Behavioural Processes 107 (2014) 15–21 Contents lists available at ScienceDirect Behavioural Processes jo ur nal homep ag e: www.elsevier.com/locate/behavproc The relationship of dominance, reproductive state and stress in female horses (Equus caballus) Carly A. York ∗ , Bruce A. Schulte Department of Biology, Western Kentucky University, Bowling Green, KY 42101, United States a r t i c l e i n f o Article history: Received 24 October 2013 Received in revised form 6 May 2014 Accepted 11 July 2014 Available online 21 July 2014 Keywords: Faecal glucocorticoids Hierarchy Stress Winner–Loser a b s t r a c t 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 domi- nance 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 require- ments 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 repro- duction 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction An animal’s social environment is one of the largest sources of information to potentially induce a stress response (Creel et al., 2013), and conspecific interactions for a social animal can serve as the preeminent source for an increase in stress level (Muller and Wrangham, 2004). A stress response is defined as a change in internal state that is induced by an external condition (Creel, 2001). Reacting to a stressful situation requires the animal to expend energy, which can ultimately decrease its relative fitness (Sapolsky, 1996; Reeder and Kramer, 2005). One type of study used to examine physiological stress response during social interactions involves pairing animals (often Mus musculus or Rattus norvegicus) in an arena, noting the winner and measuring the stress hormone levels of each contestant (Bronson and Eleftherious, 1964; Louch and Higginbotham, 1967). Because losers often have the higher stress hormone levels, low ranking animals in social groups were ∗ Corresponding author. Tel.: +1 7039995366. E-mail addresses: csind001@odu.edu, carlyanneyork@gmail.com (C.A. York). hypothesized to show higher stress than high ranking individuals, giving rise to the subordinate-stress hypothesis (Abbot et al., 2003). Dominant animals gain benefits from their social status such as access to resources and mates, but they also are subjected to costs associated with their high rank. Contrary to the results of winner–loser studies, in cooperative breeding mammals of var- ious species, dominant individuals have higher levels of stress hormones than lower ranking animals in the wild, giving rise to the dominance-stress hypothesis (see Creel, 2013 for review). This hypothesis states that although dominant individuals often have priority access to necessary resources, the hidden physiological costs could provide an indication as to why subordinates accept a lower status in the hierarchy (Morell, 1996). Similar to alterations in dominance status, the shifting of repro- ductive states also could cause a change in physiological stress. This factor was not considered in the dominance-stress hypothesis, or the subordinate-stress hypothesis. Stress levels could be elevated as a response to the highly energetic process of reproduction (Weingrill et al., 2004). In female meerkats (Suricata suricatta) concentrations of faecal glucocorticoid metabolites tripled as pregnancy progressed, indicating an increase in stress levels (Barrette et al., 2012). Because lactation is the most energetically http://dx.doi.org/10.1016/j.beproc.2014.07.005 0376-6357/© 2014 Elsevier B.V. All rights reserved.