Ten years ago, I led a field trip to Silver Lake State Park in Barnard, Vermont. A few parents and a gaggle of children – bedecked in flippers and waterwings – lugged our gear to shore. Soon some kids came up the path toting a plastic bucket.
“Something’s wrong with these frogs!”
“Yeah, some don’t have any feet.” Inside the bucket squirmed scads of young green frogs missing toes, feet, or the entire lower half of one or more legs.
“That’s gross!” said a budding scientist. “What happened to them?”
I explained theories about what causes abnormalities, but my answer didn’t satisfy because there are too many possibilities. Ten years later, we have more information but an even a greater number of questions.
Rick Levey, an environmental scientist with the Vermont Agency of Natural Resources, says, “Amphibians are hard to study without doing exhaustive field studies.” Discerning the effects on individual species requires detailed research of each possible cause and its impacts.
Frogs truly are at one with their environment and everything it contains. Their eggs grow and develop immersed in water with no shell for protection. Frog skin is highly permeable – water, air, and anything those mediums contain can pass through it.
Vermont Agency of Natural Resource researchers have collected more than 11,000 northern leopard frogs since 1996, primarily young-of-the-year froglets that hatch in early July. According to Levey, “Abnormality rates range from 0 to 10 percent. Abnormalities include truncated limbs, missing digits, or no limbs at all.”
Mark Ferguson, who is a zoologist with the Vermont Fish and Wildlife Department’s Nongame and Natural Heritage Program, says, “I have seen northern leopard frogs that were missing eyes and even some with extra limbs.”
Northern leopard frogs and green frogs are most affected. Other local amphibian species with abnormalities include the wood frog, mink frog, bullfrog, pickerel frog, and American toad.
The many possible causes of amphibian abnormalities include pesticides, fertilizers, and increased exposure to ultraviolet radiation. In addition, parasitic worms called trematodes may damage cells in tadpoles and cause abnormal legs and extra limbs to grow, but research shows that, while an issue elsewhere in the world, this is not a significant cause of deformities in our region.
Ferguson says, “It’s probably a combination of things compounding each other.”
Depletion of the ozone layer in the upper atmosphere in recent decades has increased the amount of UV-B radiation that amphibians are exposed to. High levels of UV-B can cause abnormal growth.
The algae that tadpoles eat in environments contaminated with pesticides contain toxic substances that build up in tissues. Pesticides become concentrated in dry years as the volume of water shrinks. This, coupled with higher temperatures, leads to more deformities. Levey cautions, however, that researchers in Vermont “never found levels of pesticides that were high enough to cause limb abnormalities.”
Fertilizers washing into waterways can cause the growth of certain kinds of blue-green algae that generate toxins that have a similar effect on frogs as retinoic acid – a hormone-like substance also found in pesticides that causes deformed limbs.
Pharmaceuticals and cosmetics can disrupt normal hormone function in frogs. High concentrations of compounds from personal-care products and medicines (that we flush down the drain) may cause effeminization in frogs: males have reduced or non-existing male sex organs or begin to grow female organs. Research shows that pesticide levels one-hundredth of those needed to cause limb deformities can cause sex changes.
Simply not knowing the timing of an impact can pose a dilemma when sleuthing an abnormality. In dry years, when tadpoles cluster by the hundreds or thousands in their pools, predators, and even other tadpoles, may nibble on the young and damage the “buds” from which arms and legs grow. A month later, those young frogs may have deformed limbs.
Even if abnormal frogs survive for a time, it’s hard for them to catch food and escape predators when they can’t hop effectively. If abnormalities continue to increase, population levels will likely decline. “Deformed frogs may be weak going into winter because they’re not as mobile and aren’t feeding as well,” says Levey.
Accurate, systematic records of sightings are invaluable. One schoolteacher, whose students studied frogs in the King’s Bay area near Plattsburgh, New York, in the late 1980s and early 1990s, discovered that an average of 15 percent of northern leopard frogs had deformities. Following national reports about amphibian deformities in 1996, 12 calls were made to report deformities in Champlain Valley frogs, but there had been only 12 to15 such calls in the previous 75 years, though this could be because few people were looking for them.
To report a sighting of amphibians with abnormalities in Vermont, call Rick Levey at (802) 241-1368. In New Hampshire, call (603) 271-5859. You can also enter sightings into the national database at: www.nwf.org/frogwatchusa.