Spermatophore size as determinant of paternity in an arctiid moth (Utetheisa ornatrix)

Female Utetheisa ornatrix exercise postcopulatory mate selection, favoring sperm of larger males. Larger males also produce larger spermatophores, raising the question whether males are selected on the basis of body size or the size of their spermatophore. We here present evidence, based on matings in which males were induced to deliver spermatophores disproportionate in size to body mass, that the determinant is spermatophore mass.