Abstract
Interspecific comparative studies have shown that, in most taxa, postcopulatory sexual selection (PCSS) in the form of sperm competition drives the evolution of longer and faster swimming sperm. Work on passserine birds has revealed that PCSS also reduces variation in sperm size between males at the intraspecific level. However, the influence of PCSS upon intra-male sperm size diversity is poorly understood, since the few studies carried out to date in birds have yielded contradictory results. In mammals, PCSS increases sperm size but there is little information on the effects of this selective force on variations in sperm size and shape. Here, we test whether sperm competition associates with a reduction in the degree of variation of sperm dimensions in rodents. We found that as sperm competition levels increase males produce sperm that are more similar in both the size of the head and the size of the flagellum. On the other hand, whereas with increasing levels of sperm competition there is less variation in head length in relation to head width (ratio CV head length/CV head width), there is no relation between variation in head and flagellum sizes (ratio CV head length/CV flagellum length). Thus, it appears that, in addition to a selection for longer sperm, sperm competition may select more uniform sperm heads and flagella, which together may enhance swimming velocity. Overall, sperm competition seems to drive sperm components towards an optimum design that may affect sperm performance which, in turn, will be crucial for successful fertilization.
Highlights
Comparative studies have shown that many taxa respond to increases in sperm competition by increasing sperm size which, in turn, is associated to enhanced sperm swimming speed
Paired ttests revealed that sperm head dimensions had a higher degree of variation than that observed for dimensions of flagellum components
Increases in sperm competition were associated with lower variation in the ratio between the Coefficient of variation (CV) of head length and head width (CV head length/CV head width)
Summary
Comparative studies have shown that many taxa respond to increases in sperm competition by increasing sperm size which, in turn, is associated to enhanced sperm swimming speed (mammals: [1,2,3,4], fish: [5], birds: [6]). Studies on closely related species, as well as intraspecific comparisons, have suggested that an early evolutionary step in the overall change of sperm size may be a modification of sperm head dimensions [11,12] that makes sperm movement more energyefficient by reducing the degree of resistance offered to the medium in which sperm swim [13]. Modifications in the sperm head may involve changes in elongation [11], an increase in sperm head area, or the development of appendices, such as an apical hook, as seen in rodents [12]
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