Limb Morphology in the Macaroni Penguin (Eudyptes chrysolophus) Exhibits Adaptations for Both Sub‐aqueous Flying and Locomotion on Land

Abstract

Requiring both wing‐propelled swimming and rugged overland locomotion, Macaroni penguins possess highly specialized morphology in both limbs. We dissected both the wing and hindlimb of one female and one male macaroni penguin. The distal wing exhibits very reduced musculature, and only retains very small brachialis and dorsal ulno‐metacarpal muscles. The elbow and metacarpal joints exhibit significantly decreased mobility compared to other birds. Proximal extrinsic wing muscles extend long tendons into the distal wing to control the dorso‐ventrally flattened flipper. Two sesamoid bones at the elbow joint provide a main attachment point for the proximal musculature, and probably contribute to the stiffening of the wing during underwater propulsion. The enlarged and extended sternum houses an extensive pectoralis muscle, responsible for the downstroke, which comprises nearly the entire ventral body wall. Supracoracoideus, found anatomically deep to pectoralis major and responsible for the upstroke, weighed about half of the pectoralis muscle, unlike most birds, where supracoracoideus weighs only about one‐fifth. An enlarged supracoracoideus increases the strength of the upstroke against viscous water. The tendons of latissimus dorsi cranialis and caudalis pass through a fibrous trochlea extending from the triceps complex to attach to the humerus, and possibly increases wing forces when moving the wing caudally. Unlike the sparse wing musculature, extensive leg musculature comprises about half of the dorsal body wall. The sartorius muscle, the most cranial hindlimb muscle, lies cranial to the caudal edge of the scapula, and cranial to the caudal margin of the sternum. Despite this, penguins exhibit restricted mobility during locomotion on land including apparent reduced range of motion in the knee joint, which contributes to their signature waddle. In addition, both Macaroni penguin specimens possess a unique muscle we have named the adductor tibialis. This muscle extends from the tibia of one hindlimb, across the midline where it slightly connects to the sternum, and attaches to the tibia of the contralateral limb, effectively tying the hindlimbs together. This permanent adduction probably helps create a streamlined figure while swimming to create faster speeds. In contrast to the knee, the penguin can greatly extend and flex the tarsus during locomotion. The considerable extensor system in the tarsus helps the penguin cross different terrains while walking, as well as give them the ability to jump.