Horse affairs: Factors affecting reproductive success in a feral polygynous ungulate
Polygynous mating systems result in intense competition between males for access to females. We tested hypotheses related to costs and benefits of multimale groups in free-roaming feral horses, Equus caballus , in relation to individual reproductive success. We observed horses in two populations in Utah, U.S.A. (Conger and Frisco Herd Management Areas) between 2017 and 2020. We obtained genetic confirmation of foal parentage from hair and faecal samples and had known sire status at conception for 119 foals from 35 males at Conger and 40 foals from 17 males at Frisco. We found high reproductive skew: 44% of foals in our sample were sired by 7% ( N = 5) of males at Conger and 45% of foals were sired by 12% ( N = 5) of males at Frisco. We determined four adult male statuses: harem stallion (associating with females), lieutenant (associating with a harem), tag (following a harem but not associating with them) and bachelor (associating with other males). Most males moved between statuses, including 27 castrated males at Conger. Harem stallions were sires of 86% and 95% of foals at Conger and Frisco, respectively; only 3% of foals at Conger and none at Frisco were sired by lieutenants or tags; over 96% of paternity was by single-male group harem stallions. Female reproductive success was not affected by group size or being in a multimale group. Harem stallion tenure length and reproductive success were positively correlated with affiliative interactions between adult males and females but not with agonistic interactions. We found more support for limited control than concession models of reproductive skew and partial support for the consort hypothesis explaining presence of multimale groups. Differences between horse populations relative to male roles may reflect local adaptations within polygynous systems, and multimale groups may arise from social bonds between individuals.