Testis size variation in the greenfinch Carduelis chloris: relevance for some recent models of sexual selection

Interspecific evidence that testis size responds to selection caused by spe rm competition has been obtained from many taxa. However, little is known a bout the sources of intraspecific variation in testis size, although such v ariation may have functional significance. Variation in testis size and asy mmetry was studied within and between eight geographically separated (and g enetically differentiated) populations of greenfinches Carduelis chloris. T he relationships between testis size and plumage brightness (degree of yell owness) and the prevalence of haematozoan infections were also investigated in three of these populations, as they related to the predictions of the i mmunocompetence handicap hypothesis, and Moller's hypothesis relating direc tional testis asymmetry to phenotypic quality. There were large differences between populations in testis size, with males from northern populations h aving larger testes than those from southern populations. Within population s, large testes were associated with larger body size and greater age. When the influence of these factors was removed statistically, males with large testes were more likely to be infected with haematozoan parasites, and had brighter yellow plumage. No evidence was found that directional asymmetry in testis size was related to either of these measures of phenotypic qualit y. These results are consistent with the hypothesis that males with large t estes, while signalling higher phenotypic quality as revealed by increased plumage brightness, also pay a cost in terms of reduced immunocompetence, r evealed by the increased probability of infection in these males. That thes e patterns were similar in three different populations adds further strengt h to these conclusions. Our results suggest that studying the sources of va riation in testis size among individuals can reveal interesting processes i n sexual selection.