The decline and disappearance of frog populations worldwide remains a
mystery, despite efforts by hundreds of scientists to determine the
causes. (See REHW #380, #441.) The other major problem facing frogs --
massive deformities observed since 1995 among frog populations in
California, Iowa, Minnesota, Missouri, Ontario, Quebec, South Dakota,
Texas, Vermont, and Wisconsin --is now better understood. (See REHW
During the past six months, press interviews with research scientists,
and published studies, have shed a bit of light on both problems though
true consensus has not yet emerged on either one. No one is even sure
whether the two problems are connected, though new evidence indicates
Some scientists still doubt that frogs are actually disappearing
worldwide. They prefer to believe that the simultaneous declines and
disappearances of frog populations in North and South America, Europe,
and Australia reported since 1980 are nothing more than the normal ups
and downs of any wild population. However, SCIENTIFIC AMERICAN said in
August that the "majority viewpoint" among scientists now is that the
widespread declines and disappearances are "highly abnormal." "I
think we're close to consensus now," says David Wake, a well-known frog
researcher at the University of California at Berkeley.
There are roughly 5000 species of amphibians worldwide. Of these, 242
inhabit the U.S. A recent study by the Nature Conservancy and the
Natural Heritage Network identified 92 of these 242 (or 38%) as
endangered, imperiled, or vulnerable (meaning they are likely to
become extinct within 5, 20, or 100 years if present trends continue.)
James La Clair at the Scripps Research Institute in La Jolla,
California, says, "Although amphibians have lived on this planet for
over 300 million years, nearly 120 times [as long as] modern man,
reports within the last three decades have shown that numerous
amphibian species are either suffering from serious population loss or
have disappeared altogether." La Clair says there are very likely "a
collection of causes," but one way or another they can all be traced
back to "the expansion of humankind." Loss of frog habitat --chiefly
wetlands --is probably the biggest single cause. Global warming and
accompanying droughts may contribute because frogs develop from eggs
that thrive in water. The artificial stocking of streams with trout and
bass plays a role, too. Pesticides and other chemicals certainly
exacerbate the problem (more on this below). Laboratory experiments
have shown beyond doubt that ultraviolet light from the sun can
interfere with the development of frogs' eggs. Acid rain may
contribute to the problem as well. Humans eating frogs' legs in large
quantities are not helping. And there are other causes, such as
A group of Australian researchers reported this summer that they have
identified one particular fungus that is killing frogs in locations as
far apart as Queensland, Australia and Panama in Central America.
The fungus --which has never before been reported to harm any
vertebrate species --causes changes in the skin of frogs, somehow
contributing to their deaths. The mechanism is not understood, but
frogs breathe oxygen through their skin and the fungus may cause
No one knows why an ancient fungus would suddenly start killing frogs
in places as far apart as Australia and Panama. It is conceivable that
the fungus was transported to these places only recently on the boots
or equipment of researchers studying the disappearance of frogs.
Another possibility is that the fungus has been present in these
locations for a long time but frogs are now succumbing to it because
their immune systems have been impaired by recent changes in the
environment. One candidate would be increased ultraviolet light, which
is well-known to damage the immune systems of many animals, including
frogs. In recent years, chlorinated chemicals released by humans have
thinned the protective layer of ozone in the upper atmosphere, thus
allowing about 10% more ultraviolet light from the sun to reach the
surface of the Earth.
Certain industrial chemicals released into the environment may also be
damaging the immune systems of frogs. One particular class of chemicals
--called retinoids --has come under strong suspicion because retinoids
can cause severe birth defects in many animals, including frogs and
humans. The medicine Accutane, prescribed for treating acne, is a
retinoid known to cause major birth defects in humans.
The deformities now being found in large numbers of frogs at many
locations in the U.S. and Canada are grotesque. Herpetologists
(scientists who study amphibians and reptiles) have reported finding
frogs with missing legs, extra legs, misshapen legs, paralyzed legs
that stick out from the body at odd places, legs that are webbed
together with extra skin, legs that are fused to the body, and legs
that split into two half-way down. They have also found frogs with
missing eyes and extra eyes. One one-eyed frog in Minnesota had a
second eye growing inside its throat.
Dr. David Gardiner, a research biologist at the University of
California at Irvine, has been studying retinoids for at least a
decade, and in recent years he has probed frog deformities. To him,
retinoids are the obvious culprit in the mystery of the misshapen frogs
because of the peculiar kind of limb deformities being observed. "There
is no other known mechanism for this [besides retinoids]," Gardiner
says. "Much of early development is controlled by retinoids," he says.
"Our body [and the body of a frog] is completely dependent on them," he
told a reporter.
Exposure to retinoids could also make frogs more susceptible to
infectious diseases, Gardiner says: "The kinds of chemicals that would
target development of limbs would target all organ systems," including
the immune system. Frogs with abnormal legs would also very likely have
abnormal immune systems. This could explain why some frogs are now
suddenly falling victim to infectious agents that they resisted for
millions of years.
James La Clair and his associates at the Scripps Research Institute in
La Jolla, California, recently showed that a popular anti-mosquito
insecticide, called S-methoprene, breaks down in the environment to
several different kinds of retinoids. Under laboratory conditions,
La Clair was able to show that the ultraviolet light in sunlight causes
S-methoprene to break down into half a dozen retinoids, and that these
retinoids in turn can cause frog deformities of the kind being seen now
in the U.S. and Canada.
S-methoprene was introduced in the 1970s to control mosquitoes, which
breed in water. It is sold under trade names like Altosid, APEX,
Diacon, Dianex, Kabat, Manta, Minex, Pharoid, Precor, Yuvemon, and ZR
It is also widely sold in flea powders. La Clair calculates that the
amount of flea power used to treat a ten-pound pet one time contains
enough S-methoprene to contaminate 110,000 liters of water to a level
that would cause deformities in frogs.
S-methoprene is also widely used in agriculture to treat cattle gazing
areas, tobacco, and certain grain crops. It is also sometimes added to
S-methoprene mimics a hormone that inhibits developing pupae from
molting; thus it is known as an "insect growth regulator." Because
vertebrate species do not have a pupal stage of growth, scientists
assumed S-methoprene could not harm amphibians or mammals. When fed to
mammals, S-methoprene is about as toxic as sugar.
Now La Clair's work has shown that this seemingly-harmless chemical can
be transformed into a potent teratogen by exposure to sunlight for just
a few hours. The implications of this research, which was reported in
ENVIRONMENTAL SCIENCE & TECHNOLOGY, a journal of the American Chemical
Society, are profound. For one thing, it means that once again the
pesticide regulators at U.S. Environmental Protection Agency [EPA] have
missed a key feature of a chemical whose safety they regulate.
Secondly, it shows once again that relying on risk assessment leads to
bad public health decisions. EPA's risk assessments have routinely
failed to evaluate the breakdown by-products of the pesticidal
chemicals that the agency has deemed safe enough to allow as residues
on our dinner plates. Third, it means that thousands of pesticides now
in common use need to be re-tested to see if their breakdown by-
products are dangerous to humans or other species. However, this
additional testing is unlikely to occur any time soon because EPA
currently estimates that it is at least 15 years behind schedule in
safety-testing the pesticides to which we --and the frogs --are
currently being exposed.
Indeed, the situation is worse than the agency makes it out to be.
Congress ordered EPA to re-evaluate and modernize all pesticide safety
tests in 1972, and it demanded that the agency complete the job by
1977. Since 1972 the Agency has been doing its best to comply, but each
year new revelations have come to light, new evidence showing that
pesticides can harm humans and the environment in ways that no one
imagined, so additional tests have been required. Thus La Clair's work
is just the latest surprise in a long chain of unpleasant surprises.
EPA officials in 1996 estimated that they will complete their pesticide
safety re-evaluations (which they were ordered by Congress to complete
in 1977) in the year 2011 --34 years late --IF they can keep the work
on schedule. Meanwhile the frogs and we continue to be exposed to
thousands of poorly-understood government-approved industrial poisons.
In sum, Dr. La Clair's research into the deformed frogs of North
America serves to remind us that pesticides are now too dangerous to be
safely regulated, even by the most powerful government the world has
Or is it that PESTICIDE MANUFACTURING CORPORATIONS are now too
dangerous to be safely regulated, even by the most powerful government
the world has ever known? It's a fair question.
--Peter Montague (National Writers Union, UAW Local 1981/AFL-CIO)
 James J. La Clair and others, "Photoproducts and Metabolites of a
Common Insect Growth Regulator Produce Developmental Deformities in
XENOPUS," ENVIRONMENTAL SCIENCE & TECHNOLOGY Vol. 32, No. 10 (1998),
 Rodger Doyle, "Amphibians at Risk," SCIENTIFIC AMERICAN August,
1998, pg. 27.
 William Souder, "Evidence Grows, Suspects Elusive in Frogs'
Disappearance," WASHINGTON POST July 6, 1998, pg. A3.
 Andrew R. Blaustein and others, "UV repair and resistance to solar
UV-B in amphibian eggs: A link to population declines?" PROCEEDINGS OF
THE NATIONAL ACADEMY OF SCIENCES Vol. 91 (March 1994), pgs. 1791-1795.
 Lee Berger and others, "Chytridiomycosis causes amphibian mortality
associated with population declines in the rain forests of Australia
and Central America," PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES
Vol. 95 (July 1998), pgs. 9031-9036. See also Jocelyn Kaiser, "Fungus
May Drive Frog Genocide," SCIENCE Vol. 281, No. 5373 (July 3, 1998),
pg. 23; and see Carol Kaesuk Moon, "Newly Found Fungus Tied to
Vanishing Frog Species," NEW YORK TIMES June 28, 1998, pg. unknown.
This is not the first fungus linked to frog deaths; see Andrew R.
Blaustein and others, "Pathogenic Fungus Contributes to Amphibian
Losses in the Pacific Northwest," BIOLOGICAL CONSERVATION Vol. 67
(1994), pgs. 251-254.
 J.B Kerr and C.T. McElroy, "Evidence for Large Upward Trends of
Ultraviolet-B radiation Linked to Ozone Depletion," SCIENCE Vol. 262
(November 12, 1993), pgs. 1032-1034. See also Mario Blumthaler and
Walter Ambach, "Indication of Increasing Solar Ultraviolet-B Radiation
Flux in Alpine Regions," SCIENCE Vol. 248 (April 13, 1990), pgs. 206-
 Maggie Fox, "Common chemical may be to blame for dead frogs,"
Reuters wire service August 5, 1998.
 John Wargo, OUR CHILDREN'S TOXIC LEGACY; HOW SCIENCE AND LAW FAIL
TO PROTECT US FROM PESTICIDES (New Haven, Conn.: Yale University Press,
1996), chapter 5.
Descriptor terms: frogs; amphibians; epa; regulation; pesticides; s-
methoprene; hormone disrupters; retinoids;