Functional indices and ontogeny of long bones of the limbs in the frogs

Abstract

Most studies of ontogeny in anurans focus on the premetamorphic and adult stages, but very few have focused on the postmetamorphic and juvenile stages. Juvenile frogs share the same environment as adult frogs. They are easier prey for predators, are smaller, and undergo allometric changes such as relatively longer limbs, greater muscle forces and contraction velocities, and a greater muscular mechanical advantage, muscles tending to elongate proportionally to bonelength. Locomotion in anurans involves all four limbs and differences in their proportions would facilitate functional diversity. Bones suffer the effects of gravitational forces and muscle contractions. Particularly, the long bones adapt by resisting bending and twisting during locomotion. This mechanism occurs within the geometric limits imposed by phylogenetic morphogenesis. Variation in tetrapod limbs increases proximodistally, with distal bones having the least developmental constraints and thus the greatest freedom to change in response to selective pressures. In some tetrapod groups such as mammals, morphofunctional indices (ratios) have been proposed and constructed using different linear and functional variables of the long bones during ontogeny, finding a relationship between long bone dimensions and different locomotor modes. However, such information is currently lacking in anurans. In this context, our main goal was to investigate how functional indices and bone measurements of the forelimbs and hind limbs evolve throughout the postnatal development of nine anuran species exhibiting diverse locomotor modes and habitat preferences.