change chemical' threatens frogs
frogs are the most common US frog species
of the world's most commonly used herbicides, atrazine, poses a
serious threat to frogs and other amphibians, American scientists
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.
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.
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.
the work has been questioned by the primary producers of atrazine,
the Swiss-based agrochemical giant Syngenta.
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.
are declining worldwide. Scientists believe many factors are at
play, from climate change to attack by parasites.
work by Dr Hayes and colleagues would suggest that agrochemicals
are also playing a significant part in the decline.
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.
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.
Hayes' team has now repeated the work on male leopard frogs (Rana
pipiens - one of the most common frog species in North America).
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.
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.
sites were scattered through the Corn (maize) Belt and beyond, including
in Utah, Wyoming, Nebraska and near the Iowa-Illinois border.
site with the highest concentration of feminised frogs was along
the North Platte River in Wyoming. There, 92% of male frogs showed
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.
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
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.
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.
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."
his part, Dr Hayes has criticised the studies undertaken by Syngenta
to check his work, accusing the company of not conducting its experiments