Male mating competitiveness and age-dependent relationship between testosterone and social rank in bighorn sheep
In males, the acquisition and development of behavioral and morphological secondary sexual traits typically depends on testosterone and correlates with mating success. Testosterone level could affect competition for mates and thus be a target of sexual selection. We sought to relate testosterone levels to male mating competitiveness, by teasing apart the relationships between testosterone, behavior, and growth before the mating period. We monitored 24 adult bighorn rams (Ovis canadensis) at Ram Mountain, Alberta, from 2008 to 2011. Using linear mixed models, we tested the relationships between testosterone metabolites in feces, social rank, and both growth and size of two sexually selected traits: horns and body mass. The correlation between testosterone and social rank varied with age. Testosterone and rank were weakly and negatively correlated for young rams, positively correlated for prime-aged rams, and negatively correlated for older rams. Although testosterone had an increasingly positive effect on total horn length until 8 years of age, we could not detect any effects on annual growth rate of horns or body mass. Testosterone may be related to male’s ability to compete for mates through its relationship with behaviors determining social rank, rather than by influencing the development of morphological traits. Differences in testosterone levels among competitors may be a proximate cause of variance in fitness.
KeywordsAndrogen Ontogeny Ovis canadensis Secondary sexual traits Sexual selection Social hierarchy
We are grateful to Anne Hubbs, Chiarastella Feder, and Jon Jorgenson for their invaluable support of the Ram Mountain research program. We thank all assistants and students who trapped and observed bighorn sheep over decades. Patrick Bergeron, Eric Vander Wal, Sandro Lovari, and Allan McElligott provided constructive comments on previous drafts of the manuscript. M.F.B. and F.P. are funded by NSERC Discovery Grants. F.P. holds the Canada Research Chair in Evolutionary Demography and Conservation. Our research was also supported by the Government of Alberta and an Alberta Conservation Association Grant in Biodiversity.
This research project was approved by the Animal Care Committee of the Université de Sherbrooke (MFB2009-01 and FP2012-01), an affiliate of the Canadian Council on Animal Care.
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