Deformed Frogs: The Big Mystery
Since I was born, it has been a family tradition to visit my uncle‘s home in Vermont. We always went to Vermont a number of times during the summer months. My uncle‘s house borders the Green River in Windham County. Since I can remember, the majority of my time there has been spent observing the multitude of organisms that inhabit the river.
There are many different kinds of plant and animal life that contribute to the biodiversity of this location. My brother and I have caught such organisms as crayfish, black-nosed dace (minnows), brook trout, and water striders.
We recently observed Atlantic salmon, which were stocked in the river four to five years ago. We also spent many hours observing the many different insects that inhabit the river, such as mayflies, dobsonflies, and caddis flies. This river ecosystem is also home to a variety of trees, such as white pines, red oaks, red maples, sugar maples, yellow birch, hemlocks, beeches, and gray birch. There are also many sedges and reeds that live on the edge of the river. It is this area of the river that caught my attention. For here, nestled next to the flow of the river, is a large, isolated pool of water. Every Fourth of July vacation, this pool looks as if it is set off from the main flow of the river. It was in this area that my brother and I would go to catch the tremendous number of tadpoles that developed each year. The tadpoles we caught were mostly the tadpoles of leopard frogs and the American toad.
One September, on our Labor Day weekend visit, my brother and I were doing what we always used to do—catching the frogs that had developed from the tadpoles we observed during the Fourth of July weekend. What we found this particular year was something we had never seen before—a frog with a missing back limb! How could this happen?
I had learned how metamorphosis, the change from a larval stage to an adult, occurs in a frog. Toad and frog eggs are laid in the shallow water of a flooded field, pond, or at the edge of a stream. They hatch into larvae (tadpoles) that are entirely aquatic herbivores. During metamorphosis tadpoles lose their internal gills and tail, and develop lungs and limbs. This remarkable transformation turns the previously aquatic vegetarian into an air-breathing carnivorous toad or frog which eats primarily insects.
What could have happened to cause this one frog to develop with a missing limb? This find started my quest for answers. I immediately began obtaining books and articles on deformities in frogs. I soon began to realize that the cause of deformities in frogs is a big concern among scientists today. Scientists first began noticing a decline in amphibian populations about 15 years ago, but public concern really took off in the mid-1990s. In August 1995, students from the Minnesota New Country School reported that 50 percent of the frogs they had caught had deformities of the hind legs. Many had only one undeveloped leg, others had only one hind leg, while still others had two feet on one leg.
At this point, I began to wonder if this was a widespread problem across the state of Vermont. I decided to obtain maps of the counties around my uncle‘s home in Vermont. These maps listed reported sightings of deformed frogs in the area. The two counties I looked at closely were Windham County and Rutland County, which is located north of my uncle‘s property. When I went to see if there was any information listed for Windham County, this is what I found.
I was so amazed that there were even reports of deformed frogs! Was the frog I saw part of a growing trend? Did this report mean that there is only one deformed frog in Windham County? Why have people been looking? I still had a lot of unanswered questions. I began to research some of the possible causes of these deformities.I found out that four theories have been put forward to explain the deformities:
- Chemicals such as pesticides from nearby agricultural fields
- Parasitic invasions by a trematode
- Increased ultraviolet radiation due to ozone depletion
Could any one of these theories be the cause of the deformed frogs that are occurring in the various counties of Vermont?
Many people may question why we are so concerned about frogs. Frogs are considered a "canary in the coal mine" for potential human health problems because they are so sensitive to changes in the environment. Declines in their population levels, or changes in the health of individuals, are often the first indicator that things are out of balance in our ecosystem. I am really concerned that if something in our environment is affecting the frogs in a bad way, then what would prevent people in the same environment from being harmed?
I decided to investigate each of the possible theories. I wanted to start with the parasite theory. The most prominent researcher involved in this theory is Dr. Stanley Sessions, from Hartwick College in New York. Dr. Sessions believes that the leading suspect is a natural one, a microscopic parasite. The parasite, called the trematode, is found in snails in the same kind of ponds where frogs breed. The trematodes burrow into young tadpoles and attack the buds that will develop into limbs. In doing so, they can either split the limb bud into several parts, causing multiple limbs to form, or form a massive number of cysts, preventing the complete development of limbs. The diagrams that I have included clearly show this process.
As soon as it was realized that a trematode was a possible cause, it was necessary to look at the trematode‘s life cycle.
This picture shows that the trematode lives its adult life in a garter snake. The trematode eggs are released from the snake into the water, where they are picked up by aquatic snails. Once inside the snail, each trematode egg produces thousands of trematode larvae. The trematodes then develop into a swimming stage called a cercaria. The cercaria seeks out its second host, the amphibian tadpole. It bores into the tadpole‘s tissue to form a cyst. Could these trematodes be the cause of the deformities we‘re seeing in the frogs in Vermont? And if so, shouldn‘t we also see an increase in the snail population?
It‘s also a possibility that chemicals in the environment could be the cause of the malformations. According to Dr. David Gardiner from the University of California in Irvine, it looks as if these frogs might have been exposed to retinoids. There is a pesticide called methoprene that acts like a growth hormone. Methoprene is widely used against mosquitoes, biting flies, and fleas. Did someone use methoprene in my uncle‘s river to kill the black flies? Methoprene is a synthetic compound that acts like a retinoid to switch off the growth of insects before they become adults. Methoprene is so powerful that only one teaspoon of pesticide is enough to cover an entire acre. This chemical certainly should be considered as a prime suspect. In fact, the Food and Drug Administration warns that the acne treatment Accutane, which contains retinoids, may cause severe birth defects, including cleft palate and abnormal skulls, brains, hearts, and eyes. The drug carries clear warnings that it should not be taken during pregnancy. Do retinoids have the same effect on frogs? Many of them look as if they had been exposed to retinoids.
Another possibility that needs to be looked into is the effects of increasing ultraviolet radiation from the sun. Researchers such as Dr. Andrew Blaustien from Oregon State University have discovered that too much UV radiation from the sun is permanently damaging some species of frogs. The excess radiation could possibly weaken a frog‘s immune system, which makes it more vulnerable to parasites and disease.The last theory that I would have to look into would be that of predation. If a tadpole‘s leg were bitten off early in development, it might be able to regenerate a normal limb. However, if this loss happened later in development, the tadpole might have a missing limb after metamorphosis.
I think it would be best to summarize this situation with a quote from Rachel Carson: "The more I learned about the use of pesticides, the more appalled I became," Carson recalled. "What I discovered was that everything which meant most to me as a naturalist was being threatened, and that nothing I could do would be more important." The frog deformity, to me, is a big environmental problem that so far has not been resolved. It was because of the work of Rachel Carson that people started to realize the impact we have on our own environment. Because of the research I have done on deformed frogs, I feel that we must continue in the same direction as Rachel Carson, and look for the reasons behind such deformities. Because frogs are so sensitive to their environment, they may be good early indicators of an ecosystem going bad. We need to do everything in our power as humans to prevent this.
Bernstein, Leonard, et al. Concepts and Challenges in Life Science . Upper Saddle River, New Jersey: Simon and Schuster Company, 1998.
Bury, Chris. "Freaky Frogs, Big Mystery." ABC News Nightline. August 7, 1998. (Audiotape)
Carson, Rachel. Silent Spring . New York: Houghton Mifflin Company, 1994.
Chang, K. "Yuck! Cool! And It‘s Science: Deformed Frogs Introduce Pupils to the How-tos of Research." ABC News. Retrieved from the World Wide Web on October 27, 2002: www.abcnews.go.com/sections/science/DailyNews/frogonline990428.html
Deformed Frog Controversy Forum. University of California at Berkeley.
Retrieved from the World Wide Web on November 17, 2001: http://scope.educ.washington.edu/frogs/frog-links.html
"Frog Deformities Come With the Territory, Study Finds." Environmental News Network. April 1, 2000.
"Frog Malformations in Vermont." National Wildlife Federation‘s Northeast Natural Resource Center, Montpellier, Vermont. December 1998. Retrieved from the World Wide Web on November 23, 2001: http://www.npwrc.usgs.gov/narcam/info/news/frogfix/frogfix.htm
"Frogweb: Amphibian Declines and Deformities." U.S. Geological Survey‘s Center for Biological Informatics. August 24, 2001. Retrieved from the World Wide Web on November 30, 2002: http://www.frogweb.gov/index.html
"Location Reports: Sampling Effort." North American Reporting Center for Amphibian Malformations (NARCAM), Northern Prairie Wildlife Research Center, Jamestown, North Dakota. November 16, 2001. Retrieved from the World Wide Web on November 30, 2002: http://www.npwrc.usgs.gov/narcam/reports/reports.htm
"Minnesota Country School Frog Project." Henderson, Minnesota. September 8, 1999. Retrieved from the World Wide Web on October 27, 2001: http://www.mncs.k12.mn.us/html/projects/Frog/frog.html
Sessions, Stanley. "Deformed Amphibian Research at Hartwick College." (Ongoing research in collaboration with Geffrey Stopper, Louise Hecker, Vanessa Homer, Adam Franssen.) Department of Biology, Hartwick College, Oneonta, New York. Retrieved from the World Wide Web on November 21, 2001: http://www.hartwick. edu/biology/def_frogs/
Souder, William. "New Reports of Deformed Frogs Trigger U S. Ecological Alarms." The Washington Post, January 29, 1997. Retrieved from the World Wide Web on October 27, 2001: http://www.pmac.net/frogs2.htm
Copyright © 2002 American Museum of Natural History
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