In Soay sheep, where some females are horned while others are hornless (‘polled’), horned females are more likely to initiate and win aggressive interactions than polled RXDX-106 order ones (Robinson & Kruuk, 2007) while studies of cattle show that the experimental removal of horns leads to reductions in the ability of individuals to dominate competitors in newly established groups (Boussou, 1972). Comparative studies show that the distribution of female horns and antlers in ruminants is associated with variation

in female group size (Roberts, 1996) although other factors such as the need for effective defence against predators may also be involved. In some mammals where female competition is unusually intense, females often show physiological, morphological and behavioural adaptations that increase their competitive abilities (Clutton-Brock et al., 2006) as they do in a wide range of other animals (West-Eberhard, 1983, 1984; Tobias, Montgomerie & Lyon, 2012). For example, in spotted hyenas, where females compete intensely to raise offspring, well-defined female hierarchies are associated with high levels of reproductive skew and dominant females show elevated testosterone levels, large body size and social

dominance over males (Goymann, East & Hofer, 2001, East & Hofer, 2002, 2010; Holekamp & Dloniak, 2009). Many of the same traits are found in social lemurs and are thought to be associated with intense competition between breeding females for resources Metformin purchase in a fluctuating and unpredictable environment (Jolly, 1984; Wright, 1999; Dunham, 2008). As would be expected, as a result of high levels of reproductive skew, traits likely to affect competitive ability are also unusually well developed in females of some singular cooperative breeders. For example, in meerkats and naked mole rats, females that acquire the breeding position show increased levels of circulating selleck inhibitor testosterone (Faulkes & Abbott, 1997; Clutton-Brock et al., 2006) as

well as a period of secondary growth that is reduced or absent in males and may help them to maintain their status and reproductive output (O’Riain & Braude, 2001; Russell et al., 2004; Clutton-Brock et al., 2006). Breeding females are commonly the largest individuals in their group and are socially dominant to all group members (Reeve & Sherman, 1991; Faulkes & Abbott, 1997; Clutton-Brock et al., 1998b, 2001b). Studies of domestic cattle provide additional evidence that selection in female competitiveness can lead to increased levels of aggression in females and enhanced testosterone levels. In some parts of Switzerland, domestic cattle are forced to fight with each other in tournaments before they are moved up in the summer pastures and their owners bet on their performance.