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Infanticide Among Animals: A Review, Classification, and Examination of the Implications for the Reproductive Strategies of Females Sarah Blaffer Hrdy Peabody Museum, Harvard Unirqersity Infanticide among animals is a widespread phenomenon Within the same species, infanticide may occur in some with no unitary explanation. Although the detrimental areas but not others, as evidenced by the variable outcome for the infant is fairly constant, individuals expression of infanticidal behavior among Hanuman responsible for infanticide may or may not benefit, and langurs. At present, the most obvious factor influencing when they gain in fitness there may be considerable facultative expression of the infanticidal trait is popu- variation in how they gain. Sources of increased fitness lation density. from infanticide include: (1) exploitation of the infant Where it occurs, sexually selected infanticide is a as a resource, (2) elimination of a competitor for re- significant cause of mortality. As such, it has important sources, (3) increased maternal survival or lifetime re- implications for the evolution of behavior, particularly productive success for either mother or father by elim- for patterns of association between males and females, ination of an ill-timed, handicapped, or supernumerary for female reproductive physiology, and for the pat- infant, and, finally, (4) increased access for individuals terning of sexual receptivity by females. It is hypothe- of one sex for reproductive investment by the other sex sized that the threat posed by infanticide is one of sev- at the expense of same-sex competitors. Predicted attri- eral pressures selecting for a shift among higher butes of the perpetrators (such as sex and degree of primates away from strictly cyclical estrous receptivity relatedness to the infant), attributes of the victim (i.e., towards socially determined or situation-dependent re- age and vulnerability), as well as schedule of gain, vary ceptivity. for each class. Under some circumstances, individuals commit infanticide which does not result in any prospect Key Words: Infanticide: Female reproductive strate- for gain: such instances are considered nonadaptive or gies: Population density. “pathological.” In those cases where infanticide does on the average increase fitness, selection pressures fa- voring it have arisen as a result of the extensive and INTRODUCTION time-consuming investment involved in production of young, and the extreme vulnerability that characterizes Offspring are vital to the continued survival of infancy in many animals. any species. At first glance, it seems surprising The scattered but nevertheless extensive occurrence to find selection for behavior that does not con- of infanticide among primates raises the question of tribute to the survival of infants, odder still to inter-specific variation. Factors such as seasonality in breeding, cooperation between individuals in defense of find selection for behavior that actually de- infants, marginal habitats, and low intrinsic rates of creases infant survivorship. Yet nature provides natural increase may outweigh other pressures, such as abundant examples, many of them paradigms short male tenure lengths, which select for infanticide. for the evolutionary process: natural selection at the individual level may-at least temporar- ily-preempt species advantage. Received January 18. 1979: revised May 31. 1979 The most widely reported cases of infanticide Address reprint requests lo: Sarah Blafkr Hrdy. Peabody Museum. Harvard University. Cambridge, MA 02138. involve adult males. but adult females and even Ethology and Sociobiology I: 13-40 (1979) 13 @ Elsevier North Holland. Inc.. 1979 Ol62-3095/79/010013-28/502.25 14 S. B. Hrdy hhhhhhhhhhhhhhhhhhs are sometimes implicated. In- an artifact of disturbed settings (e.g., Calhoun, fants may be killed not only by outsiders but by 1962). but rather a widely spread phenomenon members of their own group, even their parents. in natural populations. particularly among pre- As diverse as the perpetrators are the selection datory fish, rotifers. and insects which live in pressures that promote infanticidal behavior. ln- fresh water. fanticide among animals cannot be understood Rotifers such as A.splorrc/~r~m si~holtli. for ex- as a unitary phenomenon. ample, provide elegant evidence for the extent This paper attempts to classify manifold in- and duration of selective pressures from canni- stances of infanticide into categories with some balism. Male rotifers. which are smaller than explanatory and predictive potential. In general, females in this group. have evolved a variety of examples of infant-killing by either males or fe- major structural adaptations (such as proturber- males can be explained by recourse to one of antes in the body wall) and other features which five classes of explanation. The first four-ex- specifically protect them from being eaten by ploitation. resource competition. parental ma- other rotifers. including clone-mates (Gilbert, nipulation. and sexual selection-all assume that 1976). infanticidal behavior is evolved and adaptive for Lethal exploitation of an infant is not limited those individuals who perpetrate the killing. The to eating it. Among various primates, for ex- fifth class of explanation-social pathology- ample, infants may be used as “buffers” in does not assume that infant-killing is adaptive. agonistic episodes (Deag and Crook. 1971, and Cannibalism is not considered here as a separate especially. Popp, 1978) or as an object for “play category. Typically, cannibalism falls under ex- mothering” (Lancaster. 197 I ): such episodes ploitation of the infant as a resource, but it may may on rare occasions lead to injury or starva- also occur opportunistically in conjunction with tion of the infant as an incidental byproduct ohhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhf other classes of infant-killing. exploitation. Among primates. the majority of infant deaths so far attributable to adult females do not occur as a result of violence. but rather CLASSES OF INFANTICIDE starvation. This phenonmenon. known as “aunting to death.” results when nonlactating Exploitation females take an infant from its mother and for- When individuals responsible for an infant’s cibly retain it (Hrdy. 1976). As was true with death directly benefit from consumption or use the early reports of infanticide by male primates. of their victim, infanticide can be said to be a there has been a general tendency to dismiss form of “exploitation”: the infant itself becomes deaths due to “kidnap” or “aunting” as quirks a resource, either a food resource, a protective or pathologies in an otherwise benign system of buffer against aggression by third parties. or a cooperative infant care which is primarily “prop” for obtaining maternal-like experience. geared to benefit the infant. However, this view The most obvious widespread form of such accords with neither the abusive treatment exploitation is cannibalism. The victim must be sometimes dealt to borrowed infants (Hrdy, killed in order for the resource to be exploited, 1977). nor with accumulating case histories of and infants are particularly vulnerable targets. infant fatalities (Quiatt, 1977). Among some ground squirrels (Steiner. 1972). In most cases of infant transfer between fe- hyenas (Kruuk, 1972), occasionally in lions males belonging to the same group. the mother (Schaller, 1972). and other predatory creatures, is able to retrieve her infant without difficulty. particularly some fish and insects (Fox, 1975). Sometimes, however, as has been reported on cannibalism appears to be an end in itself. not multiple occasions for rhesus macaques (Hinde clearly distinguished from predation except that and Spencer-Booth, 1967: Quiatt. 1977) and immatures are more likely to be eaten than squirrel monkeys (Rosenblum. 1972) and in a adults because of the size differential among single case among wild Lowe’s guenons (CPY- animals that would otherwise be nearly equals. copirh~~crls ctrmphdli lowi) (Bourliere et al., The extensive literature on intraspecific preda- 1970) and captive colobus monkeys (Horwich tion in natural populations has been recently and Manski, 1975). the infant is taken by females reviewed by Laurel Fox (1975). Contrary to a who, because of their ability to dominate the common misconception, cannibalism is not just mother, prevent her from retrieving her infant. Infanticide among Animals 15 Where observers did not intervene. infants males may be particularly at risk. Among wild “kidnapped” by nonlactating females have died. dogs, pups born to a subordinate pack member The threat of losing an infant to a more dom- may be murdered by the dominant female, inant female may contribute to maternal pos- thereby garnering for her own offspring a larger sessiveness, and restrict the occurrence of post- proportion of the pack’s resources, including not natal infant-sharing to species with less only meat brought back by other pack members pronounced dominance hierarchies (Hrdy. 1976: but the milk of the mother who lost her pups McKenna, in press). Only among colobines, (van Lawick, 1972). Mother’s milk may also fig- where dominance relations are typically less ure in infanticide by female elephant seals who rigid than among macaques or baboons, do attack and kill infants who become separated mothers routinely give up their infants in the from their mothers: such “orphans” survive first hours after birth. only by stealing milk from other mothers and, Among colobines such