Determinants of paternity in a butterfly

Abstract

Success in sperm competition is of fundamental importance to males, yet little is known about what factors determine paternity. Theory predicts that males producing high sperm numbers have an advantage in sperm competition. Large spermatophore size (the sperm containing package) also correlates with paternity in some species, but the relative importance of spermatophore size and sperm numbers has remained unexplored. Males of the small white butterfly, Pieris rapae (Lepidoptera: Pieridae), produce large nutritious spermatophores on their first mating. On their second mating, spermatophores are only about half the size of the first, but with almost twice the sperm number. We manipulated male mating history to examine the effect of spermatophore size and sperm numbers on male fertilization success. Overall, paternity shows either first male or, more frequently, second male sperm precedence. Previously mated males have significantly higher fertilization success in competition with males mating for the first time, strongly suggesting that high sperm number is advantageous in sperm competition. Male size also affects paternity with relatively larger males having higher fertilization success. This may indicate that spermatophore size influences paternity, because in virgin males spermatophore size correlates with male size. The paternity of an individual male is also inversely correlated with the mass of his spermatophore remains dissected out of the female. This suggests that females may influence paternity by affecting the rate of spermatophore drainage. Although the possibility of female postcopulatory choice remains to be explored, these results clearly show that males maximize their fertilization success by increasing the number of sperm in their second mating.