Horses are phylogenetically distant from primates, but considerable behavioral links exist between the two. The sociality of horses, characterized by group stability, is similar to that of primates, but different from that of many other ungulates. Although horses and primates are good models for exploring the evolution of societies in human and non-human animals, fewer studies have been conducted on the social system of horses than primates. Here, we investigated the social system of feral horses, particularly the determinant factors of single-male/multi-male group dichotomy, in light of hypotheses derived from studies of primate societies. Socioecological data from 26 groups comprising 208 feral horses on Serra D’Arga, northern Portugal suggest that these primate-based hypotheses cannot adequately explain the social system of horses. In view of the sympatric existence of multi- and single-male groups, and the frequent intergroup transfers and promiscuous mating of females with males of different groups, male–female relationships of horses appear to differ from those of polygynous primates.
Polygyny Single-male/multi-male dichotomy Society Group-living animals Sexual conflict
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The field observations complied with the guidelines for animal studies in the wild of the Wildlife Research Center of Kyoto University, Japan. Special thanks are due to the municipality of Viana do Castelo for supporting our project. We are grateful to Agostinho Costinha, the director of Descubra Minho, Lourenço Almada of the Associação O Caminho do Garrano, and the villagers in Montaria for their support during our stay. The study was financially supported by grants from the Japan Society for the Promotion of Science (JSPS core-to-core CCSN and JSPS-U04 to T. M., KAKENHI Nos. 15H01619, 15H05309, and 17H05862 to S. Y.) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT No. 16H06283 to T. M.).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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