Monitoring groups of animals in the wild is a tricky business. Fixing radio transmitters to them is invasive, and can alter their behaviour in unexpected ways. Hidden cameras is an alternative, but individual beasts are hard to tell apart. And nocturnal(夜行的) creatures are difficult to see in the first place. Fortunately, there is another less obvious way to monitor animals recently: by eavesdropping(窃听) on them.
A network of underwater microphones has already been used to count and track migrating whales by identifying their individual calls. At a meeting of the Acoustical Society of America in Columbus, Ohio, earlier this month, researchers from Ohio State University explained how they have now applied a similar technique to bats.
Working in the university's Bat Lab, Stephen Burnett and Mitchell Masters recorded a total of 1, 449 echo-location calls-the high-pitched sounds that bats use to probe their surroundings - from 24 big brown bats. They then used signal-analysis software to analyse each call according to ten numerical parameters describing length, time, frequency and so on. Cluster analysis, a statistical technique that groups things together on the basis of similarity, found 29 distinct clusters of calls, which was reasonably close to the actual number of bats.
Indeed, even when presented with just two calls from each bat, rather than several dozen, the software provided a fairly accurate estimate of numbers. This suggests that, by recording bat sounds in one place over the course of a few nights, it should be possible to estimate the size of the local bat population.
Similar bio-acoustic(生物声学的) techniques are being tried on other animals. Christopher Clark of Cornell University, who pioneered the acoustic monitoring of whales, is now involved in a project to monitor elephant populations in the Central Africa. Unlike their tropical cousins in eastern and southern Africa, these animals are mostly resident in forests. That makes them difficult to count by such conventional means as flying over them with a pair of field-glasses.
Using microphones, Dr. Clark hopes to identify both the elephant making each call and the place the call was made from. It should then be possible to determine the population, track the migration of different groups of animals, and monitor their health—merely by listening.
