Intra-community infanticide in wild, eastern chimpanzees: a 24-year review

Differences in expression of male aggression between wild bonobos and chimpanzees

Researchers investigating the evolution of human aggression look to our closest living relatives, bonobos (Pan paniscus) and chimpanzees (Pan troglodytes), as valuable sources of comparative data.1,2 Males in the two species exhibit contrasting patterns: male chimpanzees sexually coerce females3,4,5,6,7,8 and sometimes kill conspecifics,9,10,11,12 whereas male bonobos exhibit less sexual coercion13,14 and no reported killing.13 Among the various attempts to explain these species differences, the self-domestication hypothesis proposes negative fitness consequences of male aggression in bonobos.2,15,16 Nonetheless, the extent to which these species differ in overall rates of aggression remains unclear due to insufficiently comparable observation methods.17,18,19,20,21,22,23 We used 14 community-years of focal follow data-the gold standard for observational studies24-to compare rates of male aggression in 3 bonobo communities at the Kokolopori Bonobo Reserve, Democratic Republic of Congo, and 2 chimpanzee communities at Gombe National Park, Tanzania. As expected, given that females commonly outrank males, we found that bonobos exhibited lower rates of male-female aggression and higher rates of female-male aggression than chimpanzees. Surprisingly, we found higher rates of male-male aggression among bonobos than chimpanzees even when limiting analyses to contact aggression. In both species, more aggressive males obtained higher mating success. Although our findings indicate that the frequency of male-male aggression does not parallel species difference in its intensity, they support the view that contrary to male chimpanzees, whose reproductive success depends on strong coalitions, male bonobos have more individualistic reproductive strategies.25.

Flexible grouping patterns in a western and eastern chimpanzee community

Primate social organizations, or grouping patterns, vary significantly across species. Behavioral strategies that allow for flexibility in grouping patterns offer a means to reduce the costs of group living. Chimpanzees (Pan troglodytes) have a fission-fusion social system in which temporary subgroups ("parties") change in composition because of local socio-ecological conditions. Notably, western chimpanzees (P. t. verus) are described as showing a higher degree of bisexual bonding and association than eastern chimpanzees, and eastern female chimpanzees (P. t. schweinfurthii) are thought to be more solitary than western female chimpanzees. However, reported comparisons in sociality currently depend on a small number of study groups, particularly in western chimpanzees, and variation in methods. The inclusion of additional communities and direct comparison using the same methods are essential to assess whether reported subspecies differences in sociality hold in this behaviorally heterogeneous species. We explored whether sociality differs between two communities of chimpanzees using the same motion-triggered camera technology and definitions of social measures. We compare party size and composition (party type, sex ratio) between the western Gahtoy community in the Nimba Mountains (Guinea) and the eastern Waibira community in the Budongo Forest (Uganda). Once potential competition for resources such as food and mating opportunities were controlled for, subspecies did not substantially influence the number of individuals in a party. We found a higher sex-ratio, indicating more males in a party, in Waibira; this pattern was driven by a greater likelihood in Gahtoy to be in all-female parties. This finding is the opposite of what was expected for eastern chimpanzees, where female-only parties are predicted to be more common. Our results highlight the flexibility in chimpanzee sociality, and caution against subspecies level generalizations.

Evolutionary and neuroendocrine foundations of human aggression

Humans present a behavioural paradox: they are peaceful in many circumstances, but they are also violent and kill conspecifics at high rates. We describe a social evolutionary theory to resolve this paradox. The theory interprets human aggression as a combination of low propensities for reactive aggression and coercive behaviour and high propensities for some forms of proactive aggression (especially coalitionary proactive aggression). These tendencies are associated with the evolution of groupishness, self-domestication, and social norms. This human aggression profile is expected to demand substantial plasticity in the evolved biological mechanisms responsible for aggression. We discuss the contributions of various social signalling molecules (testosterone, cortisol, oxytocin, vasopressin, serotonin, and dopamine) as the neuroendocrine foundation conferring such plasticity.

Puberty initiates a unique stage of social learning and development prior to adulthood: Insights from studies of adolescence in wild chimpanzees

In humans, puberty initiates a period of rapid growth, change, and formative neurobehavioral development. Brain and behavior changes during this maturational window contribute to opportunities for social learning. Here we provide new insights into adolescence as a unique period of social learning and development by describing field studies of our closest living relatives, chimpanzees. Like humans, chimpanzees have a multiyear juvenile life stage between weaning and puberty onset followed by a multiyear adolescent life stage after pubertal onset but prior to socially-recognized adulthood. As they develop increasing autonomy from caregivers, adolescent chimpanzees explore and develop many different types of social relationships with a wide range of individuals in a highly flexible social environment. We describe how adolescent social motivations and experiences differ from those of juveniles and adults and expose adolescents to high levels of uncertainty, risk, and vulnerability, as well as opportunities for adaptive social learning. We discuss how these adolescent learning experiences may be shaped by early life and in turn shape varied adult social outcomes. We outline how future chimpanzee field research can contribute in new ways to a more integrative interdisciplinary understanding of adolescence as a developmental window of adaptive social learning and resilience.

Dead-infant carrying by chimpanzee mothers in the Budongo Forest

It has been suggested that non-human primates can respond to deceased conspecifics in ways that suggest they experience psychological states not unlike humans, some of which could indicate they exhibit a notion of death. Here, we report long-term demographic data from two East African chimpanzee groups. During a combined 40-year observation period, we recorded 191 births of which 68 died in infancy, mostly within the first year. We documented the post-mortem behaviour of the mothers and describe nine occasions where Budongo chimpanzee mothers carried infants for 1-3 days after their death, usually until the body started to decompose. We also observed three additional cases of extended carrying lasting for more than 2 weeks, one of which was followed by the unusual extended carrying of an object and another which lasted 3 months. In each case, the corpses mummified. In addition, we report four instances of recurring dead-infant carrying by mothers, three of whom carried the corpse for longer during the second instance. We discuss these observations in view of functional hypotheses of dead-infant carrying in primates and the potential proximate mechanisms involved in this behaviour.

Infanticide by Adult Females Causes Sexual Conflict in a Female-Dominated Social Mammal

Infanticide by adult females includes any substantial contribution to the demise of young and inevitably imposes fitness costs on the victim’s genetic fathers, thereby generating sexual conflict with them. Few if any studies have quantified the impact of infanticide by females on male reproductive success, the magnitude of sexual conflict this causes and possible counterstrategies males use against infanticidal females. We examine these topics in spotted hyena (Crocuta crocuta) clans, where females socially dominate breeding males and strong female mate-choice is independent of male social status. We consider two causes of infanticide by females, violent attacks on cubs and fatal maternal neglect. Violent attacks are predicted during periods of social instability at the top of the female linear dominance hierarchy and victims are expected to predominantly have mothers above median rank. Fatal maternal neglect, when starving litters are abandoned, is associated with monopolization of food in clan territories by high-ranking females, and victims are predicted to have mothers below median rank. Female perpetrators of violent attacks are expected to reduce the reproductive success of the fathers of their victims more than perpetrators of fatal maternal neglect. We tested these predictions using 30 + years of data (54 recorded violent attacks, 43 cases of fatal maternal neglect, DNA profiling of 1,671 individuals). Using long-term observations at communal dens we investigated whether males use counterstrategies against infanticide reported in other mammals. Due to female social dominance over breeding males, strong female mate-choice and prolonged offspring dependence on lactation in spotted hyenas, we predicted that these counterstrategies were unlikely to be used by males against females, thus no incidences of them were likely to be observed. Our results revealed that breeding males lost cubs to violent attacks at all stages of their reproductive tenure and to perpetrators with whom they did not sire offspring. Amongst known sources of paternity loss, violent attacks comprised 12.2% and maternal neglect 9.8% of cases. Violent attacks significantly reduced offspring production rates of breeding males, suggesting that infanticide by females generates sexual conflict. As predicted, no evidence of males using counterstrategies against infanticide by females were observed.

Female Dispersion Is Necessary, but Not Sufficient, for Pairbonded Monogamy in Mammals

Explanations for the evolution of social monogamy in mammals typically emphasise one of two possibilities: females are overdispersed (such that males cannot defend access to more than one female at a time) or males provide a service to the female. However, the first claim has never been formally tested. I test it directly at three levels using population-level data from primates and ungulates. First, I show that the females of monogamous genera do not have territories that are significantly larger, either absolutely or relatively, than those of polygynous genera. Second, using two indices of territorial defendability, I show that, given their typical day journey lengths, males of most monogamous species could easily defend an area large enough to allow them to monopolise as many as 5–10 females if they ranged solitarily. Finally, I use a model of male mate searching strategies to show that the opportunity cost incurred by pairbonded males is typically 5–10 times the reproductive success they actually obtain by being obligately monogamous. This suggests that the selection pressure dissuading them from pursuing a roving male strategy must be very considerable. At present, the evidence is undecided as to whether mitigating predation or infanticide risk is the primary function, but estimates of their impacts suggest that both are in fact plausible.

Pathways to paternal care in primates

Natural selection will favor male care when males have limited alternative mating opportunities, can invest in their own offspring, and when care enhances males' fitness. These conditions are easiest to fulfill in pair-bonded species, but neither male care nor stable "breeding bonds" that facilitate it are limited to pair-bonded species. We review evidence of paternal care and extended breeding bonds in owl monkeys, baboons, Assamese macaques, mountain gorillas, and chimpanzees. The data, which span social/mating systems and ecologies, suggest that there are multiple pathways by which conditions conducive to male care can arise. This diversity highlights the difficulty of making inferences about the emergence of male care in early hominins based on single traits visible in the fossil record. We discuss what types of data are most needed and the questions yet to be answered about the evolution of male care and extended breeding bonds in the primate order.

Intercommunity interactions and killings in central chimpanzees (Pan troglodytes troglodytes) from Loango National Park, Gabon

Intercommunity competition in chimpanzees (Pan troglodytes) has been widely studied in eastern (P. t. schweinfurthii) and western (P. t. verus) communities. Both subspecies show hostility towards neighboring communities but differ in rates of lethal attacks and female involvement. However, relatively little is known about the territorial behavior of the two other subspecies, central (P. t. troglodytes) and Nigeria-Cameroon chimpanzees (P. t. ellioti). Here, we present the first insights into intercommunity interactions of individuals of a community of central chimpanzees living in the Loango National Park in Gabon. The presence of individuals of neighboring communities in the Rekambo home range was assessed using 27 camera traps. Information was compiled on intergroup interactions recorded before (2005-2016) and after (January 2017-June 2019) the habituation of the community. Individuals from neighboring communities entered the core area, where nine out of 16 recorded intercommunity encounters occurred. Males were the main participants in territorial patrols and intercommunity aggressions. Females were part of all six territorial patrols recorded and dependent offspring participated in five patrols. Females were involved in intercommunity aggression in five out of twelve recorded encounters in which there was visual contact between communities. While the intercommunity encounter rate was lower than that reported across most other long-term chimpanzee sites, the annual intercommunity killing rate was among the highest. These results suggest that the frequency of lethal attacks at Loango is comparable to that reported for the eastern subspecies. In contrast, female involvement in intercommunity interactions mirrors that of the western subspecies.

A machine learning approach to infant distress calls and maternal behaviour of wild chimpanzees

Distress calls are an acoustically variable group of vocalizations ubiquitous in mammals and other animals. Their presumed function is to recruit help, but there has been much debate on whether the nature of the disturbance can be inferred from the acoustics of distress calls. We used machine learning to analyse episodes of distress calls of wild infant chimpanzees. We extracted exemplars from those distress call episodes and examined them in relation to the external event triggering them and the distance to the mother. In further steps, we tested whether the acoustic variants were associated with particular maternal responses. Our results suggest that, although infant chimpanzee distress calls are highly graded, they can convey information about discrete problems experienced by the infant and about distance to the mother, which in turn may help guide maternal parenting decisions. The extent to which mothers rely on acoustic cues alone (versus integrate other contextual-visual information) to decide upon intervening should be the focus of future research.

Death is common, so is understanding it: the concept of death in other species

Comparative thanatologists study the responses to the dead and the dying in nonhuman animals. Despite the wide variety of thanatological behaviours that have been documented in several different species, comparative thanatologists assume that the concept of death (CoD) is very difficult to acquire and will be a rare cognitive feat once we move past the human species. In this paper, we argue that this assumption is based on two forms of anthropocentrism: (1) an intellectual anthropocentrism, which leads to an over-intellectualisation of the CoD, and (2) an emotional anthropocentrism, which yields an excessive focus on grief as a reaction to death. Contrary to what these two forms of anthropocentrism suggest, we argue that the CoD requires relatively little cognitive complexity and that it can emerge independently from mourning behaviour. Moreover, if we turn towards the natural world, we can see that the minimal cognitive requirements for a CoD are in fact met by many nonhuman species and there are multiple learning pathways and opportunities for animals in the wild to develop a CoD. This allows us to conclude that the CoD will be relatively easy to acquire and, so, we can expect it to be fairly common in nature.

Patterns of urinary cortisol levels during ontogeny appear population specific rather than species specific in wild chimpanzees and bonobos

Compared with most mammals, postnatal development in great apes is protracted, presenting both an extended period of phenotypic plasticity to environmental conditions and the potential for sustained mother-offspring and/or sibling conflict over resources. Comparisons of cortisol levels during ontogeny can reveal physiological plasticity to species or population specific socioecological factors and in turn how these factors might ameliorate or exaggerate mother-offspring and sibling conflict. Here, we examine developmental patterns of cortisol levels in two wild chimpanzee populations (Budongo and Taï), with two and three communities each, and one wild bonobo population (LuiKotale), with two communities. Both species have similar juvenile life histories. Nonetheless, we predicted that key differences in socioecological factors, such as feeding competition, would lead to interspecific variation in mother-offspring and sibling conflict and thus variation in ontogenetic cortisol patterns. We measured urinary cortisol levels in 1394 samples collected from 37 bonobos and 100 chimpanzees aged up to 12 years. The significant differences in age-related variation in cortisol levels appeared population specific rather than species specific. Both bonobos and Taï chimpanzees had comparatively stable and gradually increasing cortisol levels throughout development; Budongo chimpanzees experienced declining cortisol levels before increases in later ontogeny. These age-related population differences in cortisol patterns were not explained by mother-offspring or sibling conflict specifically; instead, the comparatively stable cortisol patterns of bonobos and Taï chimpanzees likely reflect a consistency in experience of competition and the social environment compared with Budongo chimpanzees, where mothers may adopt more variable strategies related to infanticide risk and resource availability. The clear population-level differences within chimpanzees highlight potential intraspecific flexibility in developmental processes in apes, suggesting the flexibility and diversity in rearing strategies seen in humans may have a deep evolutionary history.

Responses to death and dying: primates and other mammals

Although some definitions of thanatology-broadly definable as the study of death and dying-exclude nonhumans as subjects, recognition of the scientific value of studying how other species respond to sick, injured, dying and dead conspecifics appears to be growing. And whereas earlier literature was largely characterized by anecdotal descriptions and sometimes fanciful interpretations, we now see more rigorous and often quantitative analysis of various behaviors displayed towards conspecifics (and sometimes heterospecifics) at various stages of incapacitation, including death. Studies of social insects in particular have revealed chemical cues that trigger corpse management behaviors, as well as the adaptive value of these behaviors. More recent research on other taxonomic groups (including aquatic and avian species, and mammals) has sought to better document these animals' responses to the dying and dead, to identify influencing factors and underlying mechanisms, and to better understand the physiological, emotional, social and psychological significance of the phenomena observed. This special issue presents original short reports, reviews, and full research articles relating to these topics in New World monkeys, Old World monkeys and apes, as well as equids and proboscids. The range of events, data, hypotheses and proposals presented will hopefully enrich the field and stimulate further developments in comparative evolutionary thanatology.

Fertility as a constraint on group size in African great Apes

Gorillas and chimpanzees live in social groups of very different size and structure. Here I test the hypothesis that this difference might reflect the way fertility maps onto group demography as it does in other Catarrhines. For both genera, birth rates and the number of surviving offspring per female are quadratic (or ∩-shaped) functions of the number of adult females in the group, and this is independent of environmental effects. The rate at which fertility declines ultimately imposes a constraint on the size of social groups that can be maintained in both taxa. The differences in group size between the two genera seem to reflect a contrast in the way females buffer themselves against this cost. Gorillas do this by using males as bodyguards, whereas chimpanzees exploit fission–fusion sociality to do so. The latter allows chimpanzees to live in much larger groups without paying a fertility cost (albeit at a cognitive cost).

Maternal cannibalism in two populations of wild chimpanzees

Maternal cannibalism has been reported in several animal taxa, prompting speculations that the behavior may be part of an evolved strategy. In chimpanzees, however, maternal cannibalism has been conspicuously absent, despite high levels of infant mortality and reports of non-maternal cannibalism. The typical response of chimpanzee mothers is to abandon their deceased infant, sometimes after prolonged periods of carrying and grooming the corpse. Here, we report two anomalous observations of maternal cannibalism in communities of wild chimpanzees in Uganda and Ivory Coast and discuss the evolutionary implications. Both infants likely died under different circumstances; one apparently as a result of premature birth, the other possibly as a result of infanticide. In both cases, the mothers consumed parts of the corpse and participated in meat sharing with other group members. Neither female presented any apparent signs of ill health before or after the events. We concluded that, in both cases, cannibalizing the infant was unlikely due to health-related issues by the mothers. We discuss these observations against a background of chimpanzee mothers consistently refraining from maternal cannibalism, despite ample opportunities and nutritional advantages. We conclude that maternal cannibalism is extremely rare in this primate, likely due to early and strong mother–offspring bond formation, which may have been profoundly disrupted in the current cases.