The Trouble With Frogs

When alarming numbers of deformed frogs turned up five years ago, scientists rushed to investigate the cause.

By William Souder

Under heavy gray skies one morning in August 1995, eight schoolchildren and their nature-studies teacher set off across a bean field in south-central Minneaeota and walked straight into the middle of an environmental mystery. They were headed for a woodlot where they planned to spend the day sketching trees, but got no farther than a large pond halfway there. As the kids walked along an overgrown field road that skirted the edge of the pond, they encountered large numbers of young leopard frogs fleeing into the weeds as they approached. Scampering after the animals, the students caught first one and then another and then yet another that turned out to be grossly deformed.

During the next 12 months, a wave of similar discoveries all but overwhelmed worried officials in St. Paul. By the fall of 1996, the Minnesota Pollution Control Agency had received more reports of deformed frogs than it could handle--more than 200 sightings around the state. The MPCA, which had hurriedly persuaded the Legislature to fund a preliminary study of the problem and had assumed responsibility for the investigation, was soon joined in its search by the U.S. Environmental Protection Agency, the National Wildlife Health Center, the Natural Resources Research Institute in Duluth, several independent academic scientists, and an influential but little-known federal toxicology laboratory called the National Institute of Environmental Health Sciences, which investigates threats to human health.

Even before that fateful field trip five years ago, the scientific community knew that frogs and other amphibians were imperiled around the world. Frog populations were declining or disappearing, and the causes were not always apparent. Habitat destruction, ecosystem modification, global warming, pollution, ozone depletion, exotic species introductions, new diseases, and acid rain were all understood to be threats, though not equally discernible ones. Some frogs seemed to be prospering in heavily developed and polluted environments; others were going extinct in what appeared to be pristine surroundings.

But the mere mention of a "human health risk" raised the stakes in everyone's eyes. From the outset, Minnesota's deformed frogs suggested that a dramatic alteration in the local ecosystem had taken place, one that had to be considered a potential problem for other species.

A Single Alphabet

Life on earth, the only life we know, is notable for both its diversity and, oddly enough, its consistency. There are about 1.7 million identified species of life. The myriad dissimilarities among the species are obvious. A walnut tree. A microbe. An elephant. An eagle. Crabgrass. A bluegill. These and most other living things are readily distinguishable from one another. Yet all are subject to an immutable biochemistry. There is but one genetic code to write the genome for each species, just as there is a single alphabet to make all English words. The chemistry of life is constant. The more closely evolved two species are--that is to say, the more recently they had a common ancestor--the more alike they will be in every way.

Among vertebrates--animals with an internal skeleton--a singular body architecture prevails: four limbs. The first land vertebrates were amphibians. They appeared about 350 million years ago and evolved from a line of bony-finned fishes, which over time made their way out of the water on four legs, two attached at the shoulder and two at the pelvis. And so it has been ever since. Every vertebrate species, including humans, shares this same limb configuration, though in some species, arms or wings replace legs at the shoulder. Even snakes had four legs long ago.

Limbs appear late in the development of a vertebrate embryo. In amphibians, it happens late in the larval stage. The exact biochemistry of this amazing process is only now being deciphered. But it is certain that what happens is largely the same regardless of the species involved. The same hormones, genes, and chemical growth factors initiate limb development and regulate its progress in all vertebrates--frogs and people, to name two.

And therein lay the initial concern over the deformed frogs found all over Minnesota in 1995 and 1996. The abnormalities being reported were primarily limb deformities, mainly of the hind legs. The majority involved missing and partial legs. But there were also reports of multiple or "extra" legs, as well as a gruesome assortment of limbs that were bent, shriveled, split into two or more segments, or bound up in strange skin webbings. Sometimes extra appendages appeared that could not quite be called legs, though they contained leg elements. Seemingly overnight, the basic vertebrate architecture laid down 350 million years ago was being undermined.

Development is a sensitive biological process--birth defects occur in all species at low frequencies and for a variety of causes. Frogs may be unusually sensitive creatures. Despite having survived for eons during which many other kinds of organisms--dinosaurs, for example--have come and gone, frogs may be a kind of early warning system of environmental degradation.

Frogs live a multiphasic existence. They inhabit both land and water. They begin life as vegetarians and turn into carnivores at metamorphosis. Frogs drink through their permeable skin and can also breathe through it, even underwater. This spongelike quality, plus their intimacies with every aspect of the environment, suggest to many scientists that frogs are "sentinel species." Anything adverse that happens to a frog is a report from the front lines of environmental change.

If Minnesota's deformed frogs were telling us something, then what was it? The scientific literature abounds with examples of birth defects resulting from exposure to chemicals--from alcohol to thalidomide, the infamous sedative prescribed for morning sickness in Great Britain in the early 1960s that caused horrific developmental abnormalities, including extreme shortening of the limbs, in several thousand human births. More recently, chemicals have come under suspicion as "hormone mimics" that can wreak havoc with development and other metabolic processes by tricking the body's endocrine system. Such interference has been tentatively linked to increases in birth defects, cancer, and the "feminization" of male animals, as well as decreases in sperm counts and intelligence.

Among the several families of compounds thought to be potential hormone mimics are pesticides. Pesticides are the most ubiquitous chemical contaminant in Minnesota's agriculture-intensive landscape.

No Conclusions

In the early days of the inquiry, the MPCA's lead biologist speculated that the investigation could last "at least several months." At the time, the agency and its collaborators assumed that some common factor--most likely a contaminant in the frog's breeding ponds--would be identified as the unifying link among all the outbreaks.

But no such specific link has yet been found, although both EPA and NIEHS have tested specific, suspect compounds as well as raw water samples. Water from ponds where the deformities occur appears to be more heavily contaminated with chemicals, and the same samples have also shown a capacity for causing abnormal development in laboratory assays of frogs. In addition, the water appears to impact proper thyroid function--an intriguing finding given the important role of thyroid hormone in frog development and Minnesota's higher-than-normal incidence of thyroid disease in humans.

But all of these results remain more tantalizing than conclusive. Now, five years after it began, the great deformed frog investigation remains a work in progress. Yet a great many questions have been answered. We know more about Minnesota's frogs today than we did five years ago--a lot more.

One of the first surprises came when scientists checked the scientific literature and found that limb deformities in frogs were not a new phenomenon. In fact, frogs with extra legs have been known to science for more than 200 years. The inescapable inference is that at least some of the abnormalities now turning up seemingly everywhere are "natural" aspects of normal frog ecology.

Even so, the current outbreaks appear different in important ways. Historical reports suggest the occurrence of leg deformities in frogs is widespread but relatively infrequent. In Minnesota, it appears that the incidence of deformities has risen sharply. This was eventually confirmed by herpetologist David Hoppe of the University of Minnesota?Morris, who examined thousands of frog specimens collected in the 1960s and found the incidence of deformities to be less than 1 percent.

It also turned out that the published historical record nationally concerns extra legs almost exclusively. Although frogs with multiple hind legs generated a great deal of media attention, this was not the most common deformity being observed in the new outbreaks; missing legs, partial legs, and otherwise misshapen legs have been far more prevalent.

Hoppe's field work--primarily at a single hot spot near Brainerd--showed that the deformities occurred more frequently among species of frogs that were more aquatic in their habits than other species. Like the students, Hoppe found abnormal legs in significant percentages among northern leopard frogs, Minnesota's most recognizable--and, along with toads, most commonly encountered--amphibian species. He also found deformities in five other species. Green frogs and mink frogs, two species rarely found far from the water, showed the highest occurrence of deformities.

Plausible Causes

Compelling findings came from an unexpected quarter: Canada. U.S. investigators were shocked when they learned that similar outbreaks of frog deformities had been discovered in southern Quebec fully three years before they were confirmed in Minnesota. The Canadians had launched their own investigation. And it had yielded important results.

Working under the auspices of the Canadian Wildlife Service, scientists in Quebec conducted extensive surveys of frog populations in both agricultural and nonagricultural settings. They found a strong statistical correlation between the deformities and exposure to agricultural pesticides. But, like their U.S. counterparts, the Canadians have been stymied in their attempts to lay the blame for any of the deformities on a single compound, or to develop a coherent theory of the mechanisms of exposure, dose, and developmental interference. They assume some pesticide (or combination of pesticides) causes deformities, but what it is and how it works is still unknown.

One hypothesis is that the mystery chemical belongs to a family of compounds called retinoids. Derived from vitamin A, retinoids include hormones that are important in various developmental and metabolic processes--especially limb development. Many laboratory experiments have shown that retinoids can cause limb deformities. Indeed, almost 40 years ago investigators in Italy showed that vitamin A by itself can generate extra legs in frogs.

Another possible culprit that has been shown to cause deformities in the lab is an aquatic flatworm parasite from the genus Ribeiroia. Parasites have long been suspected of causing limb deformities by infecting a tadpole and forming cysts in its developing legs. This is advantageous to the parasite, the hypothesis goes, because leg deformities cripple frogs, making them more susceptible to predators--including the aquatic birds that are the primary hosts for Ribeiroia. Handicapping an intermediate host--a frog--is a way for the parasite to get to a primary host where it can reproduce.

Another intriguing lead involves UV light--the ultraviolet segment of the spectrum of ordinary sunlight, which has increased due to ozone depletion in the earth's atmosphere. Experiments conducted by the EPA in Duluth have shown that normal levels of sunlight alone can cause limb truncations--even completely missing legs--in native Minnesota frog species. It's not completely certain that this actually happens in the wild--where tadpoles may seek refuge to escape heavy doses of sunlight--but it seems likely.

Investigators' Fault Line

With so many plausible causes for the deformities on the table, it would seem there's plenty of room for scientists to calmly pursue several lines of investigation. That has proved about half right. The pursuit has been anything but calm.

The frog investigation has become a contentious, at times openly hostile, fellowship of scientific adversaries. The disagreements--over what research to fund, which way to proceed, what assumptions to make--have gone in all directions, but the most fundamental division has been along one basic fault line. One group of investigators, particularly those who have worked on parasites and suggested that injuries by predators cause the many missing legs, has argued that virtually all of the deformities are the result of natural ecology--and that the increase in deformities is either an illusion or the result of altered interactions between parasites and hosts, or predators and prey. On the other side of this sharp divide are scientists who are convinced that some artificial factor or factors have created new categories of unnatural deformities that have now been superimposed over nature's own complement of abnormalities.

For now, the situation is murky, the forecast for future research cloudy. The MPCA is still in the field. Developmental biologists in California are working on retinoids. The Canadians continue to study pesticide sites. Parasite experts are attempting to repeat experiments implicating Ribeiroia. The world turns, and, as always in biology, every answer raises a new set of questions.

One thing all researchers agree on is this: Deformed legs in frogs are "maladaptive." Plainly put, deformed frogs don't survive to adulthood. No one has ever found an adult frog with a major hind limb deformity. Unable to forage for food, incapable of evading predators, and possibly beset by other, less visible internal problems, deformed frogs die as juveniles.

The word from the field for the past several seasons has been that some deformities hot spots have cooled--not because the abnormalities have decreased, but because the frogs themselves are disappearing. At places where biologists just a few years ago easily collected frogs by the hundreds, more recent collection efforts have turned up only handfuls of frogs. Or none.

The frogs--so long on earth yet so demonstrably in trouble--are sending new, ever more dire messages to us. Their last and most critical warning may be a silent one. For a third of a billion years, frogs have called to each other--their raucous choruses part of the music of warm summer evenings. Now a strange, bereft silence may descend on the summer night sooner than we can imagine, leaving us to worry and wonder why.

William Souder lives in Grant. He covered the deformed frog issue for several years for The Washington Post. His book, A Plague of Frogs: The Horrifying True Story, was published last year by Hyperion.