Comparison of the communicative vocalizations and behaviors of group ranging in eastern gorillas,chimpanzees and pygmy chimpanzees

Field observations were carried out on chimpanzees, pygmy chimpanzees and eastern gorillas. Since the communicative behaviors which appear in group ranging are closely related to the grouping of the animals and to the social structure, the communicative behaviors of group ranging were compared in the above three species in order to elucidate the common and different characters of their three much diversified social structures. The following conclusions were drawn: (1) the common ancestor ofGorilla andPan had the territorial call “hoot” and behavior of display, and males were antagonistic between each other in the society; (2) the common ancestral species did not have any special long distance cohesive calling: the society was a small compact one, moving on the ground; and (3) the social structure of the pygmy chimpanzee is very different from the common ancestral social structure when compared with those of the gorilla and chimpanzee, in that the pygmy chimpanzee has lost the behaviors of strong antagonistic character between adult males.     

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.     

The social group of pygmy chimpanzees (Pan paniscus) of Wamba

This is a sociological study of the pygmy chimpanzees of Wamba which have been provisioned sufficiently since 1977. The society of the pygmy chimpanzee is basically similar to that of the common chimpanzee in the flexibility in forming parties and in the existence of a definite social unit, i.e.,unit group. It also resembles in that young females play an important role in exchanging members among unit groups, thus, as transmitters of genes from group to group, while males do in integrating groups, delimiting the social boundaries of unit groups through their inter-group antagonism. Pygmy chimpanzees differ from common chimpanzees in fission and fusion pattern of parties. The former generally forms parties larger in size and more homogeneous in composition than the latter. Any party of the former has both reproductive and nursing functions, while the parties of the latter are categorized into various types according to functions and compositions. This uniformity in pygmy chimpanzee parties seems to be well maintained by the females' high sexual ability to manifest semi-continual estrus.     

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.     

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.     

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.     

Sex differences in play among western lowland gorilla (Gorilla gorilla gorilla) infants: implications for adult behavior and social structure

Mammalian play is believed to improve motor skills as well as facilitate the development of social relationships. Given the marked sexual dimorphism in gorilla body size and the role assumed by the male in protecting the group from conspecifics and predators, the motor-training hypothesis of play predicts that male infants should exhibit higher frequencies of social play than female infants, and that males should prefer to play with other males. Given that adult female gorillas are strongly attracted to adult breeding males and form only weak social bonds with unrelated adult females, the social-relationship hypothesis of play predicts that female infants should prefer to play with males. These hypotheses were tested in a 22-month study of 12 gorilla infants, aged between 0-5 years, living in three zoological parks in Chicago and Atlanta. Consistent with the hypotheses, male infants played more than female infants did, and both male and female infants preferred to play with males rather than with females. These findings suggest that sex differences in play in the great apes and other primates can be predicted by the characteristics of adult behavior and social structure above and beyond the patterns of sex-biased dispersal or coalition formation with same-sex kin.     

Ranging and grouping patterns of a western lowland gorilla group at Bai Hokou,Central African Republic

The ranging and grouping patterns of a gorilla group were studied during 27 months from 1990–1992 at the Bai Hokou study site, Central African Republic. The study group ranged far daily (average = 2.3 km/day) and had a large home range (22.9 km²), relative to mountain gorillas, and ranging patterns differed between years. During 1990–1992, the bimale study group foraged less cohesively and had more flexible grouping patterns than mountain gorillas. The study group sometimes split into two distinct foraging subgroups, each led by a silverback, and these subgroups occasionally slept apart (mean = 950 m apart). Lowland gorillas rely on many of the same fruit resources as sympatric chimpanzees, and under certain demographic situations gorillas, like sympatric chimpanzees, may adapt their foraging group size to reduce intragroup feeding competition. However, the fiber content of the lowland gorilla diet likely relaxes constraints on foraging party size and facilitates group cohesion relative to chimpanzees. Am. J. Primatol. 43:111–133, 1997. © 1997 Wiley-Liss, Inc.     

Female contributions to the peaceful nature of bonobo society

Although chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) are closely related, females of the two species show surprisingly large differences in many behavioral aspects. While female chimpanzees tend to range alone or in small parties during non-estrous periods, female bonobos aggregate even more often than do males. Female chimpanzees do not have frequent social interactions with other females, whereas female bonobos maintain close social associations with one another. Although the ranging patterns of chimpanzee parties are generally led by males, female bonobos often take the initiative in ranging behavior. While female chimpanzees usually do not exhibit estrus during postpartum amenorrhea or pregnancy, female bonobos exhibit a prolonged pseudo-estrus during such non-conceptive periods. Studies of these two species have also shown great differences in agonistic behaviors performed by males. Male chimpanzees frequently fight with other males to compete for estrous females, but male bonobos seldom do so. While there are many records of infanticide by male chimpanzees, there is no confirmed record of such an event among bonobos. Several cases of within-group killing among adult male chimpanzees have been reported, but there is no such record for bonobos. While intergroup conflicts among chimpanzees sometimes involve killing members of the other group, intergroup conflicts among bonobos are considerably more moderate. In some cases, bonobos from two different groups may even range together for several days while engaging in various peaceful interactions. I will address two important questions that arise from these comparisons, exploring why females of such closely related species show such clear differences in behavior and whether or not the behavioral characteristics of female bonobos contribute to the peaceful nature of bonobo society.     

The social group of wild chimpanzees in the Mahali Mountains

There are more than six large groups of wild chimpanzees in the study area, which is in the north-eastern part of the Mahali Mountains of Western Tanzainia. One of these groups was provisionized, that is, customarily fed sugar cane and bananas. The characteristics of the social group of wild chimpanzees are clarified by long-term observation of the baited population. The chimpanzees live in a clear-cut social unit which consists of adult males, adult females, and immature animals. The permanency, stable membership, and integrative nature of the unit-group were confirmed during the course of this study. The size of unit-groups ranges from 30 to 80 head.The unit-group generally splits up into temporary subgroups that repeat joining and parting. The size of the subgroups of the baited population ranges from one to 28 head, the mean being 8.1 head. The centralization of a unitgroup is mainly sustained by the high sociability of adult males. The random nature of the membership of subgroups is emphasized in this paper, although subgroups are usually composed by social bonds on the basis of similar age, sex, blood relationship, and/or sexual attraction.The inter-unit-group interaction is peaceful; the subordinate unit-group avoids the dominant one. The home ranges of unit-groups overlap each other extensively, the overlapping areas being used flexibly by both unit-groups on the basis of dominance-subordination relationship. The member-exchange among unit-groups may sometimes occur, but the extent of openness or closedness of a unit-group has not been well elucidated.The research has been financed by the Scientific Research Fund of the Ministry of Education and in part by the Wenner-Gren Foundation.     

Factors influencing the formation and maintenance of all‐male gorilla groups in captivity

The social housing of males is of particular importance for captive managers of polygynous species. In this work, we highlight nine areas we believe are important to the successful formation and maintenance of all‐male gorilla groups. Although more data are still needed to complete our understanding of these groups, the general picture that emerges is that all‐male groups can be a functional social unit for male gorillas, the longevity of which might be improved by careful planning. It appears that the best time to form all‐male groups is when the animals are still immature, as males in this age class have been observed to form some of the most stable groups. Efforts should be made to diversify the groups in terms of rearing history, particularly with respect to hand‐reared males, and to limit the amount of hand rearing experienced by males. Groups probably should contain no more than three or four adult males. Exhibits should be designed to provide visual barriers, refuges for subordinate animals, the means to separate individuals, and possibly the means to isolate all‐male groups from mixed‐sex groups. Additionally, because it is likely that some males will have to be removed from all‐male groups, zoos need to design facilities that provide state‐of‐the‐art housing for solitary individuals. It is hoped that by identifying bachelor males early in life and establishing cooperative management plans across institutions, zoos will be able to provide beneficial, long‐term social situations for all male gorillas in captivity. Zoo Biol 23:189–203, 2004. © 2004 Wiley‐Liss, Inc.     

The socioecology of fission-fusion sociality in Orangutans

Fission-fusion systems can have the group or the individual as their primary unit. In group-based fission-fusion systems, predation risk reduction is the major benefit to grouping, in the individual-based ones the benefits are likely to be primarily social. Orangutans, like chimpanzees, are examples of an individual-based fission-fusion species. The orangutans inhabiting a Sumatran swamp forest (Suaq Balimbing) are more likely than elsewhere to form travel parties. As elsewhere, the main benefits of grouping are social: mating opportunities, protection from harassment and socialization of infants. Most animals also incur costs, but these are relatively low at Suaq Balimbing due to the high productivity of the swamp. Costs seem to be disproportionately high for females with mid-sized infants, who avoid parties.     

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

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

Phrase structure of wild chimpanzee pant hoots: Patterns of production and interpopulation variability

Recordings and behavioral observations of wild chimpanzees were made over a 2-year period in the Kibale National Park, Uganda (Kanyawara and Ngogo communities) to investigate patterns of acoustic variability in long-distance calls. The phrase structures of pant hoots, the species-typical loud call given predominantly by adult males, were analyzed. Analysis revealed that the build-up phrase was frequently absent from the pant hoots of Kibale chimpanzees. By contrast, analysis of published data on Gombe and Mahale chimpanzees (Tanzania) indicated that these animals typically included the build-up in their calls. These results were interpreted as evidence for phrase-level differences between populations in the acoustic morphology of this compound call. Data on age and sex differences in the context of production of pant hoots were analyzed, and their relevance to the possibility that aspects of pant hoot acoustic morphology are learned is discussed. Adult males initiated pant hoots more than subadult males, and adult males joined other pant hooting individuals with pant hoots more than subadult males did. It is suggested that younger males may pant hoot with specific adult males preferentially and that this may affect the development of their pant hoot acoustic morphologies. A peculiar pant hoot variant previously reported from Gombe, the whimper hoot, is described from Kibale. The production of this call by low-ranking individuals suggests that there are social constraints on pant hooting in the chimpanzee community. Ideas concerning the effect of social relationships on interpopulation variability in vocal signals are briefly discussed. © 1996 Wiley-Liss, Inc.     

Mahale chimpanzees: grouping patterns and cycling females

The social system of chimpanzees (Pan troglodytes schweinfurthii) is characterized by the fission-fusion of social groups. Several studies have reported that females are less gregarious than males. In the current study, adult female gregariousness depended on their reproductive state. Noncycling adult females (pregnant, lactating, or post reproductive) were observed in large bisexual parties less often than cycling adult females. On the other hand, cycling adult females were observed in large bisexual parties as often as males, regardless of their estrous state. More males were in parties that included cycling adult females with maximal genital swelling (estrous females) than in parties without them. Moreover, a bisexual party including more estrous females contained more males. These results suggest that large bisexual parties of chimpanzees are constructed by a dual mechanism. First, cycling adult females are attracted to parties that consist of the top ranking male and large numbers of adult and adolescent males. Second, adult and adolescent males that did not belong to parties originally are attracted by estrous females and join them. Thus, in Mahale Mountains National Park, Tanzania, bisexual parties of chimpanzees can be characterized as "parties for reproduction."     

An attempted within-group infanticide in wild chimpanzees

Two high-ranking adult male chimpanzees (Pan troglodytes) of M group in the Mahale Mountains National Park, Tanzania, tried to get a newborn female infant of an adult female who was born in M group. The mother was not seen to mate with these adult males during the probable conception cycle of the infant, but she disappeared from the group during the later cycle, when she may have been inseminated by a male of one of the neighboring unit groups. The adult males failed to get the infant because the grandmother of the newborn and her female friend cooperated to protect the mother and infant from attacks by the males. The sexual selection hypothesis for infanticide by adult male chimpanzees holds for this observed case. The sudden disappearance of another infant, a healthy female, strongly suggests the killing of female infants too. Therefore, the asserted male-biased infanticide in chimpanzees appears to be less tenable.     

A preliminary study of selective visual attention in female mountain gorillas (Gorilla gorilla beringei)

Visually attending to conspecifics can give group-living primates important ecological information, help them to anticipate the behavior of others and to regulate interactions with them, and provide other valuable social information. Variation in the importance and quality of social relationships should influence the way individuals selectively attend to fellow group members. Preliminary data on visual monitoring of conspecifics by wild female mountain gorillas (Gorilla gorilla beringei) show that selective attention mirrors variation in social relationships. Social bonds between males and females are central to gorilla society; correspondingly, females are more likely to stop feeding, and focus their attention on males who walk into view than on females, especially when males give dislays. Females are more likely to focus on other females with whom they have antagonistic relationships than those (mostly close relatives) with whom they have affiliative, cooperative ones. Further research on the context and consequences of visual monitoring could help to address questions about the regulation of social relationships and about social cognition in gorillas.     

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.     

Stability and Change of Social Relationship Quality in Captive Chimpanzees (Pan troglodytes)

In social animals an individual’s fitness depends partly on the quality of relationships with others. Qualitative variation in relationships has been conceptualized according to a three-dimensional structure, consisting of relationship value, compatibility, and security. However, the determinants of the components and their temporal stability are not well understood. We studied relationship quality in a newly formed group of 20 captive chimpanzees made up of several previously existing social groups. We assessed dyadic relationship quality 2?yr and again 7?yr after grouping. We confirmed the existence and stability of three relationship components and labeled them value, compatibility, and approach symmetry. Previously familiar dyads had a higher value than unfamiliar dyads, especially when they were maternally or paternally related. Compatibility was higher in dyads with only females than in dyads containing a male, but familiarity did not influence compatibility. Approach symmetry was initially higher, but later lower, in familiar than unfamiliar dyads, indicating that approach symmetry of familiar dyads decreased over time. Dyadic value and compatibility were highly stable over time, which is similar to the long relationship duration found in wild chimpanzees. In sum, relationships formed earlier in life became more valuable than those formed in later adulthood, whereas nonaggressive, compatible relationships could be formed throughout life. This suggests that for immigrating individuals, high-value relationships may be relatively difficult to establish, partly explaining why wild female chimpanzees have relatively few high-quality relationships with other females. Our study supports the multicomponent structure and durability of relationships in social species.