EXTRA-PAIR PATERNITY, SPERM COMPETITION AND THE EVOLUTION OF TESTIS SIZE IN BIRDS

Sperm competition should select for increased sperm production if the probability of fertilization by a specific male is proportional to the relative number of sperm inseminated. A review of the literature gene rally supports the predicted positive association between sperm produc tion or allocation and various measures of the presumed intensity of s perm competition. However, it is not clear how increased sperm competi tion is related to extra-pair paternity, and it remains unknown whethe r certainty of paternity should be associated with relative testis siz e. Based on a large sample of bird species with information on extra-p air paternity gathered from the literature, we demonstrate that testis mass is related positively to the level of extra-pair paternity, afte r controlling for body size and phylogeny. Although large testes may b e necessary to avoid sperm depletion in species in which males frequen tly engage in multi-pair copulations, we argue that selection has favo ured increased testis mass in situations of more intense sperm competi tion in order to retaliate against copulations by rival males. The fac t the males are not always successful in retaliating against rival eja culates further suggests that females may largely control the allocati on of paternity in birds and that increased sperm production by males may simply be a male strategy to make the best of a bad situation.