Altitudinal variation in somatic condition and reproductive investment of male Yunnan pond frogs (Dianrana pleuraden)

Comparisons between populations that differ in resource availability and in environmental variables affecting the intensity of male-male competition allow insights into the mechanisms shaping reproductive allocation. Here, we studied inter-population variation in male investments in testis size and sperm length across four populations of the Yunnan pond frog (Dianrana pleuraden) along an altitudinal gradient in western China. Male body condition, relative testis size and sperm length did not increase with altitude despite an increasing male bias in the operational sex ratio (OSR) in high altitude was recorded. Our results were inconsistent with previous prediction that increasing latitude and/or altitude should lead to a declining male reproductive investment in anurans due to shortening breeding seasons and declining resource availability. We found a positive correlation between body condition and testes mass along the altitudinal gradient, which was indicative of a condition-dependent expression of testis size. By contrast, we recorded a non-significant correlation between testes mass and OSR, suggesting that male-male competition did not result in increasing levels of sperm competition. A non-significant correlation between sperm length and testis mass suggested that the intensity of sperm competition can account for the variation in sperm length.