Ecomorphology of the pectoral girdle in anurans (Amphibia, Anura): Shape diversity and biomechanical considerations

Abstract

Frogs and toads (Lissamphibia: Anura) show a diversity of locomotor modes that allow them to inhabit a wide range of habitats. The different locomotor modes are likely to be linked to anatomical specializations of the skeleton within the typical frog Bauplan. While such anatomical adaptations of the hind limbs and the pelvic girdle are comparably well understood, the pectoral girdle received much less attention in the past. We tested for locomotor‐mode‐related shape differences in the pectoral girdle bones of 64 anuran species by means of micro‐computed‐tomography‐based geometric morphometrics. The pectoral girdles of selected species were analyzed with regard to the effects of shape differences on muscle moment arms across the shoulder joint and stress dissipation within the coracoid. Phylogenetic relationships, size, and locomotor behavior have an effect on the shape of the pectoral girdle in anurans, but there are differences in the relative impact of these factors between the bones of this skeletal unit. Remarkable shape diversity has been observed within locomotor groups indicating many‐to‐one mapping of form onto function. Significant shape differences have mainly been related to the overall pectoral girdle geometry and the shape of the coracoid. Most prominent shape differences have been found between burrowing and nonburrowing species with headfirst and backward burrowing species significantly differing from one another and from the other locomotor groups. The pectoral girdle shapes of burrowing species have generally larger moment arms for (simulated) humerus retractor muscles across the shoulder joint, which might be an adaptation to the burrowing behavior. The mechanisms of how the moment arms were enlarged differed between species and were associated with differences in the reaction of the coracoid to simulated loading by physiologically relevant forces.

Comparative osteology of the fossorial frogs of the genus Synapturanus (Anura, Microhylidae) with the description of three new species from the Eastern Guiana Shield

“…The fusion of the scapula and the coracoid have been reported in several lineages of frogs. Notably, in the fossorial Hemisus (Engelkes et al, 2020) and in the semifossorial Hamptophryne (de Sá & Trueb, 1991) and may provide a strong, fixed arch against which the muscles of the shoulder and arm brace to enable digging. These characteristics are seemingly accompanied by a reduction of the size of phalanges, more developed fringes on the fingers, smaller eyes and an increase in body size, altogether suggesting an overall increase of the fossorial habits of these species (Emerson, 1976;Mendoza et al, 2020;Thomas et al, 2020).…”

Functional Analysis of Anuran Pelvic and Thigh Anatomy Using Musculoskeletal Modelling of Phlyctimantis maculatus

“…Further, the present study highlights the impact that femoral angle can have on moment arm magnitude of thigh muscles. note also that muscle function changed due to hindlimb configuration, Engelkes et al (2020) also show that humerus position impacts muscle moment arms in the pectoral girdle. It is unlikely that the full range of limb configurations was explored during the excitation study.…”

Diversity and function of the fused anuran radioulna

“…In FEBio, we converted each STL mesh into a 3D tetrahedral mesh (average element volume = 2.69E‐7 cm 3 ). We modeled the material properties of the meshes based on previous work on amphibian finite element models: isotropic elastic material with a Young's modulus of 10 GPa and a Poisson's ratio of 0.35 (Engelkes et al, 2020 ). We tested three different loading conditions for each of our 10 models: long‐axis, landing (from a jump), and amplexus (a mating behavior in frogs in which males grasp a female's body with their forearms).…”

“…It is then possible to compute the performance of the 3D mesh under specified loading conditions. FEA is well established as a way to model biomechanical systems in vertebrate animals, including amphibians (Bates et al, 2021 ; Engelkes et al, 2020 ; Gilbert et al, 2016 ; Mielke & Nyakatura, 2019 ; Porro et al, 2013 ).…”