SONG, SPERM QUALITY AND TESTES ASYMMETRY IN THE SEDGE WARBLER

The phenotype-linked fertility hypothesis states that functional male fertility covaries with male phenotype and that females engage in extr a-pair copulations with males with more attractive phenotypes than the ir partner to obtain direct fertility benefits. We tested this idea in the sedge warbler, Acrocephalus schoenobaenus, a species in which fem ales prefer males with larger song repertoires, and in which extra-pai r copulations occur. In this study paired males had larger song repert oires than unpaired males. However, we found no significant relationsh ips between song features and any measure of sperm numbers or sperm qu ality. We also examined the hypothesis that directional asymmetry in t estes mass covaries with the expression of secondary sexual traits, su ch as song features, but again found no significant effects. However, older males tended to have larger repertoires and testes and a greater degree of directional asymmetry in testes size than young males. (C) 1997 The Association for the Study of Animal Behaviour.