According to the boffins, it shouldn't have left the ground

Awareness of Britain's bat population is very much improved. But, writes Daniel Butler, there's still much to learn about these strangest of mammals.

"Take a good look at your local bats now. You're not going to see much of them for the next five months: they're going torpid." Chris Thain, of the Radnorshire Wildlife Trust, peers through the narrow window built into the gate closing off a disused railway tunnel on the Gilfach Nature Reserve. "I found a natterer's bat settling down here last week," he observes. "It's an ideal hibernation site: humid, frost-free and with a steady temperature."

His interest is indicative of a transformation in our attitudes to the largest but least-known group of British mammals. Given this country's reputation for pioneer zoology, it seems remarkable that until recently bats were almost totally overlooked; the first specialist book on British bats was not published until 1946. It was the Wildlife and Countryside Act of 1981 that really got things going: "This gave protection for the first time," explains Tony Hudson, senior conservation officer at the Bat Conservation Trust. "There was a huge boost in interest, and dozens of local bat groups were formed almost overnight."

The result was that amateurs began to add significantly to scientific understanding: "Bats used to be considered hard to study but in fact they are easy if you track them down to their roosts - particularly if you use ultrasound," says John Messenger, of the Vincent Wildlife Trusts.

He illustrates the shortfall in knowledge by pointing out that when he started his present job in 1984 there was only one bat species on the Radnorshire mammal list; 10 others have since been discovered. Indeed, it was this sudden interest that led an amateur bat watcher to notice that his local pipistrelles had two ultrasonic call ranges. This was followed by further tests: "They look identical, but DNA testing shows them to be more different than tigers and lions," says Messenger. This means Britain's 14 resident bats will soon become 15, once the newcomer has been formally named and catalogued.

Similarly, it was enthusiasts, such as Ginni Little, of the Penzance Bat Hospital, who noticed that "vagrants" (blown in from Europe by storms), were significant: "A local tree surgeon brought in an immature male Nathusius pipistrelle," she explains. "Because it takes two years for these to reach sexual maturity, this meant it must have been born here." Other records soon confirmed that it was relatively widespread: now it's an official migrant.

Although many bats don't hibernate properly - becoming active during warm spells throughout the winter - most are now becoming torpid at the end of a frenetic autumn spent hunting insects. Those still visible are likely to be this year's youngsters, trying desperately to build up body fat for the lean winter months ahead. However, as temperatures fall and insect numbers plummet, the animals will spend more and more nights resting in hibernation sites. Abandoned mines, caves and culverts are favoured by most species, but others, such as the noctule, prefer hollow trees.

Such hibernation patterns are probably better understood than bats' mating habits. Although gestation is only three months long, births occur in June. This is thanks to delayed fertilisation, whereby females store and nurture male sperm cells within their bodies through the winter. How they overcome rejection by their own immune systems is still not understood.

The heavily pregnant females also manage to hunt while carrying a foetus that can weigh more than a quarter of their own body weight. Once born, the youngster is raised on the richest milk of any terrestrial mammal and, depending on the species, within three to six weeks it is hunting alongside its mother.

For most people the pipistrelle is much the most likely bat to be encountered. This is mainly because of its fondness for modern buildings. Any bat seen emerging from the eaves on a summer evening is almost certain to be a pipistrelle (although such roosts are abandoned in winter for more thermostatically stable subterranean lairs). Like all our bats, they are insectivorous, hunting with ultrasound, principally at dawn and dusk, making use of high-frequency echo-location to detect their prey.

Although the principles are now well understood, until recently it was a puzzle how such a tiny creature manages to fly while carrying a powerful echo-location device. This was calculated to be as aerodynamically "impossible" as the flight of the bumblebee, until it was discovered that the same muscles are used for both flight and sound-generation. Even so, scientists still do not fully understand how bats predict when hunting forays will burn up more calories than they produce. Thus, even on a summer evening bats may choose to roost if insects are scarce, yet they may hunt at midday during a mild January. So if your local bats have already disappeared, it is temporary - you may well see them before Easter.

Where to find out more about bats: 'Bats' by Phil Richardson (Whittet Books, pounds 7.99) is an excellent general guide. The Bat Conservation Trust produces a free leaflet: send sae to 15 Cloisters House, 8 Battersea Park Road, London SW8 4BG. Anyone finding an injured bat should contact the Penzance Bat Hospital, which runs a 24-hour help line (01736-365687).