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Disappearing Amphibians: Frog Deformities May Warn of Human Risk
By Mary Losure
June 4, 1998
Click for audio RealAudio 2.0 14.4

Other reports from the series:
Multiple Threats to Amphibian Populations
Vanishing Frogs of the Panamanian Rainforests

In the years since the first reports about the disappearance of frogs worldwide, many researchers have warned that the plight of amphibians may be an early sign of environmental problems that could affect humans. More recent reports of deformed frogs have added to these concerns.

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AROUND THE WORLD, FROGS ARE DISAPPEARING for reasons scientists don't understand. Now, that mystery has a new twist - the deformed frogs first reported in Minnesota in 1995, and later from many other states.

In a darkened lecture hall at the Milwaukee Public Museum, amphibian biologists from across the Midwest have gathered for a scientific meeting. Tom Johnson of the Missouri Department of Conservation is showing slides of grotesquely deformed frogs.

Johnson: There were two extra limbs on this very young bullfrog. We did have several of the missing eye reports; the right eye is missing in this young dwarf American toad from Clinton County.

There have been isolated reports of frog deformities for many years. But University of Minnesota amphibian biologist David Hoppe is convinced what's happening now is different from anything in the past. Before 1995, he had spent decades studying Minnesota frogs and had never seen deformity rates comparable to what he now shows the scientists.

Hoppe: Here are my data from 1996-97 from a site where 85 percent of the green frogs are malformed, mink frogs have been above 50 percent for two years now, northern leopard frogs above 10 percent for three years at that site, and we see it to a lesser degree in some other species.

Hoppe examined thousands of Minnesota museum specimens and found only a narrow range of deformities in a small percentage of the frogs. The wide variety of deformities he now sees routinely were absent from the museum collections.

He tells the scientists that the deformed frogs may be an indication that something has changed in the environment. If that's so, it will not be the first time wildlife species have acted as environmental sentinels. The story of peregrine falcons, bald eagles, and DDT is a famous example.

At the top of an office tower in downtown St. Paul, amid the din of the building's air-handling system, a plywood box sits on a ledge. Inside is a nest with four newly-hatched peregrine falcons and their fierce, wild-looking mother.

Harrison Tordoff: Occasionally they lay five eggs, but four is the standard full clutch, and many pairs only lay three, so she's doing very well.

Biologist Harrison Tordoff, now retired from the University of Minnesota, spent a good part of his career bringing peregrine falcons back to the Midwest after they were nearly wiped out by the now-banned pesticide DDT.

When Tordoff first began reading newspaper reports of the deformed frogs, he was reminded immediately of the plight of the peregrine falcons. In the 1950s, the birds simply stopped reproducing. No one knew why. Like the case of the deformed frogs, it was an environmental mystery.

Tordoff: And people started to look closely and discovered that even though they laid eggs, the eggs didn't hatch.... And it was really a good many years before people realized that it was population-wide, it wasn't just a pair here and a pair there that were failing to reproduce, but essentially all the peregrines in eastern North America, and other parts of the world, were having reproductive problems.

Scientists discovered DDT was thinning the eggshells of falcons and also bald eagles. The thinning eggshells were a warning sign - the tip-off that alerted scientists and the public that DDT was building up in the environment and spreading throughout the natural world. Tordoff says scientists today are right to be concerned that, like the falcons, the deformed frogs are a warning.

Tordoff: The furor over the frogs is not overblown in any way, according to my standards. We need to pinpoint what's going on.

Although scientists still don't know what's causing the deformities, as the investigation goes on, one scary scenario has been getting an increasing amount of attention - the possibility that a waterborne chemical is acting as a hormone mimic.

Hormone mimics are chemicals that disrupt the body's normal function by acting like hormones. There is growing evidence that man-made chemicals are disrupting the hormone systems of wildlife. Gary Ankeley is a toxicologist with the Environmental Protection Agency laboratory in Duluth. He says male fish in some stretches of the Mississippi are exhibiting signs of feminization that may come from effluents that mimic estrogen.

In Lake Ontario, lake trout - which once made up a thriving commercial fishery - now are failing to reproduce, and Ankeley says scientists are starting to realize why.

Ankeley: The emerging picture now is quite likely what is causing those fish not to reproduce was related to TCDD, or dioxin as it's commonly called, affecting early survival of the fish, so any fish that were there and reproduced had embryos that just didn't live very long.

Other examples include Great Lakes water birds with bill deformities caused by PCBs, and Florida alligators whose hormone systems were disrupted by a chemical spill.

Environmentalists worry that if hormone mimics are affecting animals, they may also be harming humans. They fear declining sperm counts worldwide and increases in breast cancer may be linked to hormone mimics. Developmental biologist David Gardiner of the University of California at Irvine says although that's still not clear, it's highly possible something that disrupts animal hormones could also affect ours - because the hormone systems of humans and animals are essentially the same.

Gardiner: These hormone-signalling pathways have only evolved once, so we all share common signalling pathways, and certainly, when you look at the kind of hormones we have that control development in our bodies, are indistinguishable from the equivalent hormone in something like a frog.

Gardiner believes the deformed frogs are the latest addition to the list of wildlife species whose systems have been disrupted by a hormone mimic. He thinks so because what scientists are finding in wild frogs is nearly identical to deformities that can be induced in laboratory animals by disrupting their hormones with a chemical called retinoic acid.

Gardiner and his colleagues suspect something in the water of the ponds where the frogs live is acting like retinoic acid. Although their hypothesis is far from proven, even the possibility of a retinoic acid mimic loose in the environment, in quantities sufficient to deform wildlife, is alarming. Gardiner says we already know how seriously retinoic acid can disrupt the development of laboratory animals - even though we don't know what that means for humans.

Gardiner: First of all, you don't do experiments on humans... you can't test the effects of retinoic acid on learning and memory in young children. But if we are exposing ourselves to these things then we are actually doing that experiment, whether we like it or not.

If a chemical acting like retinoic acid is indeed loose in the environment, and causing the frog deformities, it may be difficult to find out what that specific chemical is. One of the few known retinoic acid mimics found outside the lab is the insecticide methoprene, which is used to kill mosquitoes in wetlands in the Twin Cities and other places, and for pest control in dairy cattle. No one knows how many of the tens of thousands of other pesticides on the market and their breakdown products may also be retinoic acid mimics. Little is known about how retinoic acid mimics act in the environment. It may be that what scientists like Gardiner are looking for is not just one chemical, but a complex interaction of chemicals.

The deformed frogs are a complicated riddle, like the larger mystery of the worldwide decline of frogs. Researcher David Wake of the University of California at Berkeley says in a world under increasing environmental stress, problems like the plight of frogs are unlikely to have single, simple explanations.

Wake: I think this a story of modern environmental biology. This is the kind of story we're going to face increasingly in the future... very complicated explanations. There may be many explanations, not one.

In the days when DDT was thinning eggshells, scientists were able to identify a single, persistent pesticide as the cause of the disappearance of the peregrine falcons. Frogs, with their permeable skin and watery lives, are ideally suited to detect today's more complex problems... but their warning comes in a code we have not yet broken.

Related Links
Minnesota Pollution Control Agency's Live Deformed Frog Cam
MPCA's Closeup Frog Cam Images
Froglog: Newsletter of the Declining Amphibian Populations Task Force

Funding for our series on vanishing frogs was provided by the listeners of Minnesota Public Radio, and by a grant from the Society of Environmental Journalists. Special thanks to researcher Karen Lips and field assistant Liese Greensfelder for their help with the Panama segment.