Abstract
In most mammalian species, females regularly interact with kin, which is expected to reduce aggressive competitive behaviour among females. It may thus be difficult to understand why infanticide by females has been reported in numerous species and is sometimes perpetrated by groupmates. 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. By contrast, infanticide is not immediately modulated by the degree of kinship among females, and females occasionally sacrifice closely related infants. 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. This article is part of the theme issue 'The evolution of female-biased kinship in humans and other mammals'.
Highlights
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 [1 –3]
Our findings establish that female infanticide is widespread across mammals and our comparative analyses support the idea that this behaviour is adaptive, even when the target may be related
Within each type of social organization, we do find that females, like males, appear to commit infanticide when the presence of the victim might otherwise limit their own reproductive success
Summary
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 [1 –3]. 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 common in polygynous societies where one or a few alpha male(s) monopolize mating opportunities over short periods before losing dominance to others [13,14,15]. We test core predictions generated by each hypothesis for the potential adaptive benefits across species and investigate population-level information on the traits of killers and victims to assess whether females have been observed to commit infanticide when they are most likely to benefit from such killings. We assess whether: (i) instances where females kill offspring in neighbouring ranges (breeding space hypothesis) are most likely explained by competition over breeding space; (ii) instances where females kill offspring born in the same breeding association are most likely explained by competition over milk (milk competition hypothesis); (iii) instances where females kill offspring in groups where usually only a single female reproduces are most likely explained by competition over offspring care (allocare hypothesis); and (iv) instances where females kill offspring born in groups with multiple breeders are most likely explained by competition over social status or group membership (social status hypothesis)
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