Sperm viability and sperm competition in insects.

Sperm quality plays an important role in vertebrates in determining which male has the advantage when two or more males compete to fertilize a female's ova. In insects, however, the importance of sperm quality has never been considered, despite sperm competition being widespread and well studied in this group. We tested the hypothesis that sperm viability, measured as the proportion of live sperm, covaried with the intensity of sperm competition in insects. In a pairwise comparison of seven closely related species pairs, each comprising a monandrous and a polyandrous species (i.e., with and without sperm competition, respectively), we found that in all cases the polyandrous species had a higher proportion of live sperm in their sperm stores. The distribution of the percentage of live sperm showed considerable inter- and intraspecific variation, suggesting that, all else being equal, males will vary in their ability to fertilize ova on the basis of sperm viability alone. Our results suggest that sperm viability is one of a suite of male adaptations to sperm competition in insects.