CHICAGO, Nov. 17ùUltraviolet radiation, entering the environment in increased doses because of ozone layer depletion, may be a cause of frog deformities that have concerned and puzzled researchers, scientists said today.
The Environmental Protection Agency's Mid-Continent Ecology Division in Minnesota said the finding was a preliminary one and it still believes the deformities -- most commonly involving missing, misshapen or misplaced limbs -- are due to a combination of factors.
In tests at the facility, batches of fertilized leopard frog eggs were exposed to ultraviolet radiation and to water containing various levels of a mosquito control pesticide. Other batches of eggs were exposed to the pesticide but not the radiation.
When the tadpoles hatched and developed into frogs, almost half of those that had been exposed to the ultraviolet radiation for more than two weeks had deformed limbs. Those that were exposed to the pesticide but not the radiation showed no abnormalities.
"We've demonstrated in the laboratory that we can cause some kinds of abnormalities in amphibians, but we have not demonstrated that it occurs in nature," said Steven Hedke, acting director of the EPA facility in Duluth, Minn.
"That is our next step," he said in an interview.
The finding may not have a direct implication for human embryos because they are protected in the womb, unlike those of frogs, which are left floating in water and relatively unprotected, he added.
"We still believe there's probably a combination of events that are causing the effect in nature," Hedke said.
The lead researcher in the experiment, Gary Ankley, published a previous paper in which he said documented increases in ultraviolet light blamed on a thinning of Earth's ozone layer often peak in late spring and early summer in the Northern Hemisphere.
That time period coincides with a critical amphibian reproduction window, he said.
Some amphibian populations have been reported in decline around the world. Certain species of frogs in Australia and Costa Rica, for instance, are thought to have become extinct in the last 20 years. Some reports have indicated that a parasite may be causing increasing mortality in some parts of the world, though the cause of the phenomenon remains under investigation.
Large-scale frog deformities were first detected in Minnesota in the summer of 1995.
The issue concerns scientists because frogs are considered a "sentinel" species, which reacts swiftly to environmental changes.
Last month the Minnesota Pollution Control Agency said its research on two wetland sites in the state indicated that one or more still undetermined substances in the water were the likely culprits.
The North American Reporting Center for Amphibian Malformations -- which posts its findings on the Internet at www.npsc.nbs.gov/narcam -- has received reports of malformations from three Canadian provinces and 38 U.S. states.
The new EPA research was to be discussed Thursday at a symposium on frog deformities during a meeting in San Francisco of the Society of Environmental Toxicology and Chemistry.
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