4.3 Flying foxes

Gliding from branch to branch was a comparatively small step for tree living mammals, but there was one group of them that made a truly gigantic leap. Their arms changed into wings. The shoulders, the elbow, the wrist remain much the same, but the hand and the fingers changed dramatically. Flying foxes, fruit bats in Australia, they and their insect eating cousins are the only mammals that have developed true powered flight. They're so big that they can't roost in holes. Instead, they sleep out in the open, in colonies that may be hundreds of thousands strong. The thumb on each hand is free of the wing and has a hooked claw. Using that and the claws on the toes, fruit bats are surprisingly nimble, clambering about in the branches. Wings may have solved the problem of getting from one tree to another, but landing is still a challenge. As a fruit bat approaches its chosen perch, it goes into a glide. Then it lowers its toes and hooks them onto a branch. This is a textbook example of how it's supposed to be done. But some perches are more difficult to reach than others. Wings need regular grooming. They're also very delicate. But small tears quickly heal. The wing membrane is among the fastest growing of all mammalian tissues. They also fan their wings to keep themselves cool. It can be very hot hanging unprotected in the baking sun. Takeoff, too, requires a special technique. Two or three wing beats lift the body to the horizontal, and only then should the feet be unlatched. That way, you don't lose too much height. It's hard work, particularly if you're carrying a baby which is a third of your own weight. Once in the air, however, fruit bats are extremely strong flyers. They can travel great distances, as much as 30 miles, 50 kilometres, in a single night, if that's necessary to find food. They may have lost a lot of moisture hanging around in the midday sun, so their first call is often to a nearby lake, to get a drink. They do this in a rather unusual way. First, they dip their chests in the water. Then, they return to their roost and lick the moisture from their fur. But there are hazards - crocodiles. The bats only touch the water for less than a second, and usually, the crocodiles are just not quick enough to catch them. But if one miscalculates and comes down on the water, it's a different matter. They're surprisingly good swimmers. The worst danger comes when they get to land. Without being able to drop into space as they can from a perch, they find it very difficult to get airborne. Now, the crocodiles have the advantage. But a few individuals lost to crocodiles makes little impact on the bat colony. This roost alone contains a staggering five million. Living together in these vast numbers brings several important advantages. Flying foxes collect fruit and nectar of many different kinds. But knowing which species of fruit tree is in season at any particular time is not easy, and some are very unpredictable. If a few individual bats return smelling of a particular fruit, the news that this food has just come on the market spreads quickly through the whole colony. Each bat knows where trees of the various species can be found. So the next night, it'll go to its own favourite patch to collect the new fruit. That is why the whole five million don't follow one another to the same tree. Huge wings may be good for long distance flying, but they don't give great manoeuvrability in the air. And when the bats return in the dawn, hunters are awaiting them. Eagles know exactly where the bats' blind spots are and attack from below. Powerful though eagles are, fruit bats are big animals, and a hit isn't necessarily a kill. Raids like these are another reason why an individual bat finds it an advantage to roost in a colony. Since it's surrounded by tens of thousands of others, there's a good chance that an eagle will pounce on someone else. Most colonies have a resident pair of eagles that nest nearby. A breeding pair will take half a dozen or so bats a day, but that still makes little impact on bat numbers. Skilled though the eagles are in taking bats on the wing, their most successful strategy is to snatch them as they hang on the branches.