Sperm in competition: not playing by the numbers

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The outcome of sperm competition is mediated largely by the relative numbers of sperm from competing males. However, substantial variation in features of sperm morphology and behaviour, such as length, longevity and motility, exists and researchers have suggested that this variation functions in postcopulatory sexual selection. Recent studies have determined the effect of these sperm-quality traits on fertilization success and a synthesis of this literature reveals that they are important in both sperm competition and cryptic female choice. To understand how postcopulatory sexual selection influences sperm traits, future research should determine sex-specific interactions that influence paternity, identify genetic correlations between ejaculate characters, quantify the relative costs of producing different sperm traits, and test assumptions of models of sperm quality evolution. Such research will shed light on what evolutionary pressures are responsible for the diversity in sperm morphometry and behaviour.

Section snippets

Sperm size and competitive success

Interspecific variation in sperm size is astounding (e.g. [19]). For example, within Drosophila, sperm length exhibits over a 100-fold difference between species 20, 21. Moreover, sperm length evolves rapidly 13, 19. The functional significance of sperm length variation has proved a conundrum to evolutionary biologists; the production of long sperm can be costly 8, 22, and the advantage of sperm of various sizes remains enigmatic (e.g. 1, 7, but see [17]). Both sperm competition and cryptic

Sperm longevity and viability

Measures of sperm quality, including longevity and viability, have long been studied in animal reproductive sciences as indicators of fertility [39]. However, fertility is measured in non-competitive situations so how these traits contribute to ejaculate competitiveness is generally unknown. Similarly, although fish species with longer sperm had shorter-lived gametes [40], the influence of these longer sperm on competitive fertilization success is untested. A pairwise comparative study across

Sperm velocity and mobility

Sperm motility, measured as mobility, velocity and percentage of mobile sperm, might also influence paternity. Intuitively, faster sperm should be more competitive during sperm competition because they could reach the egg more quickly than could slower sperm. Moreover, increased flagellar length is predicted to increase swimming speed, taking into account drag ([46], but see [47]). A non-phylogenetically controlled comparative study of a few mammal species found that sperm length was positively

Reconciling theory and observation

Sperm competition models predict that sperm length can confer a fertilization advantage only under restrictive conditions depending on either positive or negative relationships between other sperm traits (Box 1). Recent results have generally failed to support the predicted positive association between sperm length and sperm competition and the predicted relationships between sperm length, speed and longevity in both external and internal fertilizing species (Box 2).

The discrepancy between

Conclusion

Recent studies of postcopulatory sexual selection reveal that sperm numbers are not the only predictor for competitive fertilization success. Males have an extensive arsenal of sperm traits that they use in paternity contests, and females influence the outcome of sperm competition and drive the evolution of these traits. Thus, both males and females are playing sophisticated games, the dynamics and consequences of which are only beginning to be discovered.

Acknowledgements

I thank Andrew Beckerman, Tim Birkhead, Matt Gage, David Hosken (several times over), Craig LaMunyon, Scott Pitnick, Tommaso Pizzari and David Woolley for helpful discussions and comments about the article, and Helen Crudgington for help in the laboratory. I also thank Chris Boake for encouraging me to apply for the NSF support that currently funds my work, and Therese Markow for continued inspiration.

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