Determinants and life-history consequences of social dominance in bighorn ewes

Studies of social rank in female ungulates consistently suggest that dominance increases with age, perhaps because dyadic relationships are established early in life, when the older female is always larger than the younger one. This relationship then remains unchanged, even if for fully grown adults size and age are not correlated, suggesting that typically female ungulates normally gain little from being dominant. In contrast, social interactions among 64 marked known-age bighorn sheep ewes (Ovis canadensis) over 3 summers at Ram Mountain, Alberta, Canada, suggest that the effect of age on social rank weakened substantially for ewes older than 6 years. Mass was strongly related to rank for ewes age 7 years and older, whereas horn size had no effect on dominance. Once they reach asymptotic mass, bighorn ewes appear to challenge older but lighter females to whom they were formerly subordinate. Although these results suggest that bighorn ewes may benefit from high social rank, we found no effect of rank on reproductive success, lamb sex ratio or lamb birth date.