In August 1995, schoolchildren found deformed frogs in a wetland near Henderson, Minnesota. Some frogs had extra legs, others no legs at all. Some had missing or extra eyes, toes, or feet. And some also had problems with their internal organs. By the fall of 1996, there were over 200 reports of freakish frogs, from two-thirds of Minnesota's counties. Deformed frogs have since been found in 44 states.

Deformed frog: This frog has two right back legs. Others have been found with missing legs, missing parts of legs, or legs in unexpected places.
Deformed frog: This frog has two right back legs. Others have been found with missing legs, missing parts of legs, or legs in unexpected places.Courtesy Minnesota Pollution Control Agency

A 1997 study raised frogs in the lab, mixing pure water with water from two Minnesota sites that had lots of deformed frogs. The more pond water that was used, the more likely the lab frogs were to be deformed. Water from sites with healthy frogs produced healthy animals in the lab. The scientific conclusion was, "There's something in the water." But what could it be? Since then, several researchers have been hunting for the cause.

Scientists have proposed several explanations for the deformities. It may be parasites, chemicals, ultraviolet light, or some combination of the three. Lab studies have shown that all of these factors, alone or in combination, can cause some deformities. But no single cause seems to explain it all. The research doesn't yet add up to a neat and tidy answer, so scientists continue to puzzle out the story.

Who cares about frogs? You should. If there's something wrong with the water, it may eventually hurt all of us. But it will hurt frogs first. Frogs have thin skins, and easily absorb any contaminants in the water. Frogs seem to be in trouble all around the world. There are more and more reports of deformities. And some species have disappeared, or no longer live in their old habitats. It's a wide-spread problem that may affect us all.

Your Comments, Thoughts, Questions, Ideas

bryan kennedy's picture

I wonder if the deformities in the different frogs affects how their behvior. Does a three legged frog get along well with her other frogs? Does this kind of mutation make the frogs easier prey?

posted on Thu, 02/03/2005 - 8:01am
Anonymous's picture
Anonymous says:

we need information of deformed frogs made by pollution.

posted on Tue, 04/18/2006 - 1:18pm
Anonymous's picture
Anonymous says:

I know little about bullfrogs but this last summer my son has a 3500 gallons pond in his back yard. Over several years of having just one frog suddenly there was two of them. Later on his pond was full of eggs. When the eggs hatched out there was regular tadpoles and there were also (around 40%) tadpoles with ivory color with pink/red eyes.
Now the tadpoles are growing and they are still the same color. I got several of the ivory colors that has grown to 2-3 inches and the colors has not change at all. Could these tadpoles be an albino. The 60% are growing at a faster pace then the ivory ones. If you have any information about these odd color bullfrogs please e-mail me back

Thanks, Missmollie

posted on Fri, 04/28/2006 - 5:12pm
Anonymous's picture
Anonymous says:

wow, this is really interesting! creepy, but very interesting. my family and i have found deformed frogs in our 6 ponds. most of them have 5 legs.

posted on Wed, 04/05/2006 - 8:58am
Camearea's picture
Camearea says:


posted on Fri, 02/04/2005 - 11:00am
Victoria's picture
Victoria says:

I did not know that there were so many deformed frogs in Minnesota!
I feel bad for the frogs and hope that scientists can solve this problam. Also, I think polution has a part in this.

posted on Sun, 02/13/2005 - 1:47pm
Liza's picture
Liza says:

Atrazine, a widely-used pesticide, is one of the chemicals implicated by some research into frog deformities. (This research is contested. To find out more, see the Freaky Frog mini-exhibit in the Science Buzz space on level 5 at the museum.)

State Representative Karen Clark introduced legislation this session that would track the sources of environment-related health problems linked to chemicals such as atrazine by merging pollution data from the Minnesota Pollution Control Agency and disease data from the state Health Department.

State Senator John Marty has proposed a bill that would ban the use of atrazine altogether.

posted on Thu, 03/17/2005 - 3:21pm
Liza's picture
Liza says:

On Wednesday, March 16, the MN House of Representatives Agriculture and Rural Development Committee rejected three bills that would have restricted or banned the use of atrazine in the state. Similar bills have been proposed in the Senate, but are unlikely to be heard this legislative session.

To read more, click here.

Many farmers, while they agree that more research may be warranted, are concerned that restrictions or bans on the use of atrazine in Minnesota but not in other states could be economically devastating.

posted on Thu, 03/17/2005 - 3:35pm
Anonymous's picture
Anonymous says:

I think the frogs with 3 legs will get made fun of by the other "normal" frogs. This could be a problem in the class room. The "special" frogs won't be able to learn and in the end won't get accepted to the colleges of their choice.

posted on Thu, 05/19/2005 - 9:32pm
bryan kennedy's picture

While your comment might seem a little silly at first glace, I think you are actually on to something here. Animals of all kinds have complex social structures that allow them to act as a group. Undoubtedly, deformities in individuals will have some effect on how they are accepted and interact with others of their species. Physically, they will be at a disadvantage as well.
bryan kennedy
Science Buzz Site Admin

posted on Fri, 05/20/2005 - 9:00am
a frog with legs behind his head, named n+1 eyes's picture
a frog with legs behind his head, named n+1 eyes says:

instead of being called "4 eyes", they'll be called, "5 eyes", "6 eyes", "n+1 eyes"

posted on Fri, 01/21/2011 - 12:55pm
Anonymous's picture
Anonymous says:

I live in Canada, and there have also been many deformed frogs found in Southern Ontario.

posted on Sun, 02/19/2006 - 10:09am
Sammy White's picture
Sammy White says:

In my Native American culture we are taught to respect all aspects of mother earth and appreciate all life outside (in the Universe).

posted on Sun, 02/19/2006 - 4:21pm
Gry's picture
Gry says:

In my country, Norway, Atrazine is not an approved pesticide because it scientists in Europe assume there to be a link between the chemical and hormon abnormalities in living creatures. It has been that way for many years now. Corn is not an important product for Norway and Atrazine is important for corn. There is probably a link between the economical interests of your country and why this pesticide still is allowed in your country. I don't think any bill will be past against Atrazine as long as no other pesticide can promise the same profits to corn producers....

posted on Fri, 09/08/2006 - 12:31pm
Louise's picture
Louise says:

Frogs are personally my fav. animal and to protect them we need to really start to do something about it NOW!!!!!!!!!!!

posted on Thu, 03/29/2007 - 8:57pm
Anonymoussssss.......etc.'s picture
Anonymoussssss.......etc. says:

awesome article

posted on Sat, 04/21/2007 - 10:27pm
GrandO's picture
GrandO says:

This is really strange I did not know that deformed frogs where so usual.
MY family has a pond wich reacently has been getting less and less water in it so our frog population is dying out :( but as far as I know we have never found a deformed frog
ps. I am from Sweden

posted on Wed, 05/09/2007 - 3:00pm
Tom Bowen's picture
Tom Bowen says:

I was born and raised in baltimore,md,I attended elmentary school at Belle Grove Elementary 1956-1962,this school and a portion of the pond or ponds are still their ,whether or not the Deformed Frogs maynot still exist. Across the street from these ponds was a Chemical Trucking Facility called Matlock Trucking, Additionally, I captured a two-headed snapping turtle in this very pond and had it from the 4th grade until my 10th grade biogoloy class. These very ponds drained directly into the Patasco River which entered directly into the Chesapeke Bay.FYI: Belle Grove Elementary School,Belle Grove Road,Brooklyn Park,Baltimore,Maryland.

posted on Mon, 07/02/2007 - 4:22pm
Anonymous's picture
Anonymous says:

frogs r good

posted on Mon, 09/10/2007 - 3:50pm
Anonymous's picture
Anonymous says:

I'm doing a research project for my biology class and I was told to research the government and how they are affecting the frogs.
Why did the government stop funding ATFF?
Is the government responsible?
Will they start funding again?

anyone know what ATFF is? or if the gov't actually stopped funding and why?

posted on Thu, 09/13/2007 - 1:56pm
Liza's picture
Liza says:

The only "ATFF" I know of is "Alliance for a Toxic-Free Future," in New York.

I don't think the government ever provided any funding to the organization, and so can't have stopped. The front page of their website says,

"The Alliance for a Toxic-Free Future is a non-partisan collaboration comprised of environmental, labor, environmental justice, academic, health, community, faith-based, and other groups working together for a future in which our children are not exposed to toxic chemicals."

I also don't think that "the government," as an entity, has anything to do with the appearance of "freaky frogs." But there are several scenarios that researchers are looking into (see below).

posted on Thu, 09/13/2007 - 3:09pm
Liza's picture
Liza says:

Are worms deforming the frogs?

Pieter Johnson, a researcher at the University of Wisconsin, Madison, has shown that parasites can wreak havoc with frog legs. A worm called a trematode burrows into tadpoles’ skin, causing cysts which disrupt the growth of hind legs.

Experiment #1: worms in the water
Over two years, Johnson’s team surveyed 35 ponds in California. Four of these ponds had frogs with severe deformities.

Johnson tested the water from those four ponds. He found no chemicals. Frog eggs from those ponds hatched and developed normally in the lab, so there was nothing wrong with the eggs. But one thing those four ponds did have was parasites—lots of ‘em. And when Johnson dissected deformed frogs from those ponds, he found parasite cysts in the tissue around the back legs.

What the heck is a trematode?
A trematode is a flatworm that lays eggs in the intestines of herons or other frog-eating animals. When the heron poops, it releases worm eggs into the pond. After hatching, these parasites first live inside snails, and then burrow into tadpoles, where they interfere with limb development. The tadpoles grow into deformed frogs, which are easily caught by wading birds. Herons eat the infected frogs, and complete the worm’s life cycle.

Experiment #2: worms in the lab
Back in the lab, Johnson exposed frogs to the parasites. Many died. The survivors developed defects—the more parasites, the more deformities.

Johnson concluded that the worms cause many of the deformities seen in frogs. Parasites may also be responsible for declines in frog populations. Sometimes the worms kill the frogs outright. Other times, they lead to frogs with missing or deformed limbs that are easier for birds to catch and eat.

What's the catch?
Scientists have found deformed frogs in lots of places that have no trematodes. And some kinds of deformities—misshapen eyes, twisted internal organs, and some types of missing limbs—don’t seem to be caused by the parasites.

Sixty years of deformed frogs
Reports of multi-legged frogs are increasing, but they aren’t new. They go back to the 1700s. Johnson wondered if parasite infections explained these earlier freakish frogs, too. His team tracked down preserved specimens from nine ponds in several states, dating as far back as 1946. When they dissected the frogs, they found parasite cysts in specimens from six of the ponds.

(This was part of our "Freaky Frog" exhibit in early spring, 2005.)

posted on Thu, 09/13/2007 - 3:13pm
Liza's picture
Liza says:

Is weed-killer whacking the frogs?

Tyrone Hayes, a biologist at the University of California, Berkeley, thinks that pollution may be causing the frog deformities. Atrazine—a weed killer—changes the chemical balance of the frogs. Males develop like females, with smaller vocal chords, eggs and even ovaries.

Experiment #1: chemicals and tadpoles
Hayes exposed tadpoles to low levels of atrazine and then examined them for defects after they’d become frogs.

At all but the lowest concentration, atrazine interfered with sexual development in the frogs. Up to 20% of the exposed animals had multiple sex organs. Some had both male and female parts. The male frogs also had smaller, female-like vocal chords. None of the unexposed animals had these abnormalities.

What is atrazine?
Atrazine is one of the most widely-used herbicides in the US. Farmers use it to control weeds.

Experiment #2: chemicals and adult frogs
Hayes also exposed adult frogs to atrazine. Adult male frogs lost 90% of their testosterone—the body chemical that makes a male a male. In fact, their testosterone levels were the same as normal females’.

Atrazine disrupts hormone production in mammals and reptiles, but only at high doses. Hayes’ research surprised scientists by showing that much lower levels could harm frogs. (The Environmental Protection Agency allows a small amount of atrazine in drinking water, since low doses don’t seem to hurt humans. But frogs became deformed at only 1/30th the “safe” level.)

What's the catch?
Atrazine didn’t cause all the deformities seen in the wild frogs. Hayes experiments didn’t produce any extra or missing legs. And other independent researchers have not yet duplicated his results. Until they do, it might just be a fluke.

Balancing frogs versus farms
People need to eat. And farmers need fertilizer and weed killer to grow enough food. But these chemicals may be factors in the worldwide declines of amphibians. Farmers use most of their atrazine during the spring—frogs’ breeding season, when tadpoles are developing.

Some damage is hard to see. Whatever the cause, damage to internal organs is hidden. Unlike extra legs, you can’t see an obvious change. The damage may go unnoticed until the frog populations are reduced or even extinct.

(This was part of Science Buzz's "Freaky Frog" exhibit from spring, 2005.)

posted on Thu, 09/13/2007 - 3:19pm
Liza's picture
Liza says:

Maybe parasites and pesticides work together to deform frogs

Joseph Kiesecker, a biologist at Pennsylvania State University, thinks a combination of factors is causing the deformities. His research suggests that frogs exposed to pesticides are more vulnerable to parasites.

Experiment #1: raising tadpoles in the wild

Kiesecker tracked tadpoles in six ponds known to have trematodes—a parasitic worm that causes deformities in frogs. Each pond had a different level of chemical pollution: some had none, others had a lot. Kiesecker caught tadpoles in each pond and placed them in containers that either let the worm in, or kept it out.

None of the tadpoles protected from the parasites had defects. On the other hand, tadpoles exposed to the worm, whether the water was clean or polluted, developed abnormally. But the worst case was when both problems were present. Tadpoles exposed to parasites in the polluted ponds developed many more deformities than tadpoles in clean ponds.

What are these chemicals?

Kiesecker’s research focused on three chemicals commonly used in farming:

  • Atrazine is one of the most widely-used weed killers in the US.
  • Malathion controls mosquitoes and insects that eat crops.
  • Esfenvalerate also kills a variety of insect pests.

Experiment #2: testing the frogs’ blood

After noticing that chemicals seemed to make worm infections worse, Kiesecker wanted to know why. He measured the white blood cells in the frogs’ blood. He found that even very small amounts of these chemicals altered the frogs’ immune systems.

Kiesecker found that, for some pollutants, even very low levels can harm frogs. His research suggests that the recent increase in freaky frogs may be due to pesticides weakening their immune systems. The frogs become vulnerable to diseases they would normally be able to resist.

What’s the catch?
Kiesecker’s research didn’t look at how pesticides impact the parasites or the snails they live in. Some scientists think that the parasites and snails might be even more sensitive to pesticides than the frogs. And while Kiesecker did see leg deformities in his frogs, he didn’t see missing eyes or malformed jaws or other deformities seen in the wild.

(This was part of our "Freaky Frog" exhibit from early spring, 2005.)

posted on Thu, 09/13/2007 - 3:23pm
Liza's picture
Liza says:

Why are only back legs deformed?

Stanley Sessions of Hartwick College found that frogs’ arms actually develop inside the tadpole’s gills. Protected by layers of skin, the growing arms are safe from parasitic worms called trematodes. Instead, the worm targets the tadpoles’ tush, disrupting tissue that would normally become the frogs’ legs.

Experiment #1: looking for infections

In 1986, Sessions’ team collected specimens of Pacific tree frogs from a California pond. All of the frogs studied, deformed or not, showed evidence of infection. And frogs with abnormal limbs were most likely to have cysts caused by the worms.

Experiment #2: faking an injury

Sessions surgically implanted plastic beads into healthy tadpoles. He focused on tissue that normally develops into hind legs—the same area where researchers find cysts caused by the worms. Even though he only inserted a few beads, Sessions found in his test frogs the same kinds of deformities the occur in wild frogs infected with the parasite cysts.

What’s the catch?

Parasite infection neatly explains deformed frogs at some sites. But other sites have the worms, and the frogs are healthy. Still other sites have deformed frogs, but few or no parasites. And some kinds of defects—misshapen eyes, twisted internal organs, and some cases of missing limbs—don’t seem to be associated with parasites.

(This was part of our "Freaky Frog" exhibit from early spring, 2005.)

posted on Thu, 09/13/2007 - 3:24pm
Liza's picture
Liza says:

Is too much light frying the frogs?

Andrew Blaustein of Oregon State University thinks too much ultraviolet light may be hurting amphibians. UV light is usually blocked by ozone in the atmosphere. But thinning ozone may be letting in too much UV, hurting sensitive animals.

Experiment: made in the shade

Blaustein’s team collected newly-laid salamander eggs from a single pond. They placed some in enclosures that blocked most UV-B light. Other eggs went in containers that allowed most UV-B to pass through. They then tracked the growth of the embryos.

Salamanders shielded from UV light were more likely to hatch, developed more quickly, and had fewer deformities than those left exposed. Those animals had twisted tails, blisters and swollen, fluid-filled areas of their bodies, especially the tops of their heads.

What does it mean?

The high number of deaths and deformities to salamanders in this study suggests that UV radiation may be to blame. Other amphibian species may also be at risk, especially those that lay their eggs in open, shallow water. In laboratory studies, UV-B radiation also caused defects and deaths in western toad tadpoles.

UV-B may act directly on amphibians. Or it may work indirectly, weakening their immune systems and leaving them more vulnerable to diseases and parasites.

What’s the catch?

UV radiation varies from place to place. Some sites with deformed frogs have very low levels. Also, the deformities caused by UV light in the lab don’t match those most commonly seen in the wild. There were no extra legs or altered organs.

(This was part of our "Freaky Frog" exhibit from early spring, 2005.)

posted on Thu, 09/13/2007 - 3:26pm
Candice_318's picture
Candice_318 says:

I think frog a are more nasty than a snake or a spider. There just nasty looking aslimy and give you wartz. Or is that a toad? Either way their nasty

Have a great day

Sponsered in part by
IM AWESOME Productions

posted on Thu, 03/06/2008 - 10:06am
Liza's picture
Liza says:

The human papilloma virus (HPV) causes warts, not contact with frogs or toads. You still may not like them, but you don't have to worry about warts!

posted on Mon, 12/15/2008 - 10:30am
shanee's picture
shanee says:

Ick! thats all i really have to say... frogs are gross! no matter what kind it is.

posted on Thu, 03/06/2008 - 10:09am
Nae-Nae's picture
Nae-Nae says:

well one day i was wlakin but i was with my friends and we see a pond and some frogs and we go to it just to relax so we did and i havin to see this frog that stands out i looked at it and it had like at least six legs then i put it in a container and took it homed and showed everyone and they thought it was cool so i fed it some leaves and bugs.Then the next dayi took it to school to show my teachers every one was so suprised even the teachers then they sowed it on the morning annoucments then other schools and stuf kept calling asking bout it like where i got it and the next thing you know it was on the news and thats my story.

posted on Mon, 10/13/2008 - 6:09pm
Atrazine's picture
Atrazine says:

Atrazine is the second-most common pesticide in the world. It is used to kill broad-leaf and grass weeds in crops including corn, sorghum and sugar cane, and is also used on commercially grown Christmas trees.

posted on Mon, 08/09/2010 - 2:40am
Anonymous's picture
Anonymous says:

why is this happining to the frogs there so inisent

posted on Thu, 02/24/2011 - 10:01am
KEkilleen's picture
KEkilleen says:

Maybe this is a form of adaptation? Maybe this is what the frogs need to survive in an easier/better way?

posted on Fri, 06/10/2011 - 7:44am
Shana's picture
Shana says:

In order for this to be viewed as an adaptation, it would need to be selected for via natural selection over time...these mutations are more random, they aren't spreading from a single population, and they aren't a single trait, as would be the case with an adaptation. So my guess would be that this isn't adaptive.

posted on Fri, 06/10/2011 - 9:33am
Anonymous's picture
Anonymous says:

how can we help

posted on Wed, 02/01/2012 - 12:46pm
Anonymous's picture
Anonymous says:

I wonder if there was urine or human chemicals in the water which surrounded the frog. Tgis could have explained the side effects to the frog.

posted on Wed, 04/11/2012 - 4:19pm
Anonymous's picture
Anonymous says:

This is actually due to a parasite infection of the flatworm RIBEIROIA ONDATRAE. Surprisingly not due to pollution, yay thats one thing that wasn't us.

posted on Mon, 06/22/2015 - 10:57pm

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