Testing the Role of Natural and Sexual Selection on Testes Size Asymmetry in Anurans

Simple Summary Testis asymmetry is particularly common in animals and it has been explained by two main hypotheses: the packaging hypothesis and compensation hypothesis. We studied the variations in testes size asymmetry among 116 anuran species associated with natural and sexual selection to test the two hypotheses. We found that the positive correlation between testes size asymmetry and livers mass followed the prediction of the packaging hypothesis, while the postcopulatory sperm competition (e.g., residual testes size) and the degree of testes asymmetry was positively related, supporting the positive role of sexual selection on testes size asymmetry. However, we did not find any effect of developmental stress on variation in testes size asymmetry among anurans, which was inconsistent with the compensation hypothesis. Directional asymmetry in testes size is commonly documented in vertebrates. The degree of testes size asymmetry has been confirmed to be associated with natural and sexual selection. However, the role of natural and sexual selection driving variations in testes size asymmetry among species of anurans are largely unknown. Here, we studied the patterns of variations in testes size asymmetry and the factors shaping its variations among 116 anuran species. The results indicated that the left size-biased testes in 110 species (94.83% of 116 species) is more common than the right size-biased testes in six species. For all studied species, the degree of testes size asymmetry was positively associated with relative livers and body fat mass, following the prediction of the packaging hypothesis. We also found that the postcopulatory sperm competition (e.g., residual testes size) was positively associated with the degree of testes asymmetry. However, environmental stress (e.g., high latitude, precipitation seasonality and temperature seasonality) did not promote more symmetrical testes for all species. Our findings suggest that both natural selection for larger livers in body space and sexual selection for rapid increase in testis mass for most species during the breeding season can play key roles in driving in testes size asymmetry across anuran species.