Sexual behavior of captive orangutans.

Opposite-sex pairs of orangutans were tested for sexual behavior during the intermenstrual period of the female. The male orangutan was the primary initiator of sexual interactions and initiated copulation forcefully on a daily basis, irrespective of female resistance. However, although single copulations occurred daily, copulations beyond the first occurred most frequently during midcycle. Other evidence of cyclicity in behavior was the midcycle decrease in female avoidance of the male and increased grooming, proceptivity, and masturbation by the female. Comparative analysis suggests that differences in sexual cyclicity among the great ape species are related to interspecies differences in sexual assertiveness of males and females. Sexual activity in the cycle is relatively brief when the female controls mating, more prolonged when the male is in control. This finding in man's closest taxonomic affiliates suggests that similar social factors may also influence the distribution of sexual interactions in the human cycle.