Sperm competition shapes anuran testicular architecture through tubule and tissue adjustments.

Testis morphology may act as a determinant of sperm size, and the evolutionary pressure to produce longer sperm may impose functional constraints on testicular architecture. Postcopulatory sexual selection, particularly sperm competition, is widely recognised as a primary driver of the remarkable interspecific variation in sperm size and number across animals. Theoretical hypotheses predict that longer sperm require wider seminiferous tubules and that intense sperm competition favours an increased proportion of sperm-producing (seminiferous) tissue relative to non-spermatogenic components, independent of overall testis size. Using a phylogenetic comparative approach across 78 anuran species, we tested these two predictions. Our analyses revealed that the cross-sectional area of the seminiferous tubules was significantly and positively correlated with sperm length, thereby supporting the proposed morphogenetic constraint. Furthermore, relative testis size, which was used as a validated proxy for sperm competition intensity, was positively associated with the proportion of seminiferous tissue. In contrast, absolute testis size did not predict this tissue proportion. Our findings suggest that sperm competition shapes anuran testicular architecture through two distinct evolutionary pathways: indirect selection for wider tubules mediated by longer sperm, and direct selection for increased investment in seminiferous tissue. This tissue optimisation extends beyond simple testicular enlargement and reveals sophisticated evolutionary adaptations to postcopulatory sexual selection.