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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'.

Male mammals often kill conspecific offspring. The benefits of such infanticide to males, and its costs to females, probably vary across mammalian social and mating systems. We used comparative analyses to show that infanticide primarily evolves in social mammals in which reproduction is monopolized by a minority of males. It has not promoted social counterstrategies such as female gregariousness, pair living, or changes in group size and sex ratio, but is successfully prevented by female sexual promiscuity, a paternity dilution strategy. These findings indicate that infanticide is a consequence, rather than a cause, of contrasts in mammalian social systems affecting the intensity of sexual conflict., (Copyright © 2014, American Association for the Advancement of Science.)

Female-female competition over paternal care has rarely been investigated in promiscuous mammals, where discreet forms of male care have recently been reported despite low paternity certainty. We investigated female competition over paternal care in a wild promiscuous primate, the chacma baboon (Papio ursinus), where pregnant and lactating females establish strong social bonds (friendships) with males that provide care to their offspring. We tested whether pregnant and lactating females interfere with the sexual activity of their male friend to prevent new conceptions that might lead to the subsequent dilution of his paternal care. We found that pregnant and lactating females were more aggressive towards oestrous females when they had recently conceived themselves, and when the oestrous female was mate-guarded by, and showed greater sexual activity with, their male friend. This aggression also reduced the likelihood of conception of the targeted female. These findings indicate that females can aggressively prevent further conceptions with their offspring's carer through reproductive suppression. Competition over access to paternal care may play an important and underestimated role in shaping female social relationships and reproductive strategies in promiscuous mammalian societies. [ABSTRACT FROM AUTHOR]

Multimale–multifemale groups, where both sexes mate promiscuously and the operational sex ratio is male biased, represent a classical mammalian society. Theory predicts low mating competition between females in such societies, but this is inconsistent with the frequent occurrence of female sexual signals. This study explores the determinants of female competition under such conditions by testing 3 hypotheses relating to patterns of aggression over the reproductive cycle in wild chacma baboons (Papio ursinus). Primarily, we expect the frequency of aggression to be highest among 1) lactating and pregnant females, who experience the greatest energetic demands, if females compete mainly over food, 2) lactating females, if females compete mainly over paternal care of infants, or 3) sexually active (swollen) females, if females compete mainly over mates. Data were collected from 27 females in 2 groups over 18 months and analyzed using mixed models. Our results provide most support for the mating competition hypothesis: aggression increases with the number of swollen females in a group, swollen females receive the most aggression, and mate-guarded swollen females receive more aggression than when unguarded. However, our analyses further indicate that such aggression rather than arising from direct mating competition, most likely reflects reproductive suppression and/or an increased exposure of swollen females to incidental aggression. These findings reveal the importance of sex in shaping social relationships among females in large primate groups where they were traditionally considered to be determined primarily by access to resources. Aggression associated with access to mates represents an extra cost of sociality to females. [ABSTRACT FROM PUBLISHER]

Recent research reveals that female reproductive competition is common and may shape the social and reproductive strategies of female mammals. This study explores the determinants and intensity of female intrasexual conflicts in a wild promiscuous primate, the chacma baboon, Papio ursinus . We tested a suite of hypotheses to assess whether female–female aggression was primarily driven by instantaneous competition for food, mates or paternal care, or aimed at reducing future competition among offspring via reproductive suppression. Behavioural data were gathered from 53 females in two groups over two contrasting 2-year periods (2005–2006, 2013–2014): the first characterized by stability in the male dominance hierarchies, the second by instability induced by several immigration events and male take-overs. In both periods, we found that sexually receptive females received high levels of aggression from other sexually receptive females, consistent with competition over mating opportunities. In the unstable period, females exchanged higher rates of aggression than in the stable period, regardless of reproductive state, but lactating females received most aggression, consistent with higher competition over social access to male partners in response to increased infanticide risk. There was no evidence that aggression between females reflected either competition over food or reproductive suppression. These findings indicate that patterns of aggression between females fluctuate with sociodemographic factors affecting sexual and social access to males and reflect reproductive competition more closely than resource competition in this promiscuous primate society. [ABSTRACT FROM AUTHOR]