The evolutionary origin of feathers.

Previous theories relating the origin of feathers to flight or to heat conservation are considered to be inadequate. There is need for a model of feather evolution that gives attention to the function and adaptive advantage of intermediate structures. The present model attempts to reveal and to deal with, the spectrum of complex questions that must be considered. In several genera of modern lizards, scales are elongated in warm climates. It is argued that these scales act as small shields to solar radiation. Experiments are reported that tend to confirm this. Using lizards as a conceptual model, it is argued that feathers likewise arose as adaptations to intense solar radiation. Elongated scales are assumed to have subdivided into finely branched structures that produced a heat-shield, flexible as well as long and broad. Associated muscles had the function of allowing the organism fine control over rates of heat gain and loss: the specialized scales or early feathers could be moved to allow basking in cool weather or protection in hot weather. Subdivision of the scales also allowed a close fit between the elements of the insulative integument. There would have been mechanical and thermal advantages to having branches that interlocked into a pennaceous structure early in evolution, so the first feathers may have been pennaceous. A versatile insulation of movable, branched scales would have been a preadaptation for endothermy. As birds took to the air they faced cooling problems despite their insulative covering because of high convective heat loss. Short glides may have initially been advantageous in cooling an animal under heat stress, but at some point the problem may have shifted from one of heat exclusion to one of heat retention. Endothermy probably evolved in conjunction with flight. If so, it is an unnecessary assumption to postulate that the climate cooled and made endothermy advantageous. The development of feathers is complex and a model is proposed that gives attention to the fundamental problems of deriving a branched structure with a cylindrical base from an elongated scale.