抄録
Male competition for mates has a great impact on their behavior, physiology and social system. Bonobos (Pan paniscus) are well known for their peaceful and mild characteristics compared with those of chimpanzees. One hypothesis explaining their peaceful nature is their high operational sex ratio. In other words, female bonobos are highly receptive lending to milder mating competition than seen in chimpanzees. Female bonobos have the longest period of maximal sexual swelling among species in which sexual swelling is found. Many researchers agree that prolonged periods of maximal swelling might help females conceal their peri-ovulatory period. This infers that male bonobos cannot detect a female ovulatory period, or that they do not seriously care about producing offspring. However, considering the fact that ovulation is usually situated in the later half of swelling periods, it is possible that male mating activity has been selected to occur more frequently around peri-ovulatory periods. Moreover, many previous studies have not fully considered the reproductive history of females as an important factor in determining the resumption and continuation of her menstrual cycle. In this study, we tested hypothesis that males can determine peri-ovulatory periods of females. We predicted that males may exclusively follow females with maximal swellings in the party during daily ranging. In particular, the alpha male, who can displace all other males in the party, can follow a female with maximal swelling all day, thereby monopolizing copulation opportunities. When there are several females in maximal swelling, group males may choose to follow the female with older dependent offspring, and whose maximal swelling phase started earliest and has continued for more days. This tendency will be most clear for the alpha male. Hormonal profiles of these females are currently being analyzed, and are expected to provide needed evidence to test our hypothesis in a more quantitative way.
