Surviving Brazil's Snake Island

by Brian Dunning
December 5, 2023

Its proper name is Ilha da Queimada Grande, Island of the Great Forest Fire — a name acquired long ago when people once tried to burn its jungles to clear land for a banana plantation. But few call it by its official name. To most, it's simply Snake Island, a small rocky isle about 33 km off the coast of Brazil, not far from São Paulo. As you may be able to guess, its unofficial name comes from its best-known residents: the world's only population of the golden lancehead pit viper, a snake whose venom is said to be one of the world's deadliest. But more frightening than their venom is their sheer number: by some estimates, as many as one snake per square meter (!!!!!) are found on the island. By any reasonable measure, Snake Island should be the world's most dangerous place.

There is no mystery how the island got like this. At the last ice age, sea levels were low enough that Snake Island was connected to the South American mainland. Snakes traveled freely back and forth. But when sea levels rose and the island became isolated, competition for resources quickly whittled the snake population down to a single species. Today the golden lancehead is found only on Snake Island, and nowhere else.

It is a very small island — only 43 hectares in size (a hectare being a square 100 meters on each side) — equal to 106 acres in all. Early estimates of snake density, 1 per square meter, is how the island's traditionally-listed snake population was calculated, at 430,000. The island is quite high and steep and rocky; the sides are mostly bare rock cliffs, most of the highlands are rainforest, with patches of grasslands here and there (the results of the ill-advised efforts to burn space for banana trees). And everywhere are those snakes. It is, for real, so dangerous to go there that the only humans allowed on the island are the Brazilian Navy who periodically maintain the automated lighthouse, and the occasional party of biologists who have received the necessary permits.

Let's have a listen to a few reports to see just what it's like there. Here's a snippet from an article in Smithsonian Magazine titled "This Terrifying Brazilian Island Has the Highest Concentration of Venomous Snakes Anywhere in the World":

These vipers' venom can kill a person in under an hour, and numerous local legends tell of the horrible fates that awaited those who wandered onto the shores of "Snake Island." Rumor has it a hapless fisherman landed onto the island in search of bananas — only to be discovered days later in his boat, dead in a pool of blood, with snake bites on his body. From 1909 to the 1920s, a few people did live on the island, in order to run its lighthouse. But according to another local tale, the last lighthouse keeper, along with his entire family, died when a cadre of snakes slithered into his home through the windows.

Here's from some guy's YouTube short:

I illegally went to Snake Island in Brazil, the second most dangerous island in the world. It's completely uninhabited. The only thing that's there is this poisonous snake called the golden lancehead viper. And if it bites you, you basically die of blood loss, 100% mortality rate. They're only native to this island. There's one snake per square meter, or one snake per one and a half square foot. I went to the island in a full suit of medieval armor.

In a VICE online documentary, several local Brazilian fishermen were interviewed and told these stories (according to the subtitles):

I've heard about it for years, since my father's time. People are afraid. They will not set foot there.

These folks on a boat saw a bunch of bananas and got onto the island to cut them. But both of them died.

In my mind, I believe they were introduced by pirates to protect the gold galleons. So I believe there's gold there. How else would these snakes appear there? Snakes that don't belong to any continent?

And so on and so on. By now you can probably spend full time reading articles and watching videos online repeating these same basic stories and claims. And, as you might be able to guess, since you're listening to Skeptoid, that a lot of this consists of exaggeration combined with a large helping of hogwash.

First of all, let's dispense with the "one snake every square meter" claim. Think how ridiculous that would be. How would they even eat? What food supply could possibly keep up with such a dense population? There are a number of answers to this.

First, there aren't nearly that many snakes. The best number comes from a 2008 survey where a team of biologists spent a long time on the island, and came up with an estimate of between 2,000 and 4,000 total snakes on the island, with greater confidence in the lower end of that range. That gives each snake a bit more breathing room, closer to 200 square meters, about the size of a modest single family home. Most of their diet consists of birds. There are some 40 species of birds that visit the island on regular migration paths, but the lanceheads eat only two of them. This food supply is continually refreshing itself.

Second, the snakes are pretty small. They are a classic case of insular dwarfism, the tendency for isolated species (often on islands) to grow smaller, as the selection pressure favors individuals who are naturally smaller and have lower resource requirements. Adults average some 70-90 cm, down from their nearest relative on the mainland, the jararaca, which averages 90-120 cm, though some as large as 180 cm have been observed.

Backup food sources for the golden lancehead include various insects plus frogs and lizards, while the jararaca eats a diet mainly of big fat rodents, which are not available on Snake Island. This difference in the size of the food available is the primary driver of the evolved size difference. The venom has evolved too, with the golden lancehead's having adapted to be most effective against birds — over thousands of generations, those whose venom killed birds most effectively lived incrementally longer enough to breed more often.

Third, the snake population appears to be diminishing. The biologists report that they often find trash and waste. The boat operators who ferry them to the island report having taken other people there who were posing as biologists from Brazil's Instituto Butantan, and the biologists have even been approached by unsavory individuals offering large amounts of cash for a golden lancehead specimen. Apparently they can sell for US$30,000 or more, so there's plenty of motivation for people to go out there collecting.

But what about the extreme danger of going to Snake Island? Doesn't that discourage the poachers? Well, it turns out that's part of the story that's been very heavily fictionalized for the YouTube and TikTok audience. According to the sources I used, there's actually never been a recorded incident of someone being bitten by a golden lancehead, much less killed by one. So the mortality rates described by the TikTokkers are just made-up nonsense.

We can, however, infer that its venom is still very dangerous, based on that of related species like that jararaca. Medical literature has plenty of examples of severe injuries and deaths caused by jararaca bites. Running my eye down a list of articles, I see compartment syndrome, thrombotic microangiopathy, acute kidney injury, occipital infarction, femoral vessel entrapment, and death. The venom causes your blood to lose its ability to clot, lending some credibility to some of the old stories about victims bleeding to death and being found drained of blood. However, of the few people who go to Snake Island, most are experienced around snakes and know how to be careful and avoid cornering them. Golden lanceheads, like most other snakes, would rather slither far away from you than bite you.

But it's not just illegal poaching that may be reducing the numbers of the golden lancehead. After some 11,000 years of isolation, genetic diversity within their species has diminished, a tiny bit with each generation; until today when they are beginning to face the most severe effects of inbreeding. Homozygosity, the inheritance of the same gene from both parents, means each new generation of golden lancehead will have more and more genetic defects.

Put it all together, and we can understand why the golden lancehead has a conservation status on the IUCN Red List of Critically Endangered (CR), the very last status before Extinct in the Wild (EW). This means that things are as bad as they can possibly get for the golden lancehead. The poachers' interest thus stands explained. This is why it's illegal to land on the island; it's to protect the critically endangered snake, not to protect yahoos from getting bitten.

Allow me, if I might, to present an alternative depiction of Snake Island from the one you may have heard online. In this era, which some say is the beginning of a global warming driven mass extinction, lives a lonely population of a very few thousand individuals of a unique species. They got that way, through no fault of their own, by rising ocean levels at the end of a natural ice age, which cut them off from a whole continent of opportunity. They did their best to survive; their venom adapted through natural selection pressures to be most effective against the two species of birds they live on, leaving them poorly adapted to hunt any other prey. Their progressive loss of genetic diversity means their days are numbered. Census counts of lanceheads on the island are never going to go up anymore. It is, ultimately, a doomed population; unless biologists can successfully invigorate the species with genes from the jararaca on the mainland. This process would involve removing individuals from Snake Island, cross breeding them on the mainland with diverse jararaca, and reintroducing resultant healthy individuals to the island.

If you've ever read an article online about Snake Island that talked about how dangerous it is, and how the snakes are the most vicious there are, and how anyone who gets near the island is likely to die instantly; if you got your jollies from that article in any way, then I ask you to consider the following. We've only told one-fifth of the story today. For the golden lancehead, Bothrops insularis, is only one of five snake species that are unique to Brazilian islands, and all five face similar pressures. We also have the yellow-necked pit viper (Bothrops alcatraz), Victoria's jararaca (Bothrops otavioi), the French jararaca (Bothrops sazimai), and the gizzard jararaca (Bothrops germanoi). All have stories that are not too different from that of the golden lancehead, but perhaps lacking the TikTok influence that warps them into some sort of evil monster.

Whenever you hear such a story, keep in mind that you're probably not getting the context that's usually so important. Whatever you heard about the golden lancehead probably didn't include its critically endangered status, and even if you did you probably didn't hear that it's only one of five such species struggling with a dwindling island population. Always be skeptical, and always dig further to learn the whole truth.

By Brian Dunning
Follow @BrianDunning