Elevated spring testosterone increases parasite intensity in male red grouse

The expression of testosterone-dependent sexual traits might signal the ability of their bearers to cope with parasite infections. According to the immunocompetence handicap hypothesis (IHH), such signals would be honest because physiological costs of testosterone, such as a reduced ability to control parasite infections, would prevent cheating. We tested whether testosterone would affect the outcome of a standardized parasite challenge in red grouse, using a main parasite of the species, the nematode Trichostrongylus tenuis. We caught males in spring, removed their nematode parasites, and implanted them with testosterone or empty implants, as controls. After 1 month, they were reinfected with a standard dose of infective T. tenuis parasites. When challenged, testosterone males had relatively less globulin relative to albumin plasma proteins than control males, an indication that they had experienced increased physiological stress. Testosterone-treated males had significantly more T. tenuis parasites than controls in the next autumn and also had more coccidia and lost more weight than controls. Testosterone-treated males nevertheless benefited from their elevated spring testosterone: they had bigger sexual ornaments than controls both in spring and autumn, and they tended to have a higher pairing and breeding success than controls. Our results supported the IHH in showing that elevated testosterone impaired the ability of males to cope with a standardized challenge by a dominant parasite. Testosterone thus plays a key role in mediating trade-offs between reproductive activities and parasite defense, and testosterone-dependent comb size might honestly signal the ability of red grouse to control T. tenuis infection.This work was funded by a NERC grant (NER/A/S/1999/00074) and was carried out under a U.K. Home Office license (PPL80/1438).Peer reviewe