(Beyond Pesticides, March 9, 2007)
New research shows that frogs are more sensitive to hormone-disturbing environmental pollutants than was previously thought. Male tadpoles that swim in water with environmentally relevant levels of such substances become females, according to the study that will be published in the scientific journal Environmental Toxicology and Chemistry (ET&C) in May (See University Press Release). The results may shed light on at least one reason that up to a third of frog species around the world are threatened with extinction, suggests the study.
In a laboratory experiment by researchers in Sweden, two species of frogs, the European common frog (Rana temporaria) and the African clawed frog (Xenopus tropicalis), were exposed to levels of oestrogen (estrogen or chemicals mimicking the effect of estrogen) similar to those detected in natural bodies of water in Europe, the United States and Canada. The results were startling: whereas the percentage of females in two control groups was under 50 percent – not unusual among frogs – the sex ratio in the groups of tadpoles who matured in water dosed with different levels of oestrogen were significantly skewed. Even tadpoles exposed to the weakest concentration of the hormone were, in one of the two groups, twice as likely to become females. The population of the two groups receiving the heaviest dose of oestrogen became 95 percent female in one case, and 100 percent in the other.
Some of sex-altered males became fully functioning females, but other had ovaries but no oviducts, making them sterile, Dr. Berg explained.
“The results are quite alarming,” said co-author Cecilia Berg, Ph.D., a researcher in environmental toxicology. “We see these dramatic changes by exposing the frogs to a single substance. In nature there could be lots of other compounds acting together.”
Earlier studies in the United States, Dr. Berg explained, linked a similar sex-reversal of Rana pipiens male frogs – one of the two species used in the experiment – in the wild to a pesticide that produced oestrogen-like compounds.
“Pesticides and other industrial chemicals have the ability to act like oestrogen in the body,” Berg said. “That is what inspired us to do the experiment,” she said, referring to her collaborator and lead author of the article, doctoral candidate Irina Pettersson, also a researcher at Uppsala.
The study does not measure the potential impact of pollutant-driven sex change for frog species, but the implications, said Dr. Berg, are alarming. “Obviously if all the frogs become female it could have a detrimental effect on the population.”
Amphibians are declining at alarming rates across the globe, and many scientists believe that industrial chemicals and pesticides may be partially to blame. Numerous scientific studies have definitively linked pesticide use with significant developmental, neurological and reproductive effects on amphibians. Recent studies by Dr. Tyrone Hayes at the University of California have strengthened the case for banning atrazine, the most common contaminant of ground, surface, and drinking water. Dr. Hayes demonstrated that atrazine is an endocrine disruptor that chemically castrates and feminizes male amphibians.
The environmental impacts of endocrine- disrupting chemicals has been well-established; in addition to hermaphroditic deformities in frogs, pseudo-hermaphrodite polar bears with penis-like stumps, panthers with atrophied testicles, and intersex fish have all been documented as the probable result of endocrine-disrupting chemicals in the environment. Many scientists believe that wildlife provides early warnings of effects produced by endocrine disruptors, which may as yet be unobserved in humans.