Alexander begins with some of the varied features of animal flight — speed, height, distance, size, possible vertical takeoff, combination with swimming and sex and combat and eating — and a brief introduction to phylogenetics, to how we analyse relationships between different groups of organisms.
A survey of some of the similarities and differences between fliers emphasizes the constraints of biomechanics: the importance of "control systems", trade-offs in stability versus maneuverability and in speed versus range, and different approaches to wing design. It may have been for something other than flight, but "in order to evolve a wing in the face of historical constraints, natural selection must have acted on some structure that had at least slightly wing-like properties, perhaps a larger surface area or something extending out on each side of the body". Birds use modified arms and feathers, bat wings are supported by hand and finger bones, and pterosaur wings were supported by enormous fingers along with membrane stiffeners; in contrast, insect wings are not modified limbs.
Next comes an introduction to the basic mechanics of flight, looking at lift, drag and the drag-lift ratio, gliding and flapping, and the importance of scaling and size.
"Knowing how size affects the physiology and aerodynamics of modern flyers allows us to understand the flight of ancient animals. ... This 'biomechanics' approach has, in recent decades, greatly expanded our understanding of how extinct animals lived, and has helped us understand what behaviors would have been probable, possible, or impossible for these ancient animals."
Turning to gliding, Alexander looks at the lizard Draco formosus and a range of other living and extinct animals, including some that are only marginally gliders: "ants as well as several other kinds of wingless arthropods perform a rudimentary kind of gliding when they 'skydive' back to the tree trunk after being dislodged or jumping from a branch". Overall, "gliding is a sophisticated adaptation that involves changes in anatomy, sensing, behavior, and judgment". And, rather than there being a hard divide between gliding and flapping, "nothing physical or aerodynamic prevents a glider from taking advantage of minimal attempts at flapping".
Chapters on each of the groups follow, looking in turn at insects, birds, bats, and pterosaurs.
The fossil record becomes rich in winged insects starting around 318 million years ago, but so abruptly that it is not very helpful for understanding the origin of insect flight. Neopterans have evolved a wing hinge which allows the wings to be folded, some groups later lost the ability to fly, and some have effectively reduced the original two pairs of wings to one. The anatomical origins of insect wings are debated: the two most popular theories are that they derive from larval gills and that they derive from paranotal lobes, flat plates extending sideways from the top of the thorax. Alexander attempts to evaluate all of this using model tests and observations of living insects:
"The direct ancestors of flying insects were probably rather silverfish-like and at least a centimeter or so long, perhaps much bigger. They probably fed ... at the tops of stalk-like plants and routinely jumped to the ground to escape predators or to speed up travel ... They probably had good vision ... the ability to reorient and land right-side-up during a jump or fall."
Alexander gives a rapid overview of the history of bird fossils, from Archaeopteryx through to the wealth of recent fossils that has come out of China. This covers the identification of birds as dinosaurs and their physiological adaptations for flight, including the unique bird lung with its one-way airflow system. A central debate has been between arboreal ("gliding from trees") and cursorial ("running") theories for the origin of bird flight. Drawing on the latest fossils and the work of flight biomechanists, he argues that wing-assisted incline running (the use of flapping to assist running up slopes) is "much more difficult to evolve than gliding", and he is "quite satisfied with the idea of an arboreal evolution of flapping flight occurring in maniraptoran dinosaurs".
In contrast to the situation with birds, "scientists are in near-universal agreement that bats evolved from arboreal, gliding ancestors, as first proposed by Darwin". It has been suggested that microbats and megabats evolved flight separately, but modern genetic analyses confirm a single origin. Another uncertainty is the relationship between the origins of powered flight and echolocation, which are "linked by nocturnality and by a literal physical linkage between the wingbeat and call production" (which requires considerable muscle power); some have suggested they evolved simultaneously. Between them these gave bats the key to a niche not already occupied by birds, which were already well-established and diverse.
The extinction of pterosaurs and a limited fossil record leave uncertainties even over such basics as their body mass (estimates for the largest known species range from 70 to 540 kilograms) and how their wings worked and how they walked. "If the earliest pterosaurs were quadrupedal walkers rather than bipedal runners, then a cursorial origin for pterosaur flight does not seem logical. ... most researchers have returned to the view that pterosaurs must have evolved through a gliding stage".
A good few birds and insects but no bats or pterosaurs appear to have lost flight: the last two groups perhaps constrained by using all their limbs in flight. Birds have most commonly lost flight on islands, insects as ectoparasites or in unusual environments.
As unifying themes Alexander suggests an "arboreal" origin in directed aerial descent, connections with perception, nervous system modifications for control and maneuverability, and limited structural variation and diversity resulting from the demands of flight (the last less so in insects).
On the Wing is slender and unpretentious, a straightforward and accessible presentation of the science. Alexander doesn't attempt reduction to single simple answers, and doesn't let his appeals to biomechanics swamp a broad account of the origins of powered flight. There's no dramatisation or exaggeration — which would be entirely unnecessary given the intrinsic excitement of the subject. And while some background history is given, especially where it is necessary to understand competing theories, it never takes over and there's no resort to biography. For me this is science writing at its best, with fascinating subject material presented clearly and without fuss.
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- Related reviews:
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- Mark Witton - Pterosaurs: Natural History, Evolution, Anatomy
- Pat Shipman - Taking Wing: Archaeopteryx and the Evolution of Bird Flight
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