Examining the costs and benefits of male-male associations in a group-living equid

In feral horse (Equus caballus) populations, females can associate in year-round social groups with one male (single-stallion (SS) groups) or multiple males (multi-stallion (MS) groups). Given the expectation of intense male competition for mating opportunities, male-male associations raise questions regarding their supporting mechanisms and adaptive consequences. The function of MS groups has been highly debated which led to a series of conflicting hypotheses attempting to describe the relationship between the males, and the costs and benefits of this association in relation to SS groups. Based on previous studies, our goal was to compare the group size and stability, males’ defence behaviour against group outsiders and access to females, levels of male aggression to group members, and the activity budget and body condition of horses in MS and SS groups. Between the months of May to August (breeding season) from 2016 to 2021, we collected demographic and behavioural data from feral Garrano horses living in northern Portugal, where wolf (Canis lupus signatus) predation is an important factor. Contrary to mutualistic hypotheses, our results indicated that SS males had more breeding females than MS groups and group type did not influence female membership stability. However, dominant males in MS groups may be benefitting from the presence of another male via decreased effort in group defence and more resting time. We recorded less aggressive behaviours from dominant to subordinate males in MS groups when subordinates displayed more defensive behaviours against outsiders, but we found little evidence that subordinates were being rewarded with access to females, as proposed by reciprocal-altruism hypotheses. Females in MS groups did not suffer higher levels of male aggression when compared with SS females, and the number of males in the group had no significant influence on the time females spent grazing and resting, and ultimately on their body condition. Future research should explore how MS male associations shape male-female relationships, which could provide further insights to the existence and maintenance of subordinate males. This study is the first to report on predation impact by wolves in MS and SS groups, highlighting the need to further explore how different ecological pressures shape horse behaviour and social organization. Furthermore, our results provide information that can help improve the welfare of domestic horses and can make informed management decisions of feral and free-ranging horse populations, particularly in instances where males are targeted for removals. Considering the advantages, as well as costs, provided by extra males may be useful to managers and horse owners, who can then act accordingly and with the least disturbance to horse populations