"Instead of talking about wild types, we're talking about real wild animals," said Louis Guillette of the University of Florida's Department of Zoology, "and when we talk about knockouts we don't normally mean gene knockouts."

Florida fishermen began detecting irregularities in the local American alligator (Alligator mississippiensis) population almost 20 years ago, and brought in Guillette to investigate the reproductive biology of these animals.

Guillette duly set upon the intrepid task, which involves hanging off the front of airboats in Florida's lakes and capturing reptilian subjects for investigation. "It's a little different to the recruitment that you all do for your patients," Guillette told an audience of clinical endocrinologists.

He found that male and female alligators in some lakes had abnormal levels of circulating sex hormones. Males in these lakes had elevated levels of estradiol and reduced levels of testosterone - similar to levels normally measured in females.

Females also had elevated levels of estradiol, almost twice what is normally seen, but once they reached puberty they had depressed levels of the hormone. Many of these females also had polyovular follicles, which are associated with increased rates of infertility and embryonic loss, and are generally the result of estrogen exposure during embryonic or neonatal life.

"The problem is, my animals aren't taking pharmaceutics," said Guillette. "So where in the world is this estrogen signal coming from?"

He has been detecting the most profound changes in alligator hormone levels and fertility at Lake Apopca, which lies just outside Orlando and is surrounded by an area of heavy agricultural activity; it also suffered a pesticide spill just over 20 years ago.

Chemical analysis of the water there has revealed relatively high levels of persistent organofluorine pesticides, including two major metabolites of the notorious pesticide DDT, widely banned around the world but still used in many tropical countries.

In unpublished work, Guillette has now begun to measure elevated levels of these contaminants, and others, in alligator eggs collected from nests on the lakeside. One DDT metabolite in particular, named p,p'-DDE, was detected at 10-15 parts per million, putting it on a physiologically relevant microgram scale. p,p'-DDE turns out to compete with estrogen binding for its receptor, says Guillette, making it a potent anti-androgen.

Sex differentiation in alligators has one marked difference to sex in mammals. Temperature determines alligator gender - there is no X and Y chromosome or equivalent. If you incubate an egg at 30^oC for a three-day period, at six weeks gestation, says Guillette, you get 100% females. If you incubate at 33-35^oC you get 100% males.

He has found that if he incubates alligator eggs at the "male-producing temperature", but exposes them to an estrogen (by applying it to the shell), he can redirect sex determination and end up with a female. "This is an all or none," he said. "You don't get hermaphrodites."

His team has now incubated eggs at 32.5^oC, which normally gives a low frequency of females, and treated them with increasing doses of the apparently estrogenic contaminants identified in Lake Apopca. His results are surprising. "We're getting these weird dose-response curves," he said.

Increasing contaminant concentration does not necessarily result in the production of more females. "These compounds aren't acting as full agonists, they appear to be producing both an estrogenic and an anti-estrogenic response," he said.

In addition, Guillette has found that when eggs are treated with levels of contaminants, such as p,p'-DDE, that are insufficient to cause a change of sex, there are nonetheless changes in gonad function that persist well beyond gestation. The testes of these individuals may still look like testes, he says, but they can secrete hormones as though they were ovaries.

"We're starting to get some idea that early exposure to contaminants that have the potential to act as estrogens have long-term consequences beyond just the developing embryo," he said.

The alligator provides a very useful way of monitoring endocrine consequences of environmental contaminants. "No matter how much estrogen you give a developing baby boy you're not going to turn it into a baby girl, like I could do with an alligator," said Guillette.

"The phenotype that we see in organisms, and in this case we've shown you alligators but I would argue that it's probably a large number of organisms including ourselves, is not just the product of our genes and it's not just the environment that we traditionally think about, like nutrition and maybe photoperiod etc., but the environment is also composed of contaminants that have endocrine action."