Primate "Infanticide"

A current 'hot' topic in primate studies today is the issue of infant killing. Our readings contain three articles on the subject. The first was the 1977 Hrdy article "Infanticide as a Primate Reproductive Strategy," in which the evolutionarily selected trait of infant killing is first discussed; in essence she postulates that there is a high male turnover rate in gray langur troops, which are primarily single male and multiple female. Thus it is evolutionarily effective for males to kill all infants that are not their own, as this will bring the infant-deprived females more quickly into estrous, giving the male a higher chance of effectively impregnating the females of the troop before he too is ousted by another male.

The second article was "Primate Social Behavior in a Changing World," written by Curtin and Dolhinow in 1978; it primarily discusses normal social life for the gray langur, and notes that the troops from which Hrdy collected data (and which demonstrated what Hrdy asserts is normal behavior) live under environmental conditions that are far from normal. It also notes several theoretical holes in the Hrdy article. Noteworthy among those is the improbability of the male gray langur's ability to keep track of either his past mating success, or gestation periods in the females of his species. Curtin and Dolhinow also spend some time discussing probable errors in Hrdy's data collection and subsequent hypotheses, e.g., of the many infant deaths ascribed to infant-killing males, only 4 were actually witnessed; the langurs are environmentally over-crowded, and are regularly harassed by humans and dogs; females occasionally came into estrous before the deaths of their infants.

There are numerous problems with Hrdy's hypothesis from a life history viewpoint. She assumes active males and passive females, for a start, and this is clearly not the case, as examination of langur social behavior shows. Also she did not study the troops all year round, but only for two or three months per year, for five consecutive years. This means Hrdy was observing the troops for only 8% to 15% of the five years she was there. She could not state definitively what had been happening to the individuals, but instead resorted to anecdotal evidence to fill in her temporal gaps.

Amusingly enough, Hrdy states at the beginning of her article the undesirability of relying on anecdotal evidence by quoting some rather lurid Victorian accounts of langur social activity; yet she does not seem to see any inconsistency in her later reliance on native accounts of infant killing. Finally, she could not identify all the individuals in the troops, nor had she done any DNA fingerprinting to determine paternity of infants. The ability to individually identify each troop member is considered of primary importance for accurate data collection, as stated by Altmann in her seminal article. Hrdy did not seemed to feel this was necessary for her studies.

The final article we read was by Bartlett, Sussman, and Cheverud, written in 1993, and titled "Infant Killing in Primates: A Review of Observed Cases with Specific Reference to the Sexual Selection Hypothesis." This most recent article is a well-written, objective examination of all the available data on primate infant killing. Several noteworthy points can be gleaned from this article.

The first thing I noticed was the use of 'infanticide' (an emotionally loaded and imprecise term) frequently seemed to coincide with description of the female members of the gray langur troops as 'harems' - another poorly chosen word, since it describes a human institution rather than the actual behavior demonstrated by the langurs.

Secondly, assumptions regarding paternity and identity of the killers were often being made by observers who could not even identify every individual in the troop, nor state definitively who fathered which infants.

Thirdly, the environment often included frequent inter- and intra-sex battles, marauding animals, and disease; yet all missing infants were attributed to malevolent newcomer males.

Fourthly, the attackers were not always and only newcomer males, nor were these attackers necessarily only attacking females with infants; there were instances where females without infants, juveniles, and (most damning, to me) females in estrous were being attacked. If the trait of infant killing is supposed to hasten estrous in infant-deprived females so that the new male can mate with her, for what conceivable reason would he attack the very females he wishes to impregnate?

Finally, the statistical basis for Hrdy's hypothesis is very poor. Of all observed infant killings by all primates, almost half are attributable to a single species: the gray langur. Of all the observed infant killings supposedly committed by P. entellus, over half are attributable to a single site: Johdpur. Thus a hypothesis that supposedly applies to all primates is being set forth based primarily on the killings, deaths, or disappearances of some 14 gray langur infants at a single site, under questionable circumstances. As the article itself concludes, under such a lack of evidence "the concept that infanticide in nonhuman primates is a widespread, adaptive phenomenon must be approached with the appropriate caution."