Multiply mated males show higher embryo survival in a paternally caring fish

We explored the effects of multiple mates on male reproductive success in a species with male parental care in which an increase in the number of female mating partners does not increase the number of eggs received. The broad-nosed pipefish (Syngnathus typhle) has a polygynandrous mating system. In this species, the male cares for embryos in a specially developed brood pouch. During brooding, some embryos fail to develop. We experimentally mated males with either one or two females while keeping brood size similar. We found that broods of singly mated males showed significantly lower embryo survival than those of doubly mated males. Furthermore, larger broods showed relatively lower levels of embryo survivorship independent of number of mates. We conclude that embryo survival is affected by postcopulatory processes that appear to result in higher fitness of multiply mated males. We discuss our results in the light of parental care, sibling competition, genetic benefits, and kin selection.