The effects of female group-changing behavior on female-female aggression, female rank, and female hierarchy stability

It is unclear how habitat features alter animal response 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 1) female-female aggression, 2) rank within female dominance
hierarchies, 3) stability of female hierarchies (in the groups they joined
and/or left), and 4) 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.