MIAMI — Walking gingerly through an overgrown field on the edge of the Everglades National Park in South Florida, wildlife biologist Skip Snow listened closely to the pinging sounds of a portable radio receiver.
As the electronic beeping grew louder, Snow knew he was getting closer to his prey — a 10-foot Burmese python lurking somewhere in the tall grass.
But as he walked on, the radio sounds began to soften, and Snow stopped suddenly. "Now the snake is no longer in front of us, it's back behind us," he warned.
Retracing his steps, he slowly circled a thick clump of vegetation, then froze, pushing apart the grass.
There, curled in the shade, was the long black and brown reptile that had been surgically fitted with a small radio transmitter. Park officials hope this so-called "Judas animal" will lead them to other invasive snakes, so they can be captured and killed.
Unwelcome park visitors
In the vast park, with its subtropical mystique and exotic species, the non-native Burmese pythons have found a new home and are flourishing.
But they have made themselves so unwelcome that wildlife officials are aggressively fighting back.
"They're eating pretty much everything in Everglades National Park," said Superintendent Dan Kimball. "They seem to be eating machines."
Burmese pythons are native to Southeast Asia and are among the largest snakes in the world. They can grow longer than 20 feet.
Thousands of these snakes are imported every year into the United States, and they are also raised domestically to be sold as pets.
A problem is that they can grow very quickly, especially in captivity — upwards of four feet a year.
"We can have a 12-foot snake in under three years and have breeding animals in three to five years," said Todd Hardwick, the owner of Pesky Critters, a Miami animal pest-control company.
Over the years, as the large constricting snakes outgrew their cages and became more difficult to handle, many pet owners released them into the wild.
In the warm Everglades, the freed Burmese pythons found suitable habitat and began to breed. More than 200 of them have been found in just the last few years, many of them along the main road used by tourists and fishermen visiting the park.
"I think they're going to be breeding as fast as we're capturing them," said Hardwick.
Skip Snow, the park biologist who runs the program to study and eradicate the snakes, said, "We've found Burmese pythons in more places each year than we did before. We're also finding more size classes."
A threat to native species
Wildlife officials worry that in Florida the Burmese pythons have no natural enemies to control their spread.
They are believed to be a serious threat to native birds and mammals, which they either eat or crowd out of their nesting and hiding places.
Stomach analyses prove the snakes eat wading birds, rodents, rabbits, raccoons and even bobcats.
On a few occasions now, park visitors have witnessed fierce battles between pythons and the Everglades' top predator, the alligator.
Last fall, scientist found a 13-foot python that had ruptured and died after swallowing a six-foot alligator.
Pictures of the two entangled animals were circulated widely on the Internet, and some people concluded the python had "exploded."
Opinions vary on how the rupture might have happened, but some park officials believe the alligator's sharp back claws tore through the snake's skin after it was swallowed.
No one can be certain if the alligator was dead or alive at the time.
High-tech surgery and ‘Judas’ snakes
In fighting back, scientist have gone both low-tech and high-tech.
Park officials are training a beagle named "Python Pete" to sniff out unwanted snakes. And they are educating schoolchildren to the dangers of freeing pet reptiles, and are sponsoring a "Don't Let it Loose" campaign.
At Davidson College in North Carolina, biologists anesthetized four pythons that were captured in the Everglades and surgically fitted them with antennas and radio transmitters.
One of the snakes was a 16-foot giant that aggressively bared its teeth as it was prepared for the procedure. It took quite a few students to carefully hold it tight.
When the snakes recovered from their operations, they were flown back to Florida, and were released back into the national park.
Tracking their constantly emitting signals, scientists have collected valuable information about the snakes' habitat and travel patterns and so far have been able to capture and kill 12 other snakes that were drawn to the "Judas animals."
Florida considers python law
The python problem is of such concern that a number of Florida state agencies and schools have gotten involved, including the South Florida Water Management District, which maintains some of the land where pythons have been found.
Florida lawmakers are considering a bill that would make it a third-degree felony to release Burmese pythons and other large reptiles or to negligently allow them to escape.
Pet owners would also have to register and photograph their large snakes and pay a $100-a-year fee to keep one.
State Rep. Ralph Poppell, who sponsored the bill, says too many casual python buyers have no idea about the serious problems they could face, and the damage they could do to the environment.
"What we're trying to do is bring some common sense and reality into something that some people look at as being harmless, until they have an encounter with it," Poppell said. "Please be careful with what you're bringing into our state."
Poppell says most in the reptile pet industry support the legislation and are concerned about the ecological impacts of non-native snakes’ being introduced into the wild.
Some pet store owners, however, worry that the annual fees will chase away many of their buyers.
"There's no way somebody's going to pay an extra $100 for a $50, $60, $70 snake," said Rian Gittman, owner of the Underground Reptiles store in Deerfield Beach.
He also argues that the proposed law, if adopted, would have the unwanted effect of making it even more difficult to control the snakes. "It will force a lot of people to just buy them out of state and go underground with it," he said.
Threat to humans?
Although the Burmese pythons are quite large, and to most people can appear quite frightening, wildlife experts say they don't normally target people as prey and pose only a limited threat to humans.
A few deaths have been reported but typically involve careless pet owners attacked by their caged reptiles.
"I think there's a better chance of somebody getting hurt swerving a car around a python crossing a road and having an accident, than having an encounter with one," said Kimball, the Everglades National Park superintendent.
But biologist Snow, and pest control expert Hardwick agree that people should not confront or pester these big snakes should they stumble across them in the outdoors.
"They do have the tools to kill people," says Snow. "They clearly have the ability to give you a very nasty bite. They have a mouthful of teeth, double rows of teeth in the top that are backward pointing, and very sharp."
It's yet another concern for scientists and park officials, as they confront a very large intruder that poses an ecological threat to South Florida — its new home.
Mark Potter is an NBC News correspondent based in Miami, Fla.