The frog, hallmark of the amphibian class, lives on land and water and breathes through its skin.  Therefore, disruptions to the environment, be they climate or pollutant related, are often discovered first in amphibians like frogs.  This has given the frog the unofficial designation as a signal species, representing how knowledge of environmental effects on the frog can be extrapolated to potential effects on humans in the future.

As OSU zoologist Andrew Blaustein told USA Today in 2000:

"At this point, we can say for sure that there are several causes of amphibian declines, which include rising levels of UV-B radiation in sunlight, pathogens, pollutants, habitat destruction, introduced predators, and, most recently, crop fertilizers. But the overall result is that this group of animals, which has been around since the time of the dinosaurs, is now in serious decline all over the world. And some of the things that are killing frogs almost certainly have implications for other animal species, including humans."

Recently, scientists at UC Berkeley have discovered that the pesticide Atrazine, an herbicide and widely used weed killer, causes 1 in 10 frogs to turn from male to female.  In the abstract, published online by PNAS this week, the authors say:

The herbicide atrazine is one of the most commonly applied pesticides in the world. As a result, atrazine is the most commonly detected pesticide contaminant of ground, surface, and drinking water. Atrazine is also a potent endocrine disruptor that is active at low, ecologically relevant concentrations. Previous studies showed that atrazine adversely affects amphibian larval development. The present study demonstrates the reproductive consequences of atrazine exposure in adult amphibians. Atrazine-exposed males were both demasculinized (chemically castrated) and completely feminized as adults. Ten percent of the exposed genetic males developed into functional females that copulated with unexposed males and produced viable eggs. Atrazine-exposed males suffered from depressed testosterone, decreased breeding gland size, demasculinized/feminized laryngeal development, suppressed mating behavior, reduced spermatogenesis, and decreased fertility. These data are consistent with effects of atrazine observed in other vertebrate classes. The present findings exemplify the role that atrazine and other endocrine-disrupting pesticides likely play in global amphibian declines.

The CDC says the following about Atrazine, and includes a disclaimer on their website that findings in species such as frogs may not be relevant for humans given the great differences between their reproductive biology:

The general population is probably not exposed to atrazine. However, exposure to atrazine may occur at farms where it has been sprayed. Atrazine may affect pregnant women by causing their babies to grow more slowly than normal. Birth defects and liver, kidney, and heart damage has been seen in animals exposed to high levels of atrazine. This chemical has been found in at least 20 of the 1,636 National Priorities List sites identified by the Environmental Protection Agency (EPA).

 However, as time and research progress, more and more data have been compiled to implicate Atrazine as an endocrine-disrupting chemical (affecting estrogen and testosterone) in not only amphibians,  but also mammals like rodents and humans.  So, the ‘signal species’ designation may not be a misnomer after all.

According to the UC Berkeley website:

As a result of these studies, the Environmental Protection Agency (EPA) is reviewing its regulations on use of the pesticide. Several states are considering banning atrazine, and six class action lawsuits have been filed seeking to eliminate its use. The European Union already bars the use of atrazine.

In 2004 the NRDC reported that the EPA would not restrict use of Atrazine despite some compelling studies linking the herbicide to ill health.  Since then, amendments have been made to the EPA website on Atrazine; including the announcement of  a review of scientific findings beginning in the fall of 2009, and a meeting on the initial findings of this review in February 2010.  From the EPA website:

EPA is launching a new comprehensive evaluation of atrazine. This review is based on transparency and sound science, including independent scientific peer review, and will help determine whether a change in EPA’s regulatory position on atrazine is appropriate. Based on this evaluation, the Agency will decide whether to revise its current atrazine risk assessments and whether new restrictions are necessary to better protect health and the environment.

Further Reading:

 RELATED ALIQUOT POSTS:

Female or Male by default: why our ovaries could be testes

http://open.salon.com/blog/aliquot/2009/12/16/female_or_male_by_default_why_our_ovaries_could_be_testes

Endocrine disrupting chemicals, new scientific understanding

http://open.salon.com/blog/aliquot/2009/10/21/endocrine_disrupting_chemicals_new_scientific_understanding

USA Today 2000:

http://findarticles.com/p/articles/mi_m1272/is_2661_128/ai_62685292/

UC Berkeley press release:

http://www.berkeley.edu/news/media/releases/2010/03/01_frogs.shtml

PNAS advance online publication:

http://www.pnas.org/content/early/2010/02/12/0909519107.abstract

CDC Atrazine:

http://www.atsdr.cdc.gov/tfacts153.html

EPA Atrazine:

http://www.epa.gov/testmike/oppsrrd1reregister/atrazine/

image from murmur.com