Evolution is saving elephants in Africa by producing herds with tiny tusks or none at all - which provides no profit for poachers and thus ensures the survival of the species.

The phenomenon has been noticed in all parts of Africa where hunting has been going on longest, with both trophy hunters and poachers always shooting the elephants with the biggest tusks.A survey in the Queen Elizabeth National Park in Uganda in the 1930s showed that only 1 percent of adult elephants were without tusks. Then it was regarded as a rare mutation.

This year Eve Abe, of the Ugandan wildlife authority, found that 30 percent of adult elephants in the same area were without tusks.

Richard Barnwell, World Wide Fund for Nature conservation officer for Africa, said the trend toward elephants having smaller tusks or none had been noticed all over the savannah area of West Africa, where elephants had been hunted longest.

"All the elephants with genes that produce big tusks have been taken out of the population. Those that remain either have small tusks or none at all."

He said it was now rare to find a big tusker in Cameroon, Nigeria, Ghana, Ivory Coast, Niger or Mali.

Another attribute aiding elephant survival is bad temper. Elephants were hunted almost to extinction in South Africa at the turn of the last century. One small herd in what is now the Addo national park on the edge of the Indian Ocean survived, however. Barnwell said this was partly because these elephants were known to be very bad-tempered and did not have particularly large tusks. "Elephants are very intelligent and can be very dangerous if they are prone to bad temper. Hunters decided that trying to kill them was not worth the risk, so being bad-tempered is a survival technique, too."

Poaching in the Queen Elizabeth park reduced elephant numbers from 3,500 animals in 1963 to 200 in 1992. Now the population is 1,200 and is growing quickly. The difficulty of finding an elephant with large enough tusks is defeating commercial poaching.

Lack of tusks is not all good news for elephants, however. Bulls fight for the right to mate with females, and in this respect large tusks are a big advantage. This is why bulls with big tusks developed in the first place.

An additional advantage is that tusks are used as tools, particularly in the dry season for digging in river beds looking for water. Campbell said this did not particularly matter in the Queen Elizabeth national park because water was plentiful, but for the dry savannah elephants it could be crucial.

In parts of central Africa, elephants are hunted for their value as meat, so even being without tusks is no help.

He added: "The fact is that elephants with big tusks would come back if we stopped hunting them. Large tusks are an adaptation that took place to help survival. The message of all this is that we are forcing a change in elephants which is not necessarily to their advantage. If they are to survive, we need to look after them."