The immunocompetence handicap hypothesis: testing the genetic predictions

The immunocompetence handicap hypothesis suggests that the immune system co mpetes for resources with sexually selected ornaments; variation in ornamen ts might reflect genetic variation for immunocompetence. We tested this gen etic prediction by mating scorpionfly females to males differing in the exp ression of a condition-dependent ornament trait, saliva secretion, and then comparing offspring immunocompetence. We found several indications of an i mmunocompetence handicap in our study: females had superior immunocompetenc e compared with males, the different immune traits were positively correlat ed, and there were indications of genetic variation in immune traits. Howev er, we found no significant difference in the immunocompetence of offspring derived from males differing in ornament expression, only a tendency for s ons of ornamented males to possess slightly better immunocompetence. The es timated effect of fathers on offspring immunocompetence was rather small, b ut it might be a sufficient benefit of female choice, provided that the cos ts of choice are small. We conclude that the genetic benefit of female choi ce is small concerning offspring immunocompetence. but the immunocompetence handicap principle might nevertheless work in scorpionflies.