The Physics of Leaping Animals and the Evolution of Preflight

After careful examination of the physical demands of pre-powered flight, we agree with Ostrom's (1974) arguments that Pro-avis developed the capabilities for powered flight from a bipedal cursorial habit. It is, however, difficult for us to envision how a structure such as an insect net could have developed lift, thrust, and drag capacities sophisticated enough for powered flight. We propose that a running and jumping bipedal animal that used its forelimbs for balance could be the precursor of animals with powered flight. The evolution of the modern avian wing is best explained as having passed through a series of adaptive stages concerned with the body orientation of a jumping insectivore. Extended forelimbs possessing minute increments of lift offer significant advantages to this jumping animal. Mass movements used for control of the body during a jump are very similar to the aerodynamic movements for control and propulsion used during powered flights. Thus, there is no need for the development of new and different neuromuscular pathways to perform powered flight as must be the case if an arboreal glider is to make the transition. Our calculations predict that a compact, rigid, lightweight body would be most responsive to distally developed lifting surfaces. Our model predicts that lift and thrust developed distally prior to the medial development of lifting surfaces. Primary feathers, therefore, developed before secondary feathers. Our model also predicts that the rate of evolution to powered flight was rapid. Our calculations can also be used as an argument against an arboreal, gliding precursor to powered flight. Clearly, an arboreal pathway to modern flying birds would have to commence with a quadrupedal, planar animal using stable landing mechanisms. This hypothetical Pro-avis would then have to make the following morphological changes: (1) increase its aspect ratio; (2) change to a metastable bipedal type of landing; (3) lose its planarity; (4) thicken dorsoventrally. Greenewalt's (1960, 1962) conclusions suggest a short-limbed glider would have to develop an extremely rapid wing beat to develop significant lift. This lift would need to be generated close to the animal's center of mass. No modern gliding animal can even approach the conditions necessary for flapping flight. It is highly unlikey that an ancestral glider could do likewise. Thus, gliding animals do not fit a plausible model (based on physical conditions and characteristics) to explain the evolutionary pathway leading to powered flight. In fact, it is highly probable that use of an arboreal habitat was one of the last (and most recent) adaptive zones to be occupied by birds. Our jumping insectivore model can also be used to explain the evolutionary pathway to flight in any group of terrestrial vertebrates with certain physical and energetic prerequisites (fig. 7).