Locomotion and Biomechanical Adaptations in Hind Limb Muscles of Three Mexican Anuran Species

Abstract. Anurans are anatomically built for jumping, requiring the muscles to contract to generate sufficient mechanical force to launch their bodies. A lot of energy is needed in the hind limbs, where the majority of the force is produced, whereas the front legs and both girdles serve mainly to support the frog's weight at rest as well as to absorb the shock of landing. Different species of anurans have different locomotion mechanisms, evidenced by comparing the mechanical contractile properties of hind limb muscles. In this study, we compared two muscles in arboreal (Agalychnis dacnicolor), terrestrial (Incilius marmoreus), and aquatic (Lithobates sierramadrensis) species of Mexican anurans. We performed low frequency in situ stimulations of the muscles and found that A. dacnicolor showed higher stresses than L. sierramadrensis and I. marmoreus in the two studied muscles, the cruralis and the gastrocnemius. Regarding the cruralis, the stress generated by the aquatic and terrestrial anurans was up to 23% less than that generated by the arboreal anuran. There was also a difference of up to 30% in the ratio of performance between low frequencies and twitch in L. sierramadrensis compared to the other anurans. These differences in muscle physiology suggest adaptive differences among species, although more comprehensive studies are needed to corroborate this hypothesis.