Takeshi Furuichi

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.

Sexual swelling, receptivity, and grouping of wild pygmy chimpanzee females at Wamba, Zaïre

The sexual swelling and copulatory behavior of ten pygmy chimpanzee (Pan paniscus) females in a wild group were studied at Wamba, Republic of Zaïre. Since the maximum size of the sexual skin revealed great individual variations depending on age and adolescent females showed little cyclic fluctuation in size, the sexual swelling was measured according to its firmness which periodically fluctuated in all age classes. The duration of maximum swelling and the cycle length were longer inP. paniscus than inP. troglodytes. Although pregnant females and those with newborn infants were sexually inactive, females with infants older than 3 years copulated as frequently as those without dependent infants. Contrary to previous reports on the sexuality ofP. paniscus, copulation was mostly restricted to the maximum swelling phase. All females were usually found in a large mixed party containing both sexes and offspring, regardless of their sexual receptivity.

Agonistic Interactions and Matrifocal Dominance Rank of Wild Bonobos (Pan paniscus) at Wamba

I studied dominance relations in a wild group of bonobos at Wamba, Democratic Republic of Congo. Although agonistic interactions between males occurred frequently, most of them consisted only of display, and physical attacks were infrequent. Dominance rank order seemed to exist among males, but its linearity is unclear. Dominant males rarely disturbed copulatory behavior by subordinate males. However, high-ranking males usually stayed in the central position of the mixed party and, so, would have more chance of access to estrous females. Among females, older individuals tended to be dominant over younger individuals. However, agonistic interactions between females occurred rather infrequently, and most consisted of displacement without any overt aggressive behavior. Dominance between males and females is unclear, but females tended to have priority of access to food. The close social status between males and females may be related to the prolonged estrus of females and their close aggregation during ranging. Existence of a males mother in the group and her dominance status among females seemed to influence his dominance rank among males. Young adult males whose mothers were alive in the group tended to have high status. In some cases, change in dominance between high-ranking males was preceded by a corresponding change in dominance between their mothers. As the dominance status of females is similar to that of males, mothers may be able to support their sons to achieve high status, stay in the center of the mixed party, and so have greater access to females, which may maximize the number of descendants of the mothers.

Lethal aggression in Pan is better explained by adaptive strategies than human impacts

Observations of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) provide valuable comparative data for understanding the significance of conspecific killing. Two kinds of hypothesis have been proposed. Lethal violence is sometimes concluded to be the result of adaptive strategies, such that killers ultimately gain fitness benefits by increasing their access to resources such as food or mates. Alternatively, it could be a non-adaptive result of human impacts, such as habitat change or food provisioning. To discriminate between these hypotheses we compiled information from 18 chimpanzee communities and 4 bonobo communities studied over five decades. Our data include 152 killings (n = 58 observed, 41 inferred, and 53 suspected killings) by chimpanzees in 15 communities and one suspected killing by bonobos. We found that males were the most frequent attackers (92% of participants) and victims (73%); most killings (66%) involved intercommunity attacks; and attackers greatly outnumbered their victims (median 8:1 ratio). Variation in killing rates was unrelated to measures of human impacts. Our results are compatible with previously proposed adaptive explanations for killing by chimpanzees, whereas the human impact hypothesis is not supported.

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.

Fruit availability and habitat use by chimpanzees in the Kalinzu Forest, Uganda : Examination of fallback foods

We studied seasonal change in habitat use by chimpanzees in the Kalinzu Forest, Uganda. The forest comprises various types of vegetation. For each vegetation type, we compared number of chimpanzees (per km2) that used the vegetation with fruit availability in different census periods. We estimated the number of chimpanzees by nest count and fruit availability via density of fallen fruit. The mixed mature forest contained a large amount of fruit during the high-fruiting season, but it decreased rapidly in the low-fruiting season. The number of chimpanzees also decreased in mixed mature forest in approximate proportion with fruit availability. In the Parinari-dominated mature and secondary forests, both fruit availability and number of chimpanzees were very low throughout the study. In the Musanga-dominated secondary forest, the number of chimpanzees increased toward the low-fruiting season, though the fruit availability decreased slightly. A multiple regression analysis showed that various fruits had significant effects on the number of chimpanzees during the high-fruiting season, while only Musangaleo-errerae had a significant effect during the low-fruiting season. The results suggest that the fruit of Musanga leo-errerae functions as a fallback food, and a combination of different vegetation types supports the chimpanzees in the Kalinzu Forest.

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.

What Factors Affect the Size of Chimpanzee Parties in the Kalinzu Forest, Uganda? Examination of Fruit Abundance and Number of Estrous Females

We examined factors affecting the size of chimpanzee parties in the Kalinzu Forest, Uganda. We found that the number of individuals in a party increased with observation time. Therefore, we employed two methods to reduce this bias: (1) we evaluated party size via the mean number of individuals observed in each 1-h period during the observation of a party (1-h party size), and (2) we used the number of all individuals observed in a party (1-day party size) and performed an analysis of covariance, with observation time of the party as the covariant. We examined factors that might affect party size: fruit abundance, fruit distribution, and number of estrous females. There was no relationship between party size and fruit abundance or distribution. Conversely, the number of males has a significant positive correlation with the number of estrous females, though there is no correlation with the number of anestrous females. These results suggest that males tended to join parties with more females in estrous, irrespective of differences in fruit availability.

Reproduction of Wild Japanese Macaque Females of Yakushima and Kinkazan Islands: A Preliminary Report

Wild Japanese macaque females of the Yakushima and Kinkazan populations exhibited similar reproductive features. (1) Births/female/year (BR: 0.27–0.35) was lower than those of provisioned troops, but (2) infant mortality (IM: 0.23–0.25) was higher than those of provisioned troops. (3) The interbirth interval (IBI) following the death of infants was 1.5–1.6 years, shorter than that following surviving infants (2.2–2.4 yrs). (4) Birth sex ratio (BSR) did not differ from 1∶1. There was no consistent correlation between (5) female age and IM, (6) maternal rank and offspring BSR, or (7) maternal rank and reproductive success. On the other hand, (8) BR of Yakushima females was significantly lower than that of Kinkazan females. In particular, (9) Yakushima females stopped reproduction earlier than Kinkazan females, although (10) the first birth of Yakushima females was about one year earlier than Kinkazan females. (11) BR exhibited a humped curve against female age in Yakushima, but it was uncertain whether old-aged females of Kinkazan exhibited a post-reproductive life span (PRLS). (12) The survivorship for female juveniles was lower than that for male juveniles in Yakushima, whereas the survivorship for male juveniles was lower than that for female juveniles in Kinkazan. These data may indicate that Yakushima females more severely compete for resources than Kinkazan females, because of high population density, whereas the population density of Kinkazan might be limited by climate (e.g. heavy snow) rather than density dependent ecological effects.

Matrilineal kin relationship and social behavior of wild bonobos (Pan paniscus): Sequencing the D-loop region of mitochondrial DNA

Matrilineal kin-relations among wild bonobos (Pan paniscus) were studied by DNA analysis. Subject individuals were the members of E1 group, living at Wamba, Zaire, which has been studied since 1974. DNA samples were extracted from wadges that bonobos spat out when feeding on sugar cane. The D-loop region of mitochondrial DNA was amplified by the PCR method, and a nucleotide sequence of 350 base pairs was determined for 17 individuals. Nucleotide variations were found at 44 positions of the sequence. Based on these variations, 13 matrilineal units were divided into seven groups, and the mother of an orphan male was determined among several females. These genetic analyses, together with behavioral observation to date, revealed the following facts. High sequence variation in the target region indicated that females transfer between groups of bonobos, which is in agreement with supposition from long-term field studies. For females, there was no relationship between genetic closeness and social closeness that is represented by frequencies of proximity or grooming. After immigration into a new group, females form social associations with senior females without regard to kin relationship.