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

Abstract

Anuran limbs offer a valuable model for exploring how selective pressures, developmental constraints, and morphological evolution interact. While previous research has largely focused on total limb length, little is known about ontogenetic changes in the proportions of individual limb bones. This study introduces a novel approach using morphofunctional indices to investigate the functional roles of frog limbs, linking morphology to habitat use and locomotion. We present morphological and ontogenetic data from anurans in northwestern Argentina, emphasizing functionally relevant bone indices. Measurements were taken of 18 long bone dimensions (humerus, radioulna, femur, tibiofibula, tarsals) across 79 specimens at metamorphic, juvenile, and adult stages. Eight morphofunctional indices were calculated, including five newly proposed ones. Indices such as radioulna robustness, groove position, femur and tibiofibula robustness, and tarsal fusion differed among ecological and locomotor groups. Terrestrial hopper-walkers such as Rhinella arenarum and Rhinella spinulosa have more robust hind limb bones and shorter, less-fused tarsals than jumpers like Boana riojana. These traits may be functionally associated with short-distance jumping and shock absorption. We also observed a slenderness–robustness gradient across species, reflecting locomotor strategies. Stylopodial elements show negative allometry, while zeugopodial elements exhibit positive allometry, indicating increased precision and flexibility in limb function throughout development.