Differing effects of size and lifestyle on bone structure in mammals

Eli Amson,Faysal Bibi

doi:10.1186/s12915-021-01016-1

Eli Amson, Faysal Bibi

Open Access

https://doi.org/10.1186/s12915-021-01016-1

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Abstract

BackgroundMammals are a highly diverse group, with body mass ranging from 2 g to 170 t, and encompassing species with terrestrial, aquatic, aerial, and subterranean lifestyles. The skeleton is involved in most aspects of vertebrate life history, but while previous macroevolutionary analyses have shown that structural, phylogenetic, and functional factors influence the gross morphology of skeletal elements, their inner structure has received comparatively little attention. Here we analysed bone structure of the humerus and mid-lumbar vertebrae across mammals and their correlations with different lifestyles and body size.ResultsWe acquired bone structure parameters in appendicular and axial elements (humerus and mid-lumbar vertebra) from 190 species across therian mammals (placentals + marsupials). Our sample captures all transitions to aerial, fully aquatic, and subterranean lifestyles in extant therian clades. We found that mammalian bone structure is highly disparate and we show that the investigated vertebral structure parameters mostly correlate with body size, but not lifestyle, while the opposite is true for humeral parameters. The latter also show a high degree of convergence among the clades that have acquired specialised (non-terrestrial) lifestyles.ConclusionsIn light of phylogenetic, size, and functional factors, the distribution of each investigated structural parameter reveals patterns explaining the construction of appendicular and axial skeletal elements in mammalian species spanning most of the extant diversity of the clade in terms of body size and lifestyle. These patterns should be further investigated with analyses focused on specific lifestyle transitions that would ideally include key fossils.

Highlights

Mammals are a highly diverse group, with body mass ranging from 2 g to 170 t, and encompassing species with terrestrial, aquatic, aerial, and subterranean lifestyles
We focused our analyses on lumbar vertebrae and the humerus, these elements being generally conserved across mammals, and having been previously associated with locomotor adaptations reflected in their gross morphology [16, 22, 23]
Mean vertebral global compactness and trabecular architecture within the vertebral body—the Connectivity and BV/TV show a strong positive correlation with size, even when lifestyle is accounted for, as indicated by phylogenetically informed ANCOVAs and phylogenetically informed regression of the parameter against body mass

Summary

Introduction

Mammals are a highly diverse group, with body mass ranging from 2 g to 170 t, and encompassing species with terrestrial, aquatic, aerial, and subterranean lifestyles. The skeleton is involved in most aspects of vertebrate life history, but while previous macroevolutionary analyses have shown that structural, phylogenetic, and functional factors influence the gross morphology of skeletal elements, their inner structure has received comparatively little attention. We analysed bone structure of the humerus and mid-lumbar vertebrae across mammals and their correlations with different lifestyles and body size. The extent to which lifestyle, phylogenetic heritage, body size, and other factors influence bone structure at a broad macroevolutionary scale is poorly understood. This study investigates bone structure and its correlates across therian mammals (placentals + marsupials). The most extreme are the aerial, fully aquatic, and subterranean lifestyles which were each acquired convergently on several independent occasions in marsupials and placentals [19,20,21]

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