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Night had fallen in Cabo Rojo, a wildlife refuge along Puerto Rico’s southwestern coast, by the time we started our hike. Insects hummed from the grasses, green lizards slept in the trees, and resting water birds, spooked by our approaching footsteps, squawked and flew away.

I scanned the canopy and the ground with a flashlight as my companions — a group of research biologists from a local university — had told me to. Hundreds of eyes reflected back at me from all directions: spiders.

Moments later, as I neared the rocky coastline, my beam caught something even more unnerving. A few feet from where I stood, a large snake slithered along the forest floor. It was about 3 feet long and armored with a kaleidoscope pattern of green, black, and yellow scales.

The snake was a boa constrictor. And it wasn’t supposed to be here.

That late-night sighting was a glimpse into a much bigger problem in Puerto Rico: In recent years, three species of large invasive constrictors have been spreading across the island. Boa constrictors, which are native to South and Central America, are now common on the west side of the island. Meanwhile, reticulated pythons, the longest snakes in the world known to reach 30 feet, are abundant in the central mountains. In their native range of South and Southeast Asia, retics, as snake enthusiasts call them, have swallowed humans whole. Yet another invasive constrictor — the ball python — is starting to spread, too. 

This is highly troubling for the island’s native animals, as well as for pets. 

Boas and reticulated pythons are apex predators in Puerto Rico, meaning they are at the top of the food chain. And big snakes have big appetites. “It’s very, very bad,” said Alberto R. Puente-Rolón, a biologist at the University of Puerto Rico at Mayagüez and a leading authority on invasive snakes in the region. “We have a serious problem and a serious threat to the bird species here,” said Puente-Rolón, who was with me that night in Cabo Rojo. 

This problem is especially clear in the wildlife refuge. Cabo Rojo is considered the most important stopover site for migratory species and shorebirds — including rare plovers and warblers — in the eastern Caribbean. These birds are critical pieces of complex and ancient island ecosystems. They help control the number of insects and other small animals that they consume, and they spread nutrients throughout the Caribbean (through their feces). 

Invasive snakes are similarly threatening outside of Cabo Rojo and across the island, where there are thousands of other native species. Dozens of them are endemic, meaning they’re found nowhere else on Earth. These include birds like the native Puerto Rican parrot, one of the world’s rarest avian species. Because many trees rely on native birds to spread their seeds, losing the parrot would send ripples of destruction through the island’s native forests. 

Scientists are also concerned that invasive constrictors will introduce diseases that harm the island’s native snakes, including the Puerto Rican boa, a federally endangered species that’s found only on the island.

Other regions in the tropics have experienced the devastation wrought by invasive snakes. In Guam, the venomous brown tree snake — which is native to Papua New Guinea and Australia — wiped out 10 of the island’s 12 native forest birds after it was introduced to the territory in the mid-20th century. That loss is now threatening the future of Guam’s forests; like in Puerto Rico, many of the island’s trees need birds to spread their seeds. In south Florida, meanwhile, scientists have linked the spread of Burmese pythons to the severe decline of some native mammals like rabbits and foxes.

The situation in Puerto Rico isn’t this extreme yet. While invasive constrictors are already widespread in some parts of the island, they are only just starting to fan out across Puerto Rico, scientists told me. That means local experts and environmental officials still have an opportunity to limit the destruction they can cause. 

The big question now is whether Puerto Rico, an island in the Caribbean and a US territory, will act fast enough to stem the spread. It faces ongoing financial troubles — rooted in a long history of colonialism — as well as frequent natural disasters, which together stand in the way of progress. And as epicenters of the extinction crisis have demonstrated (see, Hawaii), the US often fails to spend money on interventions until native species have all but disappeared. 

“Do we have to wait to press the panic button?” Puente-Rolón said. “Or can we be proactive?”

On that April night in Cabo Rojo, which is about three and half hours from the capital of San Juan, we spotted two more invasive boas in the next hour. They were larger and wrapped around tree branches several feet above the ground. 

Encountering three snakes in three hours is not normal. “Now you can understand the problem,” said Puente-Rolón, who has a handful of serpent tattoos on his upper body. (One is of a coral snake that he says almost killed him on a trip to the Amazon.)

The boas weren’t far from a flock of shorebirds that nest by salt flats in the refuge. Last summer, researchers found three of them in the nesting area of least terns, small seabirds with black caps and smoky gray plumage that are declining in parts of the US. Two of the snakes were captured and dissected. One of them had the feathers of a young tern in its stomach. 

“There is an ecological imbalance,” said Ana Román, who manages the Cabo Rojo National Wildlife Refuge. “These invasive species don’t belong.”

Like nonnative species everywhere, the invasive snakes are here not because of their own actions but because of human beings. Pet traders have been selling constrictors and pythons in Puerto Rico for decades, although owning boa constrictors and retics without a permit is illegal. (The law, scientists told me, is not strictly enforced.) Pet snakes escape, experts warn. Plus, reckless owners sometimes release their animals into the wild when they get too big and hard to care for. 

There are also potentially stranger routes of entry. In the ‘90s, a zoo just north of Cabo Rojo was robbed and, like a plot point in a cheap horror movie, a reptile cage was damaged and baby boas escaped, according to Puente-Rolón. (I couldn’t identify any Spanish or English news reports from the time to verify this claim, and the zoo has since closed. Puente-Rolón told me that he was at the zoo the day after the alleged break-in because he was studying one of its native snakes.) 

In the last four months, a team of surveyors led by Fabián Feliciano-Rivera, a wildlife biologist, has captured more than 150 invasive boa constrictors in Cabo Rojo. Puente-Rolón estimates that there are roughly 13 of them per hectare (meaning more than 5 per acre) in the refuge, which is something close to extraordinary, he said.   

It’s not just the number of snakes that’s surprising but that they’re in Cabo Rojo at all. The habitat here is extreme — it’s hot and dry, and the forest is sparse, leaving snakes with few places to hide. It’s certainly not like the humid forests full of fresh water that these snakes tend to prefer. To Feliciano-Rivera, that suggests boa constrictors are so abundant that they’re spreading to more challenging habitats. 

As snakes disperse across the island, they’re showing up in backyards, chicken coops, and even cars. When people find them, they typically kill the animals or call local authorities, who retrieve the snakes and hand them over to DRNA, Puerto Rico’s wildlife agency. DRNA then brings them to a place called Cambalache. 

A holding facility for exotic animals, Cambalache gets invasive snakes almost daily. Many of them come from the wild, though others are confiscated from breeders who sell them illegally. 

Cambalache looked like a rundown zoo when I visited the facility one afternoon in April. A few dozen metal cages scattered around outside held nonnative monkeys. I saw large tubs of alligator-like reptiles called caimans. Cages inside of a small concrete building, meanwhile, were full of sugar gliders, adorable, palm-sized possums with large skin flaps that allow them to glide from tree to tree. Rangers had confiscated more than 50 of them from a breeder earlier in the week.

Then there were the snakes. Tons of them.

Outside in a wooden pen, roughly the size of a small storage shed, were some 30 writhing boas and reticulated pythons. One of the pythons was 11 feet long. 

Timothy Colston, an evolutionary biologist at the University of Puerto Rico, picked up one of the pythons. The snake coiled itself around his arm and, like a blood pressure cuff, caused the skin around it to bulge. (Colston, who’s been bitten by dozens of snakes, said it felt like a “little hug.”)

No one knows how many invasive snakes there are in Puerto Rico. Hardly any scientific literature or public government documents have been published on the topic. That’s partly because the invasion is still new. It’s also because the island’s government and scientific institutions lack resources to study it (for a number of complicated reasons).  

But there’s no doubt that invasive snakes are spreading. 

One sign is the sheer number of boas in Cabo Rojo and Cambalache. Puente-Rolón has also noticed a surge in social media posts and news stories about sightings. In the last few years, scientists have also received intel from a small number of python hunters on the island, civilians who volunteer their time to capture invasive snakes. Just seven or so python hunters, also known as reticuleros, can catch 20 snakes a month, according to Jean P. Gonzalez Crespo, a doctoral researcher and invasive snake expert at the University of Wisconsin. 

I talked to a group of these reticuleros on a recent afternoon. They work normal jobs by day — one runs a pizzeria, another works in recycling — but by night they’re hunting snakes. It’s a way to safeguard their local community and the island’s rich biodiversity, said Odalis Luna, a reticulero who hunts with a small crew that includes her husband and their friend Wilson Maldonado.

In the last three years, Luna said, they’ve captured around 170 snakes across several counties. Their reptile bounty includes babies, which suggests that invasive pythons are now breeding in the wild. 

Finding the snakes is relatively easy, said Luna, who once caught a 17-foot reticulated python in front of her house. “We need to find more, because my cats are gone, my chickens are gone,” Luna said. “It’s a problem.”

Scientists at the University of Puerto Rico are now racing to study the spread of invasive snakes. They still have a lot of unanswered questions — including where they are and which native animals they threaten most. 

Those studies start by wrangling these scaly reptiles. When we’d come across an invasive boa in Cabo Rojo, one of the biologists would grab it using a specialized pole with a hook on the end and then put it in a pillowcase. The researchers also collect snakes from Cambalache, the DRNA holding facility, and from reticuleros. 

Most of those snakes are then taken to a lab at the University of Puerto Rico. 

Visiting the lab was a shock to the senses: Fluorescent lights lit up several tables, on which a handful of euthanized snakes were stretched out. It smelled of alcohol and rotting flesh. I watched as Colston and a group of students began slicing open the animals using surgical scalpels and poking around inside.

On the most simple level, the researchers are trying to figure out what the snakes are eating. In some cases, it’s obvious: In late 2020, they pulled a cat out of a boa constrictor’s stomach, like some kind of sick magic trick. 

But often, the team has to analyze the snakes’ feces. That morning, Mia V. Aponte Román, an undergraduate, squeezed poop out of a snake’s intestines and into a strainer. When she ran it under water, a handful of claws appeared. “Green iguana,” said Puente-Rolón, who was standing next to her, peering into the sink.

Puente-Rolón’s team has examined the guts of more than 2,000 invasive boas, he said. That analysis — which hasn’t yet been published — suggests that the snakes are most frequently eating rats and mice, followed by a variety of birds and lizards, including iguanas. 

When I first learned this, I wondered if the panic about invasive snakes was overblown. Rats and iguanas are invasive species, too. Aren’t the snakes just doing their own version of pest control? 

This is not how ecology works. “The rats are going to end at some point,” Puente-Rolón says, meaning their numbers will eventually dwindle. “What we have learned from Guam with the brown tree snake is that mammals are going to disappear and then birds are the next target.” (Other places have learned the same lesson. In Hawaii, enormous colonies of free-ranging cats eat rats, but they’ve also decimated endangered birds.)

Cutting open snakes also serves another, deeper purpose: helping scientists understand how exotic species adapt to their new homes once they arrive.

Typically, scientists try to predict the harm that invasive animals will cause by looking at what they do in their native range. Where do they live? What do they eat? But according to Colston, who studies evolution, invasive species can also evolve after they move in, picking up new behaviors. Importantly, some of those behaviors may make these animals more damaging invaders.

In their homeland, boas and pythons have to contend with other large snakes and predators, such as big cats. These are constraints that shape their behavior, and their evolution. Here in Puerto Rico, however, invasive constrictors have no natural predators and few competitors. Under these conditions, it’s possible that they may evolve traits that help them thrive in all kinds of habitats on the island.

Colston’s team at the University of Puerto Rico will analyze DNA from snakes captured across the island to try and figure this out. They’re looking for ways in which the genome is changing — and how those changes might manifest in the animal’s body and behavior.

There are actually hints that some of this evolution may already be underway. In Cabo Rojo, boa constrictors are smaller than those elsewhere on the island. Miniaturization could be an adaptation to drier conditions; smaller bodies retain water more easily. (It’s not clear whether the snakes are actually evolving to be smaller, generation after generation, or just failing to reach a larger size within their lifetimes. Colston’s work will likely provide answers.)

Sure, there are loads of giant snakes slithering through the forests and grasslands of Puerto Rico right now, not far from homes and rare species. But so far, the damage to the island’s native species has been minor.

“We are in the phase that the impact is not that bad on our species,” Puente-Rolón said.

To stop the snake problem from becoming a crisis, the state needs to act quickly, scientists say. Authorities — or an educated public — need to quickly ramp up efforts to remove snakes that are already in the wild and clamp down on the illegal pet trade. 

To date, DRNA, the island’s wildlife agency, has done frustratingly little on both accounts, according to a number of biologists I spoke to for this story including Puente-Rolón, Feliciano-Rivera, and Gonzalez Crespo. They say the biggest issue is a lack of personnel and funding, they said. “They don’t have biologists, they don’t have the money,” Puente-Rolón said of DRNA.

Instead of proactively removing snakes from the forest, DRNA rangers typically just respond to calls about sightings, and often only if someone is available, the scientists told me. Meanwhile, the wildlife holding facility is falling apart from a lack of upkeep and damage from hurricanes, a constant and worsening force of destruction. 

Remarkably, it’s likely that snakes have actually escaped from Cambalache, several biologists told me. This isn’t difficult to fathom: On the morning I visited Cambalache, a monkey that had apparently broken out of its enclosure was rattling some of the other cages.

What’s more is that officials at the municipal level, who are often the first to get calls about invasive snakes, have been slow to share information about where, exactly, they’re picking up the animals. That information would help scientists map the spread. “We really don’t get any help from the local government,” Gonzalez Crespo said. 

I brought this up with Ricardo Lopez-Ortiz, who leads DRNA’s commercial fisheries division and is one of the few people at the agency who focuses on invasive species, including snakes. He acknowledged that there’s a lot to do, starting with getting more information. “We don’t know much,” he said of the spread of invasive snakes, adding that it’s possibly the “worst scenario” of any invasive species on the island. “We need to do more,” he told me.

A lack of money isn’t the main issue, he said; the agency can get grants from the US Fish and Wildlife Service. But staff shortages have indeed been a serious problem, he said. “We don’t have enough personnel,” he told me. (More than a decade ago, when the country faced a financial crisis, the agency lost a large number of employees in an effort to cut government spending, he said. It’s been slow to refill the positions ever since, he added.) 

“If you have a forest infested with snakes and you only have one manager,” Lopez-Ortiz said, “what can he do?” 

(DRNA did not respond to a request for comment regarding the state of Cambalache facility. Lt. Ángel E. Atienza Fernández, a DRNA employee who oversees the Cambalache facility, also did not respond to direct requests for comment.)

Puerto Rico also faces a number of forces that work against efforts to eradicate invasive species that are largely out of DRNA’s control, from natural disasters to the island’s much broader financial hardship. That leaves wildlife conservation lower on the government’s list of priorities.

DRNA isn’t doing nothing. Lopez-Ortiz says the agency is developing a project in collaboration with biologists to study a number of invasive species including reticulated pythons and boas. That will involve gathering data from municipal authorities who are often the first to respond to snake calls. The agency is also working with government employees who manage state forests to help them identify and monitor invasive species. 

“We have plans and we are working,” he said. 

In the meantime, the heaviest burden of managing Puerto Rico’s snake problem falls on academic scientists — and the reticuleros, the python hunters. “This is going to be a problem in the long run,” Luna, one of the reticuleros, said.

On my last night in Puerto Rico, Colston took me python hunting. 

Like most of my experiences in Puerto Rico that week, it was the stuff of nightmares. Colston had gotten a tip earlier in the day from a DRNA official that we might find snakes in an abandoned sports stadium in the mountains south of San Juan. We drove to the stadium and, after dark, went inside. 

The building was enormous, a huge ring of concrete surrounding a large, covered arena. Clumps of moss and plants grew in the stands. Old mattresses were strewn about. Bats flew overhead. And giant toads hopped around the stadium floor. Invasive snakes would fit right in.

This left me feeling conflicted. I honestly wanted to see a python in the wild, mostly for the thrill of it. At the same time, I knew there was hope in their absence. Certainly one python-free night means nothing; snakes avoid people and can be hard to spot, even in areas with loads of them. Still, it was a subtle reminder of something important: It’s not too late to act.