Sperm size of African cichlids in relation to sperm competition

We compared pairs of closely related taxa of cichlid fishes from Lake Tanganyika to examine the relationship between sperm size and the presumed intensity of sperm competition. In contrast to previous reports of relatively short sperm in polygamous fishes across a variety of taxa, we found that polygamous cichlids had significantly longer sperm than their closest monogamous relatives. In addition, sperm length was significantly related to relative testis size (controlling for body size and phylogeny). The site of fertilization may also be correlated with sperm length, as species that fertilize in the female's buccal cavity had significantly shorter sperm than those that fertilized eggs on the substrate. Assuming that relatively large testes and polygamous mating are indicative of more intense sperm competition, these results indicate that sperm length is related to the intensity of sperm competition in this clade of cichlids, as has been found previously in insects, birds, and mammals.