Introduction to ecosystems
Introduction to ecosystems

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Introduction to ecosystems

2.4.4 Flying foxes

Many species of flying fox (fruit bat) have important roles in ecosystems, dispersing seeds, pollinating flowers or providing food for predators. As they have evolved not only have they acquired adaptations that enable them to exploit aerial and forest habitats, but they have also evolved alongside plants in a process called co-evolution.

What are the likely advantages to flying foxes of their particular form of roosting, taking into account vulnerability to predators, the location of food and temperature regulation?

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Watching 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.
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Colonies of flying foxes may comprise as many as a few million individuals (five million is David Attenborough’s estimate), each with a wingspan of about 1.4 m, with the entire ‘camp’ perched on often denuded trees and engaged in intense social activity.

It’s little wonder that witnessing such a site has been described as a ‘memorable auditory and olfactory experience’. Such concentrations of flying foxes are ‘visible, audible and smellable for miles’ and therefore inevitably attract predators. But congregations of this type may decrease the likelihood of any one individual falling prey to predators, such as eagles. Communication between members of the camp may also increase the efficiency of locating suitable food. But the fact that food sources are depleted so comprehensively when visited en masse raises questions as to the degree of benefit of group living.

Another possible benefit of roosting is that foliage might be protective, shading these mammals from wind, rain and sun, though trees that become camps lose many of their leaves. Fruit bats, for instance, regulate their body temperature, partly by behavioural means. Huddling together in groups should in theory reduce the rate of heat loss in cooler conditions, and decrease the rate of warming when it’s very hot. In both circumstances, the surface area that each individual exposes is lessened by contact.

As you saw in the video, eagles (and owls) take a toll of flying foxes in transit, and the largely nocturnal habit of these species once again probably reflects selection pressure of this type. Flying foxes living on islands (more than 60 per cent of species do so) tend to venture forth in the daylight and in such environments predators are often less evident. Flying foxes can devastate crops, but they can also maintain ‘the fertility of the rain forest’. Flying foxes can certainly help disperse trees by transporting their seeds to new locations, either through their messy eating of fruits or by seeds passing intact through the gut. The seeds of the commercially important West African iroko tree depend on the straw-coloured flying fox for their dispersal. Flying foxes also help in the recolonisation of deforested areas and in the establishment of plants on land newly formed or recently devastated by volcanic eruption.

Flying foxes are also important pollinators; many island species occupy the ecological niches taken over elsewhere by insects or humming-birds, for example. The transfer of pollen from one flower to another on a different tree (i.e. cross-pollination) can confer a significant advantage to the species because it promotes genetic diversity of the next generation. So the development of mechanisms that promote cross-pollination are very advantageous to trees. In Australia, pollination of some eucalyptus species depends almost entirely on visits from flying foxes. The flowering process of the Kajeng Jaler tree from Malaysia is intimately geared to the feeding habits of the dawn bat. Its flowers open just two hours or so after dusk and drop before dawn, coincident with the bat’s feeding time. The size and shape of the flower opening ensure that only the dawn bat can enter; as its long tongue reaches down to access the nectar, the position of the pollen-producing parts of the flower (the stamens) is such that pollen is deposited on the animal’s fur.

This is a further demonstration of the way in which the evolution of one species can increase its dependence on another, often reflecting some form of mutual advantage. This phenomenon is known as coevolution.

Figure 8 Flying fox

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