Background: The degree of postcopulatory sexual selection,comprising variable degrees of sperm competition and cryptic female choice,is an important evolutionary force to influence sperm form and function.Here we investigated the effects of mating system and spawning location on the evolution of sperm morphology in 67 species of Chinese anurans.We also examined how relative testes size as an indicator of the level of sperm competition affected variation in sperm morphology across a subset of 29 species.