Long-bone microanatomy in elephants: microstructural insights into gigantic beasts

TL;DR

This study explores the microanatomy of elephant long bones, revealing adaptations for weight support and similarities to sauropods, highlighting unique structural features related to their massive size and columnar posture.

Contribution

It provides the first detailed microanatomical analysis of elephant limb bones, comparing them to other heavy mammals and sauropods to understand structural adaptations.

Findings

Elephants have thick cortex and trabecular bone filling medullary cavities.

Trabecular orientation reflects mechanical load distribution.

Elephant bones are less robust than expected for their size, similar to sauropods.

Abstract

One of the greatest challenges of terrestrial locomotion is resisting gravity. The morphological adaptive features of the limb long-bones of extant elephants, the heaviest living terrestrial animals, have previously been highlighted; however, their bone microanatomy remains largely unexplored. Here we investigate the microanatomy of the six limb long-bones in Elephas maximus and Loxodonta africana, using comparisons of virtual slices as well as robustness analyses, to understand how they were adapted to heavy weight-bearing. We find that the long bones of elephant limbs display a relatively thick cortex and a medullary area almost entirely filled with trabecular bone. This trabecular bone is highly anisotropic with trabecular orientations reflecting the mechanical load distribution along the limb. The respective functional roles of the bones are reflected in their microanatomy through variations of cortical thickness distribution and main orientation of the trabeculae. We find microanatomical adaptations to heavy weight support that are common to other heavy mammals. Despite these shared characteristics, the long bones of elephants are closer to those of sauropods due to their shared columnar posture, which allows a relaxation of morphofunctional constraints, and thus relatively less robust bones with a thinner cortex than would be expected in such massive animals.