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.     

A contribution to tropical rain forest taphonomy: retrieval and documentation of chimpanzee remains from Kibale Forest, Uganda

The absence of Pan and Gorilla fossils from Africa has led some to suggest that African rain forests are not conducive to bone preservation. The absence of fossils is unfortunate as it hampers phylogenetic and socioecological interpretations on the divergence of the earliest hominids. For the most part, taphonomic studies have been restricted to cave and open country contexts. With this in mind, we have initiated a taphonomic project in a tropical rain forest, the Kibale Forest of western Uganda. In the course of bone gathering activities over the past 4 years, we have documented the retrieval of skeletal remains representing nine chimpanzees (Pan troglodytes schweinfurthii). Crania and mandibles are most commonly recovered, followed by elements of the axial skeleton, and finally, appendages. Vertical segregation of small compact bones can occur in areas with a soft substrate. Scavenging activity suggests the role of suids, but this has not been proven. Geochemical tests suggest that Kibale soils are neutral and may be conducive to bone preservation. Our independent preliminary data from bone weathering/survival experiments indicate that bones appear undamaged after several years and are able to accumulate on the forest floor. These results contrast with popular assumptions on the potential of African rain forests as potential fossil reservoirs.     

Analysis of geophagy soils in Kibale Forest, Uganda

Four soil samples from the Kibale Forest, Uganda, representative of material regularly ingested by chimpanzees, were studied for their mineral, chemical, and geochemical composition. These geophagy soils have a high content of metahalloysite, a partially hydrated clay mineral that may act much like the pharmaceutical Kaopectate™. Among the elements that may act as a stimulus or stimuli for geophagy behavior, only iron is very high (total iron ranges from 6% to 17%); other possibilities such as calcium, chromium, cobalt, bromine, and iodine are either relatively low or are below their detection limits. Chlorine is below detection limits which eliminates sodium chloride as a possible stimulus. Depending on relative availability in the gut, iron offers the most likely chemical stimulus for geophagy and given the mineral composition of the samples, metahalloysite is the most likely mineral stimulus. Iron may play a role in replenishing hemoglobin which would be important in chimpanzee physiology at high elevations near the flanks of the Ruwenzori Mountains. Metahalloysite, which in this case exists in a relatively pure crystalline form, may well act to quell symptoms of diarrhea and act similarly to Kaopectate™. Organic chemical analyses indicate only traces of organic matter and no humic acids in the K14-E14 sample.     

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.     

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.     

Censusing chimpanzees in the Budongo Forest,Uganda

It is difficult to make valid comparisons of chimpanzee densities among sites because observers calculate them using different methods. We argue that nest count estimates of density are preferable to densities from home range estimates because of the problems of defining home range. There are many problems associated with nest count methods, some of which have not been addressed in previous studies. In 1992, we censused chimpanzees in the Budongo Forest using three methods;the standing crop nest count (SCNC), the marked nest count (MNC), and visual sightings of the animals (VS). Each method is based on standard line transect techniques. Of 96 nests monitored for decay rate,those constructed in the dry seasons decayed faster than those in the wet seasons. All- day follows of individual chimpanzees and observations of nesting chimpanzees at dusk showed that about 15.8% of night nests were reused,17.5% of the population did not build nests, and 18.8% of nests were first constructed as day nests. Given the variability in nest decay rates, we argue that MNC is a better method than SCNC because it avoids having to calculate decay rates.     

Interspecific interaction of the primate groups in Kibale Forest,Uganda

An ecological survey on the influence of interspecific interaction of the primates upon the distribution of their group ranges was carried out in 100 ha of the isolated forest northern outskirts of Kibale Forest in western Uganda, Africa. The study period of 105 days was from the 12th of November, 1970 to the 24th of February, 1971, including a preliminary survey of about two months. The subjects of this study are five species of primates, i.e., black and white colobus (Colobus polykomos), red colobus (Colobus badius), red tailed monkey (Cercopithecus ascanius), blue monkey (Cercopithecus mitis), and vervet monkey (Cercopithecus aethiops), which inhabited the study area. The red colobus group is thought to be the most influential of the five in the interspecific interaction.     

Soils Consumed by Chimpanzees of the Kanyawara Community in the Kibale Forest,Uganda

We previously reported on a study of 4 soils that chimpanzees of the Kanyawara community in the Kibale National Park, Uganda consumed on a near-daily basis. We suggested that iron was a possible chemical stimulus in association with high quantities of Si:Al = 1:1-dominated clay minerals in the consumed material. To test our initial findings, we analyzed 18 samples from the same general area including 7 samples that the chimpanzees did not eat. Among the chemical elements, As, Au, Br, Ca, Cl, Dy, Mg, Ni, Sb, Sr, and I are below detection limits. Only Fe stands out as a potentially important nutritional element present in sufficient quantity to provide a physiological stimulus for chimpanzees living at high elevations near the flanks of the Ruwenzori Mountains. Along with Fe, metahalloysite is present in high amounts in these soils. In its pure crystalline form as a pharmaceutical grade clay mineral metahalloysite may well counteract the debilitating effects of diarrhea, with an effect similar to what is achieved with kaolinite (cf. Kaopectate^TM). An unexpected result, the relatively high nitrogen and carbon in the eaten samples relative to the uneaten group, indicates the chimpanzees may have a higher threshold for organic-rich material than previously believed. Contrarily, the color of the ingested material, depicts a material with less humus than in the uneaten group, a finding that is compatible with previous work reported at other geophagy sites in Africa. Of all the choices of soil available to them, the chimpanzees appear to be selecting highly homogeneous chemical natural earths with well-leached and uniform mineralogical material similar to the uneaten group, but with higher relative amounts of clay size material.     

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.     

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.     

Social behavior of chimpanzees in the Budongo Forest,Uganda

The author studied the social organization and behavior of wild chimpanzees in the Budongo Forest, Uganda, between September, 1966, and March, 1967. A population of more than 50 chimpanzees became habituated to the author and was followed continuously, by means of individual identification, throughout the study period. The present article deals with the chimpanzee's social behavior, with special reference to social organization.The central organization which controls and maintains chimpanzee society cannot be rigid dominance rank order, since agonistic interactions among chimpanzees were not frequent. On the other hand, friendly or associative social behavior was frequently observed, some of which was unique in non-human primates. Greeting behavior, especially, and begging-and-food-sharing behavior show the existence of the chimpanzee's unique communicative system, which is backed by symbolic function. This kind of communicative system appears to have an important role in the maintenance of the unique social organization of chimpanzees, which has a loose regional integration, though the animals frequently gather into parties and later split up.Sexual activity was observed throughout the study period, but the peak was seen between December and February, during the dry season.The field work necessary for this study was carried out as a part of the Kyoto University Africa Primatological Expedition and was financed mainly by a Grant for Scientific Research from the Ministry of Education and partially by the Wenner-Gren Foundation for Anthropological Research and by the economic world of Japan.     

Social organization of chimpanzees in the Budongo Forest,Uganda

A population of wild chimpanzees in the Budongo Forest, Uganda, was studied between September, 1966, and March, 1967, by means of habituation and identification of each individual. This article deals with the grouping patterns, social units, and social organization of the chimpanzees in their natural habitat. Although parties of chimpanzees form groups of many patterns according to each particular situation and have no permanent membership, many chimpanzees live within a certain restricted area and gather to form parties consisting, in the main, of chimpanzees acquainted with each other. More than 50 individuals combined by social bonds in a loose regional population which is separated from other populations without recourse to geographical or physical barriers, though it maintains friendly contact with neighboring chimpanzee residents and with strangers. No particular individual group leader can be found, but any adult, especially a male, may act as a nucleus of each party.This study was carried out as a part of the Kyoto University Africa Primatological Expedition; the field work was financed mainly by a Grant for Scientific Research, Ministry of Education, and in part by the Wenner-Gren Foundation for Anthropological Research and by the economic world of Japan.     

Variation in diet and ranging of black and white colobus monkeys in Kibale National Park,Uganda

Recently, considerable intraspecific variation in the diets and ranging behavior of colobine monkeys has been described, although in most cases this has involved documenting variation between, not within, sites. Some African colobines, such as guerezas (Colobus guereza), are relatively abundant in disturbed habitats that are very heterogeneous, raising the intriguing possibility that even groups with overlapping home ranges may exhibit large behavioral differences. If such differences occur, it will be important to understand what temporal and spatial scales adequately portray a species’ or population’s diet and ranging behavior. This study documents within-site variation in the diet and ranging behavior of guerezas in the habitat types in which they are described to be most successful—forest edge and regenerating forest. We collected data on eight groups of guerezas with overlapping home ranges for 3–5 months each in Kibale National Park, Uganda. The guerezas were highly folivorous, with leaves constituting 78.5–94.0% of the groups’ diets. The percentage of mature leaves and fruit in the diet varied widely among and within groups. We show that differences among groups in the intensity with which they fed on specific tree species were not just related to phenology, but also to differences in the forest compositions of groups’ core areas. Range size estimates varied more than fivefold among groups and the minimum distance from groups’ core areas to eucalyptus forest (which all groups regularly fed in) was a better predictor of range size than was group size. These results reveal considerable variation in the diet and ranging behavior among groups with overlapping ranges and have implications for comparative studies, investigations of within- and between-group feeding competition, and the potential for populations to adapt to anthropogenic or natural environmental change.     

Patterns of frugivory of the Budongo Forest chimpanzees, Uganda

Frugivory patterns of the chimpanzees in the Budongo Forest Reserve, Uganda were studied between June 2000 and August 2001. Chimpanzee feeding habitats, movement, group size and food eaten were assessed using focal and scan sampling. It was found that fruits were scarce during the dry season, when chimpanzees appeared and moved in large groups over long distances and raided farms at the forest edge. Chimpanzee movement out of the forest to forage was influenced by seasonal fluctuations in availability of preferred foods as some cultivated crops are perennial. Presence of chimpanzees in a specific feeding habitat was related to the availability of edible fruits both within and between months, suggesting that the presence of food may influence chimpanzee movement patterns. Therefore, a good understanding of patterns of frugivory is essential for making informed decisions about conservation of chimpanzees and other frugivores like birds and monkeys in Budongo as different forest habitats are under varying human pressure because of logging and other forms of utilization.