Beautiful sounds from foul singers

Bats may be ugly and smelly but, Daniel Bennett says, they sound wonderful.

Just after dusk we were on a canoe in the Mozambique Bay. Hundreds of bats were flying over the sea and the sound was almost deafening: squeaks, grunts, rattles and whistles punctuated with long, plaintive screeches. None of us had heard the bats calling before, although some of my companions had seen them every night of their lives. The bats were not singing, they were shouting into the dark and listening for tiny echoes bouncing off insects. Their calls were at frequencies well above the range of the human ear but, using a time expansion ultrasound detector, we were able to render them audible by slowing them down to a tenth of their original frequency.

Later, on the shore, we erected a net between two poles and took energetic sweeps at the creatures as they flew between us, around us, over us, all directions except netwards. Only when the bats retired shortly before dawn did we admit defeat. If bats can detect tiny insects on the wing, they have no trouble evading nets, however fine they are.

Close up, bats are not for the faint hearted. Even people who love them admit some are hideously ugly. Evolution has given them ridiculous ears and grotesque noses that aid their bizarre foraging tactics. Their insect diet creates a glut of protein which is secreted as various unpleasant ammoniacal com£. Nevertheless, insectivorous bats are important to people because of the huge numbers of harmful invertebrates they destroy. And as many bat scientists will confirm, in the dark, plugged into the high frequency world of bat song and at sufficient distance to negate the stench, bats are extremely beautiful creatures. They fly with the grace and strength of birds of prey, and with the speed and agility of swallows. While their songs are not exactly melodic, they are unusual and many people find the sound extremely soothing.

In the 1790s, Lazaro Spallanzani and Charles Jurine made the discovery that bats seemed unable to avoid obstacles if their ears were sealed with wax. Only much later it emerged that insectivorous bats had evolved sophisticated ultrasound, not only to navigate but also to find and chase their tiny prey. Like birds, different species have their own distinctive calls. Unlike birds, however, it is virtually impossible to watch bats while they are calling. For many years, scientists have listened to bats in Europe, Australia and the Americas, but those of Africa and Asia have been almost completely neglected. Our team of 24 from Madagascar, the United Kingdom and Russia has been building the first library of bat calls in the region over the past two years. Madagascar has an interesting collection of bats; species from Africa and Asia as well as many endemic species. However, given their tiny size, nocturnal habits and deft flying skills, virtually nothing is known about their ecology and conservation status. Furthermore, they do not like intrusive scientists as they are easily disturbed and find capture and handling traumatic. Although our study had to involve some handling of bats, once the calls of different species have been characterised, it should be possible to determine which bats are active in an area at any time, without impinging on the creatures' liberties.

Nevertheless, in order to compile the library, it was necessary to catch representatives of each species, record their calls, and use these clues to find and record individuals foraging normally. We selected sites in the rainforests of the Masoala Peninsula, the central high plateau and the deserts of the southwest. Initially, we blanketed large areas with nets, waved traps in the air and tried to make sense of the elaborate racket of whistles, clicks and groans that filled the higher airwaves. Gradually, as we were able to distinguish one call from another, we evolved from mere bat catchers to bat hunters, stalking individual animals night after night, learning their activity patterns and finally pouncing on them with cunning ambushes. We learnt the calls of 22 species, including the sucker-footed bat Myzopoda aurita , named for the adhesive disks on its wrists and ankles that allow it to roost inside rolled up leaves. It is the only species of its kind and it emerged that this creature is more common than previously thought. On the other hand, we found no trace of another three species that are either extinct or have restricted ranges.

We hope our sound library will encourage researchers to use our methods to investigate other endangered bat communities in Asia and Africa. We also hope our work will help people to appreciate these special animals. We cannot deny that bats are ugly and that they smell bad. But if we can appreciate bats from a distance they are transformed into elegant and sophisticated animals with more than an air of mystery about them.

Daniel Bennett is a zoologist at the University of Aberdeen, who travelled to Madagascar this summer after winning a BP Conservation Award. You can learn more about the Madagascar Bat Project