Special thanks are due to Viana do Castelo city for supporting our project. We are also grateful to Agostinho Costinha, the director of Descubra Minho, Lourenço Almada of Associação O Caminho do Garrano. We also thank the villagers in Montaria for their support during our stay, Tetsuro Matsuzawa for the generous guidance throughout the study, and Dora Biro and Valéria Romano for helpful comments on an earlier version of our manuscript. The study was financially supported by grants from the Japan Society for the Promotion of Science (JSPS core-to-core CCSN and JSPS-LGP-U04 to Tetsuro Matsuzawa, KAKENHI Nos. 15H01619 and 15H05309 to Shinya Yamamoto) and the Ministry of Education, Culture, Sports, Science, and Technology in Japan (MEXT No.16H06283 to Tetsuro Matsuzawa). We thank Lilly Gray and Adam Phillips, PhD from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

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.

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