Fecal testosterone in bighorn sheep (Ovis canadensis): behavioural and endocrine correlates

Abstract

Noninvasive endocrine techniques allow repeated sampling of the same individual to study causes and consequences of variation in individual behaviour and physiology. In this study, radioimmunoassay was used to measure fecal testosterone and to assess the repeatability of the testosterone assay for bighorn rams (Ovis canadensis). Fecal samples were collected from marked males during the pre-rut and the rut over 2 years. Results were highly repeatable for samples of the same ram within a day (r = 0.93). Fecal testosterone peaked during the pre-rut (when social relationships are established) and then declined from the pre-rut to the rut. For both years of study, fecal testosterone was correlated with social rank (2001: r = 0.73, P < 0.0001; 2002: r = 0.54, P = 0.007) and age (2001: r = 0.65, P = 0.002; 2002: r = 0.53, P = 0.008) of individual rams. When age was accounted for, however, the relationship between social rank and testosterone was no longer significant. Aggressiveness (measured as hourly interaction rate) was weakly correlated with fecal testosterone (r = 0.44, P = 0.039). There was no association between aggressiveness and social rank (r = 0.13, P = 0.591). To our knowledge, this is the first report of an association between testosterone levels and individual social rank in wild ungulates.