Responses to a stranger mother-son pair in the wild chimpanzee: A case report

A stranger mother-son pair of the chimpanzee was observed twice interacting with conspecifics of a neighbouring unit-group: first, when the mother and son accidentally encountered them within the core area of the former; second, when the mother and son temporarily immigrated for about one week. On both occasions, the mother and son were severely attacked by adult males of the neighbouring unit-group, and would have been killed had it not been for human intervention. The main target of the aggression was not the infant, but the mother. Some adult males intervened and prevented other males and females from attacking the mother-son pair. Moreover, most adult males displayed an ambivalent attitude since they showed aggression towards them on one occasion, but groomed, reassured and played on another. The reasons for the variable responses of adult males to a stranger female are discussed in terms of possible differences in their mating strategies.     

A case of unusually early postpartum resumption of estrous cycling in a young female chimpanzee in the wild

A case of unusually early postpartum resumption of estrous cycling (<7 months) was recorded for a young, presumably primiparous female in the M group of chimpanzees (Pan troglodytes schweinfurthii) in the Mahale Mountains National Park, western Tanzania. The female showed estrous cycling while lactating her infant, and mated with young and low-ranking males as well as with the alpha male.     

Predation on mammals by the chimpanzee (Pan troglodytes)

With respect to prey selectivity and predation frequency, chimpanzees (Pan troglodytes) show local differences as well as diachronic variability within the same population. When data on predation from three long-term studies at Mahale, Gombe, and Tai are compared, some differences and similarities emerge; Mahale is more like Gombe than Tai in regard of prey selection but features of hunting at Tai with respect to predation frequency are not conspicuous. The most responsible factor for diversity in prey selectivity is a distinct “prey image” maintained by chimpanzees of different populations, although it is necessary to clarify in future studies why and how such tradition develops. Relative body size of chimpanzees to prey species and/or the degree of cooperation among members of a hunting party may explain the variability in prey size selected at each site, the latter influencing the frequency of successful hunts at the same time. Although various degrees of habituation and different sampling methods including artificial feeding might have obscured the real differences, recent data from the three populations do not seem to be biased greatly by such factors. Nevertheless, it is still difficult to make strict comparisons due to the lack of sufficient standardized data across the three populations on the frequency of hunting and predation. It is suggested that the size or demographic trend of a chimpanzee unit-group, especially the number of adult males included, necessarily influences its hunting frequency as well as its prey profile. It is also suggested that factors which bring these males together into a party (e.g. fruit abundance, swollen females, conflict between unit-groups etc.) strongly affect theactual hunting and kill rates. Other possible factors responsible for the local differences are forest structure (e.g. tree height), skilful “hero” chimpanzees, and competition with sympatric carnivorous animals. A total of at least 32 species have been recorded as prey mammals of chimpanzees from 12 study sites and the most common prey mammals are primates (18 species), of which 13 species are forest monkeys. Forest monkeys, colobine species in particular, are often the most common victims of the predation by chimpanzees at each site. We may point out a tendency toward selective hunting for the forest monkeys in terms of the selectivity of prey fauna among all three subspecies of chimpanzees, including populations living in drier environment. The mode of chimpanzee hunting seems to correspond to the highest available biomass of gregarious, arboreal monkeys in the forest, colobine species in particular. In contrast, bonobos (P. paniscus) are less carnivorous than chimpanzees, only rarely preying on a few species of small mammals. The sharp contrast of the two allied species in their predatory tendencies appears to have something to do with the differences in the structure of primary production between their habitats.     

Inversion effect in perception of human faces in a chimpanzee (Pan troglodytes)

Three experiments investigated the inversion effect in face perception by a chimpanzee (Pantroglodytes) under the matching-to-sample paradigm. The first two experiments addressed the inversion effect in the perception of human faces. In Experiment 1, the subject received identity matching using 104 photographs of faces and houses presented in four different orientations. The chimpanzee showed better accuracy when the faces were presented upright than when they were inverted. The inversion effect was not found for photographs of houses. In Experiment 2, the subject received rotational matching in which the sample and comparisons differed in orientation. The subject showed a clear inversion effect for faces but not for houses. Experiment 3 explored the hemispheric specialization of the face inversion effect with chimeric (artificially composed) faces. The subject showed no visual-field preference when the chimeric faces were presented as samples under nonreinforced probe testing, while the inversion effect was evident when the discrimination was based on the left part of the chimeric sample. The results suggested that the face-inversion was specific to the left visual field (i.e. right hemispheric processing). In general, these results were consistent with those found in humans in similar testing situations.     

Leaf-grooming by a wild chimpanzee in Mahale

During the course of systematic observations of the leaf-grooming behavior by the chimpanzees (Pan troglodytes) of the Mahale Mountains National Park, Tanzania, I recovered a louse from a leaf “groomed” by an adult male chimpanzee after a typical leaf-grooming session. During the leaf-grooming session I observed a small object on his lower lip. He picked up a leaf, transferred the small object from his lip to the leaf, folded the leaf and crushed the folded side of the leaf with his thumb. I present this observation as further evidence of the “squashing ectoparasites” hypothesis for leaf-grooming.     

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.     

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.     

Potential hominin plant foods in northern Tanzania: semi-arid savannas versus savanna chimpanzee sites

Savanna chimpanzees are useful as referential models for early hominins, and here potential differences between chimpanzee and early hominin ecology is the focus. Whereas chimpanzees inhabit only a handful of modern African savannas, there is evidence that early hominins occupied relatively more open and arid savannas than those in which chimpanzees live. In order to help expand potential models of early hominin palaeoecology beyond savanna chimpanzee-like scenarios, and to provide a basis for future modeling and testing of actual hominin diets, this study compares the types of plant foods available in modern semi-arid savannas of northern Tanzania to plant foods at savanna chimpanzee sites. The semi-arid savannas are not occupied by modern chimpanzees, but are potentially similar to environments occupied by some early hominins. Compared to savanna chimpanzee habitats, the northern Tanzania semi-arid savanna has a lower density and fewer species of trees that produce fleshy fruits. Additionally, the most abundant potential hominin plant foods are seasonally available Acacia seeds/pods and flowers, grass seeds, and the underground parts of marsh plants, as evidenced by vegetation surveys and by studies of the diets of baboons that forage in similar areas. The information from this study should be useful for framing hypotheses about hominin diets for sites with palaeoenvironmental contexts similar to those of the northern Tanzania semi-arid savannas and for contextualising tests of actual hominin diets (e.g., those based on dental microwear or isotopes).     

Polymorphic microsatellite DNA amplification customized for chimpanzee paternity testing

DNA segments containing GT/AC dinucleotide repeats in the chimpanzee (Pan troglodytes) genome were screened. Thirteen transformedE. coli colonies were identified with the (GT)₁₀ probe to have chimpanzee DNA fragments containing (GT)_n repeats. These potentially polymorphic (variable n) DNA segments were sequenced. Primers for the polymerase chain reaction (PCR) amplifying these DNA segments were designed. Six pairs of primers yielded polymorphic PCR products. Three of them revealed considerable length polymorphisms and heterozygosities in a group of captive chimpanzees. For studies on chimpanzees in the wild and in captivity, these primers should be useful for paternity testing, for investigating genetic variations, and for improving the genetic maintenance of breeding colonies. The strategy adopted in the present study to obtain PCR primers amplifying polymorphic microsatellite DNA segments may well be applicable to almost all eukaryotic organisms.     

Conflict and post-conflict behavior in a small group of chimpanzees

Chimpanzee research plays a central role in the discussions of conflict negotiation. Reconciliation, or the attraction and affiliation of former opponents following conflict, has been proposed as a central element of conflict negotiation in chimpanzees and various other taxa. In an attempt to expand the database of chimpanzee conflict resolution, conflict and post-conflict behavior were recorded for a small group of socially housed chimpanzees at the Chimpanzee and Human Communication Institute, at Central Washington University. Data were collected over six 6-week periods between 1997 and 2000, for a total of 840 hours of observation, resulting in a substantial post-conflict (PC) and matched control (MC) data set. The data demonstrate this group’s tendencies to maintain visual contact and closer proximity after conflicts. Dyadic corrected conciliatory tendencies ranged between 0 – 37.5% and averaged 17.25% across all dyads. Individual corrected conciliatory tendencies ranged between 5.8 and 32%. The results of this study combined with recent publications on captive and free-ranging chimpanzee post-conflict behavior suggest that variation in post-conflict behavior may be important to our understanding of chimpanzee conflict negotiation, and may also have implications for the design and management of captive chimpanzee enclosures and social groups, respectively.     

A lone male chimpanzee in the wild: The survivor of a disintegrated unit-group

K Group, originally one of the two major study groups of chimpanzees since 1965 in the Mahale Mountains National Park, western Tanzania, was almost extinct by 1983: at most seven individuals remained in the group at the beginning of 1983. K Group continued to exist for more than four years, but in 1987 a male was left alone at the age of 15 after all the other chimpanzees of the group emigrated or disappeared. Since then he has been observed sporadically for more than five years only within the former range of K Group, without having any contact with the many resident chimpanzees of the neighboring M Group, the other major study group. The present observations reconfirm the strong philopatric tendency of adult male chimpanzees.     

Comparison of behavioral sequence of copulation between chimpanzees and bonobos

We compared sex differences in behaviors leading to copulation of chimpanzees (Pan troglodytes) in the Kalinzu Forest, Uganda with those of bonobos (Pan paniscus) at Wamba, D.R. Congo, using the same definition. Female chimpanzees were more likely to initiate copulation than female bonobos. While most of copulations (96%) were initiated by males in bonobos, among chimpanzees only 63% of copulations were initiated by males. Female bonobos initiated an interaction leading to copulation when males approached them within a short distance. On the other hand, both male and female chimpanzees initiated behavior at a longer distance. Higher proceptivity and a higher copulation rate during the maximal swelling period of female chimpanzees might suggest that they gain greater benefits from a high frequency of copulations than do female bonobos.     

New observations on the tool-behavior of chimpanzees from Mt. Assirik (Senegal,West Africa)

The tool-behavior of chimpanzees were studied at the Mt. Assirik, in the Parc National du Niokolo-Koba, Senegal from December 1982 to June 1983 and from February 1985 to January 1986. Five leaf-stalks to obtain termites ofMacrotermes subhyalinus species and four sticks to obtain honey (this is the first report on the use of tools to obtain honey by wild chimpanzees in far western Africa) were found; we also report the first evidence of tools probably used as hammers to break open hard-shelled fruits ofAdansonia digitata. We conclude that Mt. Assirik chimpanzees provide evidence of a certain cultural behavior related to the use of stones.     

A note on the chimpanzee ecology of the Ugalla Area,Tanzania

Results of a nine-day survey of the chimpanzee habitat in Ugalla area of western Tanzania in 1975 is reported. The most dominant vegetation of the area wasJulbernardia globiflora woodland. Evidence of chimpanzees, both indirect (such as beds, faeces, and food remnants) and direct (calls and actual observation), suggests that chimpanzees are sparsely distributed throughout the area, but that they disproportionately useBrachystegia bussei woodland along escarpments andCynometra-Albizzia riverrine forests. The hilly country of Sisegwa/Mnyangwa was the only area where chimpanzees appeared to be comparatively abundant. Mammal hair found in faeces indicates mammal-predation by the chimpanzees in this area. Hard-shelled fruits ofStrychnos were apparently bitten open without the use of extra-somatic objects. The food repertoire included fruits of the generaAzanza, Canthium, Cordia, andTamarindus.     

Tool-set for termite-fishing and honey extraction by wild chimpanzees in the Lossi Forest, Congo

The use of perforating sticks and flexible stalks in combination for termite fishing and a complex tool-set of three components used sequentially (stout chiel, bodkin, and dip-stick) to penetrate melipone and ground-dwelling bee hives byPan troglodytes troglodytes are documented or, inferred from circumstantial evidence. Functionally, termite extraction tools were similar to other locations in west and central Africa, but the plants and the number of raw material species used were different. Tools varied in the degree of modification (fraying ends). Chimpanzees in the Lossi forest seem to be able to use the tools not in a stereotyped fashion, but in a flexible, insightful way. The extraction of Melipone honey using large pieces of wood as pounding tools has rarely been recorded elsewhere. The most impressive technological solution to the honey-getting problem by wild chimpanzees was shown by this study. This is the only known, use of a tool-set of three components in sequence to extract honey by wild chimpanzees.     

Hierarchy and social status in Budongo chimpanzees

The status hierarchy is fundamental in the lives of male chimpanzees. This study describes the dominance interactions and social status among adult male chimpanzees of the Sonso community in the Budongo Forest Reserve, Uganda, during the period that they were first studied (1994 and 1995). Social dominance is typically measured using the behaviour of either the subordinate or the dominant individual, but a relationship is dependent on the behaviour of both parties and this study explicitly used both subordinate and dominant behaviours to investigate the status hierarchy. Among adult males of the Sonso community, agonistic interactions occurred at a low rate and pant-grunts were rare, but males could be ranked into separate hierarchies of agonistic dominance and pant-grunting (labelled respect) using ratios of behaviour performed/behaviour received. These hierarchies were combined to form a single hierarchy of social status that divided the males among five distinct status levels. The highest status level was held by an alliance between two males who replaced the previous alpha male during the first part of the study. Neither male in this alliance partnership pant-grunted to the other, although the reason for cooperative behaviour was unclear. Although the nominally beta male was treated as such by other adult males, he achieved surprisingly little mating success. Budongo Forest chimpanzees do not warrant the sometimes-expressed view that they are non-aggressive and peaceable and the broad pattern of their status interactions matches with that seen in other chimpanzee populations.     

Establishing the presence of a body temperature rhythm in chimpanzees (Pan troglodytes) using a tympanic membrane thermometer

Seven chimpanzees (Pan troglodytes) were trained to present their ears so that a tympanic nembrane thermometer could be inserted. Temperatures were collected from both ears of each subject every 3 hours for 72 consecutive hours. The presence of a body temperature rhythm, well documented in other mammals, was established. Each ear demonstrated its own rhythm, but the rhythms in both ears generally mirrored each other. Similarities in the temperature rhythms of cagemates were found. These data are the first evidence of a body temperature rhythm in chimpanzees, and they represent a non-invasive method of measuring the 24-hr rhythms in both human and non-human primates.     

Environmental factors influencing the seasonality of estrus in chimpanzees

Although the energetics of the estrous cycle in primates is not well understood, evidence suggests that energy and nutrient acquisition influence ovulation and the timing of conception. Energy for estrus has to compete with energy allocated for cellular maintenance, thermoregulation, movement for food, and predation avoidance. While some chimpanzee (Pan troglodytes) populations do not have a seasonal birth period, evidence suggests that there is seasonality in the number of estrous females. Similarly, the onset of postpartum cycles has been reported to be seasonal. We used 33 months of data from the Taï National Park, Côte dIvoire, to examine how the number of estrous females in a given month was influenced by the abundance and distribution of food, diet, rainfall and temperature. In a second analysis, we examined if there was a seasonal effect on first estrous swellings in adolescent females and postpartum adult females. Results demonstrated that the number of females in estrous in a given month was positively related to food abundance and percent foraging time spent eating insects, and negatively related to mean rainfall in the two preceding months and the mean high temperature. The timing of first estrous swellings of postpartum females and prepartum young females was positively related to the food abundance, and negatively related to mean high temperature. These results showed that environmental conditions can seasonally limit the energetically demanding estrus cycle. The presence of estrous females increases gregariousness in chimpanzee communities, and this study identified environmental factors that affect estrus directly and hence social grouping indirectly.     

New data on the use of stone tools by chimpanzees in Guinea and Liberia

Two types of use of “hammers” for cracking nuts by wild-living chimpanzees have been distinguished: (1) Relatively small stones are used by the chimpanzee community at Bossou in Guinea to crack the nuts of oil palms growing on abandoned farmland, while no nuts of wild tree species are cracked. (2) Larger hammer stones (and, at some sites, wooden clubs) are used in a more sophisticated manner to crack the nuts of wild trees, but not of oil palms, in an area ranging from south-east Sierra Leone through Liberia to the south-west of the Ivory Coast. The first author (1986) has proposed that Type I has been copied by the chimpanzees, under pressure of food shortage, from the local human population. New data now indicate that, at Bossou, while habitat deterioration has continued, the number of hammer and anvil stones per utilized oil palm tree has approximately tripled in the last six years. The quantity of food obtained from oil palm nut kernels, however, amounts to only a few percent of the total diet. For the rest these apes depend to a large extent on many other agricultural products cultivated at Bossou which they are allowed freely to consume, including even cassava (manioc) roots and sweet potatoes dug by them from the ground. Some factors determining the chosen size of hammers were analyzed. Two abnormal hammers were found whose wear suggested a tentative, human-like manner of use. No evidence has been found to indicate the use of stone tools by chimpanzees in the adjoining chimpanzee-inhabited areas around the range of the Bossou community. Type II stone tool use was found, however, in a primary forest on a mountain≈13 km west of Bossou. This is especially intriguing because the site is separated by a wide belt of drier rain forest from the belt of very humid rain forest in the south where all the other known Type II sites are located. More research on the geographical distribution of the use of stone tools by chimpanzees and on the underlying ecological factors is recommended.     

Nut cracking by wild chimpanzees in Sierra Leone,West Africa

I report an incidence of chimpanzee (Pan troglodytes verus) cracking ofDetarium senegalense (Caesalpiniaceae) nuts on Tiwai Island, Sierra Leone. Similar behavior has been reported from other West African locations, but not from Central or East Africa.