, Volume 26, Issue 2, pp 105–120

Socio-sexual factors of troop fission in wild Japanese monkeys (Macaca fuscata yakui) on Yakushima Island, Japan

  • Juichi Yamagiwa

DOI: 10.1007/BF02382011

Cite this article as:
Yamagiwa, J. Primates (1985) 26: 105. doi:10.1007/BF02382011


The troop fissions which occurred in a wild population of Japanese monkeys (Macaca fuscata yakui) were observed from 1977 to 1979 on Yakushima Island. The fissions were initiated in the breeding season by non-troop males who established a consort relation with estrous females. In order to analyze the socio-sexual factors which accelerated the fissions, the male emigrations and immigrations before and after two successive fissions, and the copulation frequencies, competition among males and preferences of mating partners in both sexes in the 1977–78 breeding season after the first fission were examined. The results indicated that three factors (a large number of non-troop males, a shortage of troop males and the females' choice of mating partners) effectively influenced on the establishment of consort relationships between non-troop males and estrous females. It is suggested that these factors may exert different effects on the troop disorganization in relation to troop size. In small-sized troops, a large number of non-troop males and a shortage of troop males may lead to stronger competition between them, and the females' choice affected by prolonged intimate relations with the dominant TMs may reduce their priority of access to estrous females. This situation possibly stimulates fission or male emigration in small-sized troops under the natural conditions on Yakushima Island. In contrast, in large-sized troops under isolated conditions, a surplus rather than a shortage of troop males may contribute to troop disorganization, as most former studies have suggested. A higher socionomic sex ratio may decrease the mating activities of subordinate troop males and increase the competition among them. This situation possibly accelerates the fission of large-sized troops through prolonged interactions between females and subordinate or peripheral troop males. A lower ratio and the females' choice, however, raise the mating chances of subordinate troop males and may not promote the fission of large-sized troops under isolated conditions.

Key Words

Macaca fuscata yakuiTroop fissionTroop sizeFemale choiceConsort relationship

Copyright information

© Japan Monkey Centre 1985

Authors and Affiliations

  • Juichi Yamagiwa
    • 1
  1. 1.Japan Monkey CentreInuyama, AichiJapan