This was especially true for Lake Apopka near Orlando, one of Florida's largest freshwater lakes and one of its most polluted. In some years, as few as 5 percent of its alligator eggs hatched, compared with 70 to 80 percent of eggs from a nearby lake. In the lab, half the animals that did hatch died within weeks, a mortality rate many times the norm.

Much of alligator development depends on nest variables such as temperature and moisture. But after Guillette and his team eliminated those variables as sources of the reproductive problems, they refocused on the lake water and soil. The researchers suspected that pollutant toxins, possibly stemming in part from a large pesticide spill at Lake Apopka in 1980, might be affecting the newborn alligators' hormone levels and contributing to their failure to thrive.

When the scientists analyzed blood from young alligators, they found estrogen and testosterone levels that were totally out of balance. "When you see males with hormone levels that look like those of females, that's not normal and that's what we were seeing in Lake Apopka," says Guillette. These imbalances persisted as the alligators matured. Juveniles, male and female, looked like "superfemales," and when they reached adolescence, the males were still demasculinized and the females had become defeminized.

Given the abnormal hormone levels, the researchers needed to look for physical manifestations, such as effects on the animals' genitalia. Sure enough, they found that male alligators in Lake Apopka had penises that were 25 to 30 percent smaller than those of their counterparts in other lakes.

"Penis measurements have always been a tool: if this one marker is disrupted, I'm assuming others are disrupted," he says.

Although the data kept pointing to hormonal imbalances, Guillette says he didn't put all the pieces together until his "academic grandfather"—zoologist Howard Bern of the University of California, Berkeley—visited his lab and made a presentation about the impact of the synthetic estrogen called DES (diethylstilbestrol) on the ovaries of mice and women. "During his talk, a slide came up that showed the pathology of an ovary with multi-oocytic follicles," Guillette says, referring to follicles, or sacs, with more than one egg in them. "When I realized that the alligators I was working on had the same weird follicles, it was an ‘Aha!' moment."

As Guillette later said in "Fooling with Nature," a 1998 Frontline documentary, "It was one of these incredible experiences when suddenly everything adds up. I realized, ‘I have hormonal abnormalities. I have a contaminated lake. I have a top predator that accumulates contaminants.' It all just kind of came together as a hypothesis."

Once Guillette realized that he might be dealing with endocrine disrupters, he focused his research accordingly. "We had been taught, and had taught our students, that in the endocrine system the receptor was so specific it could be described as a lock, with the hormone being the key," says Guillette. "Now we realize, after an awful lot of science, that yes, the estrogen receptor is exquisitely able to recognize estrogens, but really what it was designed to do was to distinguish between estrogens and androgens, like testosterone. It wasn't necessarily designed to preclude all other keys."

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