Nonhuman Primates: Within-Group Conflicts
Aggressive conflict between individuals within a group or community in nonhuman primate species, involving violent interactions, in certain circumstances leading to death of one party and fitness advantages to aggressors.
Intragroup killings are a common phenomenon in primate species. Whereas attacks among adults have been thought to be selected due to the benefits collected by the assailants such as eliminating rivals, gaining access to material and social resources, and ascending in rank, the evolution of infanticide has mainly been attributed to males restarting the ovarian cycle of lactating females and copulating with them. Furthermore, other adaptive hypotheses interpret infanticide as means of acquiring and retaining mates, eliminating competitors, removing the genetic presence of rivals, and rising in the social hierarchy. Some of the risk factors associated with male infanticide across primate species include a slow life history, the absence of alloparents, the type of social structure, and the prevalence of male take-overs. This review will cover the empirical evidence supporting the adaptive nature of different forms of intragroup killings in primate species.
Similarly to descriptions of conspecific killings due to intergroup attacks, intragroup killings across primate societies have been hypothesized to be associated with various fitness benefits. Primatology researchers have tested them with multiple species, especially in anthropoid primates. For example, in spider monkeys (Ateles geoffroyi ornatus and A. g. yucatanesis), intragroup killings have been attributed to a male-biased sex ratio producing an increase in male competition (Valero et al. 2006). In capuchin monkeys (Cebus capuchinus), rivalry for occupying high ranks (or rank reversals) in hierarchy, as well as attempted expulsions from the community, are considered to better explain violent deaths. Despite the fact that muriquis (Brachyteles arachnoides) display low levels of within-group competition, the killing of an old male demonstrates that even tolerant species may resort to violent behavior to eliminate conspecifics due to conflict escalation (Talebi et al. 2009).
However, originally, intracommunal killings were thought to be the product of captivity (e.g., Arnhem Zoo, the Netherlands; reviewed in Watts 2004), with victims unable to flee the scene. This perspective was challenged when cases contrary to this hypothesis were described in wild chimpanzee (Pan troglodytes) communities in Uganda and Tanzania (Ngogo, Sonso, Mitumba, and the M-group; Fawcett and Muhumuza 2000). Similarly to the cases of intergroup killings, these attacks were considered by some to be maladaptive. However, rank upheavals, unequal sex ratios, and the elimination of mating competitors appear to be some of the adaptive problems underlying this behavior.
Within-group victims of attacks usually occupy a low rank in the hierarchy (Watts 2004). The imbalance of power hypothesis, describing a decrease in the risk for the aggressors due to the collective nature of the attack, has also been considered a factor affecting the likelihood of within-group killings. Thus, by targeting a defenseless individual, coalitions of males reduce any risk to themselves, obtain benefits such as protecting their ranks in the hierarchy, and increase their mating opportunities.
Whereas adults and adolescent males had a higher mortality rate due to intercommunal aggression compared to juveniles and infants in chimpanzees, these were more frequently the victims of intracommunal aggression. The primatological literature supports this pattern, with most hypotheses and studies examining the occurrence of infant killings within groups, rather than adult deaths. As such, much of the research examining the adaptive functions of intragroup killing focuses on infanticide, as reviewed below.
Infanticide has been observed in at least 51 primate species. However, despite its ubiquity, infant killings do not occur at random. Targeted younglings are fully unrelated or genetically distant to the attacker, and the identity of the attackers is biased to one of the sexes. Thus, whereas in rodents most of the perpetrators are females, in primates, unrelated males have been found to be mostly responsible. Among some of the functions of male infanticide, observations in patas monkeys (Erythocebus patas) and gray langurs (Semnopithecus entellus) suggest that by targeting the offspring of rivals, males eliminate future competitors and remove the genetic presence of the rivals in the group. In other species such as purple-faced langurs (Trachypithecus vetulus), infanticide increased the opportunities of gaining access to limited resources. Meanwhile, in chimpanzees (Pan troglodytes) and mantled howler monkeys (Allouata palliata), infanticidal males decreased the coalitional power of competitors. Observations in gray langurs also found a relationship between infanticide and rank acquisition.
Other hypotheses have also considered the use of infanticide as a strategy for mate acquisition and mate retention. Thus, by killing the offspring of a female, males demonstrate a lack of protective abilities that competitors can offer. Similarly, according to the sexual selection hypothesis, infanticide is promoted as a consequence of females entering into an anestrus phase shortly after birth, a phenomenon known as lactational amenorrhea (van Schaik and Janson 2000). Hence, by spending more time unreceptive, females restrict the mating opportunities of males, who proceed to eliminate infants as a tactic for returning females to sexual availability. Perpetrators not only have a higher likelihood of copulating with targeted females, but also conceive their offspring. Thus, overall, infant killing in primates appears to be a consequence of both intersexual conflict and intrasexual competition, and appears to serve various adaptive functions.
Social structure and social dynamics also affect the probability of infant killings in predictable ways which suggest evolutionary design. For example, primate species living in one-male units are considered to be especially at risk of occurrences of infanticide compared to other species. Males from such units sire most of the offspring in the group. Therefore, when a dispersing male migrates and takes over, most of the young will be related to the former male. Infanticide allows the new male to obtain a reproductive monopoly and is commonly observed. Infanticide also occurs in primate species living in multimale groups (e.g., chacma baboons, Papio ursinus; and olive baboons, P. anubis). In contrast to one-male unit species, the average tenure of males living in multimale groups is brief. Thus, due to the risk of being deposed before siring any offspring, males may kill the infants of other males, which has the potential to increase their reproductive output (Beehner et al. 2005).
Paternity uncertainty is expected to be a protective factor against infanticide, as killing an infant when it is possibly one’s progeny could have intense fitness costs. An interesting and complex phenomenon is observed with respect to this in primates: Females from species that carry and take care of their offspring on their own are more likely to have their offspring killed compared to species that rely on the assistance of allocarers (i.e., individuals who engage in communal caring; van Schaik and Kappeler 1997). By having the support of kin or other helpers (e.g., males in marmosets and tamarins), communal females have a shorter interbirth interval, and even display sexual receptivity and proceptivity during pregnancy and shortly after birth. Even though males that copulated during nonconceptive periods do not sire more offspring compared to those that mate during ovulation, the confusion of paternity forces males to inhibit infanticidal tendencies to avoid killing their own offspring. The evolutionary context for females taking care of their young by themselves is considerably different. Due to the high physiological demands of investing in multiple infants at the same time, females limit the opportunities of conception by being unreceptive and in anestrus during periods of gestation and lactation and by increasing the temporal gap between births. However, at the same time, these modifications restrict the mating access of males, promoting the adaptiveness of infant killings by them. Higher infanticide rates are in fact observed in these cases.
An important realization is that evidences for an arms race between the sexes in terms of behavioral strategies to implement against infanticide would suggest that that there are in fact evolutionary forces leading to adaptations involving infanticide. In fact, due to the high costs of infanticide, evidence suggests that females have evolved multiple counterstrategies to decrease the risks of losing younglings to attacking males. Depending on the social structure of each species, females can either concentrate or confuse male paternity. Thus, offspring of females who cohabitate or consort with a single male (e.g., OMUs species), receive the protection of the male, due to the copulation exclusivity increasing paternity certainty. Alternatively, paternity confusion occurs in societies with multiple males. In these conditions, concentrating paternity in a single male may not be sufficient to dissuade other males from attacking. Since takeovers in multimale groups are considerably common in multimale species, by copulating with numerous mates, females obscure the paternity of males, forcing attackers to refrain from killing the infant due to the risk of eliminating their own young. Furthermore, females can advertise their receptivity to males through visual and vocal displays, or through sexual swellings peaking at times of ovulation. Other strategies employed by females to reduce the likelihood of infanticide by males include establishing friendship with a protector; dispersing with a deposed male; forming a coalition with other females or with kin; maternal vigilance and aggression; accelerating the weaning period; abandoning the infant, fetus reabsorption, or induced abortion.
Intragroup killings are observed across nonhuman species. Most of the victims are infants, whereas attackers are frequently adult males. Adaptive benefits obtained by infanticidal males appear to include returning noncycling females to estrus, eliminating future contenders and removing their genetic presence, decreasing the levels of current competition, and gaining access to resources. Support for these hypotheses has been found in various lines of research in different species, with research effort focusing especially in anthropoids. Attacks among adults have also been observed. However, differently from infanticide, victims and attackers in lethal conflicts among adults are mostly males. Gaining access to mates, eliminating rivals, and ascending in the hierarchy are some of the benefits obtained by successful attackers.
- Palombit, R. A. (2009). “Friendship” with males: A female counterstrategy to infanticide in chacma baboons of the Okavango Delta. In M. N. Muller & R. W. Wrangham (Eds.), Sexual coercion in primates and humans: An evolutionary perspective on male aggression against females (pp. 377–409). Cambrdige, MA: Harvard University Press.Google Scholar