The relative timing of the origin of flight and endothermy: evidence from the comparative biology of birds and mammals.

Evidence from the comparative biology of living birds and mammals is used to address the question 'which came first, flight or endothermy?'. Birds and mammals have evolved different solutions to the problems of high energy flow demanded by endothermy. The heavy apparatus needed for processing food to allow the rapid assimilation of energy is housed in the head of mammals, but low down in the bird's body. The primitive inefficient tidal-flow system of ventilation is simply enlarged in mammals, but is replaced in birds by a lighter uni-flow system through air sacs and parabronchi. Birds avoid the weight problems associated with the mammalian systems of viviparity and lactation by nourishing their young with large quantities of yolk within the egg and an unprocessed diet after hatching. The apparent adaptedness for flight of the avian systems suggests that in the animals ancestral to birds the adaptations for high energy flow were constrained from the start by the need for acrodynamic stability, i.e. flight was initiated before endothermy, The implications of this conclusion for the origin of flight and feathers are discussed.