Scientists don't know how many Louisiana pine snakes exist. They're native only to Louisiana and Texas, and it's been several years since one was trapped in any of the three Texas areas where they had been caught in the 1990s and early 2000s.

That's "cause for pretty serious concern," Craig Rudolph, a scientist at the U.S. Forest Service Southern Research Station in Nacogdoches, Texas, said Wednesday. The other four populations, like the Kisatchie reintroduction site, are in Louisiana.

Another cause for concern is that plans to release 50 to 100 hatchlings a year have been stymied. Only 20 hatched in 2010 and 14 last year. Louisiana pine snakes have the largest eggs and hatchlings of any North American snake, coming out of the egg about 18 to 22 inches long, but each female lays only three to five 5-inch-long eggs.

"Some of the snakes in the zoos are getting older and not breeding," said Beau Gregory, a zoologist with the Louisiana Natural Heritage Program.