Infanticide in Primates
Infanticide is the murder of infants usually with the mother’s consent. It is a phenomenon normally seen in animals especially primates. The theories that are usually associated with infanticide in primates are the sexual selection hypothesis, the social pathology theory, the source of protein theory and the elimination-of-future-competitors theory (Hrdy et al, 1995). Parental manipulation has been indicated by some researchers.
The first to report the phenomenon of infanticidal killings was Sugiyama in 1965. Infanticide got widespread attention only after Hrdy spoke about the sexual selection hypothesis in 1974.
Darwin defined the sexual hypothesis theory as a struggle between members of one sex for access to the other and the infanticide being a means of reducing offspring for the competitor. A male would kill unrelated males in order to get the bereaved mother which he fancies to mate (Hrdy et al, 1995). The male increases his reproductive achievements by killing infants unrelated to him hopefully in the belief that the mother would be receptive to him. He thereby ensures his gene pool in 2 ways: killing to reduce others’ infants and mating to enhance his offspring (Brown, 1996). The infanticidal act would terminate the lactational amenorrhoea of the female who lost her infant and shorten the interbirth period (Sussman et al, 1995). The infanticidal male thus gets an early opportunity to mate with the female.
Sussman et al (1995) opposed the theory claiming that the evidence observed did not prove the sexual selection hypothesis and that there was no genetic basis. Their argument was that the infant deaths were accidental caused by the aggressiveness of social conflicts. Sussman also pointed out that the evidence for the postulation of the theory was not universal and concerned the results from only one species of primates, Presbytes entellus, from a single site, Jodhpur. However evidence has been found from other studies for saying that infanticide is common and is seen in five of the six varieties of primates in captivity and the field (Hrdy et al, 1995). There is a view that the females prefer to mate with an infanticidal male, believing that males who lost their children are unfit.
When suggesting that predation could be the cause of disappearance of healthy animals, the finding of remains and cries in the night, Isbell’ study (1990) claims that actual predation scenes are not found and the disappearing animals are usually females and infants. Infanticide like predation is a recurring event not witnessed. Adult males have been found to be close to the infants they have sired. Females who have very small babies reject suitors in encounters. Some mothers leave their babies with willing familiar males rather than taking them along to be in the company of new males and other similar incidents. Sussman believes that the infanticides are incidental killings describing them as ‘inconsequential epiphenomena of aggressive episodes’ (Hrdy et al, 1995). Infant killings are accepted by all researchers as occurring and doing so after takeovers by aggressive new males (Bartlett et al, 1933).
Primatologists generally believe that male infanticide is accepted as an evolutionary strategy (Brown, 1996) which gives the infanticidal male an adaptive advantage (Sussman et al, 1995). Dohlinow (an anthropologist at the University of California, Berkeley) and Sussman believe that infanticide is an abnormal phenomenon.
They go more by the social pathology theory saying that infanticide is just a by-product of aggression due to overcrowding and social disarray (Brown, 1996). Females also could kill the babies of closely related animals to feed their weaning babies.
Previously infanticidal killings were considered harmful nut now they are considered to be normal behavior (Bartlett et al, 1993). The sexual selection hypothesis has also become the accepted theory.
Conclusion
Hrdy’s sexual selection hypothesis has made a huge impact in primate studies and also on behavioral biology. Primatologists now speak of the male reproductive strategy and the female counterstrategies. Infant mortality is believed to be due to aggression among the primates and not of any evolutionary significance. Sussman believes in the social conflict theory. She believes that the infanticide is a male reproductive strategy and that the new male increases his reproductive prowess by unseating an earlier leader and that infanticide is normal and reduces the interbirth interval. Infanticides have been reported in invertebrates, fish, amphibians, birds and non primate mammals (Glander, 1984). Personally I feel that the sexual selection hypothesis is easier to understand.
References
Bartlett,TQ; Sussman, R.W. and Cheverud J.M.; (1933), “ Infant killing in primates: a review of observed cases with specific reference to the sexual selection hypothesis” American Anthropologist 95: 958-990.
Brown, S.Katheryn; (1996), “Infanticide as a way to get ahead”, Bioscience, Vol 46, No.3, pgs. 174-177.
Glander, Kenneth E.; (1984), in Infanticide: Comparative and Evolution-ary Perspectives. Glenn Hausfater and Sarah Blaffer Hrdy, eds. New York: Aldine.
Hrdy, Sarah Blaffer; Janson, Charles and Schaik, Carol van; (1995), “Infanticide: let’s Not Throw Out the Baby With the Bath Water”, Evolutionary Anthropology 3,. 151-154.
Isbell, L.; ( 1990), “Sudden short-term increase in mortality of vervet monkeys (Cercopithecus aethiops) due to leopard predation in Amboseli. National Park, Kenya” American Journal of Primatology, 21:41-52.
Sussman, RW; Cheverud, J.M. and Bartlett, T.Q.; (1995), “Infant killing as an evolutionary strategy”, Evolutionary Anthropology, 3, 149-151.