Book review – On the Wing: Insects, Pterosaurs, Birds, Bats and the Evolution of Animal Flight

8-minute read
keywords: biomechanics, evolutionary biology, paleontology

Flight fascinates me for two reasons: one is pure envy at being earthbound, and the other because it is a fantastic example of convergent evolution, having evolved not once, but on four separate occasions. Last year I was sent Lev Parikian’s book Taking Flight and in finally reviewing that, I took the opportunity to also read David E. Alexander’s 2015 book On the Wing. A very accessible popular science book that tells the intriguing story of the evolution of flight, it helpfully assumes little background knowledge of either evolution or biomechanics. This, then, is the first of a two-part review of how life got airborne.

On the Wing: Insects, Pterosaurs, Birds, Bats and the Evolution of Animal Flight, written by David E. Alexander, published by Oxford University Press in November 2015 (hardback, 210 pages)

Alexander is (or was) a researcher at the Department of Ecology & Evolutionary Biology, The University of Kansas, investigating the biomechanics of flight and arthropod swimming behaviour. Previously, he wrote a 2002 monograph on the biomechanics of flight, and a 2009 popular book that explored the differences between flying animals and flying machines. I struggle to find any publications after 2015, and his academic staff page mentions he has retired. If so, he left on a high note.

On the Wing breaks down into two parts, discussing flying and flyers. The first three chapters discuss basics, wing anatomy, and wing mechanics. Alexander focuses on powered flight (i.e. flapping, though gliding is not entirely ignored) and discusses the anatomy of not just wings, but the sensory organs and nervous system that enable flight. The section on mechanics covers a whole bunch of physics (including lift, drag, thrust, and scale effects) and the biomechanics of both gliding and flapping. Admirably, he manages to clarify all this without presenting equations or getting lost in the aerodynamical weeds. Basically, he never forgets that his goal is not to give you a full treatment of the biology and physics of flight (which he already did in 2002) but to give you the relevant basics before turning to the question of how it all evolved.

The largest part of the book is devoted to the flyers: insects, birds, bats, and pterosaurs. This core of four chapters is introduced by one chapter on gliding and concluded by an interesting short chapter on the secondary loss of flight. Alexander discusses fossil evidence, family trees, physiological and sensory adaptations, and the history of different ideas on the evolution of flight in each of these groups.

“his goal is not to give you a full treatment of the biology and physics of flight […] but to give you the relevant basics before turning to the question of how it all evolved.”

The big picture looks something like this. Insects are in a class of their own: the period during which they evolved flight is lost in a large gap in the fossil record, they uniquely did not transform limbs into wings (though what their wings evolved from remains debated), and their small size and standard pattern of four wings means the biomechanics of insect flight differs markedly from the three larger vertebrate groups. For birds, the fossil record is fantastic and now extends well beyond the iconic bird ancestor Archaeopteryx, with especially Chinese fossil beds recently adding much to the story. The debate on whether flight started from the ground up or from the trees down increasingly favours the latter; Alexander prefers it because physics (i.e. gravity) works with you rather than against you. For both bats and pterosaurs early transitional fossils are missing, complicating the story. The consensus is that bats evolved from arboreal, gliding ancestors. Traditionally, the same was said to be true of pterosaurs, with scientists at some point considering—but ultimately abandoning—the idea that flight evolved from bipedal runners taking off from the ground. This is one of the few areas where the book is somewhat outdated: Alexander oddly does not mention Mike Habib and Jim Cunningham’s idea that pterosaurs could have launched themselves from a quadrupedal posture, using their forearms to catapult themselves into the air from a standing start. Mark Witton’s Pterosaurs, published two years earlier in 2013, reviewed the idea (originally published in a 2008 paper), but it seems Alexander missed it or did not read it in time before finishing his book.

Having outlined the book’s framework, three things stood out to me. First is the book’s clear structure, using headings and subheadings to divide chapters into bite-sized sections. Alexander writes factually, keeps rhetorical flourishes to a minimum, and includes some excellent explanations. Would it not be awesome to evolve propellers or use super-light materials for your wings? Sure, but that is just not how biology works, historical constraints are king: “my ancestry limits how much natural selection can change my lineage” (p. 22). Evolution makes do with the materials and structures an animal already possesses, and readily repurposes them. He cleverly cordons off in boxes explainers that are self-evident to biologists or engineers but are included here to benefit readers from outside those fields. For instance, how can complex structures evolve gradually? What is the Reynolds number? What is the right terminology for pterosaurs or immature insects? And there is a very insightful piece on convergences (similar structures that evolved independently) and homologies (similar structures inherited from a common ancestor). For functional biologists who want to understand why different animals work in similar ways, convergences are of interest while homologies have little explanatory power; for phylogeneticists making family trees, convergences are a source of noise that imply shared ancestry where there is none while homologies are their bread and butter.

“Alexander clarifies the relevance of phylogenetics. […] More than anyone I have read so far, he shows you the biological relevance of different evolutionary scenarios, most clearly illustrating this with bats.”

Second, Alexander clarifies the relevance of phylogenetics. Every family tree is a hypothesis, the best estimate of patterns of relationship and descent given your dataset. It is quite common that, having done all your complex analyses, you end up with multiple possible trees. More than anyone I have read so far, he shows you the biological relevance of different evolutionary scenarios, most clearly illustrating this with bats. Traditionally, these were split between megabats (family Pteropodidae which includes flying foxes and fruit bats) and microbats (everyone else) that both descended from a common flying ancestor. Then, in a 1989 paper, one group of researchers argued that, based on certain vision-related features, megabats were close relatives of primates and colugos (flying lemurs). If true, that meant flight in bats would have evolved twice independently. The deadlock was broken by molecular phylogenies analysing DNA that supported a single origin for bats. Those vision-related features thus evolved convergently in bats and primates; remember what I wrote above about convergences being the bane of phylogeneticists? However, the molecular analyses also broke up the traditional split, with some microbats more closely related to macrobats. Groups were reorganised and renamed to (ultimately) Yangochiroptera (Pteropodidae plus some former microbats) and Yinpterochiroptera (the remaining former microbats). Great, but that had other consequences in turn: Pteropodidae do not echolocate whereas all other bats do. So, was echolocation ancestral to all bats and lost secondarily in Pteropodidae? Or did it evolve independently in both groups in another case of convergent evolution? This matter remains unresolved for now.

Third, Alexander nicely illustrates the history of science, charting how theories have developed and changed over time. The history of thinking on Archaeopteryx is convoluted but interesting. When Danish artist Gerhard Heilmann in the 1920s pointed out we have no fossil evidence of a wishbone in Archaeopteryx and concluded that birds did not descend from dinosaurs, it sent researchers on a decades-long wild goose chase to locate the common ancestor of birds and dinosaurs further back in time. It was not until John Ostrom in the 1980s linked Archaeopteryx to small bipedal theropods, and we found better fossils including wishbones after all, that birds were welcomed back into the dinosaur clade. This, in turn, misdirected research on flight evolution for some time. If related to bipedal theropods, did flight evolve from the ground up from a running ancestor? Alexander puts it plainly: what researchers overlooked was that “Archaeopteryx wasn’t in the process of evolving flight, it was already a flyer […] Archaeopteryx is very important for unraveling the phylogenetic relationships of birds and their relatives [but] it may not tell us much about the beginnings of flight in birds” (p. 109).

Overall, On the Wing synthesizes research from diverse areas but remains accessible throughout, avoiding equations and very technical descriptions. Except for the recent developments on pterosaur take-off behaviour mentioned above, my limited background reading on the topic suggests the book still holds up well. I recommend it for those looking for an enjoyable, factual primer on the evolution of flight. Next up I review Lev Parikian’s recent Taking Flight, with an eye towards both his take on the subject and recent developments since publication of On the Wing.

On the Wing