Abstract: 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.

Abstract: Observations of play behavior were made on a troop of Japanese monkeys for five months. The troop consisted of 125 animals during the study period. Only 104 animals were observed playing with the troop members while the other 21 animals were never observed playing with other individuals. Two-member play was the most frequent. On the average, a monkey played with 20.7 individuals. A total of 6,068 play bouts were observed. The frequency of play appeared to be affected by age, sex, and degree of relatedness. One-year-old infant males played most with other members and the frequency of play decreased with age. Between monkeys whose disparity of age was less than two years, 5,763 bouts (95.0% of the total) were observed. Moreover, among sameaged monkeys who comprised 10.6% of the possible pair combinations, 2,739 play bouts (45.1%) were observed. Juvenile males played with same-sexed peers more than with opposite-sexed peers, whereas older juvenile females appeared to play with infants of both sexes. Individuals who were related and similarly-ranked tended to play together. There was no apparent preference for animals to play with the offspring of the highest-ranking female. Dominance rank of infnats and juveniles was primarily affected by rank of their mothers and to a lesser extent by play partners. Dominance rank of older juvenile males is more likely to be affected by play partners than females. It may be a critical time for males when they leave their natal troop and join a new troop. The timing of troop shifting by males seemed to be affected by the presence or absence of play-mates. For male Japanese monkeys, play is very important in developing social bonds. Play may act to perpetuate social bonds, enhance the chance of survival, and may contribute to their future reproductive success.

Abstract: Abstract  There have been no reports of infanticide in wild gelada baboons and it has been argued that infanticide is not necessary in geladas, since the birth interval of female gelada can be shortened after takeover of a unit by a new leader male without infanticide. However, we observed an instance of infanticide in a newly-found wild gelada population in the Arsi Region of Ethiopia. After a leader male of the unit was severely wounded by a leopard attack, he was quite weakened. The second male of the unit, a young adult male, became the leader of the unit three weeks later, but the former leader continued to stay in the unit as a second male. After a week, two other adult males joined the unit which, therefore, came to include four adult males. The infanticide took place nine days later. The perpetrator was one of the immigrant males and he showed great interest in the mother of the unweaned victim infant. Although the perpetrator copulated with her after the infanticide, the usurper was found to own all three adult females after two weeks following the infanticide; i.e. the perpetrator could not own any female. The wounded former leader showed conspicuous protective behavior towards the victim's mother and the dead infant. One possible explanation for the occurrence of infanticide in this population of geladas is as follows. Gelada males in this area may be able to join units more easily to form multi-male units but then have shorter tenure in the units. Facing the unstable condition of units, they may sometimes engage in infanticide to increase their breeding opportunities, even before becoming a leader.

Abstract: Abstract  The relationships between the relative size of the neocortex and differences in social structures were examined in prosimians and anthropoids. The relative size of the neocortex (RSN) of a given congeneric group in each superfamily of primates was measured based on the allometric relationships between neocortical volume and brain weight for each superfamily, to control phylogenetic affinity and the effects of brain size. In prosimians, “troop-making� congeneric groups (N=3) revealed a significantly larger RSN than solitary groups (N=6), and there was a significant, positive correlation between RSN and troop size. In the case of anthropoids, polygynous/frugivorous groups (N=5) revealed a significantly larger RSN than monogynous/frugivorous groups (N=8). Furthermore, a significant, positive correlation between RSN and troop size was found for frugivorous congeneric groups of the Ceboidea. These results suggest that neocortical development is associated with differences in social structure among primates.