Abstract
Great apes show a wide variety of social structure, including solitary life of orangutans, one/few-male group of gorillas, and multi-male-multi-female group of chimpanzees and bonobos. In recent studies for the understanding of such variations, much attention have been paid for the food distribution and foraging strategy of females. However, it seems difficult for the adaptation for dispersed distribution of fruit foods alone to explain the evolution of male philopatry and female transfer of African great apes. Adaptation to rain-forest environment of hominoid ancestors caused a very long interbirth interval, which resulted in an extremely high estrus sex ratio (number of adult males per adult female showing estrus). Together with the food distribution in the environment of each species, this estrus sex ratio explains various types of social structure of great apes, and provides an important insight on the evolution of social structure of early hominids.
