Dr Karen McComb, an animal behaviourist from the University of Sussex on England's south coast, has recorded calls made by females and shown that individuals may recognise the calls of as many as 150 others. Their response depends on how well they know the caller.

"Such extensive networks of vocal communication have not been demonstrated in any other mammal," McComd says.

The aim of her research, carried out in collaboration with Cynthia Moss, is to understand the social structure of elephants, which is based on a series of family units consisting of adult females and their young. These families are very mobile, forming "bond groups" with closely allied families and looser groupings with other families living further away.

"Elephants seem to be doing quite complex social things," she says. "They are not just sitting there and communicationg with their immediate neighbours."

The early hunters believed that the noises made by elephants were the rumbling of their stomachs but they are, in fact, sounds produced by vocal chords so huge that the basic frequency is 20 cycles per second or less, below the range of human audability.

"We can hear them because we hear the harmonics that go up into the audible range," she says. "But we don't hear them as the elephants do."

She has made a library of recordings of the calls of individual elephants and played them back through special speakers.

Tests showed that elephants respond to the contact calls of family and bond group members, moving in the direction of the call and calling back. When they heard the call of a stranger, however, they either ignored it or became agitated. By comparing these responses with detailed records of the elephant's relationships and movements, she found that the elephants' responses depended on their familiarity with those calling.

"What we found is that the ones they see fairly regularly, they tend to ignore. They get bothered by the ones they don't see very often," she says.

Because the sounds are such a low frequency, they carry a long way and can be recognised from at least 1,5 km.

"It's amazing when you see an elephant turn towards a sound you yourself cannot even hear," she says.

Some people have argued that the elephants' alarm signals can be heard over even greater distances.

McComb recently received a grant from the British Biotechnology and Biological Sciences Research council and intends to return to Amoseli.

The question to be answered is what enables elephants to develop such large networks of communication.

"Elephants move around a lot, so they are exposed to a lot of signals," she says, "but it may also be to do with experience - do the groups with the oldest elephants have larger networks because they have had longer to learn?"

- The Times, London

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