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 elastic layer that separates the dermis from the subcutis and is responsible for returning the stretched skin to its resting position. The comparison of avian and reptilian skin morphology illustrates an example of mosaic evolution and allows the reconstruction of the evolutionary history of the avian integument from that of a lizard‐like ancestor with imbricating scales. Funding: LSU Foundation to DGH Grant Funding Source : N.A.