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.     

New Intercommunity Infanticides by the Chimpanzees of Ngogo,Kibale National Park,Uganda

Hostile intercommunity relations, including attacking and killing extra-community infants of both sexes have occurred at most wild chimpanzee sites. We describe three recent cases of intercommunity attacks on infants committed by members of the Ngogo chimpanzee community in Kibale National Park, Uganda. Two of the attacks resulted in confirmed infanticides while a third attack probably resulted in the infant's death. In common with previous accounts of chimpanzee infanticides, the attacks described here occurred during boundary patrols outside the Ngogo community's usual range, adult and adolescent males were the main participants, one infant was cannibalized after being killed, and the victims’ mothers did not accompany the attacking party back to the Ngogo range. However, the patrol parties during each infanticide were larger than before and included females from the Ngogo community. Our observations indirectly support both the range expansion and imbalance of power hypotheses, which address why and under which conditions chimpanzee intercommunity encounters lead to aggression. These cases of intercommunity infanticide add to the growing database of the phenomenon in wild chimpanzees.     

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.     

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.     

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.     

Influence of chimpanzee predation on the red colobus population at Ngogo, Kibale National Park, Uganda

Frequent hunting of red colobus monkeys (Procolobus rufomitratus) takes place at all long-term chimpanzee (Pan troglodytes) study sites where both species are present. Red colobus are the most commonly selected prey of chimpanzees even when other monkey species are more abundant. In particular, the chimpanzee community at Ngogo, Kibale National Park, Uganda, preys heavily on red colobus monkeys: the chimpanzee hunting success rate is extremely high, and chimpanzees kill many individuals per successful hunt. Census data had suggested that the red colobus population is declining and that predation by chimpanzees may be contributing to this decline. In this paper, I address the impact of hunting on the red colobus population at Ngogo. To test the hypothesis that chimpanzee hunting is sustainable, I am using demographic data collected on red colobus monkeys over a period of 3 years, as well as fecundity and mortality data from previous studies of this species. I apply matrix models and vortex analyses using a sensitivity analysis approach to project future population development. Results show that current rates of hunting are not sustainable, but that chimpanzees are neither more “noble”, nor more “savage” than humans are, but that they also hunt to ensure maximum benefit without regard for the consequences for the prey population.     

Within-group cannibalism by adult male chimpanzees

In the Mahale Mountains National Park, Tanzania, a young adult male chimpanzee was observed to feed on a 3-month-old male infant of the same unit-group. Four other adult males and an adult female shared the carcass. The mother of the victim had immigrated from a neighboring unit-group four years previously. Circumstantial evidence strongly suggests that the first-observed cannibal male also killed the infant. The adult male and the mother of the victim had been familiar socially and sexually with each other since the female immigrated. Since the mother of the victim had usually been ranging in the peripheral part of the unit-group's range, i.e., the overlapping area of the two unit-group's ranges during pregnancy and soon after birth, the infanticidal male might have had reason to suspect the paternity of her infant. Four such cases of within-group cannibalism by adult males suggest that the female range and association pattern before and after parturition are key factors allowing an infant to survive. The possibility of male-biased infanticide is also discussed.     

Tool Use by Chimpanzees at Ngogo,Kibale National Park,Uganda

Chimpanzees make and use a wide variety of tools in the wild. The size and composition of their toolkits vary considerably among populations and at least to some extent within them. Chimpanzees at several well documented sites mostly use tools in extractive foraging, and extractive tool use can substantially increase their foraging efficiency. They also use tools for hygiene and for several other purposes, including attracting the attention of conspecifics, as in leaf-clipping. Some of the interpopulation variation in toolkits results from ecological variation, but differences in the efficiency of social transmission, perhaps related to differences in social tolerance, presumably also contribute. I describe tool use by chimpanzees in an unusually large community at Ngogo, in Kibale National Park, Uganda. Researchers have described some tool use for the community previously, but this is the most extensive report and is based on observations over 11 yr. The Ngogo chimpanzees have a small toolkit and use tools rarely except in leaf-clipping displays and to clean body surfaces; notably, males often use leaf napkins to wipe their penes after copulation. Extractive tool use is rare and is limited mostly to leaf-sponging and, less often, honey-fishing. Social tolerance is not low at Ngogo, but use of tools for extractive foraging, in ways documented at other field sites, may have little potential to increase foraging efficiency. Future research will undoubtedly show more tool use by females, which were underrepresented in my observations, but will probably not document much increase in the toolkit or in the use of extractive tools.     

Parasitological analyses of the male chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, Uganda

Numerous intestinal parasites identified in populations of wild nonhuman primates can be pathogenic to humans. Furthermore, nonhuman primates are susceptible to a variety of human pathogens. Because of increasing human encroachment into previously nonimpacted forests, and the potential for disease transmission between human and nonhuman primate populations, further detailed investigations of primate ecological parasitology are warranted. For meaningful comparisons to be made, it is important for methods to be standardized across study sites. One aspect of methodological standardization is providing reliable estimates of parasite prevalence and knowing how many samples are needed to adequately estimate an individual's parasite prevalence. In this study the parasitic fauna of 37 adult, adolescent, and juvenile male chimpanzees from the Ngogo group, Kibale National Park, Uganda, were assessed from 121 fecal samples collected over a 3-month period. Twelve taxa of intestinal species (five helminth and seven protozoan) were recovered from the samples. The four most prevalent species were Troglodytella abrassarti (97.3%), Oesophagostomum sp. (81.1%), Strongyloides sp. (83.8%), and Entamoeba chattoni (70.3%). No one species was found in all samples from any one animal, and Troglodytella abrassarti, the most common intestinal organism, was found in all of the serial samples of only 69.4% of the chimpanzees. The cumulative species richness for individuals significantly increased for every sequential sample (up to three to four samples) taken per animal during this study. The results indicate that to accurately diagnose total intestinal infection and evaluate group prevalence, three to four sequential samples from each individual must be collected on nonconsecutive days. This conclusion applies only to short study periods in which possible seasonal effects are not taken into consideration. Validation of these results at different study sites in different regions with different climatic patterns is needed.     

Tool use in insect foraging by the chimpanzees of Ngogo, Kibale National Park, Uganda

Chimpanzee tool use for resource acquisition has been reported at numerous research sites. The chimpanzees of the Kibale Forest, western Uganda, have not previously been observed to use tools in foraging for insects. Here I report the first observation of tool use by the chimpanzees of the Ngogo community of Kibale National Park, Uganda, in insect foraging. Three adult females, one adolescent male, and one juvenile male were observed making and using tools to probe into a fallen dead tree to collect insect and wood pieces. I discuss the importance of this observation, and the behavioral similarities with chimpanzees from other sites.     

Hunting Behavior of Chimpanzees at Ngogo,Kibale National Park,Uganda

Chimpanzees (Pan troglodytes) prey on a variety of vertebrates, mostly on red colobus (Procolobus spp.) where the two species are sympatric. Variation across population occurs in hunting frequency and success, in whether hunting is cooperative, i.e., payoffs to individual hunters increase with group size, and in the extent to which hunters coordinate their actions in space and time, and in the impact of hunting on red colobus populations. Also, hunting frequency varies over time within populations, for reasons that are unclear. We present new data on hunting by chimpanzees at Ngogo, Kibale National Park, Uganda, and combine them with earlier data (Mitani and Watts, 1999, Am. J. Phys. Anthropol. 109: 439–454) to examine hunting frequency and success, seasonality, and cooperation. The Ngogo community is the largest and has the most males of any known community. Chimpanzees there mostly hunt red colobus and are much more successful and make many more kills per hunt than at other sites; they kill 6–12% of the red colobus population annually. The number of kills and the offtake of meat per hunt increase with the number of hunters, but per capita meat intake is independent of hunting party size; this suggests that cheating occurs in large parties. Some behavioral cooperation occurs. Hunting success and estimated meat intake vary greatly among males, partly due to dominance rank effects. The high overall success rate leads to relatively high average per capita meat intake despite the large number of consumers. The frequency of hunts and of hunting patrols varies positively with the availability of ripe fruit; this is the first quantitative demonstration of a relationship between hunting frequency and the availability of other food, and implies that the chimpanzees hunt most when they can easily meet energy needs from other sources. We provide the first quantitative support for the argument that variation in canopy structure influences decisions to hunt red colobus because hunts are easier where the canopy is broken.     

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.     

A preliminary analysis of wound care and other-regarding behavior in wild chimpanzees at Ngogo,Kibale National Park,Uganda

Primates - Caring for others is a key feature of human behavior. Mothers, fathers, siblings, grandparents, and other group members provide care in the form of provisioning, protection, and first...     

Primate abundance along five transect lines at Ngogo, Kibale National Park, Uganda

Using the line transect methods, I studied the primate density at Ngogo, Kibale National Park, Uganda for 18 months. Comparisons with other studies show that the population of red colobus monkeys (Procolobus rufomitratus) and blue monkeys (Cercopithecus mitis) is declining, whereas the populations of black-and-white colobus (Colobus guereza), red-tailed monkeys (Cercopithecus ascanius), grey-cheeked mangabeys (Lophocebus albigena), baboons (Papio anubis), and chimpanzees (Pan troglodytes) remain constant or slightly increase. In this paper, I compare data on density from this study to data from previous and recent censuses at Ngogo and with data from other sites in the Kibale forest to examine the stability of primate populations. Furthermore, I test the hypothesis that the changes in red colobus and blue monkey density are due to changes in the forest structure and abundance of their most selected feeding trees, and show that changes in forest composition cannot account for changes in their red colobus abundance, but that hunting by chimpanzees provides a reasonable explanation.     

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.     

Floristic heterogeneity at Ngogo,Kibale National Park,Uganda and possible implications for habitat use by chimpanzees (Pan troglodytes)

Tropical forest tree communities exhibit heterogeneity at multiple spatial scales, with important implications for animals relying on these resources. However, different organisms may perceive heterogeneity in the floristic community in very different ways. Here, we characterize the overall extent of heterogeneity in the floristic community at Ngogo, in Kibale National Park, Uganda. Also, using information from studies on the diet of chimpanzees (Pan troglodytes) inhabiting Ngogo, we quantify the extent to which the habitat is likely perceived as heterogeneous by this species. The forest as a whole is slightly more diverse than comparable sites in the Congo Basin, but this diversity is driven by a relatively high proportion of rare species represented by few individuals. From the perspective of chimpanzees, the forest is, unsurprisingly, even more heterogeneous. Species that provide fruit for chimpanzees during times of low overall fruit abundance and that display interindividual synchrony in fruiting were the most common chimpanzee resource in our sample, whereas species that provide fruit during times of low overall fruit abundance and that display asynchronous fruiting were the least common. We discuss the implications of the differences in density and distribution of various classes of resources for chimpanzee habitat use and foraging efficiency.     

Infanticide in chimpanzees: Review of cases and a new within-group observation from the Kanyawara study group in Kibale National Park

A prolonged attack on a mother and 2-year-old infant that resulted in the death of the infant was observed in the Kanyawara study group in Kibale National Park. The mother was a border-area resident who was first observed associating with unit-group males six years previously. The attackers were an adult male and an adult female with a 6-week-old infant clinging ventrally to her. The attack was unusual in several respects: it is the first time a male and a female chimpanzee have been observed cooperating closely in an infanticidal attack; the adult female initially attempted to intervene in the victim's behalf, but later joined in the attack after receiving aggression from the male; and the episode was longer in duration than other reported cases. In the year following the incident, the mother did not increase her association with community males, but was seen with the male who killed her infant. The relevance of these observations to sexual selection-based explanations for infanticide in chimpanzees is discussed.