Whole‐bone scaling of the avian pelvic limb

Abstract

Birds form the largest extant group of bipedal animals and occupy a broad range of body masses, from grams to hundreds of kilograms. Additionally, birds occupy distinct niches of locomotor behaviour, from totally flightless strong runners such as the ratites (moa, kiwi, ostrich) to birds that may walk, dabble on water or fly. We apply a whole-bone approach to investigate allometric scaling trends in the pelvic limb bones (femur, tibiotarsus, tarsometatarsus) from extant and recently extinct birds of greatly different size, and compare scaling between birds in four locomotor groups; flightless, burst-flying, dabbling and flying. We also compare scaling of birds' femoral cross-sectional properties to data previously collected from cats. Scaling exponents were not significantly different between the different locomotor style groups, but elevations of the scaling relationships revealed that dabblers (ducks, geese, swans) have particularly short and slender femora compared with other birds of similar body mass. In common with cats, but less pronounced in birds, the proximal and distal extrema of the bones scaled more strongly than the diaphysis, and in larger birds the diaphysis occupied a smaller proportion of bone length than in smaller birds. Cats and birds have similar femoral cross-sectional area (CSA) for the same body mass, yet birds' bone material is located further from the bone's long axis, leading to higher second and polar moments of area and a greater inferred resistance to bending and twisting. The discrepancy in the relationship between outer diameter to CSA may underlie birds' reputation for having 'light' bones.

Scaling and functional morphology in strigiform hind limbs

“…One would expect that as the birds become larger, stress in the hind limb elements would increase proportionately and the corresponding dimensional adjustments would be evident in those elements. Previous large scale studies of bird hind limb scaling have shown that tarsometatarsus length and frequently tibiotarsus length scale against body mass with strong positive allometry, while femur length scales with isometry212223.…”

Ontogenetic scaling patterns and functional anatomy of the pelvic limb musculature in emus (Dromaius novaehollandiae)

“…Considering our results, if similar cross-sectional 404 geometry is assumed along the length of the bone shafts, this would lead to an increase in strains (at 405 least in bending, due to larger moments) at the mid-shaft with increasing body mass. However, 406 changes in cross-sectional areal geometry have been shown to lead to slight positive allometry of 407 the cross-sectional geometry of avian limb bones across species (Doube et al 2012) and 408 ontogenetically (Main & Biewener 2007 this increase in muscle masses will lead to a limb that is adapted for power production and perhaps 448 (considering our less allometric tendon results) elastic energy storage/return. The former is also 449 supported by metabolic studies which found a predominance of fast fibres in the M. gastrocnemius 450 of adult emus (Patak 1993), although more studies of muscle physiology in emus and other ratites 451 would be valuable.…”

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

“…Birds fill a diverse array of niches and rely on different modes of locomotion, which impart differential forces on their legs, and these differences can be observed in the architecture of the bone (Doube et al, 2012). Therefore, cursorial species might be expected to have relatively heavier bones than volant species, an outcome that would normalise the strain.…”