Infanticide among animals: A review, classification, and examination of the implications for the reproductive strategies of females
Infanticide among animals is a widespread phenomenon with no unitary explanation. Although the detrimental outcome for the infant is fairly constant, individuals responsible for infanticide may or may not benefit, and when they gain in fitness there may be considerable variation in how they gain. Sources of increased fitness from infanticide include: (1) exploitation of the infant as a resource, (2) elimination of a competitor for resources, (3) increased maternal survival or lifetime reproductive success for either mother or father by elimination of an ill-timed, handicapped, or supernumerary infant, and, finally, (4) increased access for individuals of one sex for reproductive investment by the other sex at the expense of same-sex competitors. Predicted attributes of the perpetrators (such as sex and degree of relatedness to the infant), attributes of the victim (i.e., age and vulnerability), as well as schedule of gain, vary for each class. Under some circumstances, individuals commit infanticide which does not result in any prospect for gain; such instances are considered nonadaptive or “pathological.” In those cases where infanticide does on the average increase fitness, selection pressures favoring it have arisen as a result of the extensive and time-consuming investment involved in production of young, and the extreme vulnerability that characterizes infancy in many animals. The scattered but nevertheless extensive occurrence of infanticide among primates raises the question of inter-specific variation. Factors such as seasonality in breeding, cooperation between individuals in defense of infants, marginal habitats, and low intrinsic rates of natural increase may outweigh other pressures, such as short male tenure lengths, which select for infanticide. Within the same species, infanticide may occur in some areas but not others, as evidenced by the variable expression of infanticidal behavior among Hanuman langurs. At present, the most obvious factor influencing facultative expression of the infanticidal trait is population density. Where it occurs, sexually selected infanticide is a significant cause of mortality. As such, it has important implications for the evolution of behavior, particularly for patterns of association between males and females, for female reproductive physiology, and for the patterning of sexual receptivity by females. It is hypothesized that the threat posed by infanticide is one of several pressures selecting for a shift among higher primates away from strictly cyclical estrous receptivity towards socially determined or situation-dependent receptivity.