The effects of locomotion on the structural characteristics of avian limb bones

Abstract

Despite the wide range of locomotor adaptations in birds, little detailed attention has been given to the relationships between the quantitative structural characteristics of avian limb bones and bird behaviour. Possible differences in forelimb relative to hindlimb strength across species have been especially neglected. We generated cross-sectional, geometric data from peripheral quantitative computed tomography scans of the humerus and femur of 127 avian skeletons, representing 15 species of extant birds in 13 families. The sample includes terrestrial runners, arboreal perchers, hindlimb-propelled divers, forelimb-propelled divers and dynamic soarers. The hindlimb-propelled diving class includes a recently flightless island form. Our results demonstrate that locomotor dynamics can be differentiated in most cases based on cross-sectional properties, and that structural proportions are often more informative than bone length proportions for determining behaviour and locomotion. Recently flightless forms, for example, are more easily distinguished using structural ratios than using length ratios. A proper phylogenetic context is important for correctly interpreting structural characteristics, especially for recently flightless forms. Some of the most extreme adaptations to mechanical loading are seen in aquatic forms. Penguins have forelimbs adapted to very high loads. Aquatic species differ from non-aquatic species on the basis of relative cortical thickness. The combination of bone structural strength and relative cortical area of the humerus successfully differentiates all of our locomotor groups. The methods used in this study are highly applicable to fossil taxa, for which morphology is known but behaviour is not. The use of bone structural characteristics is particularly useful in palaeontology not only because it generates strong signals for many locomotor guilds, but also because analysing such traits does not require knowledge of body mass, which can be difficult to estimate reliably for fossil taxa.

Blood flow for bone remodelling correlates with locomotion in living and extinct birds

“…Natural selection on flight energetics may favour loss of hind-limb mass at the risk of reducing the safety margin for catastrophic failure. Consequently, volant birds have lighter femora in response to an extreme reduction in hind-limb loading (Habib and Ruff, 2008). Primarily cursorial birds are terrestrial organisms and so their bones are built strong enough to support their weight and withstand mechanical stresses during locomotion (McMahon, 1973).…”

Hind limb morphometry of terror birds (Aves, Cariamiformes, Phorusrhacidae): functional implications for substrate preferences and locomotor lifestyle

“…However, the actual information on locomotor habits that this can provide has been seriously questioned, particularly in the case of Neornithes. Several studies indicate that the ratios between the leg elements, or the hind limb total length, are not in themselves indicative of the type of locomotion (Gatesy & Middleton 1997;Zeffer et al 2003;Habib & Ruff 2008). Hind limb proportions differ between bird species: the lengths of the femur, tibiotarsus and tarsometatarsus are proportionally different (Abourachid & Hö fling 2012).…”

“…Although the basic functions of bird legs are always landing, taking-off and walking (Habib & Ruff 2008;Abourachid & Höfling 2012), between these extremes all birds have a wide variety of locomotor habits and occupy a wide variety of environments. Locomotor behaviour can also change according to the animal's activity.…”

Finite element modelling versus classic beam theory: comparing methods for stress estimation in a morphologically diverse sample of vertebrate long bones

“…The extent to which finite element models deviate from simple beam predictions of compression is not constant, but is instead correlated with normalized curvature lever arm (z). This precludes the application of a single correction factor to account for shaft curvature and implies a measure of curvature must be incorporated into simple beam equations and calculations of 'relative strength' values, where previous studies have not done so [1,5,16,40].…”

“…In many instances, relative values of such properties as cortical cross-sectional area (A cort ), second moment of area (I ) and polar moment of area (J ) are used in comparative studies, from which skeletal posture and in vivo function may be inferred [1][2][3][4]. However, in other instances, diaphyseal cross-sectional properties are incorporated into equations to predict actual skeletal stress.…”