DEBBIE ELLIOTT, host:

And now, Science Out of the Box.

Good news for fans of big reptiles. The American crocodile found in this country only in South Florida has staged a comeback. It's done so well in fact that the U.S. Fish and Wildlife Service wants to take it off the endangered species list.

NPR's Greg Allen spent time with the field biologist who's chronicled the crocs' comeback and found that the hero in this story is a nuclear power plant.

GREG ALLEN: Fifty years ago, Joe Wasilewski was just a kid growing up in Chicago with a fascination for all things reptile. It started when a neighbor put a snake down his shirt. Wasilewski kept the snake. Now he's in South Florida living out his dream. He's a wildlife biologist who travels the world helping governments and conservation groups manage endangered reptiles. He'll be in the Bahamas counting iguanas one month, in Guyana counting black Caimans the next.

Since 1989 there's one reptile population that he's followed intimately. The crocodiles that live and breathe in the extensive canal system south of Miami, next to the Turkey Point nuclear power plant.

Mr. JOE WASILEWSKI (Wildlife Biologist): And that's literally 168 linear miles, if you were to line them up, end to end, which is a pretty big chunk of habitat.

ALLEN: Joe Wasilewski is a part-time employee with Florida Power and Light, maundering the crocodile population in canals that were built to cool the water left over from generating nuclear power. Driving his company pickup over the bumpy gravel road that runs alongside the marshy canal, Wasilewski says when it built the nuclear plant, FPL wasn't thinking about crocodiles.

Mr. WASILEWSKI: It went online in the early '70s, at which time the animals, the American crocs, were really extremely rare and headed towards extinction. So they built this and the crocs came in on their own. They began nesting here. Last year for instance, there were 24 successful nests within the cooling canal system that I found.

ALLEN: At last count, Wasilewski says, there were 400 juvenile or adult crocodiles at Turkey Point. That's nearly a quarter of the entire U.S. population. The animals love the area, he says, because it provides almost perfect crocodile habitat - few people and a mixture of freshwater and saltwater canals.

Mr. WASILEWSKI: We're at now the southwest corner of this cooling canal system. In fact, here's where I'm going to stop. I've seen this many others(ph) of croc right there. Do you see him?

ALLEN: Yeah.

Mr. WASILEWSKI: That's a big one too. He's about 10 feet long.

ALLEN: The crocodile is in the water; only his snout breaks the surface. Crocodiles are lighter colored than alligators, olive green or gray. Their heads are narrower and more pointed. And in Florida, at least, there are many fewer crocs than alligators. Maybe 1,500, Wasilewski says, versus an estimated 2 million alligators. But because of their limited range in the U.S., there were never many crocodiles in North America.

Scientists say their numbers today are about what their population was in South Florida a hundred years ago. The decision to move crocodiles from endangered to threatened status - downlisting as it's called - was based on a number of factors. An important one, Wasilewski says, is the number of nesting sites.

Mr. WASILEWSKI: Back 15, 20 years ago, we said if there are 60 - that's 6-0 -viable nests in a year, we'll consider this animal out of trouble. And two years ago, I think there were 61 nests. Last year there was much more than that and we expect that number to climb. There's food here. There's shelter here. There's boys and girls here.

ALLEN: Crocodile boys and girls.

Mr. WASILEWSKI: Crocodile boys and girls, right. There's one just swimming. There's actually three here now.

ALLEN: Compared to alligators, crocodiles are shy of people. That may be one reason why there's never been a report in the U.S. of a crocodile attacking a person. In Costa Rica and Belize where he also monitors American crocodiles, Wasilewski says it's a different story. The crocs there get bigger and there are a couple of attacks every year.

When he first heard about the proposal to downlist the crocodile a couple of years ago, Wasilewski says he wasn't in favor.

Mr. WASILEWSKI: Because I thought, oh, boy, you downlist these and now it gives the developers a green light to just brrr(ph), go up and down the coast and, you know, and develop all this land. But the protection status remains the same. So once I was convinced to that from the Fish and Wildlife Service, yeah, yeah, let's go with it. It's a good story.

ALLEN: Wasilewski says he is not sure how many crocodiles the 6,800 acres here at Turkey Point ultimately can support. But one thing he's not worried about is overpopulation. As part of his work monitoring them, he implants microchips into the tails of all hatchlings. He recently found six of the microchips in the stomach of one big croc and eight in another. Evidence, he says, that crocodiles are controlling their own population.

Greg Allen, NPR News, Miami.

ELLIOTT: This is NPR News.

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