Irula tribesmen chase pythons in the Everglades
Fueled by bountiful swamps that provide a steady supply of marsh rabbits, deer, wading birds and other meals, Burmese pythons in Florida have rapidly adapted to become hardier and more resistant to cold than their Asian cousins, a new study has found.
And that supercharged evolution should serve as a warning not just for Florida, but the entire U.S.
“We’ve used pythons in southeast Asia to say where they can live in the U.S. Our data doesn’t say they can expand to New York. But our data says holy sh-t, you gotta keep an eye on this,” said co-author Todd Castoe, whose Arlington lab at the University of Texas led the research that examined the swift genetic changes published in the journal Molecular Ecology.
Although they found no direct evidence, the findings also point to another worrisome possibility: more active Florida snakes capable of growing bigger faster, and making more babies.
“We would only assume this shift would make them more efficient. And it would seem that would leave extra energy on the table,” he said. “From the perspective of a snake the results are pretty obvious: you grow and reproduce.”
The study is the latest to conclude Florida’s invasive pythons might be undergoing physiological changes that could allow the swamp-loving constrictors to become a bigger problem for Florida, which has been plundered by invasive species as contentious as Old World climbing fern and ubiquitous as iguanas. In August, U.S. Geological Survey researchers found a small number of snakes contained DNA from Indian pythons, a closely related species that prefer higher ground and could lead the snakes to inhabit a more varied landscape as they evolve.
In their home territory, Burmese pythons have evolved in a harsh monsoon climate and eat accordingly: they feast during the plentiful rainy season and fast the rest of the year. During that prolonged fasting, the snakes dramatically slow their metabolism to conserve energy. Organs shrivel. They stop producing stomach acids. Their bodies shed what they don’t need.
“They not only turn the idle way down, but they literally kill off tissue,” Castoe said.
When they resume eating, they regenerate quickly, increasing the mass of their heart, liver, kidney and other organs and ramping up their metabolism rate by as much as 40 times higher in just 48 hours.
That remarkable transformation is what initially intrigued researchers in Castoe’s lab, who were looking for ways to treat human diseases related to metabolism. The snakes were among the first to have their DNA completely sequenced and annotated in 2013, which also found that changes in some genes could be tied to the same ones that play a role in breast cancer, melanomas and childhood leukemia.
“That’s their hallmark and why they’re so cool as a lab rat, except that Florida pythons don’t seem to do it,” Castoe said.
Instead, Florida pythons never shut down, even when they’re forced to fast. That discovery was made inadvertently after the 2010 freeze. Even as they searched for clues to human diseases, Castoe said researchers were well aware of the python’s toll on South Florida’s Everglades, where the snakes have been blamed for driving down the population of small mammals, taking over as top predator and shifting the balance in a swamp that is vast but also vulnerable to small changes.
After the freeze, reports started coming in that between 40 and 90 percent of pythons were found dead.
“To us, as evolutionary biologist, that was a trigger to say hmmm, that looks like extremely strong selection. I wonder if there’s something related to freeze tolerance,” Castoe said.
So the researchers started examining genes in 97 snakes. Just under 50 were collected before the freeze, between May 2003 and June 2009. Another 49 were found between October 2012 and December 2013. They also looked at five years worth of data on stomach contents between 2003 and 2008 and found that no matter when the snakes were caught, they frequently had food in their guts.
“Those things looked like conveyor belts,” he said. “All year long they’re absolutely full of food.”
In sorting through the genes, researchers found that almost all remained the same in snakes captured before and after the freeze. But a small sequence of genes that determine the snakes’ metabolism and, at least in mice, influences how they respond to cold, had changed. The researchers also put their lab snakes on a fast to see if they began to atrophy without food like their Burmese cousins.
In fact, the Florida snakes kept metabolizing. And generating heat, leading researchers to conclude the snakes that survived the freeze had likely been on the leading edge of a Florida python evolution.
Castoe said wildlife managers will likely need to rethink how they manage the snakes. With a higher threshold for cold weather, they may no longer be bound by South Florida’s steamy temperatures.
Anecdotally, there may be some proof they’re already pushing north. The vast majority of pythons continue to be found in South Florida marshes, where Florida has hired trained hunters to catch them after confirming a breeding population in South Florida, including the Keys. But more than a few dozen have been found further north.
While an expansion could be the most obvious bad news, Castoe said the study shed light on what could be a more troubling characteristic of the Florida python: their ability to evolve so rapidly.
The snakes first appeared in marshes in the 1980s, likely escapees from a South Dade breeding facility or freed pets. By the mid 1990s, they had become full-time residents. That means in less than three decades they changed to suit their new habitat. The speed hasn’t set a new record. The Galapagos finches that first inspired Darwin’s theory are a classic example of more rapid evolution. But it’s close.
To have a few years of cold and see the genetic adaptation occur is pretty impressive.” said Maggie Hunter, a USGS geneticist Margaret Hunter, who co-authored the study and was lead author on the Indian python report.
Whether female snakes in Florida are reproducing more quickly also remains to be seen, said USGS ecologist Kristen Hart, who has documented the snakes changing habits in the Everglades.
“We are putting these basic vital rates together little by little, but it is not really known what proportion of females reproduce in back to back years,” she said in an email.
It also signals the need to pay attention for other potential changes as the snakes adapt, and ignoring how the snakes’ ancestors behaved.
“You think you’re dealing with a Burmese python and use the rules from a Burmese python in Burma to understand what this population is going to do,” Casteo said. “This is a whole different animal and that basically means incorporating evolution in our scenario for how this is going to work.”