The secret life of bats

Why do some species of this elusive mammal congregate in caves? Simon Hadlington spends an evening in the Yorkshire Dales with a team of researchers that is attempting to find out

It is dusk on a late summer's evening in Buckden, a small village on the upper reaches of the river Wharfe in the spectacularly undulating landscape of the Yorkshire Dales. Down at the old stone bridge three figures are working with quiet efficiency, suspending vertical sheets of fine-meshed "mist netting" across the river a few metres either side of the bridge, rigging up large hand-held nets and preparing the night-vision video camera.

This is a typical evening in the field for John Altringham, one of the country's foremost experts on bats, and research students Anita Glover and Paula Senior. The group, based at the University of Leeds, is shedding new light on the complex social behaviour of those species of British bat that hibernate in caves. "Ultimately we want to understand how these bats use the Yorkshire landscape and how the landscape shapes their complex ecology and behaviour," Professor Altringham says.

Here on the river Wharfe the target is Daubenton's bat, unique in the way that it is specially adapted to skimming insects off the surface of water, catching them with its large feet. As darkness closes in, the first flitting shapes appear. Within a few minutes Anita Glover is gently unpicking a bat from the mist net and placing it in a small cotton bag hung around her neck. "It's a pipistrelle," she says. "They're not what we are after, but we will make a record of its sex and age before releasing it." Within half an hour, five or six Daubenton's have collided with the net and are in the bag.

Meanwhile, Senior is standing beneath the arches of the bridge, holding a large net up against the stonework. From the droppings under the bridge she has located the position of a number of roosts - the bats hole up in crevices in stonework and trees during the day. She's waiting for the bats to wake up and drop into the net. "It's curious," says Professor Altringham. "The bats' echo-location system is easily sophisticated enough to detect the net, but when they wake up they seem simply to cry 'Geronimo' and drop out of their crevices and straight into the net."

After an hour the team gathers to examine the catch. Most of the bats have been caught previously and have a small metal ring attached to their wing, imprinted with a number. By monitoring how frequently the bats are recaptured it is possible to make an accurate calculation of the size of the population, and to discover if individual bats tend to roost in the same area each night.

Part of Senior's research project involves the construction of a genetic profile of the population of the Wharfedale Daubenton's. By taking a small sample of tissue from the wing membrane (a tiny disc is painlessly removed and the hole soon heals), a DNA fingerprint can be made. These DNA fingerprints will be used to study the parentage of young bats, to discover if bats that roost in separate communities come together to mate.

To investigate the foraging behaviour of the bats, small radio transmitters can be attached to selected individuals, allowing the researchers to track the bat over a number of weeks as it goes on nightly forays in search of a meal. By using these techniques, Professor Altringham's team has made a number of remarkable findings that are forcing ecologists to re-think previous assumptions about the lifestyles of the cave-dwelling bat species in the UK.

The most intriguing discovery has been that in the upper parts of Wharfedale - around Buckden and Kettlewell - only male Daubenton's are caught. There are no females in these roosting areas. Some 15km further downstream, at Grassington, the Daubenton's community consists of both females and males, with the females outnumbering the males by about three to one.

"Any bats that we have ringed here in Buckden or at Kettlewell have only ever been caught again up here," says Senior. "They seem to be segregated and do not integrate with the mixed community downstream."

On the other hand, newly independent juveniles from the mixed community downstream regularly pitch up to join the male-only groups further up the river. The downstream areas have richer food reserves and better feeding areas - the stretches of slow-moving water from which the bats pluck their prey are longer and more numerous. Also the climate is more stable downstream, providing a more consistent supply of insects.

Certainly it seems likely that the females remain downstream because of the more favourable conditions. During the summer they are pregnant and produce young, so have enormous energy demands.

"This segregation poses a lot of questions," says Professor Altringham. "Are these upstream males the weaker individuals, driven to the poorer feeding grounds by stronger males: sad bachelors destined to live a life of celibacy? Alternatively are they in some way better adapted to life in the more spartan environment upstream and still able to mate with females in the mating season?" To answer this last question Professor Altringham has compared the upstream and downstream males to see if there are any physical differences between them. "A few years ago we began to look at the morphology of the bats," he says. "At first I got very excited because we found differences in the wing shape that were functionally very significant. However, when we collected a bigger sample, the differences, although still discernible, became statistically insignificant.

"But there was still one interesting difference. The upstream bats have a lower wing loading. That is to say the ratio of body mass to the area of the wings is lower - the wings are carrying less load. Now some people might say that this is simply because they are skinny - there is less food about and they just go hungry. An alternative interpretation is that the bats are specifically regulating their body mass for the job they have to do. By having a lower wing loading their flight costs are lower and they are more manoeuvrable, making them better adapted to foraging on the shorterpools in the upper reaches of the river."

That the upstream bats work harder for their supper has become clear from radio-tracking studies. "We have attached small transmitters to individuals and followed them over a period of time," says Professor Altringham. "The upstream bats spend more time out when they are foraging and they travel further. One bat flew every night to Conistone, about 15km away, had a feed, then flew back. While that is an extreme example, the downstream bats will travel only a kilometre or less to a feeding patch."

Another strand of the Leeds research is to establish the role of caves in the life-cycle of the five species of UK bat that hibernate in Yorkshire's caves, including Daubenton's.

Until recently it has been thought that caves are used principally for hibernation. But work by Professor Altringham's group in the North York Moors has shown that bats congregate in huge numbers in caves well before hibernation. "Nicky Green, a PhD student, is looking at populations of Natterer's bats that swarm in a group of caves near Helmsley," he says. "Most nights in late summer and autumn tens to hundreds of bats gather in the caves. We know from ringing studies that they come from over 60km away, and many of them visit several caves in one night."

It seems entirely possible that the caves are places for socialising - nature's equivalent of a nightclub - where the bats from many disparate communities meet to mate, thereby providing a good genetic mix and preventing inbreeding.

Glover is carrying out parallel studies in caves in the Yorkshire Dales, where similar swarming events occur. By studying the movements and DNA profiles of all five bat species it should be possible to see if the bats of each dale are isolated, or whether their autumn search for mates takes them all over the national park.