Post-mating clutchy piracy in an amphibian

Multiple mating and alternative mating systems can decrease the opportunity for sexual selection. Sperm competition after multiple mating has been observed in many animals1,4-7 and alternative reproductive systems are widespread among species with external fertilization and parental care. Multiple paternity without associated complex behaviour is also known from broadcast simultaneous spawning amphibians and fishes. Here we report ¿clutch piracy¿ in a montane population of the common frog, Rana temporaria, a novel reproductive behaviour for vertebrates with external fertilization. Males of this species clasp the females and the pair deposits one spherical clutch of eggs. No parental care is provided. ¿Pirate¿ males search for freshly laid clutches, clasp them as they would do with females, and fertilize eggs that were left unfertilized by the ¿parental¿ male. This behaviour does not seem to be size-dependent, and some males mate with a female and perform clutch piracy in the same season. Piracy affected 84% of the clutches and in some cases increased the proportion of eggs fertilized, providing direct fitness benefits both for pirate males and females. Sexual selection occurs in this population as indicated by size-assortative mating. It is probably caused by a strong male-biased sex ratio, but clutch piracy may reduce its impact. This provides a good model to explore how alternative mating strategies can affect the intensity of sexual selection.