Numerous comparative studies have shown that Pan paniscus exhibits many socio-sexual characteristics which are distinct from those of Pan troglodytes. These characteristics can be considered as successful counter-strategies against potential infanticide by males.
1) Female P. paniscus show longer maximal tumescence period per cycle, larger number of cycles during pregnancy, earlier resumption of a cycle after giving birth, than those of P. troglodytes. All of these increase a female's sexual receptivity which help to mitigate male-male sexual competition, the principal motivation for infanticide.
2) Female P. paniscus appear to form a sort of alliance against males: they are more closely associate with each other than males do, both in the formation of parties and spatial distribution, and are cooperative in agonistic interactions with males. They prevent males from establishing a coalition by assisting young males to gain higher ranking positions, and also by maintaining a close association with their sons even after they mature. Thus, females, in spite of their physical inferiority to males, attain equal or even higher social positions than males. This makes it difficult for males to behave violently towards infants.
3) Female P. paniscus take the initiative in peaceful contact with different groups. They mate more frequently with males from different groups than with males of their own group during intergroup encounters. Intergroup copulations, together with obscured ovulation through prolonged estrus, make it difficult for males to determine paternity. This extreme confusion of paternity is considered to be a powerful deterrence to infant-killing by males of both the same group and between different groups.

View full abstract

Due to a prolonged period of cyclic estrus, female bonobos show a higher copulation rate over an interbirth interval than do female chimpanzees. However, if limited to an estrus period in which females show maximal swelling of sexual skin, copulation rate of female bonobos is lower than that of female chimpanzees. There are two possible reasons for this low frequency of copulation by female bonobos: 1) limited number of adult males per adult female in the maximal swelling phase, and 2) lower proceptivity of female bonobos in the maximal swelling phase. Female chimpanzees may show higher proceptivity during the maximal swelling phase because their optimal period for producing next offspring is shorter than that of female bonobos. The longer optimal period for producing next offspring in bonobos may be attributed to socio- ecological factors, such as higher social status of female bonobos, their priority to access food, and larger food patch size and feed-as-you-go foraging, which lowers the cost of staying in a mixed-sex party for copulation and the cost of nursing two offspring at the same time.

View full abstract

We examined the 20-years records of female transfer of wild bonobos at Wamba, D. R. Congo. Most females left their natal group between 6 and 9 years old, and immigrated into new groups between 10 and 12 years. Females seemed to travel several groups before settling in a new group. After settling down, they start reproduction and will not transfer to other groups for the rest of their life. We examined average of coefficient of consanguinity of males with whom females may copulate in new groups. The probability of inbreeding drastically decreases when a female transfers groups, and it decreases according to the amount of intergroup travel. Differences in female transfer pattern between bonobos and chimpanzees seem to be due to the differences in the risks of traveling alone, such as predators, social relationships between females, and high social status of females. The balance between cost and benefit of intergroup transfer may determine philopatric social structure in chimpanzees and bonobos.

View full abstract

Two cases of meat eating and one case of meat holding by bonobos were observed at Wamba Forest, DR Congo. The target species was a flying squirrel in all cases. Wamba bonobos seem to have a specific “prey image” for flying squirrels. Compared with chimpanzees, meat eating by bonobos seems to be infrequent at Wamba, as well as at other bonobo study sites. One female ate mature Palisota hirsuta leaves with the meat. In both cases of meat eating, the prey was caught by adult females. Meat sharing was observed between adult females and between mother and infant. This may reflect the close relationships between females within bonobo groups.

View full abstract