Hindlimb myology of the monk parakeet (Aves, Psittaciformes).

We studied the hindlimb myology of the monk parakeet (Myiopsitta monachus). Like all parrots, it has zygodactyl feet enabling perching, climbing, hanging, moving easily among trees, and handling food. Muscles were described and weighed, and physiological cross-sectional area (PCSA) of four flexors and one extensor was calculated. In comparison to other muscles, the M. tibialis cranialis and the M. fibularis brevis show increased development and high PCSA values, and therefore, large potential force production. Also, a large proportion of muscle mass was involved in flexing the digits. We hypothesize that these muscle traits are associated with the arboreal locomotion and food manipulation habits. In the monk parakeet, the M. extensor digitorum longus sends a branch to the hallux, and the connection between the M. flexor digitorum longus and the M. flexor hallucis longus is type I (Gadow's classification). We reaffirm the presence of the M. ambiens as a plesiomorphic condition that disappears in most members of the order. Among Psittaciformes, the M. fibularis brevis is stronger and the M. fibularis weaker in arboreal species than in basal terrestrial ones (e.g., Strigops).