Some giant snake species, including the Burmese python pictured above, would pose a threat to U.S. ecological systems if they were ever established here, a new report finds.

﻿With thousands of Burmese pythons and other giant invasive snakes devastating wildlife in the Florida Everglades, the hottest question on many minds is: How far north could they go?

New research shows the snakes can withstand surprisingly cold temperatures, leaving open the possibility that their range could extend hundreds of miles northward.

Northern exposure

Burmese pythons have been crawling amok in South Florida since at least the mid-1990s. The population's forerunners were probably released by pet owners daunted by the prospect of maintaining a predator that can grow to 20 feet (6 meters) long and weigh 200 pounds (90 kilograms).

No one knows exactly how many there are now, but estimates put their numbers in the thousands or tens of thousands. The pythons have been devouring local wildlife, indulging in mega-meals like deer, bobcats and alligators, as well as endangered species like the woodstork and the Key Largo woodrat.

So far the Burmese python invasion is restricted to Florida's southern tip, but scientists have been debating whether it could spread to more temperate parts of the United States. After all, the species' native range includes the foothills of the Himalayas, so it is no stranger to cold. One alarming study by the U.S. Geological Survey in 2008 predicted the pythons could find suitable climate in about a third of the United States, as far north as Washington, D.C.

How cold can you go?

To test those predictions, researchers recently brought 10 adult male pythons from the Everglades to South Carolina, to see whether they could survive the cooler climate. After implanting a radio transmitter and a temperature logger in each snake, the researchers let them loose in June 2009 in a snake-proof outdoor enclosure.

All 10 pythons did well through the summer and fall, and even survived 12 December nights that were no warmer than 41 degrees Fahrenheit (5 degrees Celsius). Then, in January, the region was plunged into an extremely unusual cold spell. With temperatures dipping below freezing at night for long stretches, the 10 snakes died, according to a paper published in September online in the journal Biological Invasions.

Still, said study leader Michael Dorcas of Davidson College in North Carolina, "there certainly is a possibility that pythons could survive in South Carolina and possibly even farther north."

For one thing, the subfreezing temperatures were highly unusual for the region. For another, some studies indicate that the temperatures a snake experiences during its first year determine how it regulates its body temperature for the rest of its life. Snakes born in the area might fare better than snakes transplanted in as adults.

Finally, the pythons that survived the longest were the ones that crawled into underground cavities at night, and Dorcas wonders whether they might have fared even better outside the enclosure.

"There are certainly in South Carolina much deeper retreats that they could have found if they were out in the wild, such as armadillo burrows," Dorcas said. "If we provided deeper refugia, well, would they have survived? We certainly had snakes that survived a long time and were finally killed by the extreme cold snap we had in January. But snakes had survived many nights where it got below freezing."

Florida cold snap

The same cold snap that killed the South Carolina transplants also killed many Burmese pythons in the Everglades. Nine of 10 radio-tagged pythons there died, researchers reported in another Biological Invasions paper, published online in June. That sounds like a good proportion, but extrapolating to a population of thousands leaves plenty of snake survivors, said Skip Snow, a wildlife biologist based at Everglades National Park who contributed with Dorcas and others to the study.

In the months since the cold snap, adults and 24 hatchlings have been spotted in the wild, according to Snow. That's about the same number of hatchlings found by this time last year, so clearly the wintry weather didn't set the population back much. Whether the surviving pythons have genetically based adaptations to the cold is unknown, but if so, said Dorcas, "then we just had a major selection event for cold-tolerant pythons."

What it all means for the pythons' ability to invade farther north remains to be seen, but Snow takes the long view: "The snakes are going to tell us. They're clearly here, and they're breeding and they're established and they're going to tell us over the years and over the decades just what they can put up with and how far they can go."

If Snow doesn’t sound very hopeful that the snakes can be eradicated, it's because they're so secretive, and the Everglades are vast, largely inaccessible, and full of hiding places. "We have no proven eradication tools for introduced reptiles anywhere in the world, really. It's never been done, and we have no studies to go to, that say: 'Yep, if you do these things you can eradicate an introduced reptile.' Our toolbox is empty of proven tools," he said.

Not that he and other managers aren't trying. The current strategy focuses on containing the pythons' range, stubbing out isolated populations, and targeting areas where pythons are particularly destructive, such as near bird colonies.

Public involvement is also key. The latest tactic on that front is a new smartphone app that serves as a field guide to the region's big reptiles. Eventually the public will be able to transmit sightings, photos and GPS data to help authorities track invaders.

The state of Florida allows hunters to kill pythons and other invasive snakes on certain state lands — but warns them not to eat their quarry, after the discovery that Burmese pythons contain extremely high mercury levels.

Other invaders

Compounding the problem are other giant non-native constrictors on the loose in South Florida. A smaller population of boa constrictors is known to be established, and last year researchers confirmed that African rock pythons were breeding just outside Everglades National Park, not too far from Miami. That species can grow to 20 feet and is notorious for its aggressive temper. A multi-agency effort to track and curtail the African rock python population before it can increase its range is under way, including a plan to enlist snake-sniffing dogs.

"We really don't know what the capability of that species is to spread. It seems to have similar characteristics to the Burmese python, so perhaps it could," said Christina Romagosa of Auburn University, who is helping with the African rock python survey effort.

Ban on snakes

Recent legal changes may offer some help. On July 1, Florida implemented a ban on importing or acquiring Burmese and African rock pythons and four other non-native snake species. People who owned these species before the ban went into effect can keep their animals if they microchip them and maintain a $100-per-year permit.

The U.S. Fish and Wildlife Service has also proposed listing nine species of large constrictors as "injurious wildlife" under the federal Lacey Act, which would prohibit people from importing them or transporting them across state lines without a special permit. The service has collected more than 50,000 comments, and said a final decision could come next year.

As for Florida’s pythons, the genie is already out of the bottle. But it’s not too late to prevent the next invasion, considering how popular big snakes are in the pet trade, said Snow of Everglades National Park.

"We're bringing them into the county under the idea that they’re all innocent until proven guilty. But we have historically had such a high standard of guilt, if you will, that it requires these animals to first of all escape, establish, get out in the wild, breed, and do something egregious like eat something that someone likes," Snow said. "By then it's way too late."