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This year, as Europe’s bats emerge once more from their long winter
sleep, they will find themselves protected by an international agreement
for their conservation. No group of animals is in greater need of protection.
Bats may seem successful, for they are found from the tropics to the Arctic
Circle and the 977 bat species represents nearly a quarter of all mammal
species. But worldwide bats are declining, including most of the fruit bats
of the tropics and all 30 species in Europe. It is becoming clear that bats,
even at the best of times, live very close to their physiological and ecological
limits, and even a slight mishap can push them into oblivion.

Eleven European nations have signed the Agreement on the Conservation
of Bats in Europe that came into force in January, with four or five more
poised to sign. The agreement aims to improve the protection of bats, their
roost sites and feeding habitats, and although it is a voluntary code, the
signatories have pledged to undertake conservation activities and to enact
legislation to safeguard threatened populations of bats.

The 977 species of bats have much in common, but there are two distinct
groups. The majority, including Europe’s bats, are the Microchiroptera or
‘microbats’. The other group is made up of the 162 species of Megachiroptera
or ‘megabats’ from the tropics and subtropics, epitomised by the fruit-eating
flying foxes. Apart from the fact that all microbats hunt and forage by
echolocation while megabats rely upon sight and smell, there are many differences
between the two groups. Take size: many microbats weigh less than 10 grams
and have a wingspan of 30 centimetres, while some megabats weigh 1 kilogram
and have a wingspan of 1.5 metres. Some biologists argue that the differences
between the two are so great that they must have had different ancestors:
microbats being descended from insectivores and megabats from primates.
Whatever the differences, both groups are extremely vulnerable.

Like every other animal, bats require their habitat to provide many
different elements. The first essential is a place to feed. Then – given
that all bats prefer to fly at night – they need a safe place to roost by
day, which must also be suitable for rearing young. And some species need
an alternative feeding ground to migrate to in winter, while others need
a place to hibernate.

A region is useless unless it provides all the elements in the mosaic
and, as Tony Hutson of the Bat Conservation Trust in London points out,
each element is becoming more precarious as humans encroach deeper into
the countryside. Provid-ing safe havens is tricky too. As Hutson says, bats
are not restrained by fences or by national boundaries so ‘you cannot simply
set up a bat reserve and say ‘that’s it’.’ This is why conversation strategies
need to be international.

Bats feed on all kinds of things – fruit, nectar, pollen, leaves, insects,
frogs, fish, blood. Many of the microbats, including all the species in
Europe, feed on insects on the wing. Yet the mere presence of night-flying
insects is not enough: the bats also need to be able to get at them.

In a detailed survey of bat distribution in Britain, Allyson Walsh and
Stephen Harris from the University of Bristol have found that hedgerows,
the edges of woods, ponds and rivers are good feeding grounds for bats
because there are plenty of insects and enough room to manoeuvre. But most
bats also need good daytime roosting within a few kilometres of the feeding
ground. And few bats like to fly over open ground, preferring to use landmarks
to guide them. A hedgerow stretching from day roost to feeding ground is
ideal, but a bat such as Daubenton’s (Myotis daubentonii) will not fly across
a gap in a hedge of as little as 10 metres.

European bats, it seems, prefer the kind of landscape that people associate
with Britain: plenty of woods with spaces in between, and plenty of water
and old buildings. But this, as Walsh points out, is precisely the kind
of landscape that is fast disappearing. Details, however, can make all
the difference; the layout as well as the amount of hedgerow is important,
and so is the proximity of the edges of woods to daytime roost.

Some bats, in common with urban foxes, kestrels and Canada geese, are
finding that some quirks of human existence do suit their needs. Many bats,
for example, feed on the insects that cluster round street lights. Jens
Rydell, a biologist at the University of Aberdeen, is now finding out exactly
who feeds on what and under what circumstances.

Unfortunately, the species that like the lights most – such as the northern
bat (Eptesicus nilssonii) in Sweden and the pipistrelle (Pipistrellus pipistrellus)
in Britain – tend to be the least endangered. Bats that fly more slowly
and whose echolocation system is designed to help them in cluttered woodland
are not attracted by lamps: these include species from the genera Nyctalus
(such as Leisler’s and Noctule), Plecotus (long-eared bats), Myotis (like
Natterer’s and Daubenton’s) and Rhinolophus (horseshoe bats).

NIGHT ATTRACTIONS

This touches on an aspect of precariousness that at first sight looks
very like perversity. Although the world’s bats vary enormously in their
way of life, they all prefer night-flying. As John Speakman at the University
of Edinburgh points out, fruit and flowers are no more available by night
than by day and insects, the staple food of Europe’s bats, are much scarcer
by night. Is there something really bad about the day that bats seek to
avoid?

This is difficult to test, because bats fly so rarely by day. So Speakman
and his colleagues organised a survey which, between September 1985 and
March 1988, produced 420 observations of day-flying by bats in Britain,
which at least is a start. Their records show that the bats were more likely
to fly on days that followed cold nights, and they flew more by day in spring
than in autumn.

This, suggests Speakman, is because there are very few flying insects
on cold nights, and an aerial night-hunter would go hungry. But bats, being
small, have a high metabolic rate and very few reserves and cannot afford
to starve for too long, so after a cold night they might just have to risk
a daytime foray. They are more likely to do this in spring, when they have
spent their fat reserves in hibernation, than in autumn, after they have
been feeding well for months.

The survey suggests that bats fly by day only in extremis, which implies
that it is indeed risky. But what are the risks? Speakman and his colleagues
have tested three hypotheses: that bats must compete with the insectivorous
birds if they fly by day, that day-flying bats would be caught by birds
of prey, and that they would overheat.

The swallow is one of the biggest consumers of insects on the wing in
Britain, and therefore a potential rival to day-flying bats. The 420 observations,
however, revealed no significant antagonism from swallows when the two flew
together, so competition seems not to be a factor – though it may be with
other birds in other countries. But predation was significant: there were
13 cases of bats being attacked by predators, of which five were fatal.
From these and other figures – for example, relating to total flying time
– Speakman calculates that a bat flying in Britain would be up to a thousand
times more at risk by day than by night of being eaten by hawks, falcons,
eagles and other raptors. And any one bat would be likely to be attacked
and killed once in every 14 hours of daytime flight. This puts a day-flying
bat in the same category as an airman in the First World War: it would be
lucky to last a week.

Overheating also seems to be a problem. The wings of bats have no feathers
– the bare skin is exposed to the heavens. Speakman and his colleagues showed
that only 5 per cent of the solar radiation striking the wings was reflected,
and only 13 per cent passed through; so the remaining 82 per cent was absorbed.
Astonishingly, this absorbed radiation provides up to 12 times as much heat
energy as is generated by flying. Overheating must be a severe restraint
on daytime flying.

There are other reasons why microbats in Europe seem to live on a knife
edge. In the winter their source of food, insects, all but disappears. Their
solution is to hibernate, entering a winter-long torpor. Body temperature
falls to that of the ambient temperature and they may take a breath only
once every 45 minutes. Sometimes they go for two hours without breathing.

In this torpor they use very little energy, but they must accumulate
enough fat before winter to carry them through. For females, who have reared
a baby the previous summer, there may be little time in which to accumulate
fat. But although hibernation for bats is a dramatically switched-off state,
they do not simply cool to the outside temperature and stay cold. As Dan
Thomas, an ecologist at the University of Sherbrooke in Quebec explains,
hibernation is invariably interrupted.

INTERMITTENT AROUSAL

Roughly every twenty days, the body temperature creeps up until it reaches
about 2 °C above the ambient temperature, and then metabolism quickly
returns to normal. The cause, he suggests, has to do with dehydration: they
dry out if they do not wake up now and again to drink. But whatever the
reason, arousal costs them dear because their metabolic rate increases fifty
or sixty times. The bat may be awake for only 2 to 4 per cent of the winter,
but these brief interludes consume more than 75 per cent of the total energy
of hibernation.

Such arousals are natural, and before hibernating a bat should have
built up the reserves to see it through these energy-hungry arousals. Any
additional arousals, however, can be truly dangerous. Thomas has found that
if bats are disturbed during hibernation, they will take longer to settle
than after a natural arousal. And when their temperature finally cools,
it tends to rise again soon after, which leads to yet another arousal.

Overall, Thomas calculates that the energy cost of a single forced arousal
is equivalent to 50 days of hibernation. A single unscheduled disturbance,
therefore, could cause the bat to starve in hibernation – or soon after,
because many bats continue to lose weight in the first days of spring before
there are plenty of insects. This problem of disrupted hibernation underpins
the recommendation by conservationists that hibernating bats must be left
in peace.

BIG BABIES

Reproduction also places a serious burden on bats. Common sense suggests
that pregnancy would affect flying, and some biologists argue that bats
produce only one baby at a time to save the mother from excess baggage in
late pregnancy. But that single baby is born big – at least 25 per cent
of the mother’s body weight, which is as much as the litter of any mammal,
including the large litters of mice and most other small mammals. Furthermore,
baby bats cannot fly or fend for themselves until they are almost as big
as the adults. In temperate zones, therefore, babies must be raised almost
to adult size in a few weeks, so that they can feed and fatten sufficiently
to survive their first winter.

You might suppose that the key problem for the mother is to obtain enough
energy to sustain an intensive period of lactation leading to rapid growth,
and then to recover and fatten up herself. But Robert Barclay of the University
of Calgary in Canada suggests that the true limit on reproduction may lie
not with energy but with calcium, the principal mineral of bone.

Bat bones are dense and heavy like those of other mammals – not air-filled
and girder-like, as in birds. The body weight of the baby at weaning is
usually at least 85 per cent of adult size, while the bone of the forearm
(the radius) is around 95 per cent of the adult length, and is just as dense
as adult bone. This bone, which provides the leading edge of the wing and
is subject to large stresses, must be solid, strong and long if it is to
function.

The babies must obtain the calcium to develop these almost adult-sized
skeletons from their mother’s milk, and Barclay has found that female bats
lose calcium from their own skeletons during lactation. Since females also
lose calcium during hibernation (as do males), they must make good the loss
before winter starts.

Where do bats obtain their calcium? Some species eat small vertebrates
such as frogs with their calcium-rich bones, and others eat crustacea with
their calcium-rich exoskeletons. But the food of most bats is poor in calcium.
Of the common bat foods, pollen is best for calcium, with 3.15 milligrams
per gram. Fruit contains around 2.9 milligrams per gram while insects, the
food of most temperate bats, contain a mere 1.62 milligrams of calcium per
gram fresh weight. The big brown bat from northern Europe needs up to to
23.2 milligrams of calcium per day. It would have to eat 63.2 grams of live
insects to supply this – equivalent to four times its own body weight. No
bat could eat this much. So lactating bats seem bound to be short of calcium.
This suggests that calcium, rather than energy, provides the limit on reproductive
output.

The low reproductive rate of bats, forced perhaps by this need for calcium,
raises a further important issue for conservationists. If population levels
of, say, mice are diminished, no problem: once the good times return, they
rapidly spring back again because females can produce half a dozen babies
each when they are just a few months old. But most bats multiply as slowly
as cows. To compensate, they are long lived – the record is held by a small
brown bat weighing less than 10 grams that survived for 31 years. But in
the wild their mortality is high, with about a 30 per cent chance of death
in any one year. Add this high mortality to the low rate of reproduction
and we see that populations of bats are bound to take a long time to build
up.

It remains to be seen whether the Bonn Convention is in time, and has
enough teeth, to save Europe’s 30 species of bats. Meanwhile, the megabats
on the other side of the world are even more endangered. Old World Fruit
Bats – An Action Plan for Conservation, produced by the World Conservation
Union (IUCN) in 1992, lists 17 out of the 162 megabat species as endangered,
and at least seven are known to have become extinct in recent years. The
huge flocks of the tropics, some of which numbered tens of millions, are
becoming a curiosity of the past. Explorers of the 1840s reported that the
forest of Samoa was ‘infused with the odour of bats’. Today you would be
lucky to see any. Roosts of up to 150 000 fruit bats that were common in
the Philippines as late as the 1920s are now reduced to a few hundred.

A central problem is that 86 per cent of megabats live on islands, and
island creatures are especially vulnerable because their habitats are so
rapidly transformed and they have nowhere to escape to. Among the 57 species
of Pteropus, the largest megabat genus which includes most of the ‘flying
foxes’, 55 are distributed mainly or entirely on islands. And 35 live on
only one island or a small group.

If and when megabats disappear, a huge range of tropical trees will
suffer, because megabats (and some microbats) are crucial distributors of
pollen and seeds. They are, in fact, ‘keystone species’ – species on whom
a range of other species depend. Greg Richards from CSIRO, Australia’s national
research organisation, points out that bats may not be the only vertebrates
that visit particular trees and spread their pollen. Birds and other mammals
– at least in Australia – may do so too. But experiments in Queensland in
which flowers were caged to keep out bats but not other vertebrates, show
that for many tree species – including many eucalyptuses – bats are by far
the most important pollinators.

Without bats, too, it is hard to see how many tropical trees could spread
their seeds. Ruth Utzurrum from Boston University has found that birds tend
to damage seeds as they pass them through their guts, while bats more often
spit out undamaged seeds, or pass them undigested through their guts. Elizabeth
Pierson of the University of California at Berkeley says that while birds
are great transmitters of seeds from small fruit, bats will carry fruits
that are positively enormous; a bat weighing 1 kilogram (a megabat indeed)
can carry fruits such as durian weighing up to 200 grams.

FRUIT CARRIERS

Most big fruit bats, however, feed on big fruit while it is still on
the tree, and simply drop the seed in a small ‘shadow’ around the tree.
They tend to alight on a tree in the morning, and each one occupies a small
feeding territory throughout the day. But Richards found that when there
are more bats than there are territories, the unlucky ones act as raiders,
swooping down on the food trees and carrying off as much fruit as they can
before the residents stop them. So perhaps it is the raiders who do most
to scatter the seeds. If this is so, then efficient scattering of seeds
depends not simply on the presence of bats, but on a high density of bats.
We have a vicious circle: as forest is fragmented, bat numbers are reduced,
and as bats are reduced, seed dispersal becomes less efficient.

As if forest fragmentation were not enough, every now and again in Queensland
there is drought. Then, the eucalyptuses fail to flower and to produce fruit,
and the bats are forced to raid commercial fruit crops, such as oranges
and peaches. The farmers can easily predict when the bats are likely to
invade and could protect their crops with nets, which are very effective.
But they prefer to kill the bats. This is now a political issue in Queensland,
says Richards. Recently, the state government condoned the slaughter of
more than 250 000 flying foxes over 50 000 square kilometres. In the past
decade in Australia, he says, ‘populations of Pteropus have been reduced,
some large colonies have disappeared or been fragmented, and migration pathways
have been broken’. The implications for myriads of tree species which depend
upon these bats for their reproduction has yet to be seen.

The question many biologists often ask is how it was that the bats managed
to evolve at all. Powered flight – as opposed to mere gliding, which many
vertebrates manage – requires tremendous specialisation of anatomy and physiology.
Moreover, when bats first appeared, at least 60 million years ago, the birds
had already been exploiting the air for around 100 million years ago. Speakman
provides part of the answer: they adapted to night-flying, and if they had
not, the birds would indeed have seen them off. But after diversifying to
all parts of the world and into an extraordinary variety of niches, they
have now confronted a much greater antagonist – humans. The new European
agreement may be a new beginning, at least for some of them. But there is
a long way to go.

Colin Tudge is a science writer based in London who specialises in wildlife
conservation. His latest book, The Engineer in the Garden, is published
by Jonathan Cape.

This article was researched at a symposium on Recent Advances in Bat
Biology organised for the Zoological Society of London and the Mammal Society
by Paul Racey, professor of zoology at the University of Aberdeen.