Intense poaching by ivory hunters has caused a dramatic shift in the gene pool of Asian elephants, leading to a steep rise in tuskless herds.

Asian elephants are under more intense pressure from ivory hunters than their larger African cousins. There are believed to be no more than 50,000 spread across the Indian subcontinent and Indochina, around 10% of the number found in Africa.

Male elephants usually grow tusks, but typically around 2%-5% have a genetic quirk that means they remain tuskless. By killing elephants for their ivory, poachers make it more likely that tuskless elephants will mate and pass on the quirk to the next generation.

Zhang Li, a zoologist at Beijing Normal University and a member of the World Conservation Union's Asian elephant specialist group, studied herds in China and found that up to 10% were tuskless.

"The larger the tusks the male elephant has, the more likely it will be shot by poachers," he told the China Daily newspaper. "Therefore the ones without tusks survive, preserving the tuskless gene in the species."

The illegal trade in ivory has also skewed the sex ratio of the elephants in China, with females now outnumbering the males by four to one.

Some African countries have shown that increased policing of national parks and tighter controls on ivory can reverse the decline in numbers, but poachers are devising new strategies to evade officials. Instead of using guns, which attract rangers, many have switched to crossbows and target soft areas on the elephants, such as their mouths.

"It can be a slow and painful death over a few days and the poachers will follow the elephant until it dies and then cut the tusks out," David Cowdrey, WWF's wildlife trade campaign director, said.

He added: "As long as people are willing to pay high prices for products which come from endangered species, they're going to have a price tag on them and that fuels the poaching. Unfortunately, it comes down to the markets, which are in the west."

Last November, police raids in London and Gloucester seized some 80kg (176lb) of ivory and 141 ivory products.

Later this week, British legislation will be amended to make the buying and selling of products from endangered species an arrestable offence.