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Integrative and Comparative Biology
Integrative and Comparative Biology, volume 56, number 5, pp. 1044–1046
doi:10.1093/icb/icw115
Society for Integrative and Comparative Biology
Book Review
On the Wing: Insects, Pterosaurs, Birds, Bats
and the Evolution of Animal Flight
David E. Alexander. New York, NY: Oxford University Press,
2015. 210 pp. ISBN 978-0-19-999677-3 (hardcover),
$29.95
Introduction
Flying animals have captivated public attention for at
least as long as there have been written records.
Powered flapping flight is one of the most energetically costly behaviors in the animal kingdom, yet due
to the low cost of transport of flight relative to running and walking, it can be an efficient way to move
(Schmidt-Nielson 1972). Evolutionarily, flapping
flight has evolved four times: in bats, birds, pterosaurs, and insects. Many animal flight experts are
only peripherally aware of the flying taxa they do
not study; thus, it is nice to see all four groups
being treated together in the same book.
On the Wing: Insects, Pterosaurs, Birds, Bats and
the Evolution of Animal Flight, by David E.
Alexander, is composed of 10 chapters—three introductory chapters on flight and then one chapter
apiece on gliding animals, each of the four taxa
that have evolved powered flapping flight, and secondarily flightless animals, plus a conclusion. It is
aimed at the general public and does an excellent
job of explaining difficult concepts such as phylogenetics and induced drag in layman’s terms; however,
it has enough ‘‘depth’’ to be required reading for any
individual who wishes to learn more about animal
flight—whether for personal or professional reasons.
For some flight researchers, the enjoyment of this
book will hinge on the fact that it argues for a gliding origin of powered flapping flight in all four lineages. Many scientists, particularly ornithologists and
entomologists, disagree (e.g., Medved et al. 2015;
Heers et al. 2016). Alexander uses results from
studies of theoretical modeling and flapping robots
to suggest that gliding and flapping may be opposite
ends of a continuum rather than two completely
separate modes of flight. Furthermore, bats and
pterosaurs are widely accepted to have evolved
from gliding ancestors, and there have long been gliding-origin hypotheses for birds (e.g., ‘trees-down’)
and insects. With the discovery of Microraptor gui,
a gliding dromaesaurid, and increasing evidence that
flight evolved in terrestrial, not aquatic, insects,
Alexander concludes that powered flapping flight
has always evolved from gliding. He does point out
that any such argument is controversial, because all
modern animals are either pure gliders or powered
flappers (which, if large enough, also have the ability
to glide). The lack of any extant animal that flaps
just a little bit to help control its primarily gliding
behavior is a problem with this hypothesis—but, to
be fair, there are counterarguments for all of the
current hypotheses regarding the origin of powered
flapping flight. The book does present these alternatives, particularly the wing-assisted incline running
(WAIR) hypothesis for birds and the surfaceskimming hypothesis for insects, although each of
these arguments is dismissed in favor of a gliding
to flapping transition. However, if one is openminded enough to at least consider the possibility
that powered flapping flight always evolves from
gliding, this book is up-to-date and informative.
In terms of the book’s readability, members of the
general public may have to look up a few words to
thoroughly understand it; while scientific jargon involving flight is mostly avoided (‘parasite drag’) and
explained when it cannot be avoided (‘lift:drag
ratio’), a few overly scientific terms (‘extant’) do
slip through. However, someone with an undergraduate degree in science should have no trouble with
the text. Moreover, Alexander has a clear and comfortable writing style that draws the reader in and
makes them want to continue reading.
Section 1: flying, flapping, and gliding
Although the book itself is not divided into sections,
there is a natural division between the first three
chapters and the next six chapters plus the conclusion. The book starts by exploring the ‘‘wow’’ factor
of flight: what animals fly fastest, furthest and highest. Chapter 2 briefly discusses the physiology of
flight and then provides an in-depth description of
the wing structure of the four lineages with powered
flapping flight. The third chapter examines the physics of flight, including lift, drag, camber and wing
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Book Review
stroke kinematics. It also includes a section on gliding flight that sets up the argument that powered
flapping flight always evolves from gliding.
The introductory section is unique because
Alexander does not just rely on examples of one or
two flying lineages. For example, ornithologists who
know exactly which animal is going to win the ‘how
far?’ section can still be impressed by the feats of
honeybee workers making foraging flights. It is
clear from the outset that Alexander plans to discuss
all four groups of powered flapping flyers equally,
and he does an admirable job, both in the introductory section and later in the book.
Several arguments are, however, overstated in the
introductory section, such as that ‘‘air properties do
change a bit with size but are fundamentally the
same for all animals.’’ In the book’s defense, these
arguments are softened later in the book; in the
chapter on insects, for example, Alexander discusses
the wings of tiny insects and why they do not look
anything like the wings of larger insects. Another
issue with this section is that a few aspects of
Chapter 2’s discussion of muscles are inaccurate.
Lactic acid is not a harmful waste product, as we
once thought; in fact, it can actually be a substrate
for aerobic metabolism (Mann 2007). Furthermore,
both carbohydrates and fats can be broken down to
provide the molecules that the body uses for aerobic
and anaerobic metabolism. It is thus misleading to
state that aerobic metabolism depends on fat and
anaerobic metabolism relies on carbohydrates.
Chapter 3 explains how animals fly and glide.
From clear diagrams showing that animals do not
just flap their wings straight up and down (as
many students think) to a strong, understandable
argument that camber is not the be-all and end-all
of flight (thus explaining why Bernoulli’s equations
do not tell the whole story of flight), this chapter is a
home run. People who know almost nothing about
physics will be able to understand how flight works
through Alexander’s use of well-chosen metaphors
and examples. For example, readers are invited to
‘‘fly’’ their hand out of a car window the next time
they are a passenger, and change the angle of attack
to feel changes in lift and drag.
Section 2: flighted (and flightless)
animals
Section 2 contains separate chapters for gliding animals, each of the four lineages with powered flapping
flight, and flightless animals. The gliding chapter has
just a few minor issues; in particular, it is relatively
heavy on herpetofauna and light on mammals.
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Unfortunately, it also gives flying fish relatively
short shrift, citing a paper from 1995 instead of a
more recent analysis of lift:drag ratios of the pectoral
fins (wings) in these animals (Park and Choi 2010).
In terms of the chapter on insects (Chapter 5),
non-entomologists will likely wish the relevant phylogeny (Figure 5.4) was presented earlier, when the
different taxa are first referenced. It may also be difficult for non-entomologists to tell the difference between the historical ‘‘gill theory’’ hypothesis
described here and Marden’s surface-skimming hypothesis; the latter seems to be an updated version of
the former, albeit with a more plausible mechanism
for how a gill could evolve into a wing. Aside from
these two minor issues, however, this chapter is
easily one of the best discussions of insect flight
and its evolution available.
Given how controversial the evolution of bird
flight is, it is no surprise that Alexander’s chapter
on birds (Chapter 6) contains a few controversial
statements. For example, the WAIR hypothesis is
quickly dismissed, without reference to more recent
discussions (e.g., Heers and Dial 2012). Also, a key
part of the book’s argument for a gliding origin of
avian flight is that a dromaesaurid (Microraptor gui)
was a glider; according to Alexander, it is thus inappropriate to say that all theropods (including any
direct ancestors of birds) were cursorial so birds
must have evolved from cursorial ancestors. While
this is true, it is still fair to say that the vast majority
of described theropods were cursorial, making it
more likely (although not certain) that avian flight
evolved in cursorial, and not arboreal, animals. (Full
disclosure: this reviewer feels that the WAIR hypothesis best explains the evolution of avian flight.)
Chapter 7 discusses the origin of flight in bats.
Here, few scientists question a gliding origin for
flight. Several common misconceptions about bats
are corrected in this chapter, such as that
‘‘microbats’’ and ‘‘megabats’’ evolved flight (and
other features) separately. At the end of the chapter,
Alexander provides a stimulating discussion of echolocation and its interaction with flight in bats.
Unfortunately, a study published after Alexander’s
book showed that the story of echolocation and
flight in bats may not be as clear-cut as the book
describes because of the rapid expansion of bats following their initial evolution (Hahn and Nakhleh
2016).
The pterosaur chapter (Chapter 8) is fascinating,
although Alexander shows Soemmerring’s bat-like
reconstruction of a pterosaur—odd, given that, as
he argues, it has long misled scientists into thinking pterosaurs were bat-like; why perpetuate the
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image? It would perhaps have been preferable to
show images of fossils with the wing membranes
clearly preserved as well as fossils of pterosaur
trackways showing them walking on all fours.
Chosen figures aside, Alexander does an excellent
job of calling attention to areas of pterosaur flight
that need additional study, such as the enormous
range of body size within this lineage (and within
species).
The final two chapters are on secondary flightlessness and unifying themes. The first of the two chapters is (arguably) missing a short discussion of
vampire bats’ secondarily-gained ability to ‘‘run’’
(Riskin and Hermanson 2005), but is an enjoyable
read nevertheless. In the final chapter is a wellreasoned argument why bats and pterosaurs are
generally accepted to represent gliding-to-flying transitions: they look like typical gliding animals, with a
membrane stretched in between their limbs on either
side. Birds and insects, conversely, have wings that
do not look as if they arose from a gliding membrane, which is likely one reason why there is more
controversy surrounding the origin of flight in these
taxa. At the very end of this chapter, Alexander takes
the opportunity to point new (and old) animal flight
researchers in interesting and productive new
directions.
Overview
Minor quibbles aside, On the Wing: Insects,
Pterosaurs, Birds, Bats and the Evolution of Animal
Flight, is an excellent book. It is extremely well-written, and balances discussion of all four taxa like no
Book Review
other published treatise on animal flight. Moreover,
it is accessible to the general public while still providing enough depth (and appropriate references) for
anyone who wants to further examine the evolution
of flight. This book is a must-read for all graduate
students who plan to work on the evolution or biomechanics of flight, regardless of what lineage(s) they
study.
Melissa S. Bowlin
Department of Natural Sciences,
University of Michigan-Dearborn
E-mail: [email protected]
References
Hahn MW, Nakhleh L. 2016. Irrational exuberance for resolved species trees. Evol 70:7–17.
Heers AM, Dial KP. 2012. From extant to extinct: locomotor
ontogeny and the evolution of animal flight. TREE 27:296–
305.
Heers AM, Baier DB, Jackson BE, Dial KP. 2016. Flapping
before flight: high resolution, three-dimensional skeletal kinematics of wings and legs during avian development. PLoS
ONE 11:e0153446.
Medved V, Marden JH, Fescemyer HW, Der JP, Liu J,
Mahfooz N, Popadić A. 2015. Origin and diversification
of wings: insights from a neopteran insect. PNAS
112:15946–51.
Mann T. 2007. Sporting myths: the REAL role of lactate
during exercise. SAJSM 19:114–16.
Park H, Choi H. 2010. Aerodynamic characteristics of flying
fish in gliding flight. J Exp Biol 213:3269–79.
Riskin DK, Hermanson JW. 2005. Biomechanics: independent
evolution of running in vampire bats. Nature 434:292.
Schmidt-Nielsen K. 1972. Locomotion: energy cost of swimming, flying, and running. Science 177:222–8.