The peculiar syrinx of Rhea americana (Greater Rhea, Palaeognathae)

This work studied the skeletal and muscular syringeal anatomy of Greater Rhea (Rhea americana) throughout postnatal ontogeny, by using muscle staining and differential coloring of cartilage and bone techniques. Anatomical syrinx dissections on four adults (one female and three males) and eight unsexed chicks, were made. The type of the syrinx was tracheobronchial and it was entirely cartilaginous in chicks and in the adult female but showed a partially cartilaginous and osseous pessulus in male adults. A pair of intrinsic muscles were found and the extrinsic muscles were represented by the muscles sternotrachealis and tracheolateralis, and a broad dorsal medial muscular band. The syrinx of Greater Rhea was notable for having a more complex morphology than other Paleognathae birds. Future studies on how Rhea produces vocalizations will allow the comparison with other birds, and contribute to the understanding of the evolution of soundproduction mechanisms in birds.