Bats

However, these remarks are no more than a generalization, and any generalization is an over-simplification. To offset this a few particular items of information must be included.

As far as the daily sleep is concerned, we know that all bats so far studied behave in the same way. They stay in their roosts, which may be a hollow tree, cave, roof-space or tower, from dawn to dusk, approximately. In the roost, they hang upside-down, suspended by the hook-like claws of their hind feet. While in this position their body temperature falls to that of the surrounding air, their rate of breathing drops and so does their pulse-rate. They are, therefore, in their daily sleep, in a state closely resembling that of typical hibernation. On awakening for their night's hunting they have to warm up, and these preliminaries include much wing-flapping and, in those bats occupying a roomy roost such as a roof-space, a certain amount of running around using wings and hind-legs in a quadrupedal manner.

Most bats sleep with the wings merely folded against the sides. The horseshoe bats are peculiar in that they hang freely suspended from a ceiling with their wings wrapped around the body, much as a person might wrap a cloak around his body against a cold wind. Even while speaking of bats sleeping throughout the daylight hours it must be remembered that such sleep is not necessarily unbroken. Some species, like the pipistrelle, creep so completely out of sight into cracks and crevices that it is hard to say what their daytime habits may be. Serotines are more obvious in their roosts and these we know will readily awaken at slight sounds and will look up to see what A serotine may even scamper towards the source of a sound to find out what is happening.

Two species of bats that have been fairly fully investigated in England are the serotine and the greater horseshoe bat. The serotine hunts periodically throughout the night. Its fast flight lasts for about 45 minutes after which it returns to its roost to rest. Its prey is mainly moths, and on warm nights, when moths are flying well, it may make three or more such sorties in a night. In October, about the third week, it goes into hibernation and it sleeps solidly, so far as we can tell, until late April or early May.

The greater horseshoe bat also goes to its winter quarters in late autumn but continues to hunt until about mid-December. From January onwards its feeding flights grow fewer, and hibernation then becomes more pronounced until the end of April. Even during this period, however, the sleep is anything but unbroken. The bats wake up and move around at intervals of a few days, and banding has shown that some individuals may make long flights. Flights of 15 miles have been recorded, and as much as 40 miles has been noted.

It has been suggested that this frequent interruption of the winter sleep by the horseshoe bat is due to the need to drink frequently to avoid drying up. That is, however, no more than a guess. The important point to emerge from the comparison of the habits of serotines and horseshoe bats is that, at the moment, we can only generalize widely about the sleeping habits of bats. Probably every one of the scores of species in the temperate regions of the northern hemisphere has its peculiar habits. What does emerge also is that bats, as a class, spend a high percentage of their lives asleep, or in a state of torpor, whichever is the more appropriate word to use in their case. This may account for the long lives they enjoy, up to 27 years or more, which is a very long life-span compared with other small mammals.

BAT'S READY MADE BLIND

There lives in the Island of Trinidad a bat known as the lattice-winged bat. It has a wing span of about 12 inches, is light brown in color with a white spot on each shoulder. It flies with a jerky butterfly movement with its body held vertical so that the wings are clearly seen as it flies about. These wings are marked with alternating bands of thick and thin skin, the thick bands appearing dark and the thin bands appearing light. This causes a kind of Venetian blind effect. Another unusual feature is that when the bat is asleep it covers its face with another kind of blind. Under its chin is a fold of skin which the bat is able to pull up over its face, past the mouth, the nose and the eyes to the top of the head, where there is a small knob that holds the flap in position. This holds the skin taut forming a kind of mask. The skin is hairy except for two patches of thin bare skin, and when the blind is pulled over the face these bare patches are located over the eyes. They are semi-transparent and they enable the bat to distinguish light from darkness. They also enable the bat to detect the shadow of an approaching object falling across them. This shadow may be from a passing animal that is unlikely to molest the bat, or it may be the shadow of an enemy coming in to attack. At all events, the bat takes no chances, quickly drops its mask to take a closer look, and if its fears are unfounded it pulls back the mask and continues its slumber.

ISLANDS OF SLEEP

It follows as a matter of course that, when speaking of sleep in animals, bats should receive the most attention. They sleep more than most animals and the way they do so is more varied than in any other group. Indeed, if other animals could be said to live to eat, bats could almost be said to live to sleep. Something of this idea could have been in the mind of the Roman author Lucanius who, according to the Italian naturalist, Ulysses Aldrovandus, writing in 1581, left us a description of the Island of Sleep. This island stood in the middle of a river named Nycti-potus, which being translated means Waters of Darkness. Its only inhabitants were bats. We do not know where this island was located but it could stand for any one of a large number of islands in the tropics where fruit bats roost during the day. Some of these roosts are on actual islands in rivers and others are in islands of trees growing in grasslands. Both kinds can be found in Africa and also in southern Asia. The epauletted bat of West Africa is one species of fruit bat that behaves in this way, and its behaviour is typical of fruit bats as a whole.

At dusk the bats take off and fly in a body to some place where fruit is ripening, to gorge themselves. After this they return to their roost to settle on the upper branches of the trees where they hang, during the daylight hours, like monstrous fruits themselves. Through using one island of trees habitually the bats denude the branches in the upper part of the trees of foliage. This only makes more conspicuous the sleeping bats hanging by their hind-feet, and shading their eyes from the sun by the corners of their folded wings.

JAM-PACKED DORMITORIES

Another fruit bat that roosts on islands is the straw-colored bat. It is the fruit bat most widely distributed over Africa south of the Sahara and is noted for the way it moves about over long distances as the various fruits ripen. It also eats the blossoms of some trees, even the young shoots of some of them, and will chew the soft wood of one palm, apparently to obtain moisture.

This bat is strongly gregarious and at its roost, in daytime, is often noisy and restless.

It seems likely that the sounds made by this bat at roost are directed at preventing its fellows from hanging too near to it. So, in spite of their liking company, straw-colored bats also like to be evenly spaced out. One of the favoured foods of this bat is dates, and at times so much of this fruit is eaten that measures need to be taken against it to protect the crop. On the other hand, in the Congo, where they roost in the trees on islands in the Aruwimi River, the local peoples wade across at low water to kill them in large numbers for food. Both the nuisance value of the straw-colored bat and its value as a source of food can be understood by the numbers found at roosts. Some roosts may have only a hundred or two bats, but other roosts may contain as many as ten thousand.

SLEEPING IN STRINGS

One of the most extraordinary and coordinated sleeping habits is found in the proboscis bat of tropical America. The bat is gray and brown, with gray tufts on the fore-arms. It roosts by day in small companies of a score or so, each bat clinging by all four limbs, one in front of the other, fairly evenly spaced along the underside of a branch or a tree trunk overhanging a river. The color of their fur gives them a resemblance to lichens, so that they are inconspicuous so long as they are still. Yet despite the protection of their camouflage as well as the security afforded by their choice of roosting site over water, they readily take wing when approached, all flying off together, the whole company acting as one unit, and landing again on a fresh roosting place in unison and in the same orderly formation.

TENT-BUILDING BATS

In 1888 a naturalist in Borneo reported having found nests made of moss by bats living in the caves there. There can be little doubt that what he saw were nests made by swifts, and there is no known instance of a bat having built a nest. There are, however, some bats that make a kind of tent or penthouse, by biting through the large broad leaves of tropical trees in Central America. One of them, the yellow-eared bat was first found sheltering in this way as recently as 1932. Nobody has ever caught it in the act of making its shelter but it is clear that the bat cuts across the leaf, about halfway along its length, so that the outer half bends down at an angle. The males rest singly under these shelters but the females with their young are found in groups. When disturbed at their roosts the bats fly swiftly and without hesitation to leaves on other trees that have been similarly cut through.
A palm with large fan-like leaves has been introduced into Panama, the home of the yellow-eared bat. This is also used, although the work of biting through the numerous folds of the palm leaves must require more skill.

In the same region lives a second species, Watson's leaf-nosed bat, which also cuts through palm leaves to make a penthouse roost.

HOUDINI BAT

Some years ago, when workmen were demolishing a building, they had to break a hollow block of concrete. As the concrete fell apart a bat flew out and the story was that the bat must have somehow become incarcerated many years before, when the building was first constructed. Probably there is a simpler answer: that there was a crack somewhere in the concrete, large enough for a bat to creep in. It is surprising how small a space, between tiles, or in brickwork, is sufficient to allow a bat to insert itself. But the palm for this goes to a bat living in Malaya. This is the flat-headed bat, which habitually roosts in the hollow stems of large bamboo.

In one section of the bamboo there may be one or a small group of these bats that have crept in through a crack in the stem. The head, with the skull, is the least compressible part of an animal's body, and it is a popular saying that if an animal can get its head through a hole it will be able to squeeze the rest of its body through. This bat, having a flattened head, has a great advantage over other bats, in squeezing through cracks and using bamboos for its roost. One other thing is needed. Even the flat-headed bat is unable to fly straight through the crack. It must land on the bamboo stem, the slippery surface of which offers little foothold. The flat-headed bat surmounts this difficulty by virtue of the suction pads on its feet, which enable it to cling to the edges of the crack while insinuating its body through it.

BAT'S LEAFY BEDSTEAD

There are about forty species of pipistrelle bats in the world, in North America, and in all continents of the Old World. They all have similar habits in that they appear on the wing fairly early in the evening and fly over a regular beat with a characteristic fluttering flight on a jerky and erratic course. They roost in crevices in hollow trees, among rocks, in caves or in buildings. Most of them, especially those in the colder parts of their range, use one roost in summer and another in winter, but usually they are fairly faithful to one particular roosting place during the course of a season.

The banana bat, a species of pipistrelle living in southern and eastern Africa, is forced to change its roost from time to time because it chooses dried and rolled up banana leaves in which to sleep. The bat is able to do this because the soles of its hind feet form suction pads for clinging to the surface of the leaf.

FICKLE ROOSTER

It is always supposed that the commonest bat in Europe is the pipistrelle but it is likely that the title of "common bat" should be shared by the whiskered. Certainly this is the more widespread. It is found throughout Europe, including the British Isles, across Asia even to Japan and southwards to parts of India, and it extends into Malaya and as far east as Sumatra, Java and Borneo. One reason why the whiskered bat is apt to be overlooked is that it is mainly solitary, especially at its roosts. Sometimes a colony is found roosting together, but usually whiskered bats sleep in almost any place that will give them shelter, such as in roof-spaces, behind shutters or sign-boards, in holes in walls or in trees. They may sleep in ivy or in the spaces between boulders. Their wide choice of roosting places suggests that, unlike most bats which faithfully return to the same roosts day after day, the whiskered bat changes its doss-house fairly frequently. The idea gains from what is known of the behaviour of whiskered bats in Malaya, which sleep in rolled up banana leaves. When the young leaves of the banana first appear they are rolled, just as you might pick up a sheet of notepaper and roll it round your finger. It is not unusual to find a whiskered bat asleep in one of these banana leaves. But soon after the young leaf has appeared it unfolds and its fronds remain spread. Then the bat that has slept in it must find another roosting place, either another young banana leaf or a hole or cavity similar to it. To say the least, the mere fact that the whiskered bat should use the banana leaves in this way means that it is something of a nomad so far as sleeping quarters are concerned.