Why Elephants Have Big Ears by Chris Lavers, Victor Gollancz, 拢18.99,
ISBN 0575067225
REVVED up by espresso for a tough day at work? Calmed by chamomile tea for a
quiet hour in the garden? You can switch between these opposing states in a
matter of hours, but it鈥檚 taken millions of years for the subtle interplay
between metabolism and environment to evolve into today鈥檚 wild variety of beasts
and behaviours.
Curiously, we take metabolism for granted. Every day thousands hit the
doughnut trail, blithely assuming that their bodies will somehow burn off the
extra calories. Or perhaps they hug the forlorn hope that the morning coffee and
evening beer will make up for drinking too little water. It won鈥檛. We really
need that 3.5 litres a day to stay healthy. Deviate from the dictates of our
metabolism and we run serious risks. Our civilisation may protect us from the
harshest environmental extremes, but if our metabolic engines tip badly out of
balance, we die.
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Metabolism is a powerful but vastly underrated force in fine-tuning animals
to match their environment. The polar bear is the world鈥檚 largest and most
powerful predator, but its Arctic-adapted metabolism would fail in the heat and
humidity of the tropics. Anacondas would freeze in a snap of the fingers in an
Arctic winter: nothing in their cold-blooded metabolism could defend them
against that degree of cold.
In Why Elephants Have Big Ears, Chris Lavers becomes a metabolism
detective for creatures around the globe and across time. And, yes, he does
explain the big ears elegantly. As elephants evolved, the volume of tissue
generating heat increased faster than the surface area for dissipating it. That
helped the mammoths and mastodons to conserve heat as they wandered across
frozen tundra in the ice age.
But what about the elephants of Africa, which had to contend with blistering
heat? Sunshine aside, today鈥檚 elephants generate so much internal heat that they
would cook if they didn鈥檛 shed some of it. Like the rhinoceros and the
hippopotamus, tropical elephants have lost their hair, but even that isn鈥檛
enough to keep 5 tonnes of elephant cool. Heat fins were the answer. African
elephants evolved broad, thin flaps of ears through which blood is
pumped鈥攁 cooling mechanism as efficient as that in an air-conditioning
unit. The smaller Indian elephants have smaller flaps, and ice-age mammoths and
mastodons had small ears, not flaps.
Dinosaurs also pose intriguing, but in their case unanswerable, questions.
Were they warm-blooded like modern birds, which evolved from predatory
dinosaurs? Or were they cold-blooded like their close cousins, the crocodiles?
After 30 years of controversy, the clues from the fossil bones are neither
obvious nor consistent. Indeed, as Lavers writes, 鈥淲hen pushed by inquisitive
palaeontologists, physiologists and biochemists have had to admit that they
don鈥檛 really understand how warm-bloodedness works.鈥�
A warm-blooded metabolism does have its adaptive advantages. It gives mammals
and birds the endurance needed for sustained activity, and keeps their digestive
systems at temperatures where they are most efficient. Yet generating energy
internally carries a clear cost in efficiency.
The python, for example, can curl up and relax for months after a good meal
of a goat. Mammals must eat regularly to fuel their hungry furnaces or fatten
themselves to survive hibernation. Lavers points out that a warm-blooded
metabolism gives wolves the endurance to lope after their prey all day long, but
reminds us that although a cold-blooded crocodile could never run that marathon,
it can pounce with deadly speed on nearby prey.
Cold-bloodedness is far more efficient in warm environments with uncertain
food supplies. Lavers suggests that鈥檚 why crocodiles dominate freshwater
environments, while mammals do better on land and in the open ocean. Efficiency
may also explain why Australia, with its arid, unpredictable climate, has so
many larger-than-average reptiles.
Why Elephants Have Big Ears ventures more into ecology than physiology, and
while this makes it more fun to read, it also leaves it perhaps less
comprehensive. Some of Lavers鈥檚 interpretations may be controversial, but the
only obvious mistake was his repetition of the disproved myth that polar-bear
hair acts as optical fibres to warm the animals
(鈥淧ower of the polar myth,鈥� 快猫短视频, April 25 1998, p 50).
That can be forgiven in a book that manages to be both fascinating and illuminating.