Comparative functional morphology of the avian integument: Implications for the evolutionary history of feathers

The integuments of a turkey (Meleagris gallopovo) and an Amazon parrot (Amazona ventralis) differ in the arrangement of their feather muscles, which raise or depress contour feathers; the relative size of their feather tracts and apteria; and the type, distribution, and muscle attachments of their downy feathers. Common aspects, however, include smooth dermal feather and apterial muscles; dermal and subcutaneous fat layers that serve as a hydraulic skeleton for the feather follicles; and an elastic membrane that separates the fat layers and is responsible for returning the raised feathers to their resting position. Similarly, smooth dermal muscles, dermo-subcutaneous fat tissue, and elastic membranes have been described for the integument of squamate reptiles that have imbricating scales. In contrast, the integument of crocodilian reptiles, which have plate-like, non-imbricating scales like dinosaurs, has lost the smooth dermal muscles and has reduced its dermo-subcutaneous fat, but has retained an elasti...