The evolution of vertebrate flight

Abstract

Flight-defined as the ability to produce useful aerodynamic forces by flapping the wings-is one of the most striking adaptations in vertebrates. Its origin has been surrounded by considerable controversy, due in part to terminological inconsistencies, in part to phylogenetic uncertainty over the sister groups and relationships of birds, bats and pterosaurs, and in part to disagreement over the interpretation of the available fossil evidence and over the relative importance of morphological, mechanical and ecological specializations. Study of the correlation between functional morphology and mechanics in contemporary birds and bats, and in particular of the aerodynamics of flapping wings, clarifies the mechanical changes needed in the course of the evolution of flight. This strongly favours a gliding origin of tetrapod flight, and on mechanical and ecological grounds the alternative cursorial and fluttering hypotheses (neither of which is at present well-defined) may be discounted. The argumrnt is particularly strong in bats, but weaker in birds owing to apparent inconsistencies with the fossil evidence. However, study of the fossils of the Jurassic theropod dinosaur Archaeopteryx, the sister-group of the stem-group proto-birds, supports this view. Its morphology indicates adaptation for flapping flight at the moderately high speeds which would be associated with gliding, but not for the slow speeds which would be required for incipient flight in a running cursor, where the wingbeat is aerodynamically and kinematically considerably more complex. Slow flight in birds and bats is a more derived condition, and vertebrate flapping flight apparently evolved through a gliding stage.

The Evolutionary Physiology of Animal Flight: Paleobiological and Present Perspectives

“…Most scenarios of vertebrate flight evolution presume jumping takeoffs (directed either with or against gravity), whereas jump-initiated gliding origins of flight are biomechanically more parsimonious than "ground-up" hypotheses for both volant vertebrates and insects (60,61,(186)(187)(188). Jumping via a startle response is widespread among animals (189), and one potential commonality among volant vertebrates and pterygote insects is acquisition of active flight via the pathway of jumping and subsequent gliding to escape predation.…”

Gravity-defying Behaviors: Identifying Models for Protoaves

“…Implications inferred from cladistic analyses notwithstanding, the cursorial model is untenable on mechanistic, energetic, and ecological grounds (Norberg, 1990;Rayner, 1985aRayner, , 1988. The greatest constraint on a cursorial origin of flight is the inability of small terrestrial organisms to run fast enough and jump high enough to glide in a way that could have evolved into flapping, powered flight.…”

“…Even if this hypothetical cursorial avian ancestor could run at a speed necessary to initiate flight, the immediate loss of power resulting from a leap would slow it down below the required threshold velocity. Therefore, the low forward speeds of a fluttering protobird during these initial aerial forays would have required a hovering-type of wingbeat, the most energetically and aerodynamically complex and demanding form of flight (Rayner, 1988).…”

Phenotypic flexibility in migrating bats: seasonal variation in body composition, organ sizes and fatty acid profiles

“…Migratory birds may serve as a better group for comparison. Bats and birds are both vertebrate endotherms that have evolved the capacity for true powered flight (Rayner, 1988;Maina, 2000), and thus may have converged on similar solutions to some of the physiological challenges of migration. If bat migration is indeed more physiologically demanding than normal foraging flight, we predicted that we would observe phenotypic changes similar to those previously shown in migratory birds.…”