GAINESVILLE, Fla. — A farm irrigation canal would seem a healthier place for toads than a ditch by a supermarket parking lot.
University of Florida scientists have found the opposite is true. In a
study with wide implications for a longstanding debate over whether
agricultural chemicals pose a threat to amphibians, UF zoologists have
found that toads in suburban areas are less likely to suffer from
reproductive system abnormalities than toads near farms – where some
had both testes and ovaries.
"As you increase agriculture,"
said Lou Guillette, a distinguished professor of zoology, "you have an
increasing number of abnormalities."
Guillette is one of several UF authors of a paper on the research appearing in the online version of the journal Environmental Health Perspectives.
The lead author is Krista McCoy, who did the work as part of her UF
School of Natural Resources and the Environment dissertation.
past studies have suggested a link between herbicides commonly used in
farming and sexual abnormalities in tadpoles and frogs. Such
deformities may be responsible for declines in frogs documented in
areas affected by agricultural contaminants, such as Sierra Nevada,
Calif., McCoy said. Amphibians are declining worldwide and agricultural
chemicals are considered to be one likely cause, she said. Others
include pathogenic infections and habitat loss.
has compared frogs collected from natural areas with those collected
from agricultural areas. Other efforts have pointed to specific
chemicals, including the herbicide Atrazine, as causing abnormalities.
The UF study is the first peer-reviewed study to compare abnormalities
in wild toads – toads are a variety of frogs — from heavily farmed
areas with frogs from both partially farmed and completely suburban
areas. In so doing, it highlights the difference between the impact of
agriculture versus development.
"Our study is the first to
explicitly ask, of these two areas of human disturbance, do we see a
greater proportion of abnormal animals in one versus another?"
Because the results implicate agriculture, future research can narrow the focus to agricultural chemicals, McCoy said.
we know what chemicals are used at these agricultural sites, we can
begin to pin down the chemical cause of these abnormalities by
conducting controlled experiments with each chemical alone and in
combination," she said.
The researchers gathered giant toads,
known scientifically as Bufo marinus, from five sites stretching from
Lake Worth to Belle Glade and down to Homestead in South Florida. Bufo
marinus is a very large, exotic, invasive, species known to be deadly
to small animals. Guillette said the researchers studied the toad in
part because they are easy to catch and their large size ensures enough
blood for analysis. Also, he said, "they are common in other
agricultural areas around the world," which means they are a good
One of the sites consisted almost
entirely of land devoted to sugar cane or vegetable farms. The amount
of farmland declined in three other sites, with the last being entirely
suburban. Researchers calculated the amount of farmland in each site
using Google Earth images.
Each site was 2.1 square miles,
with the toads collected at the center. That’s because the toad’s home
range is known to be about 1.2 miles, and the researchers sought only
those toads living entirely within each site. The researchers collected
at least 20 toads from each site in 2005 and 2006.
of the euthanized toads revealed a pattern: The more agricultural the
land where they lived, the more sexual organ abnormalities or so-called
"intersex" toads — toads who have both female and male internal
reproductive organs, not a normal condition for this and most species
While normal male toads’ forelimbs are thicker
and stronger than those of their female counterparts, more of the
intersex frogs only found in agricultural areas had thin, weak
forearms. Also, intersexes had fewer "nuptial pads," areas of scrappy
skin on their feet used to grip females during copulation.
a sex was clear, the male toads appeared by far the most affected.
Normal males are brown, while females are mottled with brown stripes.
However, males from agricultural areas were mottled, looking like
Internally, the more agricultural the sites, the more
feminized the males’ reproductive organs. Many had both ovaries and
testes. Not only that, both the impacted males and the intersex frogs
had less of the male hormone testosterone than normal males, suggesting
diminished reproductive capabilities, Guillette said.
and McCoy said the study’s results may have important implications not
only for other wild species, but also for people.
"What we are
finding in Bufo marinus might also occur in other animals, including
other amphibian species and humans," McCoy said. "In fact, reproductive
abnormalities are increasing in humans and these increases could
partially be due to exposure to pesticides."
University of Florida. Public release date: July 3, 2008.