Neoteny and the thyroid in ratites.

The ratites (for example ostriches, emus) are neotenous descendants of flying birds. The best studied cases of neoteny among vertebrates are in the Amphibia. In this class, whether individuals metamorphose and breed as adults, or whether they become sexually mature as neotenous aquatic larvae, is controlled by the thyroid. In this review it is argued that the thyroid may have been important in the evolution of ratite neoteny. The evidence is based on characteristics of ratites that could indicate thyroid abnormality, similarities between ratites and thyroidectomized non-ratite birds, and preliminary results from a study of thyroid function in ostriches.