Environmental stress is a powerful evolutionary force capable of influencing not just behavior, but also the size, structure, and function of organs and tissues. Among various animal species, including several birds and a few anurans, males display a fascinating trait: the left testis is larger than the right. The size of the right testis, however, is variable and, in birds, is often positively correlated with an individual's condition and overall quality.

In order to shed light on this trait, its geographical variation, and its correlation with age and male condition, a study was conducted on several populations of Rana kukunoris, an endemic species of the Tibetan Plateau. The study, published in Acta Herpetologica in 2022, focused on populations located at altitudes ranging from 2500 to 3500 meters above sea level. For each population, the authors calculated the operational sex ratio and captured a sample of males. These males were measured, weighed, and then euthanized, with their testes removed and weighed to calculate directional testes asymmetry (DTA).

The study found significant differences between populations in terms of mean snout-vent length (SVL), age, testis size, and DTA. The degree of asymmetry correlated with temperature but showed no connection with the operational sex ratio, male condition, or age. These findings suggest that environmental conditions, rather than male quality or sperm competition, drive the evolution of DTA in R. kukunoris. In particular, the population from the warmest site exhibited the most pronounced asymmetry. This pattern indicates that DTA may be a costly trait, requiring habitats with more favorable climates to allow males to allocate more energy to reproduction.

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