Dunn is an associate of the Center for Investigative Reporting

You may have heard of them: frogs so deformed that even a veteran wildlife researcher's jaw will drop in amazement. Many species of frog, toad, and salamander have been disappearing worldwide for over a decade. But widespread deformities open a new and disturbing chapter in the mystery of the frogs. Something in the pond water is causing thousands of these animals to develop defects. And some scientists believe that hormone-mimicking pesticides are to blame.

Deformed frogs were first spotted in large numbers in August of 1995 by middle school students from the Minnesota New Country School in LeSueur, Minnesota. The students were on a field trip to study a nearby wetland area. To their surprise, approximately half of the frogs they saw that day had breath-taking deformities. These included missing limbs, extra limbs, and abnormal eyes. The school notified the Minnesota Pollution Control Agency, which has since launched an extensive investigation.

Since that time, deformed frogs have been found throughout the country. One frog sported nine legs, and another had an eye growing in its throat. Problems in the digestive and reproductive organs frequently accompany the external deformities. And deformed frogs rarely survive the winter of their first year.

In all, almost forty U.S. states and three Canadian provinces are affected. Large numbers of deformed frogs have been found in Minnesota, Wisconsin, California, Vermont and Quebec. Reports like these actually date back to 1750, and deformed frogs were discovered at a pond in California a full decade ago. But the reports were rare. Historically, perhaps one among tens of thousands of frogs was malformed. Today, up to 96% of the frogs at a given site have deformities.

It appears that chemicals in the water may be to blame. While a wide range of frogs are affected, species like the northern leopard frog that spend the most time in the water suffer most gravely. Also, water taken from ponds where deformed frogs live can create deformities in the lab. Higher doses of the pond water lead to more deformities, and the effect is reproducible. Furthermore, the defects do not appear to have a genetic cause, since they strike multiple species and have surfaced suddenly in large numbers. Researchers are currently testing water, pond sediments, and frog tissues for contaminants and microorganisms that could be responsible.

So far, the top candidates are manmade chemicals like the pesticide methoprene, which is used primarily for mosquito control. Methoprene's breakdown products are powerful hormone mimics. They mimic a naturally occurring substance called retinoic acid: a vitamin-A derivative which, during development, signals where a limb should go. In other words, it instructs the embryo to "put a leg here." By mimicking retinoic acid, methoprene can disrupt the normal signals that tell the embryo where its limbs should be, and how many should form. Chickens, mice and frogs exposed to retinoic acid in laboratory experiments show similar limb deformities. UV light may strengthen the effects of methoprene, by breaking it down into its most toxic forms.

Recently, researchers have found evidence that retinoic-acid-like compounds are indeed present in one of the Minnesota lakes that produces deformed frogs. The precise chemicals have not yet been identified, and could be either natural or manmade.

Still, no one knows for sure what is causing the deformities--and theories abound. Bacteria, viruses, ultraviolet radiation, pesticides, toxic metals, leg-munching predators or some combination of these factors are among the proposed culprits. In the case of one Sierra Nevada pond, parasites called trematodes were causing multiple limbs to form. This parasitic flatworm creates cysts in a frog's tissue, forming a physical barrier to limb growth. Developing limbs will split in two around the cysts so that two legs form where one should be. Plastic beads embedded at the site of a developing limb bud can have the same effect. However, parasites and other infections have been ruled out as the cause of deformities in Minnesota.

Experts think of frogs as sentinel species, and are concerned about what the abnormalities might mean for humans. It is logical that frogs would be among the first species to feel the effects of chemicals in the water. From eggs floating on the surface of a pond, to wriggling tadpoles, to adults, frogs spend their lives immersed in water. These animals easily absorb pollutants through their skin, and are highly sensitive to hormones and other chemicals that disrupt metamorphosis. Environmental factors that affect frogs may have more subtle effects in other species.

The research is still ongoing, and it could be some time before scientists have an answer to the mystery of the frogs. Teams have collected water and sediment samples from affected and unaffected ponds to see what the differences between these ponds may be. The water will be tested for contaminants like pesticides, metals, and organochlorines. Researchers are also considering testing the water for its ability to cause birth defects in rodents.

The Governor of Minnesota has given the Minnesota Pollution Control Agency special funding to study frog deformities. And the National Institute of Environmental Health Sciences, the U.S. Geological Survey and the U.S. Environmental Protection Agency are involved nationwide. These groups have initiated monitoring programs, encouraging students and concerned citizens to report frog deformities in their area. Monitoring will help determine the extent of the problem, and identify important trends over time. So, hop to it! You could be the next to spot one of these fascinating frogs.