Sperm Competition: Mating System, Not Breeding Season, Affects Testes Size of Primates

In taxa in which several males mate with a fertile female, males have larger testes relative to their body size than do males of taxa where only one male normally mates. However, breeding seasonality could confound the association: some species with breeding seasons have unusually large testes; seasonal breeding could concentrate fertile females in time, so making them difficult to defend and thus promoting multi-male groups. Using data from 58 species of primates, and carefully accounting for phylogeny, we investigate the potential for breeding seasonality to confound the relation between testes size and mating system. No confounding effect exists. Multi-male taxa have very significantly larger testes for their body size than do single-male taxa, independently of seasonality of breeding. Seasonality has no effect whatsoever in our sample, although few primates have very short breeding seasons. While multi-male genera are equally likely to be seasonal as non-seasonal breeders, the single-male taxa of our sample are rarely seasonal, apparently supporting the difficulty of defence hypothesis but the sample by no means allows a firm conclusion.