Male Infanticide and Female Countermeasures in Nonhuman Primates Essay (Critical Writing)

The topic of animal infanticide is a complex subject explored within the zoological context. It implies the killing of a young male offspring by a fully-grown member of an animal group, which, from an outside perspective, seems cruel and unnecessary. Nevertheless, studies have shown that infanticide is an aspect of animals’ social structure as well as their reproductive strategy. It can be traced within hundreds of nonhuman species and is more prevalent in mammals in instances when grown males compete for reproduction with several females.

The growing appreciation of the biological significance of the process led to male infanticide being defined as “behavior that makes a direct and significant contribution to the immediate death of an embryo or newly hatched or born member of the performer’s own species” (Palombit 2015, p. 2). Infanticide in primates is an especially important topic of study because of its complexity in terms of the interpretation in extensive sociobiological research. While some biologists claim this issue being very common, others state otherwise, suggesting that there are numerous superficial and overgeneralized interpretations of infanticide in nonhuman primates. Nevertheless, infanticide is a voluntary and conscious action that has support in empirical research.

Background on Animal Infanticide

Male infanticide bears some undertones of sexual conflict; as described by Herodotus, this kind of behavior was demonstrated by Egyptian cats. The historian explained this as a trick used for getting sexual access to female cats that were preoccupied with their maternal obligations. From the perspective of socio-biology of the 1970s, scholars proposed the idea that male infanticide represented a strategy used for increasing the chances of reproductive success at the expense of female fitness and well-being (Gursky-Doyen & Supriatna 2010).

On the one hand, the potential benefits of infanticide of male animals are associated with obtaining both mating partners and nutritional gain. For females, the same act is linked to acquiring beneficial resources that include nest sites and food. On the other hand, the costs of infanticide among animals are linked to the increased expenditure of energy, risks of injuries, as well as the exposure of some species to predation (Eccard et al., 2018). However, the highest cost of male infanticide among animals, including nonhuman primates, is linked to the loss of fitness of a group. In order to protect themselves against male infanticide, females use a range of strategies, which are predominantly associated with changes in behavior (Agrell, Wolff & Ylonen 1998).

When discussing the background of male infanticide among primates, it is worth mentioning that it implies a result rather than a reason for sexual conflicts among representatives of the same animal group. When scholars looked at mammalian animals that have different social systems, they revealed that the behavior “appeared to be a result of the sexual conflict in social species with non-seasonal breeding” (Lukas & Huchard 2014, p. 841). The competition of male primates for their position in a group represents an issue that many groups do not know how to address, which is why exploring the range of counterstrategies females implement is high on the agenda for researchers.

Review of Literature

Due to its complexity, the issue of male infanticide was studied by dozens of researchers. Palombit (2015) explored the issue from the perspective of sexual conflict and applied the theory of coevolution when studying male strategies and female counterstrategies. The researcher provided compelling evidence for sexually antagonistic coevolution associated with the phenomenon of infanticide. Several hypotheses for nonparental infanticide by males in animal groups were presented.

For instance, to support the argument of sexual selection, such proposals as the acceleration of female return to breeding (Palombit 2014), the improved female condition and increase fertility (Morelli et al. 2009), and the acquisition of the breeding social status (Rajpurohit et al. 2008) were offered. In support of the argument associated with mate acquisition, such proposals as the improved intergroup competition (Mitani, Watts & Amsler 2010) and nutritional exploitation (Gray 2009) were mentioned.

The research conducted by Zipple et al. (2017) is notable for its contribution to the study literature on behaviors associated with male infanticide in nonhuman primates, specifically baboons. Sexually selected infanticide and feticide were expected to target male animals and occurred as a result of male behavior. In order to test this hypothesis, Zipple et al. “measured the frequency of fetal and infant murder by male baboons in the Amboseli basin,” Kenya (2017, p. 1). An important finding related to the association between the environment and the chances of male baboons killing newborns or fetuses. Specifically, the highest rates of feticide and infanticide-related deaths within baboon groups were associated with the following types of immigration:

• An immigrant male received an important position in a group;
• An immigrant male lived with group for three months;
• The availability of food along with the social range overlap was on a relatively low level;
• A relatively large number of pregnant females and dependent infants was present.

The factors mentioned above offered strong evidence to support the hypothesis that male infanticide and feticide among baboons were linked to environmental factors (Clutton-Brock 2016). The selection of males in the group is therefore associated with increased reproductive opportunities. When killing a newly born member of the group, a male causes the mother of an infant to resume the state of reproduction sooner compared with the period needed to care for a newborn (Zipple et al., 2017).

This means that a feticidal male baboon “gains even greater reproductive benefits as he is able to eliminate periods of both pregnancy and lactation. Feticide and infanticide can be considered closely related phenomena, differing primarily in the stage of development at which the developing baboon offspring dies” (Zipple et al., 2017, p. 2).

It is important to note that in groups of nonhuman primates where infanticide and feticide are both possible, committing the latter may offer even greater benefits because of the increased number of positive reproductive outcomes. Sexually selected feticide can also relate to the Bruce effect, which implies the intentional termination of pregnancy by females after being exposed to non-paternal breeding males (Eccard, Dammhahn & Ylonen 2017). However, unlike the Bruce effect, feticide based on the sexual selection results from male aggression, and females choose to end their pregnancy following a trauma that males have inflicted on them and their fetus.

When studying the results of male infanticide, Opie et al. (2013) explored social monogamy in primates as one of the consequences of males targeting male infants. Social monogamy is an occurrence that is more common in primates than on other mammals, which is why scholars proposed to explore this concept in relation to male infanticide (Kvarnemo 2018). By using “likelihood-based phylogenetic comparative methods in a Bayesian framework,” the researchers aimed to test the existence of correlated evolutions between the systems of mating and the following factors: the care of male primates for their infants, the patterns of female ranging, and male infanticide (Opie et al. 2013, p. 13328).

Due to the previous lack of agreement about the evidence to support social monogamy, it was predicted that the evidence would show a correlated evolution in relation to the infanticide of males and monogamy in primate society. After conducting the analysis, Opie et al. revealed the existence of “correlated evolution between male infanticide, female ranging patterns, paternal care, and male infanticide” (2013, p. 13330). This finding shows that the rates of infanticide among primates can be reduced through the increase of biparental care. The research is important for showing an important role that infanticide plays in the socially dependent primate evolution.

The study by Robbins et al. (2013) focused on researching the influence of male infanticide on the development of mountain gorillas’ social structures. It was revealed that the overall rates of infanticide were higher (by 2-3 times) in one-male groups than in groups with multiple males; however, the difference did not reveal any statistical significance. It was found that “infant mortality, the length of interbirth intervals, and the age of first reproduction were not significantly different between one-male versus multimale groups” (Robbins et al. 2013, p. 1). In addition to this, the scholars did not reveal any evidence that the patterns of female dispersal were associated with the preference for multimale groups. Nevertheless, the variability in male strength can reduce the risks of male infanticide, which explains why both single- and multi-male groups show common results.

Female Countermeasures

In relation to female countermeasures, the best-known strategy is associated with confusing paternity. It implies copulation with numerous males. Such promiscuity is rationalized by the increased sexual receptivity and proceptivity that leads to insemination for multiple males (Palombit 2012). Another measure is associated with the extended period of estrus that leads to an increased likelihood of copulation and insemination by multiple males (Palombit 2015).

Concealed ovulation is a countermeasure that females use for obscuring paternity due to the absence of usual signals that make it possible for males to determine whether females are available for mating (Carnegie, Fedigan & Ziegler 2006). Postconceptional sexual behavior implies involuntary copulations that occur during early gestation (Palombit 2012). When it comes to situation-dependent sexual receptivity, females use it in cases of longer receptive periods among females with menstrual cycles as well as among deceptive females that emulate sexual behavior.

While the mentioned strategies were associated with obscuring paternity among males, there are also female countermeasures that focus on copulations and/or conceptions on specific males (Palombit 2015). For instance, females choose to mate with protector males by directly limiting periovulatory mating to the representatives of a group that is more likely to act as protectors of young males that will be born. Indirectly, females may also choose to target protector males by means of advertising copulations or ovulation, thus reducing polyandry at ovulation in order to concentrate paternity in high-quality defender males (Palombit 2012).

When it comes to methods females use to manipulate reproduction, there are reported instances of pregnancy blocks and terminations (Roberts et al. 2012), superfetation, infant abandonment, weaning acceleration, temporarily reduced fecundity, and breeding synchrony (Palombit 2015). The last important finding of Palombit’s (2015) research is linked to individual counterstrategies of females to prevent male infanticide. They include chemical suppression and sexual segregation. While the study shed light on some of the most notable examples of female countermeasures to male infanticide, the specific behaviors inherent to nonhuman primates were not discussed.

Ren et al. (2011) suggested that female resistance to invading males could lead to the increase of infanticide in langurs. The researchers’ observations suggested that infanticide committed by males that invaded social groups could be more accidental than intentional. It was shown that those males had the intention of monopolizing all mothers within a group, including lactating mothers. Multiparous mothers that agreed to accept the presence of invading males avoided infanticide in most instances (Ren et al., 2011). This points to the result that mothers would prevent the occurrence of infanticide by accepting the takeovers of invading males.

In the species of langurs, voluntary abortion and mating with invading males were implemented by females as another way to avoid the killings of male infants by takeover males. From this research, it can be concluded that the aim of males that invade langur groups was to monopolize all adult males. The primary incentive of infanticide was the resistance of mothers to accepting the takeover, while those who agreed with it would avoid their infants being killed. In terms of further research, motivation analysis is needed to get an in-depth understanding of why infanticide takes place in primate species.

The study by Knott et al. (2010) is also important to consider in the discussion about female counterstrategies to infanticide as the scholars studied reproductive strategies in orangutans. The study is notable for its attention to orangutans as a species in which forced copulation is especially prevalent. This strategy is used by competitively disadvantaged males that subsequently influence female countermeasures. Knott et al. (2010) revealed that females who used concealed ovulation in conjunction with facultative association were used for avoiding the coercive attempts of males and thus decrease the instances of infanticide. The resistance of orangutan males reduced copulation time and offered an additional tool for the selection of mates.

Conclusion

The review of studies on male infanticide among nonhuman primates revealed that the behavior was associated with the intentions of male representatives to gain control over females. This main goal of infanticide is returning females to the state of mating in order to guarantee copulation and insemination. However, there are cases when males killed infants for the purpose of saving their group from starvation. To prevent infanticide occurrence, female primates implemented such strategies as concealed ovulation, mating with protector males, chemical suppression, and several others. Depending on the environment and the intentions of the male primates, females would adapt their behaviors. Nevertheless, there are instances when females intentionally initiated infanticide when subjected to male aggression and trauma.

Reference List

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