Association,ranging, and the social unit in chimpanzees of the Mahale Mountains,Tanzania

In order to characterize the social unit in chimpanzees, about which several conflicting views have been proposed, the proximity matrix among 55 recognized chimpanzees and the range covered by each of them are examined, on the basis of data obtained at the Mahale Mountains during 12 months in 1978–1979. It is shown once again that chimpanzees have a bisexual social unit (unit-group). Two such unit-groups were detected in the study area. All animals belonged to one of the two unit-groups except a few cycling females (and a juvenile male accompanied by his cycling mother) which were seen to associate alternately with members of two neighboring unit-groups, covering a whole range of one or even two unit-groups. The problem of such females is discussed in relation to the spatial relationships between the two unit-groups. Reexamining the membership of a unit-group, it is demonstrated that a unit-group was most likely patrilineal. While nulliparous females transferred between unit-groups, parous females tended to remain in a unit-group where they first gave birth to infants and to have several offspring therein. This appeared significant for ensuring recruitment of members of the next generation to a patrilineal unit-group. Although some adult males left their natal unit-group, they never joined the other. Male departure from a unit-group seemed to be forced by the other males and to be the sociological equivalent of going into exile, which is unique in nonhuman primates.     

Sex and group differences in feeding on animals by wild chimpanzees in the Mahale Mountains National Park,Tanzania

Sex differences in animal prey intake were revealed by fecal analysis among wild chimpanzees of the large-sized M-group (ca. 100 members) in the Mahale Mountains National Park, Tanzania: prime adult or old males feed more on vertebrates, while adult females more onCamponotus ants. By contrast, such differences were not obvious in the neighboring, small-sized K-group (ca. 20–30 members), despite the similar environment in which the two unit-groups lived. Such sex and group differences may be explained in terms of various factors, either ecological or social, or both, but social factors seem most responsible in particular for the group differences. It seems likely that increased capture rate of vertebrates per unit-group in the larger-sized M-group results in increased per capita intake of meat among prime adult or old males. Also, the more frequent interactions among prime adult or old males of M-group appear to reduce the frequency of theirCamponotus ant-fishing behavior.     

Demographic study of a large-sized unit-group of chimpanzees in the mahale mountains,Tanzania: A preliminary report

Long-term demographic observations on a large-sized unit-group of chimpanzees in the Mahale Mountains, Tanzania, are summarized. The unit-group, the M group, contains over 100 individuals, which makes it the largest unit-group ever reported. The age-sex composition, natality, mortality and transfers of the M group are analyzed. An attempt is made to illustrate an age-sex pyramid of the group by estimating the ages of all the individuals in the group. The results reveal that: (1) the mortality rate of the male infants within 1 year almost doubled that of female infant; (2) adult male to adult female ratio of the M group is considerably higher than any other unit-groups elsewhere; and (3) the M group contains a relatively large number of old animals over 40 years of age, suggesting that the longevity of wild chimpanzees might be greater than estimated so far.     

A case of inter-unit-group encounter in chimpanzees of the mahale mountains

In the Mahale Mountains, Tanzania, a case of encounter between chimpanzees of different unit-groups was observed in June 1979, where two young adult males and two adult females of the M-group came across four adult females and a juvenile male of the K-group. While the participants did not explicitly show any agonistic behavior but greeted and groomed each other, they repeatedly defecated during the interactions. This clearly indicated that they were put under unusual strain, which could be considered to be brought on because they were cognizant of that the opponents were not the members of their own unit-group. It can be said, therefore, that in chimpanzees, even females identify themselves with a particular unit-group, and that a chimpanzee unit-group should be regarded as a bisexual unit.     

Grouping patterns of wild pygmy chimpanzees (Pan paniscus) observed at a marsh grassland amidst the tropical rain forest of Yalosidi,Republic of Zaïre

Fixed point observation to ascertain the grouping patterns of wild pygmy chimpanzees was carried out at a marsh grassland known locally as Iyoko amidst the tropical rain forest of Yalosidi, Republic of Zaïre. The chimpanzees were seen alone or in parties consisting of up to 32 individuals. The mean size of parties which arrived at Iyoko was 7.9 (N=67), although the modal party size was 2–5. The majority (76%) of all observed parties including those that reformed after joining/parting while staying at Iyoko (N=96) was of the mixed type, i.e., consisting of adult male(s), adult female(s), and dependent individual(s). There was a tendency for parties to be composed of approximately equal numbers of adult males and females. All “social” activities such as sexual behavior and branch-dragging displays were recorded only in mixed parties consisting of more than ten individuals. The joining of parties of pygmy chimpanzees after arriving separately at Iyoko was seen 13 times and the parting of those before departing from Iyoko occurred seven times in total. In contrast, antagonistic encounters between two parties were recorded twice. These observations suggest that the joining/parting between parties is an intra-unit-group phenomenon while antagonistic encounters between parties are inter-unit-group interactions. It was assumed that at least two such unit-groups of pygmy chimpanzees consisting of 80–90 individuals in total utilized Iyoko without intermingling with each other. A comparison on grouping patterns among three pygmy chimpanzee populations (Yalosidi, Wamba, and Lomako) indicates that in terms of basic social organization they show many similarities except for the mean unit-group size, the mean party size, and the modal party size. Perhaps differences in the unit-group size were simply reflected as a whole in the differences of the mean party size as well as the modal party size observed across the three populations.     

Social interactions and the life history of femalePan paniscus in Wamba,Zaire

The unit-group of Pan paniscustends to form one large mixed party consisting of most of its members. Females usually stay in the party irrespective of their estrous state. They aggregate in the center of the party; and, older females stay in the most central part. Adult and adolescent sons of the old adult females stay in the central part more than males without mothers in the unit-group do. Females leave their natal unit-groups as older juveniles or in early adolescence and. settle in another unit-group after visiting several. Newly immigrated young females are eager to have social interactions with senior females to improve their social positions. Females become less eager to interact socially with other females when they have their own offspring. The strong bond between mother and son continues into his adulthood; and, females in old age become important members of the unit-group, both as the targets of association for younger females and as the mothers of highranking males. High social status of females seems related to their cohesive grouping tendency. The consistency of the multimale/multifemale party and the existence of prominent mother-offspring subunits are unique characteristics of P. paniscusamong the Pongidae. This social structure may provide a feasible model of the basic society from which human society evolved.     

Socioecological factors influencing population structure of gorillas and chimpanzees

Differences in distribution and density between gorillas and chimpanzees are reconsidered with special reference to population structure. Both ecological and social factors influencing population structure are compared between species and between habitats within species. Gorillas and chimpanzees respond differently to a decline in food quality, such as fruit scarcity: gorillas change diet and decrease range, while chimpanzees do not change diet but may expand range. These responses result in different effects on their grouping patterns. For gorillas the dispersed distribution and reduction of range size decreases the rate of inter-unit encounters and female transfer. The concentration of social units increases the rate of aggressive contact between units and stimulates female transfer. Social units of gorillas may crowd or disperse in order to attain the optimal density. This tendency may result in similar densities of gorillas across habitats. By contrast, the distribution patterns or range size may not affect inter-unit relationships of chimpanzees. Within a single unit-group, various reproductive strategies are adopted by both sexes. Independent travel of females and flexible grouping patterns enable them to survive at very low density in extraordinary large ranges. Density and inter-unit relationships are good criteria for a healthy population of gorillas, while the size of unit-group and inter-individual relationships are good criteria for chimpanzees. Conservation planners should consider these differences for sympatric and allopatric survival in these species.     

Group unity of chimpanzees elucidated by comparison of sex differences in short-range interactions in Mahale Mountains National Park,Tanzania

Chimpanzees (Pan troglodytes) form multi-male and multi-female unit groups with fission–fusion grouping patterns. Short-range interaction (SRI) plays an important role in the unity of these groups and in maintaining social bonds among members. This study evaluated three models of chimpanzee social structure that differed according to the emphasis each placed on social bonds between the sexes, i.e., the male-only, the bisexual, and the male-bonded unit-group model. I investigated differences in SRI between the sexes among group members in well-habituated wild chimpanzees in Mahale Mountains National Park, Tanzania. I followed six focal adult males and six females, and quantified their respective SRI with other chimpanzees. Except between subordinate males and adult females, adults in general engaged in SRI with about 60–90% of the individuals with whom they made visual contact each day, whether in large or small parties. Although the number of social grooming (SGR) partners was limited, male–male SGR networks were wider than were either male–female or female–female SGR networks among adults. The number of contact-seeking behavior (CSB) partners was also limited, but dominant males had more CSB partners. Adult females mainly interacted by pant-grunt greeting (PGG) with adult males, but tended to do so mainly with the highest-ranking male(s) within visual contact. These results indicated that the social bonds among adult males were essential to group unity. Because of clear male dominance, adult females established peaceful coexistence with all group members despite less frequent SRI with subordinate males by maintaining affiliative social bonds with dominant males, thereby supporting the male-bonded unit-group model. Adult females had many female SRI partners, but these interactions did not involve performing conspicuous behaviors, suggesting that females maintain social bonds with other females in ways that differ from how such bonds are maintained with and between adult males.     

Association partners of young chimpanzees in the Mahale Mountains National Park,Tanzania

Association partners of young chimpanzees at the Mahale Mountains National Park were analyzed. Juvenile and adolescent chimpanzees associated frequently with their mothers, although mother-offspring association decreased as the offspring grew up. Males tended to leave their mothers and associate with adult males, while females remained frequently associating with their mothers in early adolescence. In late adolescence and young adulthood, males usually associated with adult males and cycling adult females. Females may transfer into neighboring unit-groups in this stage. Although an immigrant female tended to be alone when her estrous cycle stopped, she associated with many individuals, in particular with adult males, when she resumed cycling. Some orphans were observed to associate frequently with particular adults. The findings were discussed in relation to the unique characteristics of chimpanzee social system.     

Age differences in social interactions of young males in a chimpanzee unit-group at the Mahale Mountains National Park,Tanzania

The behavior and social interactions of young male chimpanzees were studied in relation to their age change. The data were obtained at the Mahale Mountains National Park, during a four-month period in 1986. Early adolescent males, becoming independent of their mothers, spend a long time near adults of both sexes. Late adolescent males are not tolerated by the senior males. Although such animals do not stop traveling together with their seniors, they are separated from the other members including the males of their own age class, and each of them lives a relatively lonely life. Where seniors are not nearby, they perform charging displays in front of estrous females. Young adult males tend to remain in the proximity of the alpha male, and can associate with their seniors without pant-grunting. Although some young adult males dominate over some senior males, increasingly performing charging displays, they do not appear to be permitted to associate intimately with their seniors; they are not yet considered to have attained social maturity. Prime and senior males are strongly bonded with one another, being able to associate intimately with those of their own or senior age classes including the alpha male. A young adult male's rise in rank is not connected with joining the “adult male-cluster,” nor does a senior male's decline necessarily means his dropping out from the cluster: the social position of male chimpanzees cannot be understood solely from their agonistic dominance rank. The alpha male plays a leading part in integrating the males of the unit-group. Young adult males and their seniors tend to associate most frequently with him, and all the males of the early adolescent or senior age classes pay attention to his movements.     

Male-male relationships among wild bonobos (Pan paniscus) at Wamba,Republic of Zaire

Male-male relationships among wild bonobos (Pan paniscus) in two adjacent unitgroups (E1 and E2 groups), which were formed by division of the E group, were studied at Wamba, in the Central Zaire Basin, by analyzing the proximity and social interactions among males. Dominant-subordinate relationships between a male-male dyad were easily recognized from the directions of individual agonistic interactions. Male bonobos rarely joined forces in aggression. Clear differences in social status existed between adult and adolescent male bonobos in both groups, as reported in the case of chimpanzees (Pan troglodytes). The presence of mothers in the unit-group greatly influenced the dominant-subordinate relationships among males through strong mother-son bonds in both groups. However, the extent of the mother-son bonds differed between the groups. Males in the E2 group participated more frequently in agonistic or affinitive interactions than did males in the E1 group. Males in the E1 group were divided spatially into several clusters, while there were cohesive relationships among the adult males in the E2 group. The difference in intensities of mother-son bonds between the groups may be explained by the distribution of males at the time of the division of the E group. Differences in male-male relationships between bonobos and chimpanzees seem to be related to differences in intra- and inter-unit-group competition among males between the two species. Male chimpanzees may achieve coexistence by manipulating ambivalent relationships that are caused by intra- and inter-unit-group competition among them, while male bonobos may achieve coexistence by decreasing intra- and inter-unit-group competition among them.     

Dominance among male chimpanzees in the Mahale Mountains National Park,Tanzania: A preliminary study

Dominance relationships among male chimpanzees in the Mahale Mountains National Park, Tanzania, were analyzed. Although all adolescent males were unequivocally subordinate to all adult males, dominance relationships within the age classes were much less clear. Especially among adolescent males, few pant-grunts or agonistic interactions occurred. While adolescent males frequently pant-grunted at adult males, these latter males, except the alpha and the youngest, rarely pant-grunted to one another. This suggests that a difference of social status exists between adolescent and adult males. Adult males rarely display overt dominance to one another probably because the presence of other males affects their interactions. Moreover, they seem to try to keep their dominance relationship ambiguous when making it overt is not advantageous to them. This may be a political way for males to coexist with one another in a unit-group.     

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.     

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.     

Predation on mammals by chimpanzees in the montane forest of Kahuzi, Zaire

The rate of predation on mammals by chimpanzees was determined from carcasses and from fecal specimens found on fresh trails during a 16-month period in the montane forest of Kahuzi-Biega National Park, Zaire. A unit-group of semi-habituated chimpanzees, composed of 22 – 23 individuals including 8 adult or adolescent males, appeared to kill about 18 – 30 mammalian prey (16 – 28Cercopithecus monkeys) per year, if the multiple kills by chimpanzees were not considered. A juvenile l'Hoest's monkey was recorded for the first time as the prey of chimpanzees in this study. Predation occurred in the late dry and the early rainy seasons, when the diversity of ripe fruits was the highest during the year. The Kahuzi chimpanzees tended to kill mammals less frequently but to killCercopithecus monkeys more frequently than chimpanzees in other habitats. The absence of red colobus monkeys, which are the most frequent prey in Gombe, Mahale, and Tai, might be responsible for the low predation rate. However, the estimated rate of predation onCercopithecus monkeys is the highest record among various chimpanzee habitats. At least 11 – 18% of theCercopithecus population seemed to be lost annually as a result of being killed by chimpanzees. Chimpanzees may be the most important predators on these monkeys in the absence of leopards at Kahuzi. The examination of fecal samples and carcasses suggested that adult (probably male) or adolescent chimpanzees tended to eat juvenile or subadult monkeys most frequently, as is also seen for chimpanzees in Gombe, Mahale, and Tai.     

Opportunistic and restrictive matings among wild chimpanzees in the Mahale Mountains,Tanzania

The mating patterns of free-ranging chimpanzees (Pan troglodytes) of the Mahale Mountains, Tanzania, were studied. Opportunistic mating (non-competitive and temporary mating) was frequently observed in a large-sized unit-group, among young, low-ranking males, and among young, newcomer, non-ovulating females. Restrictive mating (a continuous sexual relationship between a particular pair which includes possessiveness and consortship) was frequently observed in a small-sized unit-group, among middle- and old-aged, high-ranking males, and among old, resident, ovulating females. Relations between those characteristics, such as group size and composition, ages of the individuals of both sexes, female estrous stages, and life history, and the 2 mating patterns are discussed.     

Social influences on ranging patterns among chimpanzees (Pan troglodytes verus) in the Tai National Park, Cote d'Ivoire

Group size is expected to be an important factor to predicthome-range (HR) size in social animals. In chimpanzees adultmales play an important role in defending the HR against neighbors,and therefore it has been suggested that HR size depends onthe number of adult males. In this long-term study on wild WestAfrican chimpanzees, we analyzed the relative importance ofcommunity size and composition on ranging patterns over a 10-yearperiod, using multivariate statistics. Because community sizedecreased from 51 individuals with 6 adult males in 1992 to22 individuals with only 1 adult male in 2001, we expected adecrease in HR size, which should be better predicted by thenumber of males than by community size. We further investigatedthe effect of fruit availability on monthly HRs over a 4-yearperiod. As predicted, HR size decreased during the first 7 yearsof our study but increased during the last 3 years. Overall,the number of adult males was the best predictor of HR size,whereas fruit availability did not correlate with HR size. HRuse remained stable over the entire study period, with a constantproportion of about 35% of the HR used as core area. High HRand core-area overlap values between different years indicatedstrong site-fidelity. Although the number of males within thecommunity explained the decrease in HR size, the recent increasein size remains unexplained. This finding suggests that otherfactors such as relative fighting power of males affect HR size.     

Adolescent male chimpanzees (Pan troglodytes) form social bonds with their brothers and others during the transition to adulthood

Social relationships play an important role in animal behavior. Bonds with kin provide indirect fitness benefits, and those with nonkin may furnish direct benefits. Adult male chimpanzees (Pan troglodytes) exhibit social bonds with maternal brothers as well as unrelated adult males, facilitating cooperative behavior, but it is unclear when these bonds develop. Prior studies suggest that social bonds emerge during adolescence. Alternatively, bonds may develop during adulthood when male chimpanzees can gain fitness benefits through alliances used to compete for dominance status. To investigate these possibilities and to determine who formed bonds, we studied the social relationships of adolescent and young adult male chimpanzees (N = 18) at Ngogo in Kibale National Park, Uganda. Adolescent male chimpanzees displayed social bonds with other males, and they did so as often as did young adult males. Adolescent and young adult males frequently joined subgroups with old males. They spent time in proximity to and grooming with old males, although they also did so with their age peers. Controlling for age and age difference, males formed strong association and proximity relationships with their maternal brothers and grooming relationships with their fathers. Grooming bonds between chimpanzee fathers and their adolescent and young adult sons have not been documented before and are unexpected because female chimpanzees mate with multiple males. How fathers recognize their sons and vice versa remains unclear but may be due to familiarity created by relationships earlier in development.     

Chimpanzees share forbidden fruit

The sharing of wild plant foods is infrequent in chimpanzees, but in chimpanzee communities that engage in hunting, meat is frequently used as a 'social tool' for nurturing alliances and social bonds. Here we report the only recorded example of regular sharing of plant foods by unrelated, non-provisioned wild chimpanzees, and the contexts in which these sharing behaviours occur. From direct observations, adult chimpanzees at Bossou (Republic of Guinea, West Africa) very rarely transferred wild plant foods. In contrast, they shared cultivated plant foods much more frequently (58 out of 59 food sharing events). Sharing primarily consists of adult males allowing reproductively cycling females to take food that they possess. We propose that hypotheses focussing on 'food-for-sex and -grooming' and 'showing-off' strategies plausibly account for observed sharing behaviours. A changing human-dominated landscape presents chimpanzees with fresh challenges, and our observations suggest that crop-raiding provides adult male chimpanzees at Bossou with highly desirable food commodities that may be traded for other currencies.     

Observations on the population dynamics and behavior of wild chimpanzees at Bossou,Guinea, in 1979–1980

A small population of wild chimpanzees was studied at Bossou, Guinea, for 80 days from December 1979 to March 1980 in its natural habitat. Since the beginning of the previous study in November 1976, the population decreased from 21 to 19 animals due to the disappearance of five members and three births. Three out of four adult and adolescent males disappeared, but no adult and adolescent female changed membership during the 39 months. The high social cohesion of this group and related behavior patterns changed little between the studies in spite of the disappearance of dominant and other males. In the moving range of the group, 127 mature oil-palm trees were examined; the base of 39 were used for palm-nut cracking by chimpanzees. The selection of cracking sites is discussed. A possible sign of leaf-sponging for drinking water from the hollow of the tree was found. The leaf-clipping display in which frustration or mild aggression is manifested by breaking stiff leaves with the mouth was observed throughout both study periods. A juvenile male chimpanzee was seen to catch a tree-pangolin which was later eaten by an adult female but other group members took little interest in the meat. It is clear that hunting and meateating are not habitual but accidental among the chimpanzees of Bossou.