'Sex change chemical' threatens frogs

Leopard frogs are the most common US frog species

One of the world's most commonly used herbicides, atrazine, poses a serious threat to frogs and other amphibians, American scientists claim.

The researchers studied male leopard frogs across a large area of the US and found a high proportion to be developing female characteristics where they swam in waters contaminated with the herbicide.

The survey of the wild animals, published in the journal Nature, builds on laboratory work which also suggests that male frogs can be feminised through exposure to the weedkiller.

"These studies clearly indicate that atrazine is detrimental to amphibians," said Dr Tyrone Hayes, associate professor of integrative biology at the University of California at Berkeley.

However, the work has been questioned by the primary producers of atrazine, the Swiss-based agrochemical giant Syngenta.

It says the effects noted by Dr Hayes' team have long been observed in the wild - even before atrazine was marketed - and there is no established evidence to link the two.

Wild results

Amphibians are declining worldwide. Scientists believe many factors are at play, from climate change to attack by parasites.

The work by Dr Hayes and colleagues would suggest that agrochemicals are also playing a significant part in the decline.

Earlier this year, they reported that male African clawed frogs (Xenopus laevis) raised in laboratory tanks contaminated with atrazine developed egg cells in their testes - they became hermaphrodites.

This feminisation process, they found, would occur in water with atrazine levels as low as 0.1 parts per billion (ppb), 30 times lower than the current allowable limit for atrazine in drinking water set by the US Environmental Protection Agency.

Dr Hayes' team has now repeated the work on male leopard frogs (Rana pipiens - one of the most common frog species in North America).

These, too, became demasculinised when raised in lab tanks. But the Californian researchers have gone a step further by looking for similar effects in the wild.

Comparative studies

They sampled leopard frog tadpoles in eight separate ponds, ditches, rivers and streams in the Midwest during the summer of 2001 and say they found feminised male frogs at every site with measurable levels of atrazine.

The sites were scattered through the Corn (maize) Belt and beyond, including in Utah, Wyoming, Nebraska and near the Iowa-Illinois border.

The site with the highest concentration of feminised frogs was along the North Platte River in Wyoming. There, 92% of male frogs showed sex reversal.

This area of Wyoming reports little use of atrazine, but the river is fed by streams that carry run-off from Colorado farms, which do use significant amounts of the herbicide.

Other species

Atrazine has been used to control weeds in maize and soy crops for 40 years, but concerns over its possible effects on human health have now led to it being banned in several European countries (but not the UK).

Syngenta disputes much of Dr Hayes' work. It says its experts have been unable to reproduce his laboratory work and have questioned the quality of his field work.

The company told the BBC: "The occurrence of hermaphrodites in the genus Rana has been previously observed, with the earliest reports appearing decades before the introduction of atrazine.

"Recent studies in other frog species have found no significant relationship between the occurrence of hermaphrodites and the historical and spatial usage patterns of this herbicide."

For his part, Dr Hayes has criticised the studies undertaken by Syngenta to check his work, accusing the company of not conducting its experiments properly.