1994 年 10 巻 3 号 p. 261-267
Immature bonobos genital contact behavior with individuals of most age classes. Genital contact between immature individuals was most frequently performed in the ventro-ventral posture. It was usually observed during play sessions, and males participated more frequently than females. Both males and females perform genital contact with adult males though not so frequently. On the other hand, only males frequently perform genital contact with adult females and its frequency increased with age. As they grew older, genital contact behavior of immature males with adult females became more like copulation between adults. By contrast, immature females did not have any genital contact behavior that resembled copulation. Females became very active in copulation with adult males and genital contact behavior with adult females after they had moved to non-natal groups.