Evolution of tendon shape in Diphyabatrachia is influenced by size, phylogeny and locomotion

Abstract

Tendons are viscoelastic elements with unique mechanical properties. With a close relation to movement, tendons function is mainly related to the transmission, amplification and recycling of energy. In Anura, jumping is the principal locomotor challenge, and most related anatomical specializations have been observed in pelvic girdle and hindlimbs. Despite of a greatly specialized body plan, locomotor variation in anurans goes beyond jumping, including specializations for swimming, burrowing, and climbing. On this study we apply Geometric Morphometrics method to test for morphological adaptation in three tendinous elements associated with the urostyle and hindlimb joints. The variation of 99 species of Diphyabatrachia clade was analyzed as a study case, taking in consideration functional and ecological aspects of species and their phylogenetic history. Diphyabatrachia includes jumping and climbing species of arboreal habitats (Centrolenidae) and jumping species of long and short distances with terrestrial and aquatic habitats (Leptodactylidae). We found significant differences between locomotor modes and habitat use regarding tendon shape, especially in the patellar tendon of jumping-arboreal species when compared to others. In all analyzed tendons, locomotor modes and habitat use showed interaction with size. Optimization and reconstruction of tendon shape revealed two putative synapomophies, a broad tendon of the longissimus dorsi for Leptodactylinae and an elongated patellar tendon for Centrolenidae. In addition, there was significant covariation of shape between the knee and the heel tendon shapes, which led us to propose a functional analogy between these two structures. All evidence points to an adaptive significance of tendon variation, yet the results also showed a great influence of phylogeny. Therefore, it is possible to conclude that a combination of factors, rather than one determinant variable, is influencing tendon evolution in Dyphiabatrachia.