diversity Gradient[END_REF]]. We fitted separate phylogenetic models using MCMCglmm [START_REF] Hadfield | General quantitative genetic methods for comparative biology: phylogenies, taxonomies and multi-trait models for continuous and categorical characters[END_REF] to identify the extent to which each of the predicted variables (Table 2) explains the presence of infanticide by females across species (binary response, assuming a categorical family of trait distribution). Following the recommendations of [START_REF] Hadfield | MCMCglmm: Markov chain Monte Carlo methods for generalized linear mixed models retrieved from cran[END_REF], we set the priors using an uninformative distribution (with variance, V, set to 0.5 and belief parameter, nu, set to 0.002). Each model was run three times for 100,000 iterations with a burn-in of 20,000, visually checked for convergence and for agreement between separate runs.

Results

Social organisation and infanticide by females

Infanticide by females has been observed in 89 (31%) of the 289 mammalian species in our sample (Table 1). Female infanticide (of any type) varies with the social organisation and is more frequent when females breed in groups (Figure 1): it has been observed in 43% of associated breeders, in 36% of pair breeders, and in 30% of social breeders, but only in 18% of solitary breeders. Across all species, females are equally likely to kill offspring when they are philopatric (47 of 135 species, 34%) than when they disperse to breed (17 of 59 species, 29%) (effect of female dispersal on presence of infanticide by females: -10.1, 95% p=0.34) but there are differences for two types of social organisation: across associated breeders infanticide only occurs in philopatric species; while across pair breeders infanticide is more likely to occur in species in which females disperse (Table 1). Across all group-living species (associated breeders, pair breeders with helpers, social breeders), there is no relationship between levels of average relatedness among female group members and whether infanticide by females of offspring born in the same group does (median levels of average relatedness across 10 species 0.09, range 0.01-0.38) or does not occur (median levels of average relatedness across 24 species 0.21, range -0.03-0.52) (effect of levels of average relatedness on presence of infanticide by females: -21.1, 95% CI -81. 4 -10.1, p=0.18). Across species in which groups are stable (i.e., excluding associated breeders where groups are sometimes difficult to define and can be very large), levels of average relatedness are slightly higher when infanticide occurs (see also [START_REF] Lukas | Social complexity and kinship in animal societies[END_REF]). The population-level information show instances of killers being close kin of the victim in 33% of species (22/65), with either grandmothers killing their grandchildren or aunts killing their nieces, with kin being common victims in cooperative breeders (8 of 12 cooperative breeders) but also in several social breeders (14 of 51 social breeders).

Life-histories and infanticide by females

Energetic investment into reproduction by mothers is higher in species with any form of infanticide by females compared to the remaining species (Table 1). However, these patterns encountered across all species do not reflect a general association between infanticide and all these traits, but rather the fact that each different scenario of infanticide is associated with specific life histories, with species having either larger offspring mass at birth, shorter time to weaning and between births, and/or larger litter sizes. Females in infanticidal species where there is usually a single breeding female per home range (extraterritorial infanticide and infanticide in pair breeders) have larger litters than females in species without infanticide (Table 1). Species in which females kill offspring in a breeding association are characterized by fast-growing offspring, while offspring are relatively small at birth in species in which females kill offspring in stable groups (Table 1). Table 1: The social and life-history conditions associated with the distribution of infanticide by females Females are more likely to commit infanticide in group-living species, but whether breeding females are philopatric or disperse does not appear to be associated with the distribution of infanticide. In general, species with infanticide by females are characterized by high maternal investment, which is either expressed as females having large litters, short lactational and interbirth periods, and/or relatively large offspring. For each of the main hypotheses, we tested two core predictions in phylogenetic comparisons and two predictions about the individual traits from the field observations. For the comparisons, we list the sample of species included.

Sample of species

H1: Exploitation

We find no support for predictions suggesting that females kill conspecific offspring primarily for exploitation (Table 2). Across species, infanticide by females is as likely to occur in the absence of infanticide by males (44 of 147 species, 30%) as in its presence (43 of 135 species, 32%) (effect of presence of male infanticide on presence of female infanticide 2.1, 95% CI -13.5 -16.9, p=0.74). Similarly, carnivorous species are not more likely to show infanticide by females (18 of 56 species, 32%) than species in which meat does not constitute an important part of the diet (59 of 192 species, 31%) (effect of carnivory on presence of female infanticide -0.7, 95% CI -11.2 -10.0, p=0.89). The age of victims varies from birth to beyond independence across species, but is more homogeneous within each type of infanticide (see below), so that killings do not appear simply opportunistic.

H2: Resource competition

Infanticide by females appears more likely to occur where competition over resources is expected to be more intense (Table 2). The climatic environments of species in which females commit infanticide are harsher (as estimated by a principal component reflecting low unpredictable rainfall and cold seasons) than the environments of species in which infanticide has not been observed (effect of environmental harshness on presence of female infanticide 7.0, 95% CI -0.2 -14.7, p=0.03, 54 species with infanticide and 193 without) (Figure 2a). In species where females commit infanticide, they invest substantially more energy into the production of offspring, being able to produce the equivalent of 1.0 times their own body mass in offspring mass per year (number of offspring times mass of weaned offspring; median across 41 species, range 0.05 -12.1 times) compared to 0.33 times in species in which infanticide has not been observed (median across 77 species, range 0.03 -11.1) (effect of maternal energetic investment on presence of female infanticide 25.8, 95% CI 1.5 -58.7, p<0.001) (Figure 2b).

H2.1: Competition over breeding space

Thirty-two of the 33 species in which females kill juveniles outside their own home-range keep their offspring in burrows or holes, compared to 93 of the 163 species in which infanticide by females appears absent (effect of burrow use on the presence of infanticide by females 14.4, 95% CI 6.0 -22.9, p<0.001). The exception is Semnopithecus entellus, where "females occasionally steal infants from a neighboring troop" [START_REF] Hrdy | Care and Exploitation of Nonhuman Primate Infants by Conspecifics Other Than the Mother[END_REF]Mohnot 1980] Females are more likely to kill offspring born to other females in social breeders in which they also aggressively evict other females from their group (infanticide has been observed in 6 of 10 species with evictions and 7 of 35 without evictions) (effect of occurrence of evictions on presence of infanticide by females 7.8, 95% CI -0.2 -14.4, p=0.02). In all twelve social breeders in which infanticide events have been observed, killers were old and high-ranking. Killers were never pregnant, but in all cases had dependent young of their own. Victims were not yet weaned, and victims might be related to the killer in 5 out of 12 species. There is only one species where the data suggest that females might preferentially kill offspring of one sex: in Macaca radiata (a species with female philopatry), female offspring appear to be the predominant victims.

Discussion

Our findings establish that female infanticide is widespread across mammals and that it is frequently expressed as intensely as competition among males. Females have been observed

to kill conspecific juveniles in various species and our comparative analyses provide support to the idea that this behaviour may be adaptive under a wide range of circumstances.

Infanticide is more likely to occur in species in which multiple adult females live or breed together than where females breed solitarily, and infanticide appears most frequent in species where females only associate temporarily to breed. Since infanticidal behaviour is relatively rare and may be difficult to detect, this association may reflect the fact that opportunities to commit and to observe infanticides may be greater where females live or breed together.

Within each type of social organisation, we do however find that females, like males, appear to commit infanticide when the presence of the victim might otherwise limit their own reproductive success. While infanticide by males has evolved primarily in response to mate competition across mammals [START_REF] Lukas | The evolution of infanticide by males in mammalian societies[END_REF]Palombit 2015], the evolutionary determinants of infanticide by females are apparently more complex, as females may compete over multiple resources.

Several lines of evidence indicate that female infanticide is adaptive, with females killing conspecific offspring in response to competition over resources that are critical for successful reproduction. First, infanticide appears associated with variation in ecology and life-history.

Specifically, it is most frequently observed in species facing harsh climatic conditions and making the greatest reproductive efforts; it is unlikely that such associations are due to variations in opportunities to observe or commit infanticides across species. Rather, the potential costs of sharing critical resources might outweigh the risks associated with committing infanticide in such circumstances.

Second, specific determinants of female infanticide identified at the population level by field studies also seem to predict its distribution across species. Extraterritorial infanticides were found to be most frequent in solitary species where females use burrows to give birth and territories to raise offspring, allowing killers to free-up reproductive space for their own offspring. Anecdotal reports suggest that mothers of victims in these species frequently move away after the loss of their offspring [START_REF] Sherman | Reproductive competition and infanticide in Belding's ground squirrels and other animals[END_REF]]. The strong association with burrow use might occur because burrows represent a clear defendable resource, because offspring kept in burrows tend to be altricial [Kappeler 1991] and unable to flee or defend themselves, or because infanticide is more easily observed if researchers know where offspring are. Our findings further show that female infanticide occurs in pair breeders where helpers -fathers or additional group mates -are present. A lower number of helpers per offspring reduces their weight and their chances to survive at independence, such that females might even kill offspring born to close kin, such as their grandchildren [Clutton-Brock et al. 2001;[START_REF] Hodge | Determinants of reproductive success in dominant female meerkats[END_REF]. Finally, patterns are slightly more complex in social breeders. There, infanticide preferentially occurs in species where aggressive competition among females leads to the eviction of some individuals -generally young adults -from the group, especially at times when group size increases (e.g. [Kappeler & Fichtel 2012]). In such cases, killing unrelated juveniles may limit future competition and the related risk of being evicted for the killer's offspring. In addition, in social breeders where females are philopatric, infanticide was only found to occur where female rank acquisition is nepotistic, a hierarchical system where each additional offspring may contribute to strengthen the social status of a matriline -and where infanticide may consequently weaken competing matrilines on the long term.

Anecdotal reports of female pinnipeds killing orphans as they attempted to suckle from them inspired the hypothesis that females compete over milk in species where they only associate to breed [START_REF] Digby | Infanticide by female mammals: implications for the evolution of social systems[END_REF]]. While our comparative analyses did not reveal any difference in the energy content of milk of associated breeders in which infanticide is present versus absent, associated breeders nevertheless comprise the species with the highest energy content of milk and the fastest growth rates, and we further found that offspring are larger at birth and weaned at an earlier age in associated breeders with infanticide compared to those without it. The lack of support for the milk competition hypothesis in our analyses may be explained by a noisy dataset, where the absence of infanticide in some species may be due to the fact that it goes undetected if it is hard to observe, or to the evolution of counter-adaptations that protect offspring against infanticide. Alternatively, milk is not the only resource over which these females compete. For example, in the large breeding colonies of pinnipeds, space is sometimes very restricted [Baldi et al. 1996], especially in the immediate vicinity of the harem leaders. These bulls often protect their females and calves from attacks by younger males, and may represent another source of competition for lactating females.

It is likely that, in any given species, infanticide may be triggered by more than one determinant -including some that may not be considered here. A killer may accordingly get multiple benefits from one infanticide event, but may also commit infanticides in more than one context. For example, half of the species of pair breeders committing intra-group infanticides also commit extraterritorial infanticides. It is therefore possible that different types of female infanticide -following our classification -have followed a common evolutionary path. Specifically, it is possible that infanticidal behaviour initially emerged in response to one particular pressure (e.g., competition over access to allocare) in a given species, which subsequently started to express it in other competitive contexts (e.g., competition over breeding territories). However, the limited number of species for which observational data on infanticide are available, as well as heterogeneities in the sample -such as an over-representation of group-living species -introduce uncertainty when attempting to reconstruct the evolutionary history of the trait. It is consequently hard to infer the ancestral state, whether each infanticide type has evolved independently, or how many times infanticidal behaviour has emerged across mammals. Similar difficulties limit causal inferences regarding the association between infanticide and social organization. In some lineages the risk of infanticide might prevent breeding females from forming groups, while in others the evolution of infanticide might be favoured by the constraints or opportunities of a specific social system, in line with patterns observed in males [START_REF] Lukas | The evolution of infanticide by males in mammalian societies[END_REF].

In addition to the nature of the resources that may directly limit female reproductive success in various types of social organisations [START_REF] Cavigelli | Female dominance status and fecal corticoids in a cooperative breeder with low reproductive skew: ring-tailed lemurs (Lemur catta)[END_REF], Saltzman et al. 2009[START_REF] Rosvall | Intrasexual competition in females: evidence for sexual selection?[END_REF], contrasts in the occurrence of infanticide across species reveal other broad patterns on female reproductive competition in mammals. In particular, the lack of association between female infanticide and philopatry across species (Table 1), as well as a synthesis of observations revealing that killers and victims are commonly related in some contexts, such as in pair breeders where reproductive suppression is common [START_REF] Lukas | Social complexity and kinship in animal societies[END_REF],

suggest that matrilineality and subsequent increases in average kinship among associated females does not necessarily lead to a reduction in competition among females. Some previous work suggested that mammalian females might be predisposed to behave positively and cooperatively with kin [di Fiore & Rendall 1994], such that species with female philopatry would be characterized by stable social bonds [Silk 2007]. However, the factors leading to limited dispersal and the spatial association of kin frequently also result in high local competition [START_REF] Frank | Foundations of Social Evolution[END_REF]] which can overcome the potential benefits of cooperation among kin [START_REF] West | Cooperation and competition between relatives[END_REF]. Studies of competition among males in such circumstances have shown that contrasts in levels of aggression can be explained by variation in the potential direct fitness benefits of winning [West et al. 2001], and it is likely that this also applies to the observed pattern of infanticide by females -where the direct benefits of infanticide in terms of increased access to a critical resource might outweigh its costs, including the indirect fitness costs associated with killing related juveniles.

Our study compiles five decades of behavioural data across species and within populations to elucidate the determinants of infanticide by mammalian females, which are less well understood than those of male infanticide. Our analyses suggest that the distribution of female infanticide across species reflects contrasts in social organisation; infanticide is most frequent in species that breed in groups, which probably have more opportunities for killings and also face greater breeding competition. Female infanticide occurs where the proximity of conspecific offspring directly threatens the killer's reproductive success by limiting access to critical resources for her dependent progeny, including food, shelters, care or a social position.

Finally, these data support the idea that female killers occasionally sacrifice related juvenile conspecifics, and may therefore actively harm their indirect fitness in order to maximize their direct fitness.

Supplementary File 1 for Lukas & Huchard: The evolution of infanticide by females in mammals

Parts A (citations for comparative data), B (citations for population data), and C (reference list).

A) Citations for new data in comparative analyses

The table lists the references with the observations on the presence or absence of infanticide by females for each species, and the references with details on the interactions among females in group-living species. All references are listed in full below the second table.

Species Reference infanticide by females Reference eviction/dominance hierarchy

Acinonyx jubatus

Hunter & Skinner, Transactions of the Royal Society of South Africa 56, 79 (2003). NA

Acomys cahirinus

Ebensperger , Biological Reviews 73, 321 (1998). NA

Acrobates pygmaeus

Ward, Australian Journal of Zoology 38, 503 (1990). NA

Aepyceros melampus

Averbeck, Doctoral dissertation, Technische Universität München (2002). NA

Aethomys namaquensis

Fleming, African Zoology 39, 123 (2004). NA Porter, Folia primatologica 72, 69 (2001). NA

Ailurus fulgens

Callithrix flaviceps

Digby & Saltzman, in The smallest anthropoids: the Marmoset/Callimico radiation, Ford, Porter, Davis, Eds. (Springer, New York, 2009), pp. 135-153;Hilario & Ferrari, Journal of Ethology 28, 195 (2010).

NA

Callithrix jacchus

Digby, Behavioral Ecology and Sociobiology 37, 51 (1995). NA

Callorhinus ursinus

Ebensperger , Biological Reviews 73, 321 (1998). NA

Canis dingo

Corbett , Ethology 78, 177 (1988). NA

Canis latrans

Camenzind 2000).

NA

Cercocebus atys

Range & Noë, American Journal of Primatology 56, 137 (2002).

Range & Noë, American Journal of Primatology 56, 137 (2002); Range, Behavioral Ecology and Sociobiology 59, 511 (2006).

Cercocebus torquatus

van 59, 511 (2006).

NA

Cercopithecus albogularis

Henzi & Lawes, Folia primatologica 48, 125 (1987). NA

Cercopithecus ascanius

Ebensperger , Biological Reviews 73, 321 (1998). NA 47, 817 (1978). NA

Cercopithecus campbelli

Galat

Chalinolobus gouldii

Dixon & Huxley, Mammalia 53, 395 (1989). NA

Cheirogaleus medius

Fietz et al., Behavioral Ecology and Sociobiology 49, 8 (2000); Fietz & Dausmann, Folia primatologica 74, 246 (2003).

NA

Chiropotes albinasus

Treves, Folia primatologica 69, 81 (1998).

Treves, Folia primatologica 69, 81 (1998).

Chiropotes satanas

Treves, Folia primatologica 69, 81 (1998). NA

Chlorocebus aethiops

Horrocks, Life-history characteristics of a wild population of vervet monkeys (Cercopithecus aethiops) in Barbados, West Indies. International Journal of Primatology 7, 31 (1986)

Isbell & Pruetz, International Journal of Primatology 19, 837 (1998).

Chlorocebus sabaeus

Treves, Folia primatologica 69, 81 (1998). NA

Colobus guereza

Ebensperger , Biological Reviews 73, 321 (1998). NA Teichroeb et al., Behavioral Ecology 23, 1348(2012). Wikberg et al. Behaviour 150, 295 (2013).

Colobus vellerosus

Connochaetes taurinus

Estes & Estes, Zeitschrift für Tierpsychologie 50, 45 (1979). NA

Crocidura russula

Cantoni & Vogel, Animal behaviour 38, 205 (1989)

NA

Diceros bicornis

Weladji & Laflamme-Mayer, African Journal of Ecology 49, 471 (2011). NA

Dicrostonyx groenlandicus

Blumstein, in Infanticide by Males and Its Implications, C. P. van Schaik, C. H. Janson, Eds. (Cambridge University Press, Cambridge, 2000).

NA

Didelphis virginiana

Hopkins & Forbes, The Murrelet 61, 20 (1980). NA

Dolichotis patagonum

Taber & Macdonald, Journal of Zoology 227, 439 (1992). NA

Enhydra lutris

Riedman & Estes, Biological report 90, 1126 21 (2000).

Eulemur macaco

Pereira & Weiss, Behavioral Ecology and Sociobiology 18, 141 (1991); Andrews, Folia Primatologica 69, 14 (1998); Jolly et al., International Journal of Primatology 21, 21 (2000). Andrews, Folia Primatologica, 69, 14 (1998). Bruemmer, Natural History 103, 26 (1994). NA

Eumetopias jubatus

Felis catus

Ebensperger, Biological Reviews 73, 321 (1998);

Pontier & Natoli, Aggressive Behavior 25, 445-449 (1999). 57, 157 (2002). NA

NA

Galea monasteriensis

Helogale parvula

Rasa book chapter (1994) NA

Herpestes sanguineus

Rood, Animal Behaviour 39, 566 (1989);Waser et al., Animal Behaviour 47, 289 (1994). NA

Heterocephalus glaber

Faulkes, in Cooperative Breeding in Mammals, Solomon, French, Eds. (Cambridge University press, New York, 1997).

NA

Hippopotamus amphibius

Karstad & Hudson, Mammalia 50, 153 (1986) 2000); Wagner et al., Molecular Ecology 16, 4356 (2007); Wagner et al., Animal Behaviour 75, 1131(2008).

NA

Hydrochoeris hydrochaeris

Ebensperger & Blumstein, in Rodent societies: an ecological and evolutionary perspective, J. O. 28, 1123 (2007). Berman et al. International Journal of Primatology 25, 1283(2004).

Macaca tonkeana

Muroyama & Thierry, International Journal of Primatology 17, 219 (1996).

Muroyama & Thierry, International Journal of Primatology 17, 219 (1996).

Macropus giganteus

Nave, Doctoral dissertation, The University of Melbourne (2002). NA

Madoqua kirkii

Hendrichs, Zeitschrift für Tierpsychologie 38, 55 (1975); Brotherton & Manser, Animal Behaviour 54, 1413(1997) (1990).

NA

Neotoma albigula

Topping & Millar, Behavioral Ecology and Sociobiology 43, 115 (1998). NA

Neotoma cinerea

Escherich, American Zoologist 15, 821 (1975). NA

Neotoma lepida

Blumstein 77, 179 (1996). NA

Onychomys torridus

Horner & Taylor, Journal of Mammalogy 49, 644 (1968). NA

Oreamnos americanus

Gaillard et al., Annual Review of Ecology and Systematics 31, 367 (2000). NA

Oryctolagus cuniculus

Künkele, Journal of Mammalogy 73, 317 (1992); Rödel et al., Ethology 114, 22 (2008). NA

Oryx leucoryx

Harding, Oryx 41, 478 (2007). NA Otaria byronia Campagna et al., Behaviour 107, 44 (1988). NA

Otocyon megalotis

Lamprecht, Zeitschrift für Tierpsychologie-Journal of Comparative Ethology 49, 260 (1979) 117, 317 (2003). NA Dinerstein et al., Journal of Mammalogy 69, 813 (1988). NA

Rhinoceros unicornis

Rhynchocyon chrysopygus

Rathbun, Mammalian species 117, 1 (1979). NA

Saguinus fuscicollis

Tirado- Herrera et al., American Journal of Primatology 153 (2000). NA

Saguinus mystax

Culot et al., Primates 52, 179 (2011). NA

Saguinus oedipus

Savage et al., American Journal of Primatology 38, 85 (1996). NA

Saimiri oerstedii

Zimbler-DeLorenzo & Stone, American Journal of Primatology 73, 607 (2011). NA

Saimiri sciureus

Hrdy, Advances in the Study of Behavior 6, 101 (1976). NA

Sciurus carolinensis

Lane et al., Animal Behaviour 75, 1927(2008). NA

Semnopithecus entellus

Sugiyama, in Social communication among primates, S. A. Altmann, Ed. (University of Chicago Press, Chicago, 1967), pp. 221-236;Hrdy, Folia primatologica 22, 19 (1974); Hrdy, Advances in the Study of Behavior 6, 101 (1976).

Borries, Behavioral Ecology and Sociobiology 41, 139 (1997). 134, 184 (2011).

Speothos venaticus

Symphalangus syndactylus

Morino, Doctoral dissertation, Rutgers University-Graduate School-New Brunswick (2012). NA

Tadarida brasiliensis

Kunz & Ebensperger, Acta Chiropterologica 1, 17 (1999); Knörnschild et al., PLoS One 6, e25001 (2011).

NA

Tamias striatus Bergeron et al., Journal of Evolutionary Biology 24, 1685(2011). NA

Tarsius bancanus

Roberts, International Journal of Primatology 15, 1 (1994). NA

Theropithecus gelada Ebensperger, Biological Reviews 73, 321 (1998). Dunbar, Behavioral Ecology and Sociobiology 7, 253 (1970).

Trachypithecus cristatus

Harding, Mammalian Species 42, 149 (2010). NA

Trachypithecus poliocephalus Zhao et al., Behavioral Ecology andSociobiology 65, 1535 (2011). NA Ebensperger, Biological Reviews 73, 321 (1998). NA Wielgus & Bunnell, Biological Conservation 93, 145 (2000).

Trachypithecus vetulus

Tursiops truncatus

NA

Ursus maritimus

Taylor et al., Arctic 38, 303 (1985). NA

Varecia variegata

van Noordwijk & van Schaik, in Infanticide by Males and Its Implications, Van Schaik, Janson, Eds. (Cambridge University Press, Cambridge, 2000); Baden et al., Behavioral Ecology andSociobiology 67, 1937 (2013).

NA

Vulpes lagopus Kruchenkova et al., Naturwissenschaften 96, 457 (2009). NA

Vulpes velox Kamler et al., Animal Behaviour 68, 83 (2004); Kitchen et al., Animal Behaviour 71, 1029(2006). NA

Vulpes vulpes

Packer & Pusey, in Infanticide: comparative and evolutionary perspectives., G. Hausfater, S. B. Hrdy, Eds. (Aldine, New York, 1984), pp. 31-42.

NA

Vulpes zerda

Lariviere, Mammalian Species 714, 1 (2002). NA

Xerus inauris

Waterman , Ethology 101, 130 (1995). NA

Zalophus californianus

Le Boeuf & Campagna, in

B) Citations for population observation with details on female infanticide

The table lists the references with the observations on the characteristics of the killer and the victim and the circumstances of infanticide by females for the set of species for which this information was available. All references are listed in full below.

Figure 1 :

 1 Figure 1: The distribution of the different forms of infanticide by females in relation to the social organisation across the mammalian species in our

Figure 2 :

 2 Figure 2: Factors associated with female competition and the distribution of infanticide by females Species in which female infanticide is present are, on average, characterized by (a) living in harsher environments (lower and more unpredictable rainfall and temperatures) and by (b) higher maternal energetic investment (total mass of weaned offspring produced per year relative to maternal mass). Black lines indicate the median across the species in the sample, boxes contain 75% of the values, and whiskers extend to the extremes.

  60.West SA, Murray MG, Machado CA, Griffin AS, Herre EA. 2001 Testing Hamilton's rule with competition between relatives. Nature 409,510-513. (doi: 10.1038/35054057)

Type of infanticide Infanticide is absent present absent present absent present absent present absent present

		all		all		associated breeders	pair breeders	social breeders
		any		extraterritorial	within-group	within-group	within-group
	Sample size	200	89	253	33	34	19	29	16	92	20
	Females solitary (% species)	38%	16%	39%	76%	NA	NA	NA	NA	NA	NA
	Female philopatric (% species)	73%	68%	70%	70%	65%	100%	48%	31%	64%	63%
	Maternal investment (per year relative to bodyweight)	39%	118%	47%	150%	48%	86%	110%	169%	29%	45%
	Litter size	1.6	2.6	1.7	3.9	1.5	2.1	1.9	4	1.3	1.3
	Interbirth interval (days)	365	180	340	104	308	212	308	181	453	522
	Age at weaning (days)	127	61	113	35	107	59	105	42	229	273
	Offspring weight (relative to bodyweight)	3.6%	3.9%	3.9%	2.8%	3.5%	5.9%	5.1%	2.8%	5.1%	3.0%

Table 2 :

 2 Testing the core predictions generated by the different hypotheses proposed to explain the distribution of infanticide by females

  In associated breeders with female infanticide, offspring do not seem to have greater growth rates (they gain on average 0.28% of their adult body mass per day until weaning, median across 9 species, range 0.04% -1.72%) than in species in which females have The killer was usually the dominant breeder (as was regularly the case in 9 of 12 species) and was pregnant or with dependent infants in all cases. In most instances the killer and the victim belonged to the same group (11 of 14 species), and were consequently related (10 out of 13 species). Victims were often a few days old and all dependent.

	not been observed to kill juveniles (offspring gain on average 0.17% of their adult body mass
	per day until weaning, median across 11 species, range 0.05% -0.73%) (effect of presence of
	infanticide by females on offspring growth rate 6.1, 95% CI -16.2 -35.4, p=0.59). Killers are H2.4: Competition over social status
	either pregnant or have dependent young (21 out of 21 species) and are old, with most reports Of the 27 social breeders with female philopatry (and available data), female group members
	H2.2: Competition over milk (female infanticide occurs in 68% [15/22] of cooperative breeders versus in 4% [1/23] of pair
	Across associated breeders, the energy content of milk produced by mothers in species in breeders in which there are no other helpers). Across species in which offspring receive
	which females have been observed to kill offspring within the same breeding space (2.8 allocare, the number of potential allocarers is higher in species with, compared to species
	MJ/100ml, median across 8 species, range 1.2-4.7) does not differ from that of species in without female infanticide (infanticide present: 3 allocarers per group, median across 15

. In most species in which females kill offspring in neighbouring home-ranges

(25 of 33)

, females generally appear not to tolerate other breeding females close by and most home-ranges only contain a single breeding female (solitary or cooperatively breeding species), while in most other species females form associations or groups (home-ranges contain a single breeding female in 105 out of 268 species) (effect of presence of a single breeding female per homerange on presence of infanticide by females

12.4, p=0.007)

. In all cases, the killer was either pregnant or had dependent young of her own (17 species with observations), and all offspring that were killed were not yet weaned

(17 species)

. which such killings have not been observed (2.1 MJ/100ml, median across 7 species, range 0.8-5.7) (mean effect of milk energy on presence of infanticide by females 2.0, 95% CI -66.

3 - 83.1, p=0.97)

. suggesting that they had been observed to give birth in previous years (they are not primiparous). All victims were reported to be unweaned (22 out of 22 species). In 7 out of 14 infanticide reports from associated breeders, victims were killed as they attempted to suckle from the killer.

H2.3 Competition over allomaternal care

Infanticide by females in pair breeders occurs only when fathers provide care (all 16 species) while fathers care for offspring in only 16 of the 39 pair breeding species in which this form of infanticide is absent. In 15 of the 16 pair breeders with female infanticide, additional helpers are present (cooperative breeders), while there are only a further 7 cooperatively breeding species in which females have not been observed to kill juveniles from their own group species, range 2-23; infanticide absent: 2 allocarers per group, median across 13 species, range 1-20; effect of number of allocarers on presence of infanticide

25.3, p=0.02)

.

do not form social hierarchies in three species, hierarchical rank is determined by age in five species, and rank is influenced by nepotism in 19 species. Females have been observed to kill offspring born to other group members in eight of these latter 19 species, but in none of the species where nepotism does not influence female rank (effect of presence of nepotistic rank acquisition on presence of infanticide by females 134.1, 95%

 p<0.001

).

  . NA

		Kruuk, The Spotted Hyena: A Study of Predation
		and Social Behaviour. (University of Chicago Press,
	Crocuta crocuta	Chicago, 1972); Hofer & East, in Serengeti II. Dynamics, Management and Conservation of an	White, Behavioral Ecology 16, 606 (2005)
		Ecosystem, A. R. E. Sinclair, P. Arcese, Eds.
		(University Press, Chicago, 1995), pp. 332-363.
		Clarke et al., Proceedings of the Royal Society of
	Cryptomys damarensis	London. Series B: Biological Sciences 268, 899	NA
		(2001).
		Johnsingh, Journal of Zoology 198, 443 (1982); van
	Cuon alpinus	Noordwijk & van Schaik, in Infanticide by Males and Its Implications, Van Schaik, Janson, Eds.	NA
		(Cambridge University Press, Cambridge, 2000).
	Cynictis penicillata	Vidya et al., Journal of Zoology 278, 57 (2009).	NA
		Blumstein, in Infanticide by Males and Its
	Cynomys gunnisoni	Implications, C. P. van Schaik, C. H. Janson, Eds.	NA
		(Cambridge University Press, Cambridge, 2000).
	Cynomys leucurus	Hoogland, Behaviour 69, 1 (1979).	NA
		Blumstein, in Infanticide by Males and Its
	Cynomys ludovicianus	Implications, C. P. van Schaik, C. H. Janson, Eds.	Hoogland, Science 230, 1037 (1985)
		(Cambridge University Press, Cambridge, 2000).
		Ebensperger & Blumstein, in Rodent societies: an
	Cynomys parvidens	ecological and evolutionary perspective, Wolff, Sherman, Eds. (The University of Chicago Press,	NA
		Chicago, 2007).
		Perry & Stenson, Behaviour 122, 1 (1992);
		Lydersen et al., Journal of Comparative Physiology
	Cystophora cristata	B-Biochemical Systemic and Environmental Physiology 167, 81 (1997); Haug et al., Northwest	NA
		Atlantic Fisheries Organization Scientific Council
		Studies 26, 101 (1996).
		Kraaijeveld et al., Proceedings of the Royal Society
	Dasyurus hallucatus	of London. Series B: Biological Sciences 270, S251	NA
		(2003).
		Kraaijeveld et al., Proceedings of the Royal Society
	Dasyurus viverrinus	of London. Series B: Biological Sciences 270, S251

  ; van Noordwijk & van Schaik, in Infanticide by Males and Its Implications,Van Schaik, Janson,

		Cerqueira Santos et al., Ensaios e Ciencia 14, 83
	Eubalaena australis	(2010); Payne, Reports of the International	NA
		Whaling Commission Special Issue, 161 (1986).
	Eulemur fulvus	21 (2000). Jolly et al., International Journal of Primatology 21,	Jolly et al. International Journal of Primatology 21,
			NA
		Eds. (Cambridge University Press, Cambridge,
		2000).
	Eptesicus fuscus	Kilgour, Ethology 119, 189 (2013).	NA
	Equus burchellii	Pluhacek et al., Journal of Mammalogy 87, 35 (2006).	NA
		van Noordwijk & van Schaik, in Infanticide by
	Equus caballus	Males and Its Implications, Van Schaik, Janson, Eds. (Cambridge University Press, Cambridge,	NA
		2000).
		Smith et al., African Journal of Ecology 46, 207
	Equus zebra	(2008); Lloyd & rasa, Behavioral Ecology and	NA
		Sociobiology 25, 411 (1989).
	Erythrocebus patas	Enstam et al., International Journal of Primatology 23, 85 (2002).

  Wolff, P. W. Sherman, Eds. (The University of Chicago Press, Chicago, 2007).

	Macaca nigra	A. Engelhardt, Pers. Comm.	Engelhardt, pers com; Duboscq et al., American Journal of Primatology 75, 361 (2013)
	Macaca radiata	Ebensperger, Biological Reviews 73, 321 (1998).	Silk et al. Animal Behaviour 29, 1106 (1981).
		van Noordwijk & van Schaik, in Infanticide by
	Macaca silenus	Males and Its Implications, Van Schaik, Janson, Eds. (Cambridge University Press, Cambridge,	Singh et al. 2006 Journal of Bioscience 31, 369 (2006)
		2000).
	Macaca sinica	Dittus, Animal Behaviour 36, 1628 (1988).	Dittus, Behavioral Ecology and Sociobiology 19, 257 (1986)
	Macaca sylvanus	Ebensperger, Biological Reviews 73, 321 (1998); Paul & Kuester, Behavioral Ecology and Sociobiology 39, 139 (1996)	Paul & Kuester, Behavioral Ecology and Sociobiology 39, 139 (1996)
	Macaca thibetana	Berman et al., International Journal of Primatology
			NA
	Hylobates lar	Borries et al., Behavioral Ecology and Sociobiology 65, 685 (2011).	NA
	Lagothrix lagotricha	Stevenson, Primates 47, 239 (2006); Di Fiore et al., American Journal of Primatology 68, 637 (2006).	NA

  Blumstein, in Infanticide by Males and Its Implications, C. P. van Schaik, C. H. Janson, Eds. (Cambridge University Press, Cambridge, 2000); Ebensperger & Blumstein, in Rodent societies: an ecological and evolutionary perspective, J. O. Wolff, P. W. Sherman, Eds. (The University of Chicago Press, Chicago, 2007).

		Packer & Pusey, in Infanticide: Comparative and	Packer & Pusey, in Infanticide: Comparative and
	Panthera leo	Evolutionary Perspectives, G. Hausfater, S. B. Hrdy,	Evolutionary Perspectives, G. Hausfater, S. B. Hrdy,
		Eds. (Aldine, New York, 1984), pp. 31-42.	Eds. (Aldine, New York, 1984), pp. 31-42.
	Panthera onca	Soares et al., Genetics and Molecular Biology 29, 735 (2006).	NA NA
	Panthera pardus	Hunter & Skinner, Transactions of the Royal Society of South Africa 56, 79 (2003).	NA
	Panthera tigris Rhabdomys pumilio	Hunter & Skinner, Transactions of the Royal Society of South Africa 56, 79 (2003). Schradin & Pillay, Journal of Comparative Psychology	NA
		Ebensperger, Biological Reviews 73, 321 (1998);
		Palombit, in Sexual Selection and Reproductive	Smuts, Sex and Friendship in Baboons. Routledge,
	Papio anubis	Competition in Primates: New Perspectives and	(1985).
		Directions, C. B. Jones, Ed. (American Society of
		Primatologists, 2003), pp. 367-412.
		Collins et al., in Infanticide: comparative and
		evolutionary perspectives G. Hausfater, S. B. Hrdy,
		Eds. (Aldine: New York, 1984); Wasser & Starling,
		in Primate ontogeny, cognition and social
		behavior, J. G. Else, P. C. Lee, Eds. (Cambridge
	Papio cynocephalus	University Press, Cambridge, 1986), pp. 343-354; Shopland & Altmann, International Journal of	Kleindorfer & Wasser, Behavioral Ecology and Sociobiology, 56, 328. (2004)
		Primatology 13, 61 (1987); Palombit, in Sexual
		Selection and Reproductive Competition in
		Primates: New Perspectives and Directions, C. B.
		Jones, Ed. (American Society of Primatologists,
		2003), pp. 367-412.
		Ebensperger, Biological Reviews 73, 321 (1998);
		Palombit, in Sexual Selection and Reproductive
	Papio hamadryas	Competition in Primates: New Perspectives and	NA
		Directions, C. B. Jones, Ed. (American Society of
		Primatologists, 2003), pp. 367-412.
		Ebensperger, Biological Reviews 73, 321 (1998);
		Palombit, in Sexual Selection and Reproductive
	Papio papio	Competition in Primates: New Perspectives and	NA
		Directions, C. B. Jones, Ed. (American Society of
		Primatologists, 2003), pp. 367-412.
		Ebensperger, Biological Reviews 73, 321 (1998);
		Palombit, in Sexual Selection and Reproductive	Cheney et al., 2006. In Reproduction and fitness in
		Competition in Primates: New Perspectives and	baboons: Behavioral, ecological, and life history
	Papio ursinus	Directions, C. B. Jones, Ed. (American Society of	perspectives. (Springer US 2006) pp. 147-176;
		Primatologists, 2003), pp. 367-412; Brain,	Brain, International Journal of Primatology 13,
		; Malcolm, International Journal of Primatology 13, 593	593 (1992).
		Journal of Zoology 208, 457 (1986); van Noordwijk (1992).
		& van Schaik, in Infanticide by Males and Its Blumstein, in Infanticide by Males and Its	NA
	Paraxerus cepapi	Implications, Van Schaik, Janson, Eds. (Cambridge Implications, C. P. van Schaik, C. H. Janson, Eds.	NA
		University Press, Cambridge, 2000); Wright et al., (Cambridge University Press, Cambridge, 2000).
		Journal of Mammalogy 91, 437 (2012). Blumstein, in Infanticide by Males and Its
		Buettner-Janusch, Folia primatologica 2, 93 (1964); Implications, C. P. van Schaik, C. H. Janson, Eds.
	Otolemur crassicaudatus	Welker & Schaäfer-Witt, International journal of (Cambridge University Press, Cambridge, 2000);	NA
	Peromyscus californicus	primatology 9, 509 (1988). Ebensperger & Blumstein, in Rodent societies: an	NA
	Otolemur garnettii	Welker & Schaäfer-Witt, International journal of primatology 9, 509 (1988). ecological and evolutionary perspective, J. O. Wolff, P. W. Sherman, Eds. (The University of	NA
	Ovibos moschatus	Lent, Mammalian species 302, 1 (1988). Chicago Press, Chicago, 2007). Blumstein, in Infanticide by Males and Its	NA
	Ovis aries Peromyscus leucopus Peromyscus maniculatus	Gaillard et al., Annual Review of Ecology and Implications, C. P. van Schaik, C. H. Janson, Eds. Systematics 31, 367 (2000). (Cambridge University Press, Cambridge, 2000). Festabianchet, Journal of Animal Ecology 58, 785 Blumstein, in Infanticide by Males and Its (1989); Festabianchet et al., Behavioral Ecology 5, Implications, C. P. van Schaik, C. H. Janson, Eds.	NA NA NA
	Ovis canadensis Peromyscus polionotus	21 (1994); van Noordwijk & van Schaik, in (Cambridge University Press, Cambridge, 2000). Infanticide by Males and Its Implications, Van Margulis et al., Animal Behaviour 69, 627 (2005); Schaik, Janson, Eds. (Cambridge University Press, Cambridge, 2000). Kaufman & Kaufman, Journal of Mammalogy 68, 275 (1987); Foltz, American Naturalist 117, 665	NA NA
	Ozotoceros bezoarticus	Kunz & Ebensperger, Acta Chiropterologica 1, 17 (1981). (1999). Somers et al., Acta Theriologica 40, 257 (1995);	NA
		Furuichi et al., in P. M. Kappeler, D. P. Watts, Eds., van Noordwijk & van Schaik, in Infanticide by
	Pan paniscus Phacochoerus aethiopicus	Long-term field studies of primates. (Springer-Males and Its Implications, Van Schaik, Janson,	NA
		Verlag, Berlin Heidelberg, 2012), pp. 413-433. Eds. (Cambridge University Press, Cambridge,
		Goodall, The chimpanzees of Gombe: patterns of 2000).
	Pan troglodytes	behaviour. Harvard University Press, Cambridge, (1996); Townsend et al., Current Biology 17, R355	NA
		(2007).

NA

Phascogale tapoatafa

Rhind, Wildlife

Research 29, 247 (2002)

. NA

Phoca groenlandica

Le

Boeuf & Campagna, in Infanticide and parental care, S. Parmigiani, F. vom Saal, Eds. (Harwood Academic Publishers, 1994), pp. 257-276. NA Rattus norvegicus

  Macdonald, Journal of Zoology 239, 525 (1996); van Noordwijk &van Schaik, in Infanticide by Males and Its Implications, Van Schaik, Janson, Eds. (Cambridge University Press, Cambridge, 2000). Blumstein, in Infanticide by Males and Its Implications, C. P. van Schaik, C. H. Janson, Eds. (Cambridge University Press, Cambridge, 2000). Blumstein, in Infanticide by Males and Its Implications, C. P. van Schaik, C. H. Janson, Eds. (Cambridge University Press, Cambridge, 2000). Blumstein, in Infanticide by Males and Its Implications, C. P. van Schaik, C. H. Janson, Eds. (Cambridge University Press, Cambridge, 2000). Blumstein, in Infanticide by Males and Its Implications, C. P. van Schaik, C. H. Janson, Eds. (Cambridge University Press, Cambridge, 2000). Blumstein, in Infanticide by Males and Its Implications, C. P. van Schaik, C. H. Janson, Eds. (Cambridge University Press, Cambridge, 2000).

			NA
	Spermophilus beecheyi	Ebensperger, Biological Reviews 73, 321 (1998).	NA
		Ebensperger & Blumstein, in Rodent societies: an
	Spermophilus beldingi	ecological and evolutionary perspective, J. O. Wolff, P. W. Sherman, Eds. (The University of	NA
		Chicago Press, Chicago, 2007).
	Spermophilus citellus	Ramos-Lara et al. Mammalian Species 46, 71 (2014).	NA
	Spermophilus columbianus		NA
	Spermophilus franklinii		NA
	Spermophilus fulvus		NA
	Spermophilus parryii		NA
	Spermophilus richardsonii		NA
		Blumstein, in Infanticide by Males and Its
	Spermophilus townsendii	Implications, C. P. van Schaik, C. H. Janson, Eds.	NA
		(Cambridge University Press, Cambridge, 2000).
	Spermophilus tridecemlineatus	Blumstein, in Infanticide by Males and Its (Cambridge University Press, Cambridge, 2000). Implications, C. P. van Schaik, C. H. Janson, Eds.	NA
	Suncus murinus	Rissman, Journal of Experimental Zoology 4, 207 (1990).	NA
	Suricata suricatta	Clutton-Brock et al., Proceedings of the Royal Society of London, Series B 265, 2291 (1998).	NA
	Sus scrofa	Andersson et al., Applied Animal Behaviour Science 134, 184 (2011).

Andersson et al. Applied Animal Behaviour Science

  Patterson et al., Proceedings of the Royal Society, Series B 265, 1167 (1998).NA

		Ebensperger, Biological Reviews 73, 321 (1998);
	Ursus americanus	Samson, Canadian Journal of Zoology 79, 633	NA
		(2001).
		Ebensperger, Biological Reviews 73, 321 (1998);
	Ursus arctos

  Infanticide and parental care, S. Parmigiani, F. vom Saal, Eds.(Harwood Academic Publishers, 1994), pp. 257-276.

			NA
		Le Boeuf & Campagna, in Infanticide and parental
	Zalophus wollebaeki	care, S. Parmigiani, F. vom Saal, Eds. (Harwood	NA
		Academic Publishers, 1994), pp. 257-276.

Pruetz & Isbell, Behavioral Ecology and Sociobiology 49, 38 (2000); Isbell & Pruetz, International Journal ofPrimatology 19, 837 (1998).

Acknowledgements

We thank Tim Clutton-Brock, Peter Kappeler, Oliver Höner and Eve Davidian for useful discussions.

Data Accessibility

All data are provided in the supplementary material and are deposited at the Knowledge Network for Biocomplexity doi:10.5063/F1ZG6QFR.

Funding DL was supported by the European Research Commission (grant 294494-THCB2011 to Prof. T. Clutton-Brock) and the Max Planck Society. EH was supported by the Natural Environment Research Council (grant NE/RG53472) and during the writing-up of this manuscript funded by the ANR (grant ANR-17-CE02-0008).

Authors' Contributions

Both authors contributed to the conception and design of the study, collected and interpreted the data, wrote the article, and gave final approval; the analyses were done by DL with input from EH.

Competing Interests

We have no competing interests.

Supplementary Material

This submission includes all data in Supplementary Tables 1 (comparative data) and 2 (population observations), with references for observations on infanticide by females in Supplementary File 1, and credits for the animal drawings used in Figure 1