Positive Association between Female-Biased Sexual Size Dimorphism and Male-Skewed Operational Sex Ratio in Anurans

Sexual size dimorphism (varying body sizes between males and females) and the operational sex ratio (ratio of sexually active males to receptive females) are key demographic traits influenced by complex selective pressures. Two hypotheses explain their relationship: the mating competition hypothesis posits that male-biased sexual size dimorphism intensifies with increasingly male-skewed adult sex ratios, while the mating opportunity hypothesis proposes that female-biased sexual size dimorphism escalates with greater male-biased adult sex ratios. We tested these hypotheses across 101 Chinese anuran species. Our results support the mating opportunity hypothesis, with enhanced female-biased sexual size dimorphism at more male-skewed operational sex ratios, particularly in monogamous species. We further explored the role of ecological factors and life history traits in shaping sexual size dimorphism and operational sex ratio. We found predation pressure to covary negatively with the male bias in operational sex ratios, while temperature variation, likely reflecting seasonal differences, negatively influenced both sexual size dimorphism and operational sex ratio. Our findings highlight the interplay between sexual selection, ecology, and life history in driving the evolution of sexual size dimorphism and operational sex ratio in anurans. Understanding these mechanisms is crucial for predicting how species may respond to future environmental changes.