Environmental conditions influence red grouse ornamentation at a population level

Theory suggests that condition-dependent sexual displays should be more weakly expressed under adverse conditions than under more favourable ones. Here, we tested this hypothesis in wild red grouse Lagopus lagopus scoticus using a data set of nearly 1500 individuals from nine populations over 8 years, covering varying environmental conditions. We analysed whether male and female ornament expression (i.e. comb size) in a given site and year varied with various indices of environmental conditions: population density, Trichostrongylus tenuis nematode infection at the population level, and climate conditions [measured as winter North Atlantic oscillation (NAO) index]. We found that average comb size in males, but not in females, negatively correlated with population density, parasite infection levels, and winter NAO index. Furthermore, the coefficient of variation (CV) of comb size was higher in females than in males. CVs in both males and females were not clearly associated with the studied environmental variables. Our results support the idea that the expression of condition-dependent sexual traits should be lower under more stressful environmental conditions, but only in males. We discuss the potential reasons behind the effect of environmental conditions on secondary sexual traits, and why these effects differ between sexes. (C) 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, , .