Hiroshi Ihobe

VARIATION IN MALE RELATIONSHIPS IN BONOBOS AND CHIMPANZEES

(With 7 Figures) (Acc. 20-VII-1994) Summary 1. Inter-male relationships were compared between two groups of bonobos at Wamba, Zaire, and a group of chimpanzees at the Mahale Mountains, Tanzania. 2. Although distribution of females is much different between the two species, frequencies of affinitive interactions between males (inter-individual proximity and grooming) were similar between the species. 3. By contrast, marked inter-species differences were found in agonistic aspects of male relationships. Such differences could be related to the different estrus patterns of females. 4. Male relationships of both species may have been reflecting the male-bonded strategy which was taken by a common ancestor. Within this framework, males of each species seem to have developed differentiated social tactics for the access to females showing different patterns of distribution and estrus.

Seasonal Changes in Fruit Production and Party Size of Bonobos at Wamba

Because chimpanzees (Pan troglodytes) have a unique fission-fusion social structure, many researchers have investigated the nature of foraging parties. They have reported that chimpanzees form foraging parties whose size and sex composition change flexibly, and that the sizes of parties may vary according to fluctuations in fruit abundance, the number of estrous females, or both (Wrangham 1977, Ghiglieri 1984, Isabyre-Basuta 1988, Stanford et al. 1994, Boesch 1996, Matsumoto-Oda et al. 1998, Newton-Fisher et al. 2000, Boesch and Boesch-Achermann 2000, Hashimoto et al. 2001). Researchers have also reported that females tend to join mixed-sex parties less frequently than males do, and that this likely occurs because ranging in large mixed-sex parties may not be beneficial to the feeding activities of females (Wrangham 1979, 2000, Janson and Goldsmith 1995, Williams et al. 2002, Reynolds 2005). The closest relative of chimpanzees, bonobos (P. paniscus), have a similar fissionfusion social structure. However, the size and composition of the parties are different from those of chimpanzees. Previous studies of bonobos at Wamba and Lomako in the Democratic Republic of the Congo showed that bonobos form larger and more stable parties (Kuroda 1979, Kano 1982, 1992, Furuichi 1987, White 1988), and that the size of parties may be influenced by fruit abundance to a lesser extent. This is probably because either the seasonal changes in fruit abundance are small or because some foods are available year-round (Kano 1982, Kano and Mulavwa 1984, White 1998, Malenky and Stiles 1991, Malenky and Wrangham 1994, Chapman et al. 1994). Furthermore, bonobo females tend to join mixed-sex parties

Long-Term Studies on Wild Bonobos at Wamba, Luo Scientific Reserve, D. R. Congo: Towards the Understanding of Female Life History in a Male-Philopatric Species

Long-term studies on wild bonobos began at Wamba, in the current Luo Scientific Reserve, in 1973. Except for several interruptions due to political instability and civil war, we have been conducting studies of identified individual bonobos over 35 years, providing valuable data on their population dynamics and life history. Although the number of groups and number of individuals in the northern section of the reserve decreased by half during the interruptions of the study, the number of members of the main study group has steadily increased since 2002 when we resumed the study. Our long-term data demonstrated the male-philopatric structure of the group. There is no confirmed case of emigration of males from the study group, and no case of immigration of males into the group. On the other hand, all females born into the study group disappeared by the age of 10 years, and females with estimated ages of 6–13 years immigrated into the study group. These ages of intergroup transfer are much earlier than those reported for chimpanzees. Exceptional cases of immigration of two adult males and two adult females with offspring occurred right after the war. It is likely that remnants of extinct groups joined the study group. Such integration of members of foreign groups highlights the peaceful nature of bonobo society. The study group is characterized by an extremely high tendency for female aggregation. Various factors, including high density of food patches, female initiative in ranging, prolonged estrus of females, and high social status of females, seem to be responsible for the high attendance ratio of females in mixed-sex parties. Our long-term observations therefore provided evidence for interesting behavioral contrasts with chimpanzees.

Do bonobos copulate more frequently and promiscuously than chimpanzees

The copulatory activities of bonobos (Pan paniscus) of Wamba, Zaire, were compared with those of chimpanzees (P. troglodytes schweinfurthii) of Mahale, Tanzania. The copulation rates of adult male bonobos were equal to or lower than those of adult male chimpanzees. The copulation rates of adult female bonobos were approximately equal to those of adult female chimpanzees who were in maximal genital swelling, but it should be much higher than those of the adult female chimpanzees throughout the birth interval. The copulation rates of adolescent male bonobos were lower than those of adolescent male chimpanzees, whereas the copulation rates of adolescent female bonobos were much higher than those of adolescent female chimpanzees. It was suggested that the bonobos of Wamba did not copulate more promiscuously than did the chimpanzees of Mahale. The female bonobos may show “receptivity”, whereas female chimpanzees may show rather “proceptivity”.

Non-antagonistic relations between wild bonobos and two species of guenons

Interspecific relations between wild bonobos (Pan paniscus) and two species of guenons (Cercopithecus wolfi andC. ascanius) were studied at Wamba in the Central Zaire Basin from September 1989 to January 1990. Data on the guenons were collected while following parties of bonobos or when searching for them. The guenons were observed directly 59 times during the study period. In about half of these observations, the guenons were found within 20 m from the bonobo parties. The encounters between the bonobos and the guenons sometimes lasted over an hour. The guenons mainly initiated the encounters by approaching the bonobos. During the encounters, no aggressive interactions were observed between the bonobos and the guenons. Evidence of hunting by wild bonobos has been restricted to small mammals, and there has been no evidence of hunting of primates by wild bonobos. These findings and the results of the present study strongly suggest that wild bonobos do not hunt sympatric primates.