Evolution of sperm morphology in anurans: insights into the roles of mating system and spawning location

Abstract

Background: The degree of postcopulatory sexual selection, comprising variable degrees of sperm competition and cryptic female choice, is an important evolutionary force to influence sperm form and function. Here we investigated the effects of mating system and spawning location on the evolution of sperm morphology in 67 species of Chinese anurans. We also examined how relative testes size as an indicator of the level of sperm competition affected variation in sperm morphology across a subset of 29 species.

Results: We found a significant association of mating system and spawning location with sperm morphology. However, when removing the effects of body mass or absolute testes mass for species for which such data were available, this effect became non-significant. Consistent with predictions from sperm competition theory, we found a positive correlation between sperm morphology and relative testes size after taking phylogeny into account.

Conclusions: Our findings suggest that sexual selection in Chinese anurans favors longer sperm when the level of sperm competition is high. Pre-copulatory male-male competition and spawning location, on the other hand, do not affect the evolution of sperm morphology after taking body mass and absolute testes mass into account.

Figure 1

Differences in sperm morphology between polyandrous and monogamous mating system across 67 anurans species. 1 – sperm total length; 2 – sperm head length; 3 – sperm tail length.

Figure 2

Differences in sperm morphology among spawning sites across 67 anurans species. 1 – sperm total length; 2 – sperm head length; 3 – sperm tail length.

Figure 3

Correlations between sperm morphology and relative testes size among 29 anurans species. Relative testes size was from residuals of observed testes mass minus predicted testes mass on the basis of the allometric regression.