On an August day 15 years ago, eight Minnesota junior high school students on a field trip caught 22 frogs in a farm pond. At least half of the frogs had some abnormality, mostly in their hind legs. The conscientious teacher reported the group’s finding to the state. Dutiful state scientists surveyed wetlands across Minnesota and found at least one hotspot of frog abnormality in every county in the state.
It might have ended there, with the only conclusion being “don’t be a frog in Minnesota.” But over the next few years, scientists in Wisconsin, Quebec, and Vermont found shockingly high rates of frog abnormalities in those places, too.
Scientists in New Hampshire, who surveyed the state in 1998, found the highest rate of abnormalities was 9 percent, in the Great Bay National Wildlife Refuge. The average rate of abnormalities in the state was 3.6 percent. Because the rate of abnormalities was considered to be low, there was not much further research done.
In Vermont in 1997, 35 percent of the frogs captured in a marsh on the Poultney River, in the southern Lake Champlain valley, had an abnormality of some kind.
Rick Levey, an environmental scientist with the water quality division of the Vermont Department of Environmental Conservation, was there from the beginning. “I was busy back there from ’97 until 2001 or so.”
Levey worked with Vermont Reptile and Amphibian Atlas director Jim Andrews to survey frogs throughout the state, eventually focusing on frog abnormality hotspots in the Champlain valley. He also helped other researchers gather frogs for various studies.
Although regular headlines in recent years have declared, “Frog Mystery Solved,” Levey says, “The verdict isn’t in.”
Certainly, we know more now than we did ten years ago. Between Levey and Andrews’ research, and research conducted in Vermont by Yale University scientist David Skelly, a few things seem clearer.
The frog abnormalities were mostly found in lowland locations, near farms and residences. Most of the frogs with abnormalities were northern leopard frogs, with green frogs also showing a high percentage of abnormalities. These facts make their own sense, since river floodplains are not only home to Vermont’s most productive farms, but are also the leopard frog’s breeding habitat.
Also, 79 percent of the abnormalities found in Levey’s study were in the frog’s back legs. Further, Levey says, most of those back leg abnormalities were found on only one leg. And, while frogs that had more than the usual number of legs or leg parts were among the more horrifying abnormalities found nationwide, Levey found just one of those in over 5,000 frogs captured in the Vermont study.
So when Levey read that ultraviolet radiation could cause deformities in frogs, he saw that these deformities almost always affected both legs, so that didn’t explain most of the Vermont cases. Similarly, when he read that a parasite of frogs, called a trematode, could case extra limbs to form, he knew that wasn’t the cause of abnormalities in Vermont, because he had found only one frog with that problem.
Levey found atrazine, an herbicide, in some of the wetlands he studied. But other researchers had found that this mostly causes problems with frogs’ sexual organs, and that also didn’t explain the missing legs.
Andrews’ impression of what he and Levey saw a decade ago leads him to think that the frog abnormalities seen in Vermont were due to several factors. He thinks bug predators, such as dragonfly larvae, may be nipping off the buds of hind legs as they sprout from the sides of the tadpoles, or that the tadpoles are getting crowded as floodwaters recede, and are nibbling on each other. Run-off from farms and houses may introduce nutrients that cause more, or allow different, predators to haunt the floodwaters.
Levey has studied frogs long enough to see the number of abnormalities per year spike up to 20 or 30 percent and then drop down to two or three percent, more than once. He’s also seen a collection of frogs from the New York side of the lake in the 1970s and 1980s, which suggests that this is not a new phenomenon.
He’s not ready to write the whole thing off to predators, though. He’s seen X-rays of the frogs he collected that show a few with malformed hips that don’t appear to be from predator bites. He has no explanation for those abnormalities. For most of the frogs, though, he agrees predators might be an important cause. He points to stress – the stress of losing habitat and of living in a more polluted environment – as a contributing factor.
Fifteen years after that Minnesota field trip, we still don’t know what causes frog abnormalities. Research is still being done, but the pace has slowed. Levey says, “I’m never going to forget how humbling researching the causes of these abnormalities has been.”