Environment | WATCH | As pythons hit the Florida Keys, a worrisome concern: They’re eating endangered species

In the dark hours after midnight, a tiny endangered mouse foraged its way through a matrix of subterranean pipes at the abandoned Nike Missile Base in Key Largo. Little did it know that a massive predator — one that its ancestors never faced — was waiting in the next pipe.

As the mouse sniffed about, the 7-foot invasive Burmese python struck, wrapping around the prey and later eating it.

The female python, which researchers nicknamed Dory, often targeted the endangered Key Largo cotton mice despite being able to take prey 160 times as large, such as raccoons. In other words, she was not behaving as biologists expected her to.

There’s a general rule when it comes to snakes — the bigger their mouth, the bigger their prey. “Theory suggests that the snakes shouldn’t waste their energy on the small stuff,” said Michael Cove, a mammalogist with the North Carolina Museum of Natural Sciences and one of the senior authors on the study.

“It costs more energy to catch the mice than (the snakes) are consuming calorically. The mice are so tiny it’s like, wow, why is she wasting her energy trying to eat these?”

Cove and the researcher team had hoped that larger snakes would stick to bigger meals, like raccoons and possums.

“These adult snakes could be eating these endangered rodents, when previously we’d thought that only juvenile snakes would be a threat,” said Cove.

Now they had photographic proof that endangered cotton mice were on the menu. They also know the snakes are eating the endangered Key Largo woodrats. Both species live in the upper Keys and nowhere else on the planet.

As invasive Burmese pythons, which were introduced to Florida via the exotic pet trade in the 1970s and ’80s, expand their range both north to Lake Okeechobee and south into the Keys, they’ve decimated native wildlife. There’s now concern that the snakes could eventually reach the endangered Key deer, a diminutive deer that lives only in the outer Keys.

Filming a cryptic apex predator

The research team, including Cove and Jeremy Dixon, manager of the Crocodile Lake National Wildlife Refuge in Key Largo, used camera traps to document Dory preying on endangered Key Largo cotton mice. The images are the first photos of a wild invasive python actually killing an endangered species.

They set trail cams in the underground pipes at the Nike Missile Base in Crocodile Lake National Wildlife Refuge on Key Largo.

During a 10-day period, the team documented the same tagged 7-foot female snake attacking five different cotton mice. She ate two of them, while the others escaped. He said the mice only weigh an ounce. The meals took 10 and 17 minutes to catch and consume.

“The main idea is that these endangered rodents aren’t safe, even when we thought the snakes would have outgrown them.”

Maybe the snake was just a growing girl. During the 292 days the team observed this particular python, she apparently ate well, growing from 6.9 feet to 8 feet in length, and nearly doubled her weight, from 13.5 pounds to 25 pounds.

Could the python invasion reach endangered Key deer?

Much attention has been paid to the Burmese python expanding their invasion front north.

What started in the 1990s with a few snakes at the southern tip of Everglades National Park is now a full-fledged and currently unstoppable invasion, with possibly hundreds of thousands of snakes expanding their reproductive range up to Lake Okeechobee and the outskirts of Fort Myers.

Biologists consider the invasive pythons to be established and reproducing in the upper Keys, based on the presence of various sizes of snakes, including hatchlings.

But how far south into the Keys have they traveled, and what does that mean for the endangered Key deer, which live 70 miles down in the group of islands in the Big Pine Key area?

“They can move pretty large distances,” Cove said. “I don’t think we really do have a good handle on that, on how much farther down they are in the Keys.”

Cove said that a handful of pythons started showing up in Key Largo around 2007, but it wasn’t until 2016 that researchers documented python reproduction there.

Now, the sightings and captures are going up exponentially. “That suggests to me that if they’re detecting them at low levels farther down in the Keys, it’s only a matter of time.”

And the snakes don’t need to move via land and busy bridges. They’re adept at water travel.

According to the University of Georgia’s Early Detection and Distribution Mapping System website, which tracks invasive species sightings and captures, pythons have been captured near small keys in the middle of Florida Bay, 14 miles by sea from Islamorada and 8.5 miles from the nearest shoreline in Everglades National Park.

Cove thinks 2017’s Hurricane Irma was a spreading event for pythons from the Everglades into the Keys.

Researchers don’t have a tagged snake that would prove it, but, Cove said, “if you remember, Irma sucked out the entire Florida Bay and just blasted it back at the Keys. A lot of pythons are just in those waterways, and would have been sucked up as part of that.

“And they’re super-resilient and durable. They don’t have a problem just drifting. They can go for a few weeks without drinking fresh water. They can go several months, with their fat stores, without eating.”

It’s clear the pythons are capable of finding their way down the Keys, and there have been random observations of Burmese pythons over the years in the Big Pine Key area, according to the EDDMaps.

But U.S. Fish and Wildlife Service Biologist Brian Powell, who works with Key deer, said via email that the service is not aware of python sightings in Big Pine Key.

There’s an estimated 748 Key deer remaining, he said, and their primary cause of mortality at the moment is being hit by cars.

Larger pythons love white-tailed deer.

Biologist Ian Bartoszek, who performs snake necropsies for the Conservancy of Southwest Florida, said that nearly every python over 100 pounds that he has studied has deer remains in their gut.

A 15.5-foot python caught in the Everglades in 2013 had the remains of three deer in its digestive system. Hoof and tooth size indicated that one was a 99-pound doe, and the others were 37- and 29-pound fawns. Key deer weigh 50 to 75 pounds, so would be vulnerable to a wider size range of pythons.

“Although it is likely to expect that there would be some amount of Key deer mortality associated with an established python population in Big Pine Key, it is unknown at this time if the amount of mortality would affect the viability of the overall population,” wrote Powell.

“Threats to Key deer include sea-level rise, development, wildfires and disease. The primary factor, however, remains sea-level rise due to its direct expected impact on upland habitat.”

New ways to stop the snakes

Two years ago, Michael Cove and Jeremy Dixon were using GPS collars to study the movements of raccoons and possums in the Key Largo area. They soon found that when the mammals died, they would often find the trackers in pythons’ stomachs.

Though sad, it was an “ah-ha” moment: They could track mammals to find and euthanize the biggest, baddest pythons in a given area, something that has proven nearly impossible with the highly inefficient method of cruising roads at night with spotlights.

The original GPS collars were prohibitively expensive at $2,000, but Cove and Dixon are now partnering with South Florida Water Management District and have developed cheaper $200 trackers that they’ve put on 100 local wild possums in the past nine months.

When the animal stops moving for more than four hours, the collar sends out a mortality signal and Cove’s team can find them — sometimes as roadkill, sometimes in the belly of a 15-foot snake.

What they found was fascinating. They collared 10 possums in June and the rest in September of 2023.

So far, seven collared possums have died by python. Researchers were able to catch and euthanize four of them.

Two of the snakes passed the collars before the researchers could find them — one of the snakes actually passed two collars. They found another collar in coyote scat.

In the same summer timeframe, pythons killed more of the collared possums than vehicles did.

Then they ramped up the collar program in October, just before python breeding season, which run to April.

Pythons tend not to eat when breeding, so there haven’t been any python predations on the possums since October. But Cove knows that the feeding will pick up in spring, and is eager to take more pythons out of the ecosystem.

Bill Kearney covers the environment, the outdoors and tropical weather. He can be reached at . Follow him on Instagram @billkearney or on X @billkearney6.