Bilateral Asymmetry in Sperm Quality of Bank Voles: Fluctuating, Huge, and Warranting Attention

Despite its potential significance for male fertility, bilateral testicular asymmetry in vertebrates remains poorly understood. Fluctuating asymmetry (FA), which serves as an indicator of stochastic developmental variation, constitutes a crucial yet frequently overlooked aspect of phenotypic variability. Although FA is commonly examined in morphological traits, its measurement in functional testicular traits has not been previously conducted due to methodological constraints. We analyzed FA in epididymal sperm motility (curvilinear velocity, VCL) and sperm concentration (SEC) in reproductively mature male bank voles (Clethrionomys glareolus, n = 61) from wild populations using Computer-Aided Sperm Analysis (CASA). Parallel measurements of "soft" (testis and kidney mass) and "hard" (mandibular dimensions) morphological traits were performed for comparison. CASA showed reliable reproducibility (VCL: 0.89; SEC: 0.73), enabling accurate FA quantification and confirming the absence of directional asymmetry or antisymmetry. Functional traits exhibited remarkably high FA values: 12% of the mean trait value for VCL and 30% for SEC, which is an order of magnitude greater than that of morphological traits. FA accounted for one-quarter (VCL) to one-half (SEC) of the total variation, representing two to three times the corresponding proportions of morphological traits. These elevated FA levels suggest that within-individual left/right differences may surpass between-individual variation. Our findings underscore the importance of (1) conducting repeated bilateral measurements for accurate FA quantification in testicular function and (2) utilizing averaged data from both testes for population-level analyses. These findings have significant practical implications for physiological and ecotoxicological research using rodent models, in which epididymal sperm analysis is standard practice, highlighting the necessity of considering functional asymmetry to prevent biased conclusions.