Socio-sexual factors of troop fission in wild Japanese monkeys (Macaca fuscata yakui) on Yakushima Island,Japan |
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Authors: | Juichi Yamagiwa |
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Institution: | (1) Japan Monkey Centre, 484 Inuyama, Aichi, Japan |
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Abstract: | 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.
This study was financed in part by a Grant-in-Aid for Special Project Research on Biological Aspects of Optimal Strategy and
Social Structure from the Japan Ministry of Education, Science and Culture, and by the Cooperative Research Fund of the Primate
Research Institute, Kyoto University. |
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Keywords: | Macaca fuscata yakui Troop fission Troop size Female choice Consort relationship |
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