Infanticide (zoology)

Infanticide is the intentional killing of dependent immatures by a member of the same species or a conspecific. Victims may be as young as fertilized eggs or nearing independence from their parents. The infanticidal attackers may be strangers or relatives. The motive is always selfish competition, but sometimes the young are direct competitors and sometimes indirect competitors with the killers. Infanticide occurs in every class of animal, but mammals and birds seem to have evolved the most pervasive and pernicious form, sexually selected infanticide. Wherever it is seen, infanticide reveals the darker side of evolutionary adaptation. There are three scientific explanations for infanticide, each of which applies to particular circumstances.

Infanticide by Genetic Relatives

The first circumstance occurs when related individuals kill dependent young. For example, extreme sibling rivalry occurs in spotted hyenas (Crocuta crocuta) and many raptors. The unpredictability of resources or extreme competition for parental care may lead siblings to fight to the death in the den or nest. Even parents may act infanticidally. When parents have larger numbers of offspring than they can feed, the parents themselves may neglect or inflict damage on the smallest or least vigorous young so as to reduce the number of offspring and increase each survivor’s chances of success. Sibling rivalry and parental neglect or abuse of young can lead to infanticide in extreme cases when resources, such as food or parental time and energy, are in short supply. The brutal logic of natural selection shows that selfishness and lethal competition can divide even close kin. However, in most animal species, unrelated individuals are more dangerous to the young.

Infanticide by Direct Competitors

The second circumstance in which infanticide arises is referred to as local resource competition. When immatures use resources that unrelated animals need, aggression may be severe and directed to killing the infants. All ages of infants are vulnerable, even independent juveniles. In the burying beetle (Nicrophorus orbicollis), male and female pairs compete for access to rotting meat in which to lay eggs. If defenders of such a resource are displaced by an intruding pair, the intruders proceed to kill and eat the eggs (ovicide) or larvae of the previous pair. Similarly, many birds, such as black-and-white casqued hornbills (Bycanistes subcylindricus), compete for rare tree-hole nesting sites. If adults encounter another nest with eggs in such a tree hole, they will roll them out or crush them and lay their own eggs. Finally, cannibalism of unrelated young has been seen in many amphibians, fish, reptiles, and even in chimpanzees. All of these cases represent extreme competition where resources are scarce and vulnerable young are eliminated or eaten by unrelated killers. The behavior patterns seen in this type of infanticide often resemble predation, are directed at the young of any age, and can be performed by adults of either sex.

Sexually Selected Infanticide

The third circumstance in which infanticide occurs is motivated by sexual competition among adults. In a wide range of mammals, including several genera of primates, adult males will kill unrelated infants if they can then mate with the mother. The infant's mother does not usually reject the infanticidal male even though he has inflicted a tremendous cost on her. Typically, it takes female mammals many months—even years—to nurse young to independence. During this period of infant dependency, the mother is generally physiologically incapable of reproducing. An infanticidal male benefits if he can cut short this period to fertilize the female’s next egg. Therefore, a successful infanticidal male eliminates another male’s offspring and advances his own reproductive career. Sexually selected infanticide is not restricted to mammals, although the best-documented cases come from lions (Panthera leo), langur monkeys (Semnopithecus entellus), and rodents (order Sciurognathi). Some birds may also behave infanticidally. For example, the wattled jacana (Jacana jacana) shows a fascinating reversal. In this species, the adult females may commit ovicide and infanticide against unrelated young. Adult male jacanas incubate eggs on the nest and protect the young from predators, so females strive to monopolize the parental care donated by males. To do this, a female must eliminate the young of another female and lay her own eggs in the care of the male. Typically, the behavioral patterns seen in sexually selected infanticide differ from those in local resource competition. The young are virtually never eaten, and permanent separation of mother and young is the primary goal—whereupon attacks usually cease.

Evolutionary Consequences of Infanticide

Infanticide is widespread and may reach high frequencies in certain species, but it is not universal. Siblicide and parental neglect are found in a very restricted subset of birds and very few mammals. Local resource competition is more common, found in most classes of animals. Finally, sexually selected infanticide is most common in mammals, but even here, many orders of mammals never display infanticide. Species with a very long lactation period relative to their gestation period are most vulnerable to sexually selected infanticide. The reason for this is that infanticidal attacks are risky because of maternal defense and counterattacks by allies. Therefore, infanticidal behavior will only evolve when the males gain substantially by shortening the lactational infertility of the mother.

Clearly, infants and their parents have a very strong motivation to avoid infanticide. Evolution has favored various mechanisms to reduce the risk. In general, infants avoid strangers, this is seen even in humans. In some primates, it seems that infants are born with unusual coat colorations that may impede males’ efforts to determine which infants are theirs and which were fathered by other males. Parents are normally very protective of their young, both because of predators and because of the risk of attack by conspecifics. This parental protection is expressed through frequent proximity, carrying, and physical defense. One of the most fascinating consequences of a high risk of infanticide is the tendency for the father and mother to establish a long-lasting relationship whose primary benefit is the protection of the young from conspecifics. This is seen in burying beetles and many primates. Alternatively, groups of mothers may cooperate in protection against infanticidal males, as in lions and langurs. Therefore, infanticide is one of the few evolutionary pressures that favor complex social relationships.

Principal Terms

Conspecific: a member of the same species

Genera: plural of genus, a grouping of animals above the species level

Gestation: the period when the young are nourished within the mother’s body; pregnancy

Lactation: the period when mammal mothers produce milk to nourish their infants

Ovicide: killing of fertilized eggs

Raptors: predatory birds such as hawks and eagles

Siblicide: infanticide committed by the siblings of the individual killed

Bibliography

Butynski, T. M. “Harem-Male Replacement and Infanticide in the Blue Monkey (Cercopithecus mitis stuhlmanni) in the Kibale Forest, Uganda.” American Journal of Primatology 3 (1982): 1-22.

Hausfater, Glenn, and Sarah Blaffer Hrdy, eds. Infanticide: Comparative and Evolutionary Perspectives. New York: Aldine, 1984.

Morell, Virginia. “Why Do Animals Sometimes Kill Their Babies?” National Geographic, 28 Mar. 2014, www.nationalgeographic.com/science/article/140328-sloth-bear-zoo-infanticide-chimps-bonobos-animals. Accessed 2 July 2023.

Parmigiani, Stefano, and Frederik S. vom Saal. Infanticide and Parental Care. Langhorne, Pa.: Harwood Academic Press, 1994.

Van Schaik, Carel P., and Charles H. Janson, eds. Infanticide by Males and Its Implications. New York: Cambridge University Press, 2000.