Forelimb bone curvature in terrestrial and arboreal mammals.

Keith Henderson,Jess Pantinople,Nick Milne,Hazel L Richards,Kyle Mccabe

doi:10.7717/peerj.3229

Abstract

It has recently been proposed that the caudal curvature (concave caudal side) observed in the radioulna of terrestrial quadrupeds is an adaptation to the habitual action of the triceps muscle which causes cranial bending strains (compression on cranial side). The caudal curvature is proposed to be adaptive because longitudinal loading induces caudal bending strains (increased compression on the caudal side), and these opposing bending strains counteract each other leaving the radioulna less strained. If this is true for terrestrial quadrupeds, where triceps is required for habitual elbow extension, then we might expect that in arboreal species, where brachialis is habitually required to maintain elbow flexion, the radioulna should instead be cranially curved. This study measures sagittal curvature of the ulna in a range of terrestrial and arboreal primates and marsupials, and finds that their ulnae are curved in opposite directions in these two locomotor categories. This study also examines sagittal curvature in the humerus in the same species, and finds differences that can be attributed to similar adaptations: the bone is curved to counter the habitual muscle action required by the animal’s lifestyle, the difference being mainly in the distal part of the humerus, where arboreal animals tend have a cranial concavity, thought to be in response the carpal and digital muscles that pull cranially on the distal humerus.

Highlights

The presence of curvature in mammalian limb bones appears biomechanically paradoxical, and so has been the subject of research and discussion for over 50 years
This study has demonstrated that the curvature of the ulna and humerus is qualitatively different in arboreal and terrestrial species
Arboreal and terrestrial lifestyles place quite different demands on the forelimb bones, and this is clearly reflected in their curvature

Summary

Introduction

The presence of curvature in mammalian limb bones appears biomechanically paradoxical, and so has been the subject of research and discussion for over 50 years. The most-studied curved bone is the radius of obligate quadruped species like sheep, goats and horses (Bertram & Biewener, 1992; Lanyon, Magee & Baggott, 1979). These bones have a concavity on the caudal side (caudal curvature) and strain gauge studies have demonstrated (Lanyon & Baggott, 1976; Lanyon, Magee & Baggott, 1979) that during stance phase the radius is subjected to strains that increase the existing curvature (caudal bending). It has been hypothesised that bone curvature exists to accommodate bulky musculature (Lanyon, 1980; Swartz, 1990), that it increases bending strains to stimulate remodelling and improve bone strength (Lanyon, 1980), or it may provide an early warning if bones approach their loading limits (Currey, 1984). Bertram & Biewener (1988) suggested that curved bones benefit from the predictable bending that results from curvature, but these authors did not provide examples of how soft tissue mechanisms might mitigate that predictable bending. Swartz (1990, p. 496) commented ‘‘. . . it is necessary to develop a way in which the concept of predictability can be made more explicitly operational’’

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Conclusion