This entry was written by Hayley Vassallo as part of a project done in BIAN 2133 ‘Human Reproductive Strategies’ at The Australian National University in 2019 Semester 2.
Introduction
Infanticide occurs when of an individual, generally male, kills an unweaned infant of the same species as a means to increase their reproductive fitness or social status. Although infanticide is an uncommon phenomenon in nonhuman primates (primates), low reproductive rates of females of certain species arguably results in strong selective pressure amongst the group and may ultimately result in infanticide. Although rare, infanticide has been recorded in a variety primate species by field and laboratory researchers, with several evolutionary and sociological hypotheses aiming to strengthen the understanding of the phenomenon. Most instances of primate infanticide can be explained by the sexual-selection hypothesis, where the aggressor is unrelated to the infant (Soltis et al., 2000). Chimpanzees, however, are of particular interest regarding infanticide as it occurs not as the result of sexual selection, but rather may be explained by other adaptive or nonadaptive explanations: nutritive benefits, the resource competition hypothesis and the social pathology hypothesis (Murray et al., 2007). This essay will discuss both the evolutionary and nonadaptive hypotheses regarding infanticide in varying primate species.
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Sexual Selection Hypotheses
Incidents of infanticide across most primate species can be explained by the sexual selection hypothesis. In this respect, the unrelated male aggressor engages in infanticide to increase their reproductive success by increasing the probability of siring successive offspring. This can occur as the mother is then able to resume ovulation cycling more quickly due to the cessation of lactating. The aggressor will then mate with the female, thus increasing his reproductive success. Although the sexual selection hypothesis is controversial amongst the field, most male infanticide in primate species and other species (such as lions) are consistent with the theory. In primate species specifically, the sexual selection hypothesis can explain infanticide in single-male, multiple-female groups, where an intruder male defeats the resident male, with the consequential taking over of the group of females, prior to the killing of unweaned infants. This ultimately shortens the interbirth interval of the mothers, often allowing the infanticidal males to mate and reproduce with the newly available female (Soltis et al., 2000).
Soltis et al. (2000) study supports the sexual-selection hypothesis on infanticide in wild Japanese macaques. Although rare, infanticide has been known to occur within this species. The authors documented between one and six attacks on unweaned infants, with one observed infanticide by eight residential males during the mating season, with majority of attacks occurring on unrelated infants. However, female mating behaviour was not entirely witnessed due to the difficulty of accessible terrain to observe. Further the study was lacking hormonal data and thus could not estimate ovulation timing of female macaques, which provides a limitation to the study (Soltis et al., 2000).
Nutritive Benefit Hypothesis
While the Sexual Selection hypothesis can be used to explain infanticide in a range of primate species, it fails to explain infanticide in others, such as Chimpanzees. This is due to previous studies, as depicted by Murray et al. (2007), observing various acts of infanticide amongst chimpanzees of multiple populations. The infanticide observed occurred in three specific ways; by males within and between communities, and by females within communities. Intercommunity infanticide is not explained by sexual selection as females seldom transfer into the group of the aggressor, and thus the aggressor does not receive any reproductive benefit. The nutritive benefits hypothesis assumes that infanticide will evolve, or be considered adaptive, if it provides the aggressor with key nutrients. Thus, this hypothesis cannot be rejected in the instance of the chimpanzees as cannibalism occurred in almost all instances, except where the mother retrieved the infant and escaped (Murray et al., 2007).
The Resource Competition Hypothesis
The third adaptive hypothesis related to infanticide in primates is the resource competition hypothesis. This hypothesis holds true when the killing of an infant has a direct, positive influence on the aggressors’ access to resources, ultimately by reducing resource competition within the group. Further, when resources are scarce and infanticide has occurred, this will result in higher fitness for not only the aggressor, but his descendants, too (Murray et al., 2007; Xiang & Grueter, 2007). However, in order to study and assess this hypothesis, the data required must encompass the following: that competition for a specific resource or resources limits the reproductive success and inclusive fitness of the aggressor, and if the aggressor or his descendants actually gain an increase in fitness related to the increased consumption of the resource. Unlike the first two hypothesis discussed, where research on infanticide could be adequately conducted, it is argued that there are no cases of primate infanticide with adequate data to support these assumptions. Thus, many have attempted to understand primate infanticide under this hypothesis from a theoretical perspective. From this angle, it is argued that resource competition infanticide is highly unlikely and may only occur in very small populations as it is in these environments that resource competition is extremely high, or when the costs of infanticide for the aggressors are low (Xiang & Grueter, 2007).
The Social Pathology Hypothesis
Unlike the previous hypotheses discussed, the social pathology hypothesis is nonadaptive. This hypothesis postulates that infanticide will either occur as a side effect of aggressive behaviours within a group, or caused by human disruption. Human disturbance or disruption may result in primate infanticide when human interference causes the primate population to live in overcrowded conditions, resulting in higher levels of aggressive behaviour than those living in undisturbed conditions. The conditions of this hypothesis when researching such a phenomenon are that an aggressive event must take place prior to or during the act of infanticide, and, perhaps more specifically, that the infanticide is not advantageous to the aggressor (hence the nonadaptive hypothesis). Similar to the previously mentioned resource competition hypothesis, this hypothesis is highly unlikely to occur within primates due to the majority of observed cases of infanticide in primates resulted in an increase in the aggressors reproductive success (Xiang & Grueter, 2007).
Conclusion
Although infanticide in primates is relatively rare, it seems obvious that majority of the cases are adaptive and result in higher reproductive success for the aggressor. Thus, infanticide is a tool used to increase reproductive success in primate species, and is rarely, if ever, used as to reduce resource competition or as a nonadaptive response to aggression.
Literature Cited:
Murray, C.M., Wroblewski, E. & Pusey, A.E. (2007). New Case of Intragroup Infanticide in the Chimpanzees of Gombe National Park. International Journal of Primatology, 28(1):23-37.
Soltis, J., Thomsen, R., Matsubayashi, K. & Takenaka, O. (2000). Infanticide by resident males and female counter-strategies in wild Japanese macaques (Macaca fuscata). Behavioral ecology and Sociobiology, 48:195-202.
Xiang, Z. & Grueter, C. C. (2006) . First Direct Evidence of Infanticide and Cannibalism in Wild Snub-Nosed Monkeys (Rhinopithecus bieti). American Journal of Primatology, 69(3): 249-254.