Hermaphrodite Frogs Linked to Pesticide Use
Los Angeles Times
THE NATION
March 2, 2005
By Marla Cone, Times Staff Writer
Study finds more sex organ abnormalities in 1950s, when chemicals were more widespread.
Scientists who compared frogs collected over the last 150 years have discovered a dramatic increase in hermaphrodites during the times when contamination from the pesticide DDT and other chlorinated compounds was widespread.
Frogs with both male and female reproductive organs were rare in the 19th and early 20th centuries but more common during the 1950s, when the largest volumes of the chemicals were used.
The findings, reported Tuesday in the journal Environmental Health Perspectives, add to the growing evidence that an array of pesticides and industrial chemicals can alter the sex hormones of animals.
The ability of certain chemicals to mimic or block estrogen and testosterone, which are key in sexual organ development and reproduction, is considered one of the most disturbing discoveries in environmental science of the last decade.
Scientists suspect that the phenomenon has been occurring for decades but it wasn't documented in wildlife until the early 1990s when it was first observed in Florida alligators and then among many other species.
Toxicologists and veterinarians at the University of Illinois at Urbana-Champaign, led by Amy Reeder, examined the reproductive organs of 814 cricket frogs collected in Illinois between 1852 and 2001 and stored at natural history museums.
Studying endocrine disruption of animals dating back more than a century has not commonly been done.
"It's a wonderful approach, and appropriate, because museums have this great collection of data that goes back through time," said Louis Guillette, a reproductive zoologist at the University of Florida and an expert on hormone disruption. "This is a very, very important study that suggests to us that some of the things we're seeing, even today, in frog populations may have a historical basis dating back to when we were using large amounts of these compounds." Guillette, who was not involved in the study, has linked hermaphroditic alligators to DDT.
Cricket frogs, once abundant, declined dramatically around Chicago and in other regions in the 1960s. Scientists found that the times and places with high rates of hermaphrodites, also called intersexes, overlapped with when and where the frog numbers dropped in Illinois.
The scientists theorize that DDT, industrial compounds called PCBs (polychlorinated biphenyls) and other contaminants had an antiestrogenic effect, reducing the proportion of females and causing them to develop abnormal sex organs, triggering a population crash, particularly in the Chicago region.
"I'm sure it's not the only stressor that affected cricket frogs, but the intersexes did come along and the species did disappear," said Val Beasley, a coauthor of the study and a professor of ecotoxicology at the University of Illinois' College of Veterinary Medicine.
Frogs and other amphibians have been vanishing worldwide over the last few decades.
"These guys have been around a long time, since before the dinosaurs, and they are declining all over the place," Beasley said. "Endocrine disruptors seem to be a factor, but certainly not the only factor."
Frogs are considered key specimens for studying effects of environmental degradation because they undergo a vulnerable time of metamorphosis and spend most of their time in water, where pollutants accumulate. Environmental Health Perspectives science editor Jim Burkhart said frogs "may show the effects of ecological change more quickly or more obviously than other species."
The new study has limitations. The team did not measure individual animals for contaminants because archived samples could not be reliably tested. As a result, there could be other explanations for the hermaphrodites, such as factory and vehicle emissions or other environmental changes. Also, the scientists could not compare frogs from the same lakes or ponds over time.
The highest rates of intersexes were found in frogs collected from 1946 to 1959, when larger volumes of DDT were used for mosquito control in Illinois. Some frogs had complete ovaries as well as complete testes. The proportion of females also was greatly reduced during that period. The area around Chicago had four times more intersex cricket frogs than southern Illinois.
From 1852 to 1929, one intersex frog was found in the 84 collected. In the 1930s, the intersex rate began to increase. Between 1946 and 1959, 17 out of 153, or 11%, were intersex.
Recent rates of hermaphrodites were the lowest of any period studied except for 1852 to 1929. Out of 339 collected from 1980 to 1996, there were nine intersex frogs, or less than 3%. DDT and PCBs were banned in the 1970s in the United States and although they remain in the environment, levels are low in most areas.
But, the scientists reported, "we cannot conclude that the era of endocrine disruption in cricket frogs has come to an end." The severity of the current problem is unknown, Beasley said, "because you can't collect where the inter-sex rate was high. There aren't any frogs left in those areas to collect." Also, males still outnumber females, while it was the opposite before 1930.
Atrazine, an herbicide widely used on corn, might also contribute to the sex organ abnormalities and decline of the frogs, the report says. The intersex rate in central Illinois, where atrazine is used, is twice as high as in the southern areas, where atrazine use is low, Beasley said.
The possibility that atrazine alters hormones is controversial because it is the most popular herbicide in use today and the U.S. Environmental Protection Agency decided in 2003 to allow its continued use. Some studies have found reproductive or hormonal effects from atrazine while other studies have found no effects at levels found in the environment.
Wildlife biologists agree that environmental contaminants have altered hormones of wild animals, including polar bears in the Norwegian Arctic, fish in the Potomac River and otters in the Columbia River.
But the experts don't agree on what doses are harmful and whether populations have been substantially reduced. Scientists do not know if there are human effects, although some theorize that chemicals cause reduced sperm counts, reproductive diseases and premature puberty in girls.