Sexual Size Dimorphism in Rays and Skates (Elasmobranchii: Batoidea)

Sexual size dimorphism (SSD) is a widely observed but poorly understood phenomenon in which male and female animals differ in body size (e.g., length or mass). Despite extensive research on the interspecific distribution of SSD across various lineages, the evolutionary drivers behind male-biased and female-biased SSD remain contentious. In Elasmobranchii (sharks and rays), it is hypothesised that spatiotemporal differences in reproductive effort distribution between oviparous and matrotrophic species underlie variation in the direction and magnitude of SSD. However, existing studies have focused almost exclusively on sharks, overlooking batoids (rays), which comprise over 50% of elasmobranch diversity. In this study, we analysed published size (total length and disc width) records from 187 batoid species to quantify interspecific SSD variation across batoids and tested for ecological and evolutionary drivers of SSD within a comparative phylogenetic framework. Our findings reveal that, although interspecific trends in SSD among batoids superficially mirror those in sharks, subtle differences emerge in ecological signal and modes of trait evolution between the two. These differences suggest that selection for substantial male-biased and female-biased SSD in batoids is weaker than in sharks. The underlying reasons for this remain unclear but may involve variation in fecundity selection between batoids and sharks. Further studies quantifying variation in sexual selection and fecundity selection will be essential to fully clarify the adaptive basis of SSD variation within elasmobranchs.