The relationship of dominance, reproductive state and stress in female horses (Equus caballus)

Maintaining a dominant position in a hierarchy requires energetically expensive aggressive displays and physical exertion. Lab based winner-loser studies, often conducted with individuals from non-social species, have shown that subordinates have higher stress hormone levels than dominant individuals. However, in wild studies on cooperative breeders, displays of aggression used to maintain dominance status are associated with elevated stress hormone levels. The effect of reproductive state on dominance and stress has not been addressed within either of these situations. The purpose of this study was to examine physiological stress levels in relation to dominance rank and reproductive state in a non-cooperative breeder and herbivore, the domestic horse. The social interactions and measured faecal glucocorticoids were recorded in pastured, female horses that were either lactating or non-lactating. While faecal glucocorticoid metabolite level did not differ between reproductive state and rank, activity behaviour demonstrated significant differences between reproductive states. Higher energetic requirements of lactation were reflected in significantly longer bouts of eating and significantly less time spent alert and socializing. As non-cooperative breeders, the social nature of horses does not limit their reproduction or resource acquisition based upon rank, and therefore does not fit with the dominance-stress hypothesis or subordinate-stress hypothesis and instead supports a rank-independent stress hypothesis.