Testing the use/disuse hypothesis: pectoral and leg muscle changes in captive barnacle geese Branta leucopsis during wing moult.

Previous studies on wild moulting waterfowl have demonstrated that flight and leg muscles experience periods of hypertrophy and atrophy. This is thought to be in response to the change in use of the locomotor muscles as described in the use/disuse hypothesis. We tested this hypothesis using captive barnacle geese. Forty geese were dissected before, during and after wing moult, to determine the changes in mass and functional capacity of the flight and leg muscles. Physiological cross sectional areas (PCSA) and mean fascicle lengths of functional muscle groups were calculated to ascertain the force-producing capabilities of the flight and leg muscles. At the onset of moult, flight muscle mass was at a minimum, having atrophied by 35% compared with pre-moult levels, but it returned to pre-moult levels by the end of wing moult. By contrast, the leg muscles hypertrophied during wing moult by 29%, and the PCSA of individual muscle groups increased substantially. Increases in mass, PCSA and fascicle length of individual leg muscle groups during moult suggest that, when flightless, the leg muscles are functionally adapted to provide greater force and/or manoeuvrability to the birds, to aid ground-based escape from predators. Through studying captive animals that are unable to fly, it has been possible to conclude that the major changes in leg and flight muscle in moulting captive geese cannot be explained through use or disuse. Instead, changes seem to be compensatory or to occur in anticipation of changes in locomotor patterns.