Sexual ornaments may enable females to discriminate amongst potential mates and choose those having either fewer parasites, or those which are more immunocompetent, or better able to cope with parasites. Tetraonid birds exhibit supra-orbital combs that function in both intra-sexual competition and mate choice. Through a correlative and experimental approach we investigated whether comb size of male red grouse Lagopus lagopus scoticus was related to infection intensity by their main parasite, the gastro intestinal nematode Trichostrongylus tennis, and to immune function. We first looked at the relationships between immune function, parasite infection and condition. We found that spleen mass, an indirect measure of immune response, correlated positively with T tennis load, and negatively with condition. Cell-mediated immunity, a measure of immune defence, correlated positively with condition, and although not significantly related to T tennis load, increased when nematode parasites were experimentally reduced. Secondly, we investigated whether comb size was related to condition, T tennis load or immune function. Comb size was not significantly related to T tennis infection and did not change significantly after nematode parasite removal. However, males with bigger combs were in better condition, had a lighter spleen, a lighter spleen than expected from their T tennis load and had greater cell-mediated immunity. The findings suggest that comb size relates to immune function rather than T tennis parasite infection intensity. Males with bigger combs are likely to be of higher phenotypic quality because they are more immunocompetent and might be better able to cope with the detrimental effects of parasites.
- Trichostrongylus tenuis
- avian spleen