New cases of inter-community infanticide by male chimpanzees at Ngogo,Kibale National Park,Uganda

Infanticide by males has been recorded in four chimpanzee populations, including that in Kibale National Park, Uganda. Some infanticidal attacks occur during inter-community aggression. The sexual selection hypothesis does not easily explain these attacks because they may not directly increase male mating opportunities. However, females in the attackers’ community may benefit by expanding their foraging ranges and thereby improving their reproductive success; thus infanticide may increase male reproductive success indirectly. We report two new cases of infanticide by male chimpanzees at Ngogo, Kibale National Park. Like two previous cases, these occurred during a boundary patrol and were almost certainly between-community infanticides. The patrolling males attacked despite the proximity of males from the victims’ presumed community. This probably explains why, unlike the earlier cases, they did not completely cannibalize their victims. Such attacks seem to be relatively common at Ngogo and infanticide may be an important source of infant mortality in neighboring communities. Our observations cannot resolve questions about the sexual selection hypothesis. However, they are consistent with the range expansion hypothesis: the infanticides occurred during a period of frequent encounters between communities associated with a mast fruiting event, and Ngogo community members greatly increased their use of areas near the attack site during another mast fruiting event one year later. Our observations contribute to growing evidence that lethal intergroup aggression is a common characteristic of wild chimpanzee populations.     

Infanticide and cannibalism by male chimpanzees at Ngogo, Kibale National Park, Uganda

Researchers have documented infanticide by adult males in four wild chimpanzee populations. Males in three of these have killed infants from outside of their own communities, but most infanticides, including one from Kanyawara, in Kibale National Park, Uganda, took place within communities. Here we report two new cases of infanticide by male chimpanzees at a second Kibale site, Ngogo, where the recently habituated chimpanzee community is the largest yet known. Both infanticides happended during boundary patrols, which occur at a high frequency there. Patrolling males attacked solitary females who were unable to defend their infants successfully. The victims were almost certainly not members of the Ngogo community. Males cannibalized both infants and completely consumed their carcasses. These observations show that infanticide by males is widespread in the Kibale population and that between-community infanticide also happens there. We discuss our observations in the context of the sexual selection hypothesis and other proposed explanations for infanticide by male chimpanzees. The observations support the arguments that infanticide has been an important selective force in chimpanzee social evolution and that females with dependent infants can be at great risk near range boundaries, but why male chimpanzees kill infants is still uncertain.     

Lethal intergroup aggression by chimpanzees in Kibale National Park, Uganda

Chimpanzees (Pan troglodytes) have hostile intergroup relations throughout most or all of their geographic range. Hostilities include aggressive encounters between members of neighboring communities during foraging and during patrols in which members of one community search for neighbors near territory boundaries. Attacks on neighbors involve coalitions of adult males, and are sometimes fatal. Targets include members of all age/sex classes, but the risk of lethal intergroup coalitionary aggression is highest for adult males and infants, and lowest for sexually swollen females. The best-supported adaptive explanation for such behavior is that fission-fusion sociality allows opportunities for low-cost attacks that, when successful, enhance the food supply for members of the attackers' community, improve survivorship, and increase female fertility. We add to the database on intergroup coalitionary aggression in chimpanzees by describing three fatal attacks on adult males, plus a fourth attack on an adult male and an attack on a juvenile that were almost certainly fatal. Observers saw four of these attacks and inferred the fifth from forensic and behavioral evidence. The attackers were males in two habituated, unprovisioned communities (Ngogo and Kanyawara) in Kibale National Park, Uganda. We also summarize data on other intercommunity attacks at Ngogo. Our observations are consistent with the "imbalance of power" hypothesis [Manson & Wrangham, Current Anthropology 32:369-390, 1991] and support the argument that lethal coalitionary intergroup aggression by male chimpanzees is part of an evolved behavioral strategy.     

Effects of male group size,parity, and cycle stage on female chimpanzee copulation rates at Ngogo,Kibale National Park,Uganda

Chimpanzees have complex and variable mating strategies, but most copulations occur when females with full sexual swellings are in parties with multiple males and mate with most or all of those males. Daily copulation rates for fully swollen females vary at different times of a female’s cycle, among females, and across communities and populations. Variation in female age, parity, and cycle stage underlie some of this variation, but possible demographic effects on copulation rates have not been systematically investigated. Demographic variation can affect many aspects of behavior and ecology, including the frequency and success of different mating tactics. Analysis of data from the unusually large chimpanzee community at Ngogo produces two results that are consistent with the hypothesis that demographic variation affects female copulation rates. Copulation rates were high compared with those reported from other research sites, where females had fewer potential mates available. Daily copulation rates of fully swollen females were also positively related to the number of males with whom they associated. Ngogo data also re-confirm results from other studies, of both wild and captive populations, showing that female copulation rates increase during periovulatory periods. This is consistent with the hypothesis that sexual swellings and extended receptivity and proceptivity help to protect females against infanticide by helping to ensure they mate with all potential sires. As at some other sites, parous females at Ngogo copulated at higher rates than nulliparous females. Possible effects of demography on sexual behavior should be considered in assessments of differences between chimpanzees and bonobos and of variation across chimpanzee populations.     

Attacks on neonates by a male mangabey (Cercocebus atys)

Aggressive behavior by an adult male toward selected infants and their mothers was observed during a long-term study of reproduction in a captive social group of sooty mangabeys (Cercocebus atys). The highest-ranking adult male in this group was observed to attack and bite three neonates out of a total of 13 infants born in 1982. All three attacks were directed attacks in which infants were grabbed from their mothers and bitten in the head. The first infant was fatally injured; the other two probably would have sustained fatal wounds had the male's canines not been blunted beforehand and had observers not intervened. The attacks were preceded by a pattern in which the male persistently stalked or chased the mother-infant pairs, a behavior first observed in the hours immediately following parturition. Unlike attacks in wild baboon groups following male immigration, these attacks on infants occurred in a stable social group in which the male attacker had been a lifelong resident. This male, however, had gained alpha rank only 3 months before attacking the first infant. These attacks, in the context of other evidence of aggression and wounding, are discussed relative to current models of infanticide in primates.     

Demographic influences on the hunting behavior of chimpanzees

We investigated hunting in an unusually large community of wild chimpanzees at Ngogo in the Kibale National Park, Uganda. Aspects of predation were recorded with respect to the prey, the predators, and hunting episodes. During 23 months of observation, the Ngogo chimpanzees caught 128 prey items from four primate and three ungulate species. Chimpanzees preyed selectively on immature red colobus primarily during group hunts, with adult males making the majority of kills. Party size and composition were significant predictors of the probability that chimpanzees would hunt and of their success during attempts. Chimpanzees were more likely to hunt red colobus if party size and the number of male hunters were large; party size and the number of male hunters were also significantly larger in successful compared with unsuccessful hunts. The Ngogo chimpanzees did not appear to hunt cooperatively, but reciprocal meat-sharing typically took place after kills. Hunts occurred throughout the year, though there was some seasonality as displayed by periodic hunting binges. The extremely high success rate and large number of kills made per successful hunt are the two most striking aspects of predation by the Ngogo chimpanzees. We compare currently available observations of chimpanzee hunting behavior across study sites and conclude that the large size of the Ngogo community contributes to their extraordinary hunting success. Demographic differences between groups are likely to contribute to other patterns of interpopulation variation in chimpanzee predation. Am J Phys Anthropol 109:439–454, 1999. © 1999 Wiley-Liss, Inc.     

New Case of Intragroup Infanticide in the Chimpanzees of Gombe National Park

Researchers have reported a total of 31 infanticides in 4 different chimpanzee (Pan troglodytes) populations. Though infanticide is infrequent, low reproductive rates of females likely make it a strong selective pressure in the species. We report a new incident of intragroup infanticide in Gombe National Park, Tanzania, in which a community male attacked a 3.5-yr-old male. We then consider the infanticide in terms of adaptive and nonadaptive explanations for infanticide including the social pathology, by-product of male aggression, nutritive benefits, resource competition, and sexual selection hypotheses. The incident reported here is not well explained by any of them. While the infanticide is puzzling in terms of ultimate explanations for infanticide, it provides a good context in which to consider proximate mechanisms for offspring recognition. The incident provides some evidence that males may use their mating history with the mother to assess paternity likelihood.     

Causes of death in the Kasekela chimpanzees of Gombe National Park, Tanzania

Understanding the rates and causes of mortality in wild chimpanzee populations has important implications for a variety of fields, including wildlife conservation and human evolution. Because chimpanzees are long-lived, accurate mortality data requires very long-term studies. Here, we analyze 47 years of data on the Kasekela community in Gombe National Park. Community size fluctuated between 38 and 60, containing 60 individuals in 2006. From records on 220 chimpanzees and 130 deaths, we found that the most important cause of mortality in the Kasekela community was illness (58% of deaths with known cause), followed by intraspecific aggression (20% of deaths with known cause). Previous studies at other sites also found that illness was the primary cause of mortality and that some epidemic disease could be traced to humans. As at other study sites, most deaths due to illness occurred during epidemics, and the most common category of disease was respiratory. Intraspecific lethal aggression occurred within the community, including the killing of infants by both males and females, and among adult males during the course of dominance-related aggression. Aggression between communities resulted in the deaths of at least five adult males and two adult females in the Kasekela and Kahama communities. The frequency of intercommunity violence appears to vary considerably among sites and over time. Intercommunity lethal aggression involving the Kasekela community was observed most frequently during two periods. Other less common causes of death included injury, loss of mother, maternal disability, and poaching.     

Intracommunity Coalitionary Killing of an Adult Male Chimpanzee at Ngogo,Kibale National Park,Uganda

Intercommunity coalitionary killing of adult and adolescent males has been documented in two chimpanzee communities in the wild, and it was strongly suspected in a third. It may increase survivorship for the attackers, their mates, and their offspring by reducing the combined strength of hostile neighbors and/or by increasing territory size and food availability, and it may help the attackers to attract mates. Lethal coalitionary attacks by males on other male members of their own communities would not provide these benefits and are not expected, given the importance of cooperation among male community members in contests for dominance rank and in both defense and offense against neighboring males. Nevertheless, intracommunity coalitionary killings associated with struggles for alpha rank occur in the wild and in captivity, and observers have seen serious gang attacks on maturing adolescent and young adult males at Mahale and Budongo: the victim in the Budongo case was killed (Fawcett and Muhumuza, 2000). I describe a lethal attack on a young adult male by a large coalition of males from his community at Ngogo, Kibale National Park, Uganda. The Ngogo community is the largest known for chimpanzees and has an unusually large number of males. The attack was not related to a struggle for alpha rank: the victim was low-ranking and the community had a well-established alpha at the time. However, the victim had risen substantially in the male hierarchy over the past few years and might have appeared threatening to many higher-ranking males. Simultaneously, he associated relatively little with most other adult males, had relatively few grooming partners and was not well integrated into the male grooming network, and had no influential allies. The combination of these social factors with the unusual demographic circumstances – which presumably meant that mating competition was relatively high and the cost of losing one male relatively low – might have triggered the attack.     

Fatal Chimpanzee Attack in Loango National Park,Gabon

In some populations, chimpanzees engage in lethal aggression within and between social units. We report a fatal attack on an adult male chimpanzee at a new research site in Loango National Park, Gabon. We found a fresh corpse of an adult male chimpanzee only a few hundred meters from the research camp, after noting numerous vocalizations and chimpanzee movements the previous evening. Previous contacts with chimpanzees and fresh tracks in the area around the corpse suggest that 2 communities of chimpanzees range where the attack occurred and that members of the neighboring community killed the chimpanzee. To support the conclusion, we conducted genetic analysis for 13 Y-chromosome loci and 9 microsatellite loci of fecal samples from the dead individual, 5 possible attackers, and 2 members of the other community Though we cannot exclude the possibility of an intracommunity killing, the combined observational and genetic evidence suggest an intercommunity attack. The case study adds to the growing evidence that intercommunity killings are a rare but widespread phenomenon among chimpanzees and not an artifact of human provisioning or habituation.     

Demographic influences on the behavior of chimpanzees

Recent research has revealed substantial diversity in the behavior of wild chimpanzees. Understanding the sources of this variation has become a central focus of investigation. While genetic, ecological, and cultural factors are often invoked to explain behavioral variation in chimpanzees, the demographic context is sometimes overlooked as a contributing factor. Observations of chimpanzees at Ngogo, Kibale National Park, Uganda, reveal that the size and structure of the unit group or community can both facilitate and constrain the manifestation of behavior. With approximately 150 individuals, the Ngogo community is much larger than others that have been studied in the wild. We have taken advantage of the unusual demographic structure of this community to document new and intriguing patterns of chimpanzee behavior with respect to hunting, territoriality, and male social relationships. Chimpanzees at Ngogo hunt often and with a considerable degree of success. In addition, male chimpanzees there frequently patrol the boundary of their territory and engage in repeated bouts of lethal intergroup aggression. By forming two distinct subgroups, male chimpanzees at Ngogo also develop social bonds above the level of dyadic pairs. While the sheer number of chimpanzees contributes to differences in hunting, patrolling, mating, and subgrouping at Ngogo, the demographic situation may also constrain behavioral interactions. At Ngogo, male chimpanzees who are closely related genetically through the maternal line do not appear to affiliate or cooperate with each other. Demographic constraints may be responsible for this finding. In this paper, I use these examples to illustrate how the demographic context affects the possible range of behavioral options open to individuals and ultimately contributes to the explanation of behavioral diversity in chimpanzees.     

Intra-community infanticide and forced copulation in spider monkeys: a multi-site comparison between Cocha Cashu, Peru and Punta Laguna, Mexico

We describe two cases of infanticide, two suspected infanticides, and a forced copulation by familiar resident males in two populations of wild spider monkeys (Ateles belzebuth chamek and A. geoffroyi yucatanensis). These are the first known infanticides and forced copulation in spider monkeys. Data were gathered from four neighboring communities of spider monkeys in Manu National Park at the Cocha Cashu Biological Station, Peru and two communities in the Otoch Ma'ax Yetel Kooh Reserve at Punta Laguna, Mexico, during intensive field studies of over 2,000 hr each. These are rare behaviors, but results suggest that mating history and sexual coercion are important in spider monkey social relationships.     

Primate population dynamics over 32.9 years at Ngogo, Kibale National Park, Uganda

We present census data for eight primate species spanning 32.9 years along the same transect at Ngogo, Kibale National Park, Uganda, demonstrating major changes in the composition of the primate community. Correlated with an estimated decline of ～89% in the red colobus population was an increase in encounter rates with chimpanzee parties. Our data, along with the unusually high rates of predation by chimpanzees on red colobus at Ngogo and the fact that the chimpanzee community at Ngogo is the largest ever recorded, support the conclusion that the red colobus decline was caused primarily by chimpanzee predation. This seems to be the first documented case of predation by one nonhuman primate causing the population decline in another. We evaluated disease and interspecific competition as other possible causes of the red colobus decline, but judged them to be relatively insignificant compared with predation by chimpanzees. Notable changes in encounter rates with other primate species may have resulted from forest expansion. Those for mangabeys, redtails, and black and white colobus increased significantly. Encounter rates increased for l'Hoest's monkeys too, but the increased sightings may have been an artifact of increased habituation. Sightings of blue monkey and baboon groups declined. There was no significant change in encounter rates for all species combined. The Ngogo primate community seemed to be in a nonequilibrium state, changing from one dominated by two species, a folivore (red colobus) and a frugivorous omnivore (redtails), to one dominated by three species of frugivorous omnivores (redtails, mangabeys, and chimpanzees). This study demonstrates the importance of long-term monitoring in understanding population dynamics and the role of intrinsic variables in shaping the species composition of a community.     

Wounding aggression during the formation and maintenance of captive,multimale chimpanzee groups

Although the multimale community is the natural social organization of chimpanzees, both wild and captive adult males have killed other adult males and infants in intercommunity conflicts and intragroup aggression. Despite the potential for serious aggression, the formation of captive, multimale social groups is desirable for the efficient, long-term, humane housing of chimpanzees in socially and physically enriched environments and for the education of zoo visitors. The University of Texas Science Park (UTSP) has maintained multimale groups of chimpanzees for 14 years. In the UTSP outdoor corral housing, multimale/multifemale social groups of unrelated adult and adolescent chimpanzees (42 F, 46 M) were formed by a series of 397 individual introductions. Wounding aggression was minimal during introductions of females to males or other females and upon male-male introductions of formerly single-caged adolescent and young adult males having had long-term prior visual familiarity. Serious wounding occurred during male-male introductions when there were major discrepancies in the age and social experience of the subjects or when adult, socially experienced males were reintroduced to former group mates following lengthy separations. Male wounding in the eight established long-term groups of 5–11 adults (2–7 males) averaged 1.4 episodes per male-year of residence; 14% of male wounding episodes required surgical therapy. Adult wounding was significantly associated with the presence of one or more group females with maximally tumescent genital swellings. No male-perpetrated infanticides occurred. Not all multimale groupings are successful, but the majority of formerly laboratory-housed chimpanzees may live and reproduce safely in multimale groups. Experience with all-male groups at UTSP suggests that bachelor groups are also practical for long-term housing. © 1995 Wiley-Liss, Inc.     

Diet of chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, Uganda, 1. Diet composition and diversity

Chimpanzees (Pan troglodytes) are ecologically flexible omnivores with broad diets comprising many plant and animal foods, although they mostly eat fruit (including figs). Like other ecologically flexible nonhuman primates (e.g., baboons, Papio spp.) with broad diets, their diets vary across habitats. Much data on diets come from short studies that may not capture the range of variation, however, and data are scant on variation within habitats and populations. We present data on diet composition and diversity for chimpanzees at Ngogo, in Kibale National Park, Uganda, collected over a 15-year period, with a focus on the plant components of the diet. We compare Ngogo data to those on chimpanzees at the nearby Kibale site of Kanyawara, on other chimpanzee populations, and on some other frugivorous-omnivorous primates. Results support the argument that chimpanzees are ripe fruit specialists: Ngogo chimpanzees ate a broad, mostly fruit-based diet, feeding time devoted to fruit varied positively with fruit availability, and diet diversity varied inversely with fruit availability. Comparison of Ngogo and Kanyawara shows much similarity, but also pronounced within-population dietary variation. Chimpanzees fed much more on leaves, and much less on pith and stems, at Ngogo. Figs accounted for somewhat less feeding time at Ngogo, but those of Ficus mucuso were quantitatively the most important food. This species is essentially absent at Kanayawara; its abundance and high productivity at Ngogo, along with much higher abundance of several other important food species, help explain why chimpanzee community size and population density are over three times higher at Ngogo. High inter-annual variation at Ngogo highlights the value of long-term data for documenting the extent of ecological variation among chimpanzee populations and understanding how such variation might affect population biology and social dynamics.     

Grooming Between Male Chimpanzees at Ngogo,Kibale National Park. I. Partner Number and Diversity and Grooming Reciprocity

Allogrooming serves many social functions in primates. Grooming can help individuals to service social relationships generally, sometimes reciprocally, and may be particularly important in the development and maintenance of alliances. However, time constraints limit the number of partners with whom one individual can groom enough to maintain cooperative relationships. As a result, the size of its grooming network may reach an asymptote as the size of its group increases, and it may distribute its grooming less equally among potential partners. Chimpanzees live in multimale, fission-fusion communities; males are philopatric, and commonly associate and groom with each other. Males form within-community alliances that influence dominance rank and access to mates, and allies groom with each other regularly; males also cooperate in aggression between communities. The chimpanzee community at Ngogo, in Kibale National Park, Uganda, is unusually large and has more males than any other known community. Field data show that adult Ngogo males groomed far more with other adult males than with females or with adolescent males, in contrast to a previous report (Ghiglieri, 1984). Adolescent males groomed adults much more than the reverse; males groomed and were groomed by females about equally. Individual males groomed mostly with a small number of other males. On average, males at Ngogo had only slightly more male grooming partners overall and had the same number of important partners as those of males in a much smaller community in the Mahale National Park, Tanzania, and they distributed their grooming less equitably. These results fit those expected if limits on available grooming time cause males to have a loyalty problem as the number of potential grooming and alliance partners increases. Despite differences in the extent and equitability of their grooming networks, males at both Ngogo and Mahale showed reciprocity in grooming. Grooming reciprocity has been demonstrated for captive chimpanzee males, but the Ngogo findings are the first demonstrations of reciprocity in wild communities.     

Dominance rank and fecal testosterone levels in adult male chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, Uganda

In an attempt to describe hormone-behavior interactions in a sample of wild male chimpanzees, we quantified testosterone in 67 fecal samples obtained from 22 adult male chimpanzees at Ngogo, Kibale National Park, Uganda. A mixed-model methodology that controlled for age-class identified a significant positive association between testosterone levels and dominance rank. The results are consistent with those reported from a separate, smaller chimpanzee community in the same population in a study that analyzed testosterone levels in urine [Muller & Wrangham, 2004]. As in that earlier study, our results held during a period of social stability, which is not consistent with predictions of the "challenge hypothesis." We concur with Muller and Wrangham [2004] that the challenge hypothesis requires modification to explain the chimpanzee data, because fission-fusion sociality in chimpanzees makes challenges unpredictable. We also discuss the utility of fecal samples and a mixed-model statistical method for behavioral endocrinology studies.     

New Cases of Intergroup Violence Among Chimpanzees in Gombe National Park,Tanzania

Despite considerable attention to chimpanzee intergroup violence, the number of observed cases remains small. We report 4 cases of intergroup violence that occurred in Gombe National Park, Tanzania, between 1993 and 2002. We observed (3 cases) or inferred (1 case) males from the Kasekela community to attack members of their 2 neighboring communities: Mitumba and Kalande. In 1993, Kasekela males killed and ate a female infant from Mitumba. In 1998, Kasekela males captured 2 infants (sex unknown) from Kalande, one of which escaped and the other was killed and eaten. Also in 1998, Kasekela males attacked an adolescent male from Kalande. The victim was alive but severely injured by the end of the attack. The intensity and duration of the attack are comparable to other attacks that resulted in fatal injuries. In 2002, observers found the body of an adolescent male from Mitumba following an incursion by Kasekela males into the area. The injuries inflicted on the Mitumba male together with circumstantial evidence suggest that Kasekela males killed him. The attacks support the view that intergroup violence is a persistent feature of chimpanzee societies and that the primary benefit attackers gain from them is reduced competition for resources.     

Why do chimpanzee males attack the females of neighboring communities?

Our closest nonhuman primate relatives, chimpanzees, engage in potentially lethal between‐group conflict; this collective aggressive behavior shows parallels with human warfare. In some communities, chimpanzee males also severely attack and even kill females of the neighboring groups. This is surprising given their system of resource defense polygyny, where males are expected to acquire potential mates. We develop a simple mathematical model based on reproductive skew among primate males to solve this puzzle. The model predicts that it is advantageous for high‐ranking males but not for low‐ranking males to attack females. It also predicts that more males gain a benefit from attacking females as the community's reproductive skew decreases, i.e., as mating success is more evenly distributed. Thus, fatal attacks on females should be concentrated in communities with low reproductive skew. These attacks should also concur with between‐community infanticide. A review of the chimpanzee literature provides enough preliminary support for this prediction to warrant more detailed testing. Am J Phys Anthropol 155:430–435, 2014. © 2014 Wiley Periodicals, Inc.     

Infanticide risk and infant defence in multi-male free-ranging sooty mangabeys, Cercocebus atys

For years, infanticide by males was thought to be unlikely in multi-male primate species. Recent studies have, however, presented evidence of infanticide in such species and a recent model by Broom and colleagues predicts that males’ age and rank influence the occurrence of infanticide: youngest and highest-ranking immigrant males are more likely to commit infanticide than their older and lower-ranking counterparts if putative fathers fail to protect infants. I collected data on adult free-ranging sooty mangabey females in the Taï National Park, Ivory Coast, over 11 months including a birth and a mating season. Infanticide had been previously reported in captivity for this species, but not in the wild. Several males entered the group prior to and during the mating season. As predicted by the model, only the more dominant immigrant ones attacked mother-infant pairs significantly more often than did other males. Mothers often reacted with counter-attacks. Potential fathers guarded and supported infants and mothers throughout the period of infant vulnerability. Furthermore, as only one of seven infants died despite 136 observed attacks on mother-infant pairs and unattended infants by immigrant males, we conclude that cooperation between putative fathers and mothers represents an effective protection against infanticide.