Introduction to ecosystems
Introduction to ecosystems

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Introduction to ecosystems

2.4 Life in trees

Figure 5

In considering food chains in an oak wood you saw animals adapted to a life in trees. Mammals that live in trees show a range of adaptations that make them well suited to the arboreal lifestyle.

Woods and forests present a number of problems for mammals that inhabit them. The habitat stretches vertically for a substantial distance yet for tree-dwellers to travel any horizontal distance they must either go down to the ground each time, or jump from sometimes flimsy branches over large gaps. Sir David Attenborough describes how squirrels have overcome the problem.

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Life in Trees: squirrels

SIR DAVID ATTENBOROUGH
Squirrels deal with the problem with dazzling ease. They're such lightweights that they can race along the thin twigs at the very far end of the branches, and they're spectacular jumpers. Their powerful hind legs provide the thrust. Their long tail acts as a rudder. And their shorter front legs serve as shock absorbers to cushion the landing. Superb sight enables them to judge distance with great accuracy, an essential ability when racing along this three dimensional highway. They're at their most acrobatic during the mating season, when males start to pursue the females. One male may begin the chase, but others quickly join in. Eventually, one wins, but as soon as he's claimed his prize, the chase will start all over again, and the female may mate with up to eight different males in a single day. But a gap this size is just too big, so a grey squirrel, like a tamandua, often has to come to the ground if it's to visit all the trees in its range. A grey squirrel can leap eight feet, but there's another tree dweller that can leap much farther than that. Although it's no bigger than my hand, it can jump from this tree to that tree over there, more than 50 feet away, an astonishing distance. But to see how it does it, we'll have to come back at night. Since they have an acute sense of smell and love seeds and nuts, maybe these will tempt one down from the treetops. They are flying squirrels. How do they fly? Just watch. Maybe gliding squirrel would be a more accurate name. They're nonetheless astonishing. That furry membrane stretching between wrist and ankle makes a most efficient aerofoil. Flying squirrels are not territorial, and as many as half a dozen can be foraging in the same area of woodland. Although this little squirrel may have travelled a very long distance in order to get this valuable source of food, it's such an expert glider, it's done so with a minimum of effort. And in forests like this one, where food sources are often very widely dispersed, the ability to travel fast and far, but with very little effort, is a very valuable ability indeed. There are few gaps in these forests that defeat them, but to cross really long distances, they do need height. They steer partly with their tail and partly by moving their outstretched legs so that they vary the tension of their gliding membrane. And you can see that they can steer when one squirrel uses the same takeoff point, but glides away to land on different trees. Even so, they're not agile enough in the air to escape birds of prey, so during the day, they sleep in holes and only emerge when it's dark.
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