DDT years saw boost in hermaphrodite frogs

Scientists who compared frogs collected over the past 150 years have discovered a dramatic increase in hermaphrodites during the times when contamination from the pesticide DDT and other organochlorine chemicals was widespread.

Frogs with both male and female reproductive organs were rare in the 19th century and early 20th century but abundant during the 1950s, when the largest volumes of the popular chemicals were used.

The findings, reported in the journal Environmental Health Perspectives, add to the growing evidence that a variety of pesticides and industrial chemicals can alter the sex hormones of animals.

The ability of chemicals to mimic or block estrogen and testosterone, which are critical for normal sexuality and reproduction, is considered one of the most disturbing discoveries in environmental science of the past decade.

Scientists believe that the phenomenon has been occurring for decades but wasn't documented in wildlife until the early 1990s.

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 museums.

Cricket frogs, once abundant, declined dramatically in the 1960s, and the scientists found that the times and places with high rates of hermaphrodites, also called intersexes, overlapped with when and where the frogs disappeared in Illinois.

The scientists theorize that DDT, industrial compounds called PCBs, and other contaminants had an anti-estrogenic effect, reducing the proportion of females and causing them to develop skewed sex organs, triggering a population crash.

Frogs and other amphibians have been vanishing worldwide over the past few decades.

"These guys have been around a long time, since before the dinosaurs, and they are declining all over the place," said Val Beasley, a co-author of the study and a professor of ecotoxicology at the university's College of Veterinary Medicine. "Endocrine disruptors seem to be a factor, but certainly not the only factor."

Frogs are considered a key sentinel for 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 study has limitations. The team did not measure individual animals for contaminants because archived samples cannot be reliably tested. As a result, there could be other explanations. Also, the scientists could not compare frogs from the same lakes or ponds over time.

The highest rates of intersex animals were found in frogs collected from 1946 through 1959, when large volumes of DDT were used for mosquito control in Illinois.

Between 1852 and 1929, one intersex frog was found in the 84 collected. Then, beginning in the 1930s, the intersex rate increased. Between 1946 and 1959, 17 out of 153, or 11 percent, were intersex. Recent rates of hermaphrodites were the lowest of any period studied except for 1852-1929. Out of 339 collected from 1980 through 1996, there were nine intersex frogs, or less than 3 percent. DDT and PCBs were banned in the 1970s in the U.S., and although they remain in the environment, levels are now low in most areas.

Nevertheless, 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 intersex rate was high. There aren't any frogs left in those areas to collect."