Researchers have found that such chemicals interfere with songbirds' reproduction. They also alter their brains, making females sing when they shouldn't.
"This is very significant," says David Crews, an expert on the effects of synthetic chemicals on animal reproduction at the University of Texas in Austin. He says that until now, concrete evidence on how pollutants affect animals has been scarce. "This is the first step needed to demonstrate a causal link between specific pollutants and the effects on wildlife populations," he says.
The pollutants in this case are oestrogens. Synthetic oestrogen used in birth control pills or hormone replacement therapy (HRT) for women has been finding its way into streams and rivers. Oestrogen-like compounds are also formed by the breakdown of pesticides such as DDT.
Last week the US Geological Survey reported that waterways in the US are contaminated with such compounds (Environmental Science and Technology, vol 36, p 1202). Environmental oestrogens are believed to be responsible for male alligators in Florida being born with shrunken penises. Male fish have also produced yolk protein-something usually only female fish do.
Researchers know they can change the brain circuitry of female zebra finches by injecting them with oestrogen shortly after birth. This makes them sing, normally the preserve of males. But James Millam and his team at the University of California in Davis wondered whether the birds would also be affected by eating doses of oestrogens similar to those they would ingest in the wild.
They fed hatchlings for a week with a chemical containing oestradiol, an oestrogen used in HRT. Controls were given rapeseed oil. When the birds grew into adults, the researchers found that female finches fed on oestradiol started to sing, as did the males. The team then dissected the birds' brains, and found that the regions controlling singing were highly developed in the oestradiol-fed females, though not in the controls.
More significantly, the birds' ability to reproduce was severely affected. Finches given oestradiol produced fewer eggs and they had brittle shells. The number of hatchlings fell dramatically. In another study, still to be published, Millam's team found that oestradiol also caused enlarged oviducts in hatchlings. "We've shown that oestrogens can disrupt reproduction," says Millam. "So there's a reason to go looking for the effects on wild-bird populations."
Author: Anil Ananthaswarmy
More at: Hormones and Behavior (vol 41, p 236)
New Scientist issue: 30 March 2002
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