Scientists trying to figure out what's causing frogs from California to Maine to sprout extra legs have come up with a possible answer that seems absurdly simple: vitamins.
A group of vitamin A compounds called retinoids may be at least partly responsible for the frog deformities that have been found in 32 states, said David Gardiner, a specialist in human limb regeneration at the University of California at Irvine.Retinoids are vitamin A compounds and include retinoic acid, a hormone that regulates several key aspects of development in vertebrates. Excess amounts of retinoic acid produce birth defects in humans.
Deformed frogs were noticed first by schoolchildren in 1995 as they studied a farm pond near Henderson, about 55 miles southwest of Minneapolis. They found about 200 frogs with multiple, missing or twisted legs. A few others had abnormal eyes.
Finding the cause of the deformities is important because scientists believe amphibians may be an early barometer of environmental problems.
Gardiner said he suspected retinoids last year when he saw pictures of deformed frogs taken from a Minnesota lake.
The frogs his team studied had one or more leg segments that, instead of being straight, folded nearly in half to produce a "triangulated" appearance. The deformities also included fewer fingers and one bone instead of two in the forearm.
Gardiner and Bruce Blumberg of the Salk Institute of La Jolla, Calif., found evidence of retinoids in water samples from the Minnesota lake where the frogs were found.
The substance may have come from a pesticide or pesticide derivative, or may even be a natural compound produced by an organism in the lake, Blumberg said this week.
The deformities were consistent with those seen in chickens, mice and other vertebrates exposed to retinoic acid in experiments, they said.
Though confident of the findings, Gardiner said they were only a starting point. Other factors may contribute to the deformities, he said.
Scientists around the country have been trying to unravel the mystery of deformed frogs. Chemicals or parasites were thought to be the culprits elsewhere.
Researchers at the Minnesota Pollution Control Agency said they knew from the beginning that retinoids might be to blame. But they cautioned against making too much of the findings.
"It would be a mistake on our part to stop looking at all the other possible causes," said Judy Helgen, one of the agency's scientists.