Allometry of the pectoral flight muscles in birds: Flight style is related to variability in the mass of the supracoracoideus muscle

Abstract

AbstractAvian flight is powered by a downstroke force generated by the pectoralis (P) muscle and an upstroke force generated by the supracoracoideus (SC) muscle. Some taxa, notably penguins and hummingbirds, have very low P:SC ratios associated with larger masses of the supracoracoideus. Allometric relationships of these muscles are not reported relative to body mass and presumably the two muscles will exhibit isometry in volant birds. This study collated muscle masses for over 600 species representing 28 orders of birds and used phylogenetically controlled analysis of covariance to test for the effects of taxonomic grouping on the allometry of the flight muscles in birds. The P:SC ratio and the standardized residuals for the allometric relationships, were calculated for different orders. The pectoralis scaled isometrically with body mass but the supracoracoideus muscle exhibited a slight but significant negative allometry. Both muscles showed significant effects of taxonomic group but this effect was magnified for the supracoracoideus. Wing‐propelled divers and hovering hummingbirds had disproportionately high proportions of supracoracoideus muscles but tinamous, gamebirds and pigeons, which exhibit burst flight, also had low P:SC ratios. Many taxonomic groups had a ratio around 10:1 but values of 20:1, recorded for birds of prey and owls, were associated with relatively small masses of the supracoracoideus muscle. The P:SC ratio seems to reflect differing flight styles with the greater variability being more associated with the size of the supracoracoideus, the significance of which has not necessarily been fully appreciated in the morphology and flight action of birds.