Female fish are being masculinised by gender-bending pollutants in rivers. The effect, which has been largely overlooked by scientists, seems to be a mirror image of the more commonly recorded feminisation of male fish by oestrogen-mimicking substances.
It has been speculated that it may even be having a hitherto unnoticed influence on humans.
A team from Samford University in Alabama, US, revealed that artificially high concentrations of a natural chemical, known as "plant cholesterol", are the most likely culprit, at a conference on environmental hormones in New Orleans on Monday.
Separate studies found freshwater fish swimming in effluent downstream from paper mills and a sugar beet processing plant had been masculinised.
Mike Howell, a professor of biology who led the Samford team, has been funded by the US government to identify the culprit.
"We know there have been feminising effects caused by chemicals that can act as oestrogens - now we are beginning to become aware of masculinising effects too," said Professor Howell.
"We have tentatively found some androgens in the water which may be responsible, though the evidence is not firm yet."
His original study, published in 1980, went unnoticed by the scientific community, which has subsequently focused its attention on environmental oestrogens.
He found that female mosquito-fish living in waters below paper mills grew an extra fin - used by males to transfer sperm during copulation - and chased normal females in a vain bid to fertilise them.
This work was recently supported by another team of scientists, who documented a case of masculinised female catfish in a stream below a sugar beet processing plant.
Professor Howell has found evidence that the chemical beta-
sitosterol, a vital component of plant cell membranes, may be responsible. Large quantities of this ubiquitous substance had been dumped into pools in tons of paper pulp wastes and then attacked by microbes.
The bugs turned the chemical into an androgen molecule similar to the male sex hormone testosterone. This is thought to have disrupted the development of the female fish.
"By concentrating huge amounts of decaying plant material, letting it biodegrade and then allowing the products into streams, humans have created this problem. The possible effects of environmental androgens on human populations have not been studied and should be a topic of future concern," said Professor Howell.