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.
