The more promiscuous the female cichlid, the faster the sperm from males swim, according to research published in a recent issue of the Proceedings of the National Academy of Sciences.
The study by John Fitzpatrick and coauthors studied 29 species of Lake Tanganyika cichlids whose reproductive strategies ranged from monogamy to highly promiscuous oral-brooding females that allow several males to ejaculate into her mouth.
The authors examined the sperm size and swimming speed of the male cichlids after ranking the degree of sperm competition (which measures the promiscuity of their reproductive strategies) on a scale of one (absent) to four (high).
The authors then reconstructed a phylogeny of the cichlids using mitochondrial DNA in order to correct for the degree of relatedness in their analysis, as well as to assess the most probable evolutionary route from an ancestral to a derived sperm trait.
The authors found that within Tanganyikan cichlids, large and fast sperm evolved from small and slow sperm in response to increases in the strength of sperm competition.
For more information, see the paper: Fitzpatrick, JL, R Montgomerie, JK Desjardins, KA Stiver, N Kolm and S Balshine (2009) Female promiscuity promotes the evolution of faster sperm in cichlid fishes. Proceedings of the National Academy of Sciences 106, pp. 1128"1132.