Intraspecific variation in sperm size characters.

Interspecific variation in quantitative characters of sperm is considerable and can sometimes be related to aspects of sperm competition. In contrast, continuous intraspecific variation is often ignored; for example, many studies concentrate on measuring a species-typical sperm size. However, statistically significant variation amongst males in a variety of sperm size characters, particularly total length and a number of head characters, has been reported in at least 13 species. Furthermore, sperm size has been shown to influence the outcomes of processes involved in sperm competition in two species, with larger sperm being more successful in both cases. It is suggested here that intraspecific variation may be a widespread phenomena, further examination of which could contribute considerably to our understanding of sperm evolution. Size differences between the sperm of competing males could be used in controlled matings to investigate the fates of sperm within female tracts and storage organs, but care should be taken that size characters may not be neutral markers. Furthermore, the genetic determination of sperm size is considerable (h2 = 0.56-0.92 for total length and a number of head characters) and determinants are at least sometimes sex-linked, with the result that particular care must go into experimental design if genetic effects are to be fully elucidated. Studies combining genetical and behavioural effects of intraspecific variation in sperm morphology should be particularly rewarding.