The signature waddle: Myology of the appendicular skeleton of the macaroni penguin ( Eudyptes chrysolophus )

Penguins, a highly specialized group of birds that have mastered underwater flight, evolved a unique body plan dramatically divergent from volant birds. We dissected two macaroni penguins (Eudyptes chrysolophus) to define appendicular myology in comparison to their aerial relatives. Macaroni penguins have unique and specific adaptations to underwater flight including: an m. supracoracoideus that is proportionally larger than in aerial birds, an annular retinaculum through which both mm. latissimus dorsi cranialis and caudalis pass-an unexpected configuration of the m. latissimus dorsi complex-and an unusual interconnection between several muscles of the pes. We also define a muscle that originates at the sternum and attaches on the proximal tibiotarsus, with the right and left muscles meeting at a midline raphe attaching to the distal sternal keel, functionally supporting the two tibiotarsi in adduction. This muscle has been discussed in the literature for over 100 years as either a slip from m. obliquus externus abdominus or possibly as a superficial head of m. flexor cruris medialis, historically called m. semimembranosus in birds. Our dissections reveal that this muscle is anatomically distinct and may play a specialized role underwater by adducting the hindlimbs to maintain a relatively effortless streamlined silhouette, and on land, by keeping the tibiotarsi adducted thereby supporting otherwise abducted femora. Therefore, we propose that we recognize this distinct muscle with the name m. adductor tibialis.