Ontogenetic changes of hindlimb muscle mass in Cabot's tragopan (Galliformes, Phasianidae) and their functional implications

Abstract

Cabot's tragopan (Tragopan caboti) is a vulnerable species, endemic to south-east China. It usually nests in trees and is more arboreal than other pheasants, but the myological features related to its arboreal habits are not well known. In the present study, 10 carcasses of this pheasant including hatchling chicks, juveniles, and adults, were dissected to obtain measurements of leg muscle mass, which is an important determinant of force-generation capacity. The results showed that isometry prevailed for growth in muscle mass. Scaling patterns of individual muscles were presumed to correlate with the more arboreal habits of the species. Comparison of muscle mass distribution across age groups demonstrated a distal to proximal gradient in muscle development. A higher percentage of hip and thigh muscles in the adult should be favorable for the birds to maintain an upright standing posture, and to increase speed by means of additional use of femoral retraction. Knee extensors were found to be the most massive among eight functional groups, suggesting that they have a very important role during terrestrial movement. Greater relative mass of digital flexors in hatchling chicks is correlated with breeding ecology, further revealing the importance of grasping ability in the early stages of postnatal development.