The condition dependence of a secondary sexual trait is stronger under high parasite infection level

Indicator models of sexual selection predict that the expression of sexual ornaments should be condition dependent. This is only partly supported by data, as many studies do not find positive associations between ornaments and condition. The reason for this inconsistency remains poorly understood. It has been hypothesized that environmental context may explain variation in the condition dependence of sexual traits, with stronger relationships between ornaments and condition expected in harsher environments. However, field tests of this idea are scarce. We studied 9 populations of wild red grouse Lagopus lagopus scoticus over 11 years, and compared the relationship between ornamentation and body mass (an index of body condition) in relation to environmental variability. We used the abundance of a key parasite in this system, Trichostrongylus tenuis, as an index of environmental conditions. We found that both ornament expression and body mass negatively correlated with parasite infection at both population and individual levels. More interestingly, we found that the relationship between ornamentation and body mass was stronger in populations with high parasite infection levels. Our findings support the idea that the condition dependence of secondary sexual ornaments varies in relation to environmental context. In sites and years when parasites are abundant, sexual ornaments provide better signals of condition.