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Engineering Animals
Engineering Animals
how life works
Mark Denny
Alan McFadzean
the belknap press of
harvard university press
Cambridge, Massachusetts
and London, England
2011
Copyright © 2011 by the President and Fellows of Harvard College
All rights reserved
Printed in the United States of America
Library of Congress Cataloging-in-Publication Data
Denny, Mark, 1953–
Engineering animals : how life works / Mark Denny and Alan McFadzean.
p.
cm.
Includes bibliographical references and index.
isbn 978-0-674-04854-6 (alk. paper)
1. Physiology. 2. Animals—Adaptation.
3. Animal ecophysiology. I. McFadzean, Alan, 1958– II. Title.
QP31.2.D46
2011
591.7—dc22
2010051355
Mark dedicates this book to a mammal,
Jane Denny, with love
Alan dedicates this book to his pack:
Anita, Kirsty, and Gordon
Contents
Prologue
1
pa r t o n e : Structure and Movement

Go with the Flow

Structural Engineering: The Bare Bones

A Moving Experience

A Mind of Its Own

Built for Life

Simple Complexity: Emergent Behavior
7
35
58
81
106
pa r t t w o : Remote Sensing

A Chemical Universe

Sound Ideas
178
155
130
contents
viii

Animal Sonar
203

Seeing the Light

There and Back Again: Animal Navigation

Talk to the Animals
Epilogue
Notes
289
313
317
Further Reading
References
347
351
Acknowledgments
Index
236
373
371
263
Engineering Animals
Prologue
You are a Great Ape. As it happens, both authors are also animals belonging
to this family; we share our two homes with other hominids, plus mammals
from the canine and feline families. Unintentionally we also share our homes
with a wide variety of other animals, mostly from the classes Insecta and
Arachnida. We might have begun by saying “You are a biped” and then proceeded, in a mathematically progressive way, to state how we shared our homes
with quadrupeds, hexapods, and octapods. However, referring to our readers
as “apes” rather than as “bipeds” makes a point. You were probably a little
taken aback—metaphorically slapped across the face—which would not have
happened had we called you “bipeds.”
Why is this so? Both statements are true, yet we hominids of the species
Homo sapiens (no doubt all our readers are from this species—we would be
very interested to hear from readers who are not) sometimes think that we are
superior to the rest of the animal kingdom and so resent being reminded of
our lineage. In some ways, humans are superior, in other ways we are not.
Viewed with scientific objectivity—and from a certain pragmatic perspective
more often found in engineering than in science—we can estimate how humans
stack up, in biological capability, against other animals. We have brains that
2
prologue
are uniquely capable (as far as we know) of abstract thought and language; this
ability sets us apart. We have pretty good eyesight, by the standards of the rest
of the animal world, though not the best. Some of our other senses are dull
or lacking: dogs would hold up their noses at our ability to smell (meaning
our ability to sense smell); owls wouldn’t hear of our acoustic abilities. Our
skeletons are standard issue, though perhaps not as well adapted to bipedal
locomotion as many quadruped skeletons are to four-legged locomotion.
Octopuses—if they were capable of human arrogance—might regard us as inferior, because we can’t even sense polarized light, let alone emit it. Bats might
regard themselves as occupying the top of the evolutionary tree, because their
sonar is better than the most hi-tech of ours.
This book is about the wonderfully varied and astonishing capabilities of
animals. We look at animal design from the standpoint of engineers. Skeletons
are marvels of engineering as well as of evolution. Stated differently, evolution
has provided animals with support structures that are—that have to be—well
engineered. Birds are well engineered for flying. Pigeons are flying remote sensors with capabilities we can appreciate from an engineering perspective: they
have celestial navigation, wideband acoustical receivers, hi-res optical receivers, and magnetic sensors. There is a small fly that can locate the source of a
sound very precisely, even though the fly is much smaller than the wavelength
of the sound it hears—a difficult feat. Albatrosses travel vast distances across
the southern oceans while expending very little energy—they exploit wind
shear using a technique known to glider pilots as dynamic soaring. These
last two example—two small pieces, we feel bound to say—of the extensive
research that we have pursued in writing this book have been turned into a
couple of pedagogical papers written to show university science students how
the animal world employs good (and novel) engineering principles to achieve
certain goals. In this book you will get the results of our investigations without
the math.
What qualifies us to write a book about animal engineering? Well, we are
animals—have been for decades—and we both have many years experience as
research engineers. Both of us worked for multinational aerospace companies
designing radar and sonar algorithms for military and other remote-sensing
applications. Both were trained as physicists, and this combination (science
and engineering) plus a long shared history of mathematical and computer
modeling of natural phenomena provides us with a pragmatic perspective on
how things work. Applying our engineering experience to the world of ani-
prologue
3
mals has shown us, again and again, how well adapted and well made animals
are for the roles they play—they are what engineers would call “mature technology.” Looking at animals in this way—analyzing them as engineering
structures—in no way reduces our sense of wonder at their diversity, adaptability, and astonishing capabilities. We are confident that reading our book
will refine your appreciation of Great Apes like us and of other, more distant,
relatives.
pa r t o n e
Structure and Movement
[To view this image, refer to
the print version of this title.]
1
Go with the Flow
E
nergy is a familiar concept in the everyday lives of most people, even if they
are unable, when asked, to conjure up the physicists’ precise definition. We
tire during exercise and feel refreshed after an “energy drink.” Our dog runs
along the beach for no reason, so it seems, other than the joy of life—he is “full
of energy.” Physicists tell us that energy is a real thing, like matter, but outside
8
structure and movement
the realm of Einstein’s relativity, it is not a material. Instead, it is a property of
a material and can take many forms: heat energy, chemical energy, kinetic
energy (the energy of motion), and so forth.
Energy flow—the subject of this first chapter—refers to the transmission of
energy from one place to another; in our case we will consider the chemical
energy that is contained in food flowing into an animal, or energy moving
from one ecosystem to another. Energy flows out of an animal when it dies
and into another animal or plant that eats it, or into the atmosphere as heat.
Energy flow is an important consideration when we seek to understand how
nature is organized.
Energy flow influences animals at all levels and in every aspect of existence: cells and ecosystems, behavior and structure, size and shape. This
pervasive influence may affect a single species uniquely—perhaps through a
peculiar adaptation—or it may affect all species so that they exhibit universal features with surprising consistency. In this chapter we will see how that
basic engineering concept—the flow of energy—influences different aspects
of animal life.
Solar Power
The sun is a fairly average star, located in a middle class suburb of the Milky
Way galaxy. Its third planet (the blue-green one) is warmed by a tiny fraction
of the truly prodigious power emitted by the sun: about one part in a billion.
This solar power mostly takes the form of electromagnetic radiation, and the
amount of this radiation falling on Earth’s upper atmosphere is about 330
calories per square meter per second (330 cal ⋅ m–2 s–1).1 Power is energy per unit
time. So, every square meter of Earth’s upper atmosphere is gently bathed by
330 calories of energy every second. This solar energy is absolutely vital to life
—without it, life would not have evolved on Earth. More importantly—and
here is the central message of this chapter—energy flow influences the structure of all living things; it determines the way that animals move and behave,
the way they evolve, and their physical form. Every aspect of animal (and
other) life is dominated by the flow of energy.
How much of the sun’s energy reaches Earth’s surface? Clouds reflect some
of it back into space, while the atmosphere absorbs or scatters some more.
Furthermore, for most places on Earth, the sun is not directly overhead, and
at any given time, half of Earth is sunless.
g o w i t h t h e f l ow
9
extremophiles
Immediately we are faced with an exception to the rule—life, with all its spectacular variety, is like that. There are some very odd forms of animal life at the
bottom of deep oceans, where sunlight does not penetrate. Much of life in the
benthic zone does, in fact, depend indirectly on sunlight: it feeds on detritus
raining down from the surface. However, near hydrothermal vents and cold
seeps there are animals that exist without light or heat from the sun, and they
have done so for countless generations, quite independently of life elsewhere on
Earth.
Hydrothermal vents occur at the fault lines that separate tectonic plates in
the deep oceans of the world, for example the mid-Atlantic ridge. Hot sulfurrich gases from deeper within the Earth vent in copious quantities, and the
geothermal and chemical energy they bring has been harnessed by certain bacteria that live symbiotically in the bodies of animals that populate these regions.
The best known of these strange animals is the giant tube worm Riftia pachyptila, which grows to 8 ft (2.4 m) in length. It is white with a red “head”—like
a lit cigarette. Even bigger are the 10 ft Lamellibrachia tube worms that populate cold seeps and live for 250 years. Cold seeps, again usually located so deep
below the ocean surface that light will not reach them, are regions where hydrocarbons seep up from below. At these sites, communities of animals and bacteria (more than 100 species) form ecosystems that thrive without light.
Because the hydrocarbons represent stored solar energy from previous geological epochs, the cold-seep creatures do depend indirectly on sunlight. Hydrothermal vent ecosystems are truly independent of photosynthesis, however. These
animals, living without light in regions that are very hot and very cold—within
the space of a few meters—and at depths where water pressure is enormous, seem
wholly alien. They are not: they share the same DNA code as the rest of life on
Earth, and so must share a common origin, way down the evolutionary tree. So,
even hydrothermal vent ecosystems may have their origins in life forms that once
relied on photosynthesis, although today their descendants do not.
In one year, on average, the surface of our planet receives about 10% of the
energy that reaches our upper atmosphere from the sun. How much of this
solar energy is harnessed by life? Ecologists reckon on the following rough
values. The primary producers—photosynthetic plants (on land) and phytoplankon (on the sea surface)—convert about 1% of the electromagnetic energy
10
structure and movement
that reaches them into useful stored chemical energy to fuel life on Earth.
Now we turn to the trophic pyramid and the ecologists’ 10% rule (see Figure 1).2
Photosynthesizing plants make up the basement level of our energy pyramid. Herbivores, the animals that feed on plants and constitute the first-floor
trophic level, convert about 10% of the plant energy into herbivore energy.
Small carnivores, who feed on herbivores and form the second-floor trophic
level, convert about 10% of the herbivore energy into small carnivore energy.
Large carnivores on the top floor, who feed on their smaller brethren, convert
about 10% of the energy locked up in small carnivores. You will appreciate that
the 10% rule is, like weather forecasts, only approximately true. Many thousands of food chains exist in nature, with different herbivores and small and
large carnivores. Animals use energy for respiration, circulation, digestion,
locomotion, temperature regulation, and nervous function, but an awful lot
of energy is dissipated as heat. This wastage, from one trophic level to the next,
figur e 1 The basic trophic levels, drawn approximately to scale. Photosynthesizing
plants occupy the basement floor, providing about 10% of their energy to first-level
occupants (herbivores—here represented by a caterpillar), who in turn provide 10% of
their energy to second-level occupants (secondary carnivores—here an insectivorous
bird). These carnivores in turn provide 10% of their energy to third-level occupants
(primary carnivores—here a cat).
g o w i t h t h e f l ow
11
is not always a rigid 90%; it varies from about 80% to 95%. Why so much
wastage? Physicists would blame the Second Law of Thermodynamics. Converting energy from one form to another always generates some wasted heat as
a by-product. We can see other reasons for inefficiency without invoking the
Second Law. Photosynthesizing plants are not sensitive to all wavelengths of
light emitted by the sun; herbivores cannot eat all plant material—some is
indigestible; carnivores produce energy-rich feces (which other creatures—
detritivores—may consume). As we will see, even the very act of digestion
takes energy—a significant fraction of an animal’s metabolic rate, in fact. All
these aspects of life represent inefficient uptakes of energy.3
Fox Populi
As a first illustration of how energy flow influences animal life, we take the
10% rule at face value and show how this inefficient flow of energy or power
from the sun to top-level carnivores limits carnivore population density. We
saw that about 10% of the solar energy reaching the upper atmosphere is available on the surface; about 1% of this is converted into usable energy for photosynthesizing plants; about 10% of plant energy is converted to herbivore use;
about 10% of herbivore energy is converted for use on the second floor (small
carnivores). Finally about 10% of small carnivore energy makes it up to the top
floor. So, large carnivores convert about one part per million of the solar
energy that bathes our upper atmosphere. How does this limited energy conversion restrict the large carnivore population density? Consider a 1 km square
on the surface of Earth: our calculations show that this area will yield about
29,000 kcal of energy to be shared out among the large carnivores—of all
types—each day. A 100 kg (220 lb) carnivore requires about 7,000 kcal of
energy per day, and so the maximum large carnivore population density is
about four animals per square kilometer. In fact, this number is much greater
than actual carnivore densities, for several fairly obvious reasons. Climate limits the density of photosynthesizing plants, so most regions of Earth’s surface
are not completely covered with green leaves. Plant defenses reduce the impact
of herbivores. Disease restricts animal numbers; territoriality and other modes
of behavior limit population densities. Lion density in an African game reserve
is about one per 7 km2, whereas wolf populations in northern Canada vary
between about one per 20 km2 and one per 500 km2. It is easy to see why the
actual numbers are lower than our upper limit; the main point of this calcula-
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structure and movement
tion is to show that there is a strict upper limit, and it is imposed by energy
flow.4
Carnivores that are smaller than lions (but still occupy the top floor of the
trophic pyramid) eat less, and so their population densities are higher, at
approximately 24 per km2 for 10 kg animals. We find for example that the
population density of Red Foxes (Vulpes vulpes) in Poland averages one per
km2, rising to twice that number on farmland and five times as many in suburbs. Again, these numbers are less than our strict upper limit, as expected.
Given the rough nature of our calculation, the upper limits we find are very
credible.5
The idea of relating population density to available energy may be extended
to other trophic levels. It is trickier to work out numbers, because there are
generally more species at lower trophic levels and the available energy has to
be shared out between them, so the actual population density of a given species will be much less than the estimated maximum. Furthermore, the population density of first-floor animals (herbivores) is limited by predation by second-floor species, as well as by energy availability. Energy flow is by no means
the whole story, but you can see how it is always a significant factor in determining the behavior and form of animal species. Later in this chapter we show
how the idea of energy flow leads to scaling laws amongst animals. These seemingly universal, or nearly universal, laws relate different aspects of animal form
and behavior and apply across species of many different shapes and sizes. Here
we have seen a simple example of how energy flow relates body size to population density.6
Foxes illustrate another way that energy flow influences animal life. Locomotion requires energy. For example, a study shows that the Kit Fox (Vulpes
macrotis) spends about 20% of its energy budget on moving around (they can
cover 30 km per day). These data were gathered outside their breeding season,
so we can assume that most of that movement was associated with hunting.
So, 20% of the foxes’ energy is spent obtaining more energy (and a further
20% of any animals’ energy budget is spent digesting its food). Clearly, all
carnivores have similar dilemmas: they must expend energy to gain more energy.
Natural selection has equipped them with optimized strategies to obtain the
maximum food for the minimum effort. Equally, natural selection will have
optimized their prey’s abilities to avoid being caught. The hunters improve
their chances of a successful kill if they improve their eyesight, hearing, and
their brains (for cooperative hunting, perhaps, or ambushes). They must decide
g o w i t h t h e f l ow
13
when to give up a chase and when to initiate one, weighing the odds of success
against the cost in expended energy. Prey species similarly are driven to evolve
superior predator-avoidance strategies: better eyesight and hearing, faster legs,
cooperative lookouts who communicate the presence of predators to the rest
of the herd, and so on. In most cases the length of food chains—the height of
the trophic pyramid—is limited by energy availability to two or three links.
We have seen that most of the energy at one trophic level does not make it to
a higher level: the energy cost of locomotion (in particular of catching prey)
accounts for part of this wastage.7
A recent study shows yet another way in which energy flow influences carnivore evolution. Small carnivores (less than about 15 kg) tend to hunt prey
that is much smaller than themselves—invertebrates or small vertebrates.
Large carnivores, however, tend to hunt prey roughly their own size: domestic
cats hunt mice, but lions take wildebeest. The transition weight of predators
(the point at which they switch from small to large prey) can be predicted
from energy considerations. We can readily see why this might be the case:
• Catching a mouse requires little energy, but the gain is small.
• Catching a wildebeest requires lots of energy, if you’re big enough to do
so, but the gain is large.
• A large predator uses a lot of energy to chase prey, even if the prey is small.
• A domestic cat can’t catch a wildebeest and it isn’t worth the effort for a
lion to chase a mouse.
The same analysis predicts a maximum size for mammalian land predators of
about 1 ton. The largest land predator in the world today, the Polar Bear,
weighs about half a ton. The largest extinct terrestrial mammalian predator
(the Short-Faced Bear) is thought to have weighed about a ton.8 So it seems
that the size of terrestrial predators and of their prey is determined by energy
efficiency.
Hare Today, Gone Tomorrow
So the natural world has partitioned itself, in terms of energy distribution,
into trophic layers. This distribution is far from simple and is far from static.
Had space permitted, we could show you reams and reams of charts that ecologists and biologists have put together, displaying their discoveries about the