Hindlimb muscles of the Emperor Penguin Aptenodytes forsteri (Aves, Sphenisciformes) at different postnatal ontogenetic stages.

The Emperor Penguin Aptenodytes forsteri is the largest living species of penguin, found exclusively in Antarctica, and is unique in breeding during the winter. Consequently, fewer anatomical studies have been conducted on this species over time compared to others. This study aims to provide an updated and comprehensive description of the hindlimb musculature of Aptenodytes forsteri. The muscles of the thigh, shank and foot were compared along postnatal development. The results were compared with the muscles of swimming and diving birds and also with ground-dwelling taxa. By dissecting seven specimens, the origin and insertion were described in newborns, chicks and juveniles. Of the 37 muscles recognized, 19 correspond to the thigh, 14 to the shank, and four to the foot. Notably, the hindlimb muscles appear relatively developed compared to the overall body size in the newborns, showing considerable development in independent chicks, and reached full growth in the juvenile. The m. flexor hallucis brevis, m. extensor hallucis longus and m. lumbricalis were absent in all specimens, whereas the m. femorotibialis lateralis was found in only one specimen. Additionally, a branch of the m. flexor cruris medialis, originating in the abdominal area and merging with the pelvic portion before insertion, was observed in the newborn chicks. Aptenodytes forsteri showed similarities with Numida meleagris in the insertion of m. iliofibularis, and m. flexor cruris lateralis p. pelvica. These findings, along with other comparative results, constitute the first comprehensive descriptive study of the hindlimb musculature of Aptenodytes forsteri, including different ontogenetic stages.