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AFTER a hard day鈥檚 work, the journey home, cooking, washing up and putting
the kids to bed, it鈥檚 time to collapse into an armchair for five minutes. And in
those few snatched moments, you might well envy the unbelievably easy life of
some vertebrates. Take the sloth: it sits motionless for hours up a tree in the
rainforest canopy. Or the giant python, which lies around for months waiting for
its next meal and then rests in the bushes for weeks doing nothing but
digesting. Bliss.

Well, not quite. Recent research into the behaviour and metabolism of such
seemingly shiftless animals shows that doing nothing has nothing to do with
taking it easy. These animals are operating on the very edge of survival where
doing nothing is essential for staying alive. Even more surprisingly, the
metabolism of some of the most immobile creatures may be working as hard as a
racehorse on the big day.

Mark Chappell, a biologist at the University of California, Riverside, has a
particular interest in the energy consumption of animals that live in extreme
environments. While working in the Antarctic a few years ago, he discovered that
Ad茅lie penguin chicks are not as idle as they seem. Other than occasional
brief bouts of begging food from their parents, these juveniles stay fixed to
the same spot on the ice for weeks. But when Chappell measured the metabolism of
these little birds, he got a shock.

He placed chicks in small sealed chambers with a monitored air supply to find
out how fast they were using up oxygen. Oxygen consumption correlates directly
with metabolic rate: the higher the demands made on cells, the more oxygen they
need to burn glucose and produce energy. Chicks with empty stomachs have a
metabolic rate of 1 millilitre of oxygen per gram of body weight per hour.

What really surprised Chappell was that the metabolic rate of freshly fed
chicks was twice that figure. Such an increase in metabolic rate is unusually
high among warm-blooded animals.

When resting, humans have a metabolic rate of about 0.3 millilitres of oxygen
per gram of body weight per hour. Light exercise, such as walking, can double
this, whereas sprinting can raise it as much as tenfold over short periods. But,
at best, digestion drives our metabolism to only about 50 per cent above its
resting rate. The rates for most mammals are similar. So a penguin chick
digesting its dinner is actually working its metabolism as hard as a human on a
brisk walk.

Exercise consumes energy mostly through the working of muscles, but the costs
of digestion are more diverse. With the penguin chick鈥檚 high-protein diet, about
half the energy goes on moving the food along the gut, producing digestive
enzymes to break down the food, and pumping the resulting molecules into cells
in the gut wall. The other half is used within these cells to reassemble amino
acids from the food back into proteins.

But why do penguin chicks expend so much energy on these activities? At this
early stage of life, penguins have one objective: to grow as quickly as
possible. Chicks make good snacks for skuas, predatory gulls that harass the
penguins. In this environment, a small, weak chick is a dead chick. So, building
up body mass fast is vital for survival, and rapid digestion helps this along.
If the chicks bolt their food, they can go back to their parents more frequently
for more.

Way to grow

So a souped-up metabolic link to the digestive system helps the chicks build
up their bodies quickly. This is why idle ways are so useful: energy wasted on
movement cannot be converted into body mass. And, as any couch potato knows,
doing nothing is a great way to grow fast.

Chappell found still more surprises about the balance between metabolism,
exercise and digestion when he studied another bird that lives much closer to
home. House wrens nest in tree holes throughout North America. Their chicks,
which hatch blind and helpless, sit still for two weeks, converting whatever the
parents feed them into bone, muscle, fat and feathers. In that time, they
increase their mass nearly tenfold.

To look at their metabolic capabilities, Chappell put baby house wrens in a
sealed chamber with a supply of oxygen and a meter to show how much oxygen they
used. He tried a number of different conditions, studying well-fed chicks,
hungry chicks and hungry chicks that were prodded to keep them moving around in
the nest. His results show that during the first eight days nothing the birds
did was more strenuous than digesting food.

A six-day old nestling, for example, had a resting rate of 1 millilitre of
oxygen per gram of body weight per hour. In its most energetic burst of
activity, it could raise that rate by only half again. But by simply sitting
still and digesting, a chick could double this rate and then some鈥攁n
increase even larger than that of the penguin chicks鈥�.

Like humans and other mammals, the chicks鈥� parents can double their metabolic
rate only when they physically exert themselves. By contrast, the chicks are
adapted to draw on this extra energy only when digesting food. They are lazy
little growth machines designed to work as hard at digesting food as their
parents are at bringing it. Yet after about eight days of doing nothing,
Chappell found that the chicks鈥� metabolism flipped so they could consume energy
faster during physical activity than during digestion.

In an even more remarkable example of working hard at doing nothing, the
Burmese python stays completely still for weeks at a time. Yet the metabolic
abilities of this critter rival those of a racehorse going flat out. In his
laboratory at the University of California, Los Angeles, Stephen Secor measures
the rate of oxygen consumption of young Burmese pythons while they are digesting
a meal or fasting. The more they eat, he finds, the faster their metabolism.

The largest meal one of his small pythons has eaten is five rats鈥攅qual
to the snake鈥檚 own body mass. Snakes that eat this much can increase their
metabolism a stunning 44-fold in less than a day. 鈥淚t looks like they are just
sitting there, but they are really huffing and puffing,鈥� he says. As the meal is
digested, which for big meals can take a couple of weeks, the snakes gradually
throttle back again. The only other animal ever measured with a metabolic rate
running so far above its resting rate is a thoroughbred horse at full
gallop.

Of course, in absolute terms the snake has a much lower metabolic rate than
the horse鈥攁bout an eleventh of its rate, in fact. The snake starts with
the very low resting rate of about 0.032 millilitres of oxygen per gram of body
weight per hour. Like all cold-blooded animals, a python does not have to
maintain a constant body temperature the way mammals and birds do, so it expends
much less energy at rest. And a python also feeds so infrequently that to
conserve energy it shuts down its gut in between meals.

Digestive tracts are very expensive to maintain, mainly because the cells in
contact with the food and digestive juices constantly die and slough off.
Replacing them takes energy. But the python stops the digestive juices flowing
and temporarily interrupts the cells鈥� replacement cycle. Its gut actually
deflates along its length. 鈥淭his is like turning off a car in a traffic jam to
save gas,鈥� says Secor.

Secor found that other python organs tighten their belts in lean times, too.
The liver, kidney, and heart all shrink gradually as the belly empties. But
within a few days of feeding they can grow by up to 50 per cent. The gall
bladder is the only organ found to lose weight after a meal as it empties
stored-up bile into the gut.

These metabolic adaptations are specifically suited to predators that wait to
ambush large, rare prey. For the python, which may go for months without finding
a meal, keeping still is a matter of life and death. Moving about would not
yield enough extra food to justify the expense, because prey is scarce, and
would run away if chased, or both. So a python that constantly went hunting
would probably die of starvation.

The Burmese python is not alone in adapting to a niche in which the costs of
motion outweigh the benefits, says Brian McNab, a biologist at the University of
Florida who is writing a book on the role of energy consumption in evolution.
The Texas blind salamander lives in lightless caves where the only food is
detritus that washes through in streams or leaks through the roof. The only
animal that could survive here is one with a very low energy consumption.

So the salamander is by necessity a sedentary creature. But it doesn鈥檛 just
do nothing to survive鈥攊t also sees nothing. McNab argues that the creature
is descended from lizards that could see, but that the evolutionary pressure to
save energy was so strong that it deprived the salamander of its sight.

It is possible that the animal鈥檚 ability to see evolved away through lack of
pressure to maintain it: chance mutations in genes controlling vision could have
stopped them working and wouldn鈥檛 have harmed the salamander鈥檚 prospects because
it didn鈥檛 need to be able to see. But McNab argues there was probably a
selective pressure to drive it away. It takes a great deal of energy to maintain
vision, he says, because there is a rapid turnover of cells in the retina and
cornea. So keeping the eyes when they were of no use would have been terribly
wasteful. 鈥淲hat they are really doing,鈥� he says, 鈥渋s saving energy.鈥�

Compared with a dark cave, you might think that the lush rainforests of the
Amazon contain an inexhaustible supply of tasty comestibles. And yet among the
branches live some remarkably low-energy animals. One of them is the
three-toed sloth, a creature that sits motionless so long that algae grow in its
fur.

Tough to digest

Even at its peak of activity, climbing a tree in what looks like slow motion,
the sloth鈥檚 metabolic rate never rises above 0.48 millilitres of oxygen per gram
of body weight per hour鈥攖hree times its resting rate. It simply can鈥檛
spare any more energy. Although the sloth can eat all the leaves it wants, the
energy inside the leaf is very hard to get at. It is mostly bound up in
cellulose, which takes more energy to break down than the simpler carbohydrates
in fruits and the proteins in insects.

What鈥檚 worse, as a defence against the vast array of insects in the canopy,
rainforest leaves have evolved defences to make them difficult to digest. They
contain high concentrations of tannins which bind to the leaves鈥� proteins and
prevent them from being broken down in the animal鈥檚 gut. This means that much of
the energy in the leaf is unavailable. Most leaves also contain powerful toxins
such as alkaloids that take energy to inactivate.

In the rainforest, low and high-energy animals live side by side. Howler
monkeys can often be seen hanging listlessly from trees, while capuchins swing
playfully from branch to branch stuffing themselves with fruit. Kenneth Glander
of Duke University in Durham, North Carolina, has studied New World primates for
three decades. He says the smaller, lighter capuchins easily outmanoeuvre the
howlers, which weigh nearly twice as much at between 4 and 7 kilograms.

In a sense, the capuchins are creating an extreme environment for the howlers
by grabbing all the best food. So when capuchins are around, howlers make do
with leaves and slow their metabolism down just as the sloths do. Sometimes,
they hang still for hours. Because they must spend so much more energy on
processing the leaves in their gut, howlers must do nothing just to survive. 鈥淚n
most cases,鈥� says Glander, 鈥渉owlers are operating on a minimal margin of error.鈥�
If they were more active, they would probably die of starvation.

So the next time you sit down to put your feet up, remember this: while the
chance to do nothing may seem like an evolutionary bonus that humans missed out
on, all the research into vertebrate loafers shows that being built to do
nothing is about survival鈥攁nd comes with a penalty clause. What鈥檚 good for
the Texas blind salamander could be bad news for you.