Scientists finally discovered why the platypus has a duckbill. It is a kind of antenna for picking up the weak electrical signals emitted by prey animals moving about in murky waters.

The platypus, a primitive egg-laying mammal native to Australia, is a semiaquatic species that feeds entirely on frogs, fish, crustaceans and other small animals captured underwater.

The prey's muscles, like those of all animals, operate by means of electric currents. Whenever electricity flows, it produces an electromagnetic field in the space around the current, much as does a radio transmitter's antenna.

The field produced by the human heart muscle can be detected by an electrocardiograph (ECG or EKG) machine.

According to scientists who studied platypuses in the wild and in the laboratory, the animals swim along the bottom, swinging their bills from side to side two or three times a second, apparently scanning for signals from small animals hidden in the rocks and mud. When the animal detects a life form, it snaps its bill repeatedly at the object of interest, trying to bite and swallow it.

The researchers established that the bill is sensitive to electrical fields after carrying out an elaborate series of experiments involving measured electrical currents emitted underwater. The easiest experiments to understand involved dropping small batteries in the water. The platypuses quickly found and tried to snap up the live batteries while ignoring dead ones.

Although some fish, such as sharks, and certain amphibians are known to have such electroreceptors, the new report, published in the British scientific journal Nature, is the first to find a mammal with the ability.

The findings were made by Henning Scheich and Gerald Langner of West Germany's Darmstadt Technical University in collaboration with Chris Tidemann, Roger B. Coles and Anna Guppy of the Australian National University in Canberra City.