Geographical variation in limb muscle mass of the Andrew’s toad (Bufo andrewsi)

Muscles are vital for the process of movement, mating and escape of predators in amphibians. During evolution, the morphological and genetic characteristics as well as the size of muscles in species will change to adapt different environments. Theory predicts that low male-male competition in highaltitude/ latitude selects for small limb muscles. Here, we used the Andrew’s toad (Bufo andrewsi) as a model animal to test this prediction by analyzing geographical variation in the mass of limb muscles across nine populations from the Hengduan Mountains in China. Inconsistent with the prediction, we found that latitude and altitude did not affect the relative mass of total combined limb muscles and mass of combined hindlimb muscles among populations. Meanwhile, the relative mass of combined forelimb muscles, the two forelimb muscles (flexor carpi radialis and extensor carpi radialis) and the four hindlimb muscles (e.g. biceps femoris, semimebranous, semitendinosus and peroneus) was lowest in middle latitude and largest in low latitude whereas gracilis minor was largest in high latitudes. However, we did not find any correlations between the two forelimb muscles and the four hindlimb muscles and altitude. Our findings suggest that combined forelimb muscles, flexor carpi radialis, extensor carpi radialis, biceps femoris, semimebranous, semitendinosus and peroneus are largest in low latitudes due to pressures of mate competition.