Salient features in locomotor evolutionary adaptations of proboscideans revealed via the differential scaling of limb long bones

TL;DR

This study investigates limb bone scaling in proboscideans and elephants, revealing adaptations related to locomotion mechanics and highlighting differences between species and ecological influences.

Contribution

It provides a comparative analysis of limb long bone allometry in proboscideans, integrating theoretical models to explain locomotor adaptations and constraints.

Findings

Elephants show limb bone adaptations for bending and torsion during fast locomotion.

African elephants' limb bones are adapted for gravity-induced compression.

Hindlimb bones are more compliant than forelimb bones, affecting locomotion dynamics.

Abstract

The standard differential scaling of proportions in limb long bones (length against circumference) is applied to a phylogenetically wide sample of the Proboscidea, Elephantidae and the Asian (Elephas maximus) and African elephant (Loxodonta africana). In order to investigate allometric patterns in proboscideans and terrestrial mammals with parasagittal limb kinematics, the computed slopes (slenderness exponents) are compared with published values for mammals and studied within a framework of theoretical models of long bone scaling under gravity and muscle forces. Limb bone allometry in E. maximus and the Elephantidae are congruent with adaptation to bending and/or torsion induced by muscular forces during fast locomotion, as in other mammals, whereas limb bones in L. africana appear adapted for coping with the compressive forces of gravity. Consequently, hindlimb bones are expected to be more compliant than forelimb bones in accordance with in vivo studies on elephant locomotory kinetics and kinematics, and the resultant negative limb compliance gradient in extinct and extant elephants, which contrasts to other mammals, suggests an important locomotory constraint preventing achievement of a full-body aerial phase during locomotion. Differences in ecology may be responsible for the subtle differences observed between African and Asian elephant locomotion, and the more pronounced differences in allometric and mechanical patterns established in this study. Key words: long bone scaling models; standard differential scaling; limb gradient functions; proboscideans; extinct and extant elephants