In social animals, reproductive success is often related to social dominance. In cooperatively breeding birds and mammals, reproductive rates are usually lower for social subordinates than for dominants, and it is common for reproduction in subordinates to be completely suppressed. Early research with captive animals showed that losing fights can increase glucocorticoid (GC) secretion, a general response to stress. Because GCs can suppress reproduction, it has been widely argued that chronic stress might underlie reproductive suppression of social subordinates in cooperative breeders. Contradicting this hypothesis, recent studies of cooperative breeders in the wild show that dominant individuals have elevated GCs more often than do subordinates. Here, I summarize relationships between rank, aggression, and GCs from field studies of 3 cooperatively breeding carnivores: the dwarf mongoose (Helogale parvula), the African wild dog (Lycaon pictus), and the gray wolf (Canis lupus). In all 3 species, GC levels are higher in dominants than in subordinates for 1 or both sexes. Higher GCs are associated with higher rates or severity of aggression in some cases, but not all. As studies have accumulated, the patterns observed in these carnivores are emerging as typical for cooperative breeders.
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