Biomechanical Properties of Long Bones and Degrees of Morphological Integration Between the Fore and Hindlimbs in Anuran Species With Different Habitat Uses

The mechanical loads from muscle contraction and gravity affect the biomechanical properties of long-bone limbs, varying according to the functional demands of each limb. In anurans, both limbs are used for locomotion, but the hindlimbs generate higher energy for jumping or swimming, and the forelimbs serve additional purposes (e.g., landing, amplexus, feeding, etc). This study examines the bone architecture of the forelimb bones (humerus and radioulna) and the hindlimb bones (femur, tibiafibula, tibiale, and fibulare) of 24 anuran species with different habitat uses within a phylogenetic context. Also, because of functional divergence among limbs, we investigate possible divergence in morphological integration among long bones depending on habitat use. Across all species, forelimb bones show significantly higher bone biomechanical properties values than hindlimbs, with aquatic and semiaquatic species exhibiting the most resistant bones to bending and fracture. The femur and tibiafibula of aquatic, semiaquatic, and terrestrial species showed similar and higher values, while arboreal species had the lowest values. The tibiale and fibulare bones show a unique stratified pattern across habitats, and in most species, these bones have higher values than the femur and tibiafibula. Although morphological integration varies across habitats-with terrestrial species showing the highest and aquatic and arboreal species the lowest, reflecting differences in limb specialization-the tibiale and fibulare uniquely exhibit significant covariation across all species. While phylogenetic factors may contribute to the observed variability, ecological factors play a crucial role in shaping bone geometry, highlighting the evolutionary adaptations of long bone resistance across ecological niches.