22 November 2008
| Agricultural Runoff Implicated In Sexual Abnormalities |
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| Science - Biological Sciences | |||
| Written by TS-Si News Service | |||
| Sunday, 06 July 2008 17:00 | |||
  The Frog. An amphibian in the order Anura (Gr. an-oura; without + tail), frogs are some of the oldest living creatures.
The order contains 5,250 species in 33 families. Approximately 88% of amphibian species are frogs, distributed from tropic to subarctic regions. Most species found in tropical rainforests. They are among the most diverse groups of vertebrates.
Frogs and toads. Distinguished on the basis of their appearance, people often distinguish between frogs and toads. However, this is caused by the convergent adaptation of toads to dry environments.
Technically, they are all the same and the distinction has no taxonomic basis. Many species fit equally well into either category.
• The common name toad is generally given to those with dry, warty skin and short hind legs for walking (instead of jumping). Toads generally live in dry climates and lay their eggs in long chains.
• Frogs have smooth moist skin and strong webbed hind legs for swimming and jumping. Frogs usually live in moist climates and lay their eggs in clusters.
Population decline. Certain frog species have declined significantly since the 1950s. More than one third of all species are threatened with extinction; more than 120 species are suspected to be extinct since the 1980s.
Since frogs are especially sensitive to pollution, their condition often indicates previously unknown or underestimated environmental problems.
Deformities. Malformed reproductive organs are seen as the basis for the continuing decline of frog populations. Natural malformations can occur in 5 percent or fewer of the members in a given population.
However, there are populations where 70 percent show deformities.
Researchers are working to understand what outside source causes abnormal development in the frogs. Scientists have identified several candidate factors:
• Climate changes, such as global warming and the thinning of the ozone layer, can result in overexposure to ultraviolet radiation.
• Habitat destruction.
• Pollution looms as major source of deformities sonce frogs absorb water directly through their skin. They are particularly vulnerable to water pollutants like pesticides and acid rain.
In any case, the result had been diminishing populations around the world. Deformities from agricultural contaminants may be responsible for the documented worldwide declines in amphibian populations. Many scientists consider agricultural chemicals as one likely cause. Several past studies suggested a relationship between common farm herbicides and sexual abnormalities in tadpoles and frogs. The findings pointed to specific chemicals — including the herbicide Atrazine — as the cause. Other suspected causes include pathogenic infections and habitat loss.
Previous research compared frogs collected from natural areas with those collected from agricultural areas. New research from the University of Florida (UF) is the first peer-reviewed study to compare abnormalities in wild toads (a variety of frogs) from heavily farmed areas with frogs from both partially farmed and completely suburban areas. In doing so, it highlights the difference between the impact of agriculture versus development.
 "As you increase agriculture," said Lou Guillette, a distinguished professor of zoology at the University of Florida (UF), "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 (EHP).
 The lead author is Krista McCoy, who did the work as part of her UF School of Natural Resources and the Environment dissertation. Guillette 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."
Guillette said "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, according to McCoy. "Because 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.
 Giant toad (Tampa, Florida)
Ordinarily, Bufo marinus is a very large, exotic, and invasive, known to be deadly to small animals. Researchers found that toads in suburban areas are less likely to suffer from reproductive system abnormalities than toads near farms — where some toads had both testes and ovaries.
Photo of Bufo marinus by Bill Waller, courtesy of Wikimedia Commons (GNU license).
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. 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 generalist species.
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.
Examination 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 of amphibians.
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.
Where 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 females.
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.
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| Last Updated on Sunday, 06 July 2008 10:09 |
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The TS-Si News Service is a collaboration of TS-Si staff, contributors, and corresponding institutions. Contents do not necessarily convey official positions of TS-Si, its partners, or affiliates