Mean mares? Habitat features influence female aggression in response to social instability in the feral horse (Equus caballus)

It is unclear how habitat features alter animal responses to social instability. Only by uncovering such interactions can we fully understand the evolutionary drivers and fitness consequences of sociality. We capitalize on a management-induced manipulation of social stability in an island population of free-ranging feral horses (Equus caballus), living across three distinct habitat types. We tested whether female group-changing behaviour (a reliable measure of social instability) affected (i) female-female aggression, (ii) rank within female dominance hierarchies, (iii) stability of female hierarchies (in the groups they joined and/or left), and (iv) how habitat characteristics shaped these responses. Female group-changing behaviour positively predicted aggression from other females, but only when habitat features such as visibility and freshwater distribution were considered. We found no strong association among female group-changing behaviour and the aggression initiated, female rank or the stability of female dominance hierarchies. Our work reveals that animal responses to social instability are nuanced and impacted by the surrounding habitat. A better understanding of these impacts can help elucidate evolutionary drivers of sociality and mitigate unintended effects of management practices.