Phenotypic correlates of spermatozoon quality in the guppy, Poecilia reticulata

The phenotype-linked fertility hypothesis suggests that a female can benefit directly from mate choice when the cues she uses indicate the quantity and/or quality of his spermatozoa. We tested the link between sperm quality and male body size and coloration in the resource-free mating system of the guppy, a tropical fish characterized by strong female choice. Larger males possessed larger testes and are therefore predicted to produce larger numbers of spermatozoa than smaller males. Larger male guppies also produced longer spermatozoa than smaller males. Degree of carotenoid coloration did not predict either the quantity or the quality of a male's spermatozoa. These results are consistent with a previous study that showed that female guppies in the study population prefer larger males to brightly colored males. The male-size directed increase in spermatozoon size may be the result of interplay between sperm competition and the coevolution of spermatozoon traits with the female reproductive tract.