The Mystery of the Frogs

Schoolchildren on a nature walk in rural Minnesota were at first curious, then horrified, by what they found. A pond brimming with mutants: Baby frogs with too many legs, missing legs, crippled limbs, even missing eyes, as many freakish frogs as normal ones.

Their discovery among the reeds and cattails of Le Sueur County in August 1995 was just the beginning.

Extraordinary numbers of deformed frogs--some with as many as nine legs and one with an eye growing in its throat--have been confirmed this year throughout Minnesota and other states, concentrated largely in the Midwest. In California, damaged frogs were found in a Sierra Nevada pond.

The creatures are so grotesque they make experienced biologists gasp, and they have prompted such widespread fear of an ecological crisis that the U.S. Environmental Protection Agency convened a gathering of scientists from around the country in September to trace the cause.

Some mysterious force is disrupting the fragile period of metamorphosis when tadpoles turn into frogs, and the most popular theories involve parasites, pesticides that alter hormones, viruses, ultraviolet radiation--or perhaps more than one of them working in concert.

“These frogs are sending us a vivid message that something is wrong,” said David Wake, director of UC Berkeley’s Museum of Vertebrate Zoology. “To me, it’s got to be environmental. The question is, what aspect of the environment?”

Whatever the explanation--natural or man-made--the images are haunting. Frogs have been immortalized in American folklore, and they inspire childhood memories of summer days of scooping up slithery tadpoles and watching their wondrous transformation.

Dubbed the “celebrated deformed frogs of Le Sueur County” by a local newspaper, Minnesota’s amphibians have attracted attention from Europe to Japan to Australia. The state Legislature set up a citizens frog-watch patrol and hotline, and a middle school’s Internet page dispatches pictures of the hideous creatures.

But the phenomenon is intriguing for reasons beyond macabre curiosity.

With permeable skin and no hair or scales as shields, frogs are ultra-sensitive to changes in their watery world. When nature sends out such powerful messages as seven-legged frogs, biologists say, people should listen, because it signals that the environment is so out of whack that it cannot support normal life.

“I personally find it quite alarming,” said Robert Bezy, curator of amphibians and reptiles at the Natural History Museum of Los Angeles County. “I would take this most seriously. Not only for the frogs themselves, but you can extrapolate dangers for humans.”

This year, the Minnesota Pollution Control Agency has amassed a phenomenal outpouring of reports of frogs with missing or extra legs at about 150 ponds and lakes, spreading from one end of the state to another in farm areas, cities and suburbs. “Worldwide, nobody has ever reported deformities on this level. Things have just gone crazy. They’ve just exploded, and it’s the suddenness of it,” said Ralph Pribble of the state pollution agency, which has teamed with University of Minnesota scientists to spearhead the investigation.

At one central Minnesota pond, 91 of 94 frogs captured in a survey were deformed; at another, 65% were. At the original marsh explored by the schoolchildren in a farm town southwest of Minneapolis, 30% to 40% were abnormal last summer, Pribble said. Inexplicably, most of last year’s abnormalities involved extra hind legs, while this year most had missing limbs.

High numbers of deformities this year have also been confirmed in Vermont, Wisconsin, Iowa and Quebec, while scattered deformities have been reported in South Dakota, Missouri, Kansas, Michigan and several other states.

In California last month, scientists found disfigured limbs, eyes and other defects in about 10% of the frogs in a Nevada County pond about 75 miles northeast of Sacramento. In all, more than a dozen states have reported at least a few malformed animals.

“If 60% of the children in Minnesota were born with cleft palates we certainly would be concerned about it. That’s what we’re seeing with these frogs,” said Kathy Converse, a wildlife disease specialist at the National Wildlife Health Center in Madison, Wis., which has teamed with the Minnesota researchers to investigate.

The defects don’t appear to be genetic mutations, since they have struck various species, and surfaced suddenly in large numbers instead of gradually, as they would if they were inherited.

Frogs have what is called “plastic development,” which means their growth is easily altered by their surroundings. Many things can traumatize a frog during its metamorphosis, and ones with multiple or missing limbs have occasionally been know for centuries, including some discovered in a pond in Aptos, near Santa Cruz in Northern California, a decade ago.

But until the summer of 1995 such deformities were extremely rare--perhaps one among tens of thousands. What could now be damaging such extraordinary numbers at one time, in such a severe way, and in an array of places that seem to have little in common?

“It’s incredibly important that we explore the possibilities, because it might even be something that could happen to us, too,” said Tyrone Hayes, a developmental endocrinologist at UC Berkeley who researches frogs. “They are a sentinel species, predictors of what will happen in the rest of the environment.”

The deformities are emerging at a time when amphibians are already in serious trouble.

Many species of frogs, toads and salamanders have been disappearing worldwide for over a decade. Biologists have pointed at ultraviolet radiation from the thinning ozone layer, wetlands destruction and pesticides as probable causes for the population crashes.

Teams of scientists are examining the water and the frogs for unusual chemicals, such as herbicides and metals, as well as parasites, bacteria and other clues, but preliminary tests have come up empty-handed.

Whatever the culprit is, it strikes quickly. Most tadpoles are only a couple of months old when their legs develop, and most malformed ones, unable to feed or escape predators, die young. The species that spend the most time in the water are suffering the worst rate of deformities, a sign that the damage is occurring during those weeks a tadpole spends in the water.

Parasites under a tadpole’s skin could offer at least a partial explanation. But worms and other parasites have always been ubiquitous, so if they are the cause, why are they suddenly out of control?

“It is so, so extreme, this range of enormous deformities happening all at once,” said Steve Goldberg, a Whittier College herpetologist who specializes in frog parasites. “I just don’t know if the parasites could be that efficient.

“I would think it’s something else, something in the water, but you don’t really know until you sort it all out,” he said. “I personally think it’s going to be a number of things interacting.”

Mark Jennings, a California Academy of Sciences biologist who has studied dwindling frog species, says if parasites are to blame, they might have “bloomed” because of some unusual weather pattern, because their populations are controlled by water temperature.

Since the investigators have no idea what they are looking for, unraveling the mystery could take years. Even if they narrow their search to pesticides, there are hundreds to test for--and infinite combinations.