Snake Island: what isolation did to the golden lancehead

Queimada Grande is a speck. One study calls it a protected reserve of about 43 hectares (roughly 106 acres) lying roughly 30 kilometers (about 19 miles) off the southern coast of São Paulo state, Brazil. It is the only place the golden lancehead (Bothrops insularis) lives in the wild. A mainland relative, the jararaca, is dark, lives on the ground and mostly hunts small mammals. The island snake is yellow, semi-arboreal (part of its life is spent off the ground), and as an adult feeds mostly on birds.

That is the story of isolation in a sentence: a small group cut off from the rest, living on different food, ending up different. This article follows what the sources say about how the island changed the snake, from its diet to its venom to its small numbers. It also checks one popular legend against the numbers.

The Komodo dragon is another island specialist, though its tale is about size, not diet.

Meet the golden lancehead

The golden lancehead is a pit viper, a snake in a group with heat-sensing pits on the face, like the Western diamondback rattlesnake and the Cottonmouth. It is venomous. This article is not a guide to handling venomous snakes, and the golden lancehead, like any venomous animal, is for watching from a distance. On this species, a distance usually means a photograph.

How an island snake began

A 2025 genome study from Brazil’s Butantan Institute, led by Pedro Nachtigall and coordinated by Inácio Junqueira de Azevedo, tried to reconstruct the snake’s past. According to the news reports on it, the model suggests that about 100,000 years ago a portion of a mainland population of around 140,000 snakes likely became isolated. The researchers say there may have been an initial isolation caused by rising seas, and that a second rise in sea level isolated the snakes for good.

The model then shows two sharp drops, around 50,000 years ago (to about 30,000 snakes) and about 11,000 years ago (to about 10,000), and a slower slide between 10,000 and 5,000 years ago to about 5,000. These are model outputs. They lean on an assumed mutation rate and a new generation every two years, and the researchers say it is impossible to know for sure whether an earlier isolation was followed by renewed gene exchange with mainland snakes before the final one. We found no source that settles the exact date the island separated, so treat every date here as an estimate.

A bird-eating pit viper

Diet is where the island shows most clearly. Marques and colleagues studied the gut contents of adult golden lanceheads and found that the diet is based on birds. Of the 41 bird species recorded on the island, adults relied on just two, both among the most abundant perching birds when present. The paper’s own title calls them seasonally plentiful visitors. The snakes are usually found on the ground, but when one of those prey species, the tyrant flycatcher Elaenia chilensis, peaked in number, more snakes turned up in vegetation. The authors suggest the birds forage in plants and pull the snakes upward with them.

The most abundant perching bird on the island, a resident wren, appears to avoid being eaten, which is a good deal for the wren. A 2014 study adds that the snake is potential prey for four raptors, though predation does not look like a strong pressure on it.

Venom tuned for birds?

The same genome study looked at the venom. According to the news release, the venom is rich in enzymes and proteins that cause bleeding and clotting disorders, and it may also affect blood pressure and tissue, as mainland jararaca venoms do. Experiments have long shown it is more potent in birds than in mammals. The researchers found no major differences in the toxin genes themselves and suggest that a protein or a smaller fragment may explain the extra punch against birds. That is a possibility, not a settled finding.

The bigger result was about evolution. The variation in venom genes did not fit random genetic drift, which is what many people would expect in a tiny island population. The analyses pointed instead to strong natural selection, possibly linked to diet or to the small range. Again, the authors present that as a possible explanation. The sampling was limited: one complete genome from a male from the island, less detailed genomes from eight other snakes (seven wild, one from a captive conservation program), and venom-gland data from four.

How many are there?

Sources do not agree, and the gap is worth seeing side by side.

The 2014 authors also report high temporary emigration, about 0.70, meaning many snakes were out of reach of the survey at any one time. That helps explain why counting them is hard.

Here is a quick worked example. The island is about 43 hectares, or roughly 430,000 square meters. Divide that by 4,000 snakes and you get about 108 square meters each; divide by 2,000 and you get about 215. In other words, roughly one snake for every square 10 to 15 meters (about 34 to 48 feet) on a side. An average like this says nothing about where snakes cluster, but it is a long way from a floor carpeted in snakes.

Common myths and mix-ups

Myth: the island is carpeted with snakes. The census estimate works out to one snake per 100 to 200 square meters or so, on average, by our arithmetic. Real numbers vary, and the sources do not give a density.

Mix-up: the golden lancehead is just a jararaca on an island. The sources describe clear differences: the island snake is yellow, semi-arboreal and eats birds, while the mainland jararaca is dark, ground-living and hunts small mammals. The genome study treats the jararaca as a relative, not the same animal.

Mix-up: Critically Endangered means numbers are crashing. The category reflects risk, and part of the reason given for this listing is a small range and a single location. The 2014 study could not confirm a decline and concluded the population was stable during its sampling years, while urging caution.

Mistake: calling its venom “stronger” without saying for whom. The experiments the researchers cite compare potency in birds and mammals, not a ranking of all snakes.

Conservation status

The golden lancehead is listed as Critically Endangered on the IUCN Red List of Threatened Species. That is the category as distributed by GBIF, the Global Biodiversity Information Facility, and we checked it on November 9, 2026. Critically Endangered means the species faces an extremely high risk of extinction in the wild.

The pressures our sources name are specific. The whole wild population lives on one island, and the 2014 study says the small range and single location are part of the reason for the listing. It also cites an earlier concern, from 2008, that falling encounter rates and suspected illegal trade pointed to a decline. On the hopeful side, the 2014 study could not confirm a decline over its 2002 to 2010 sampling, the snake appears to face little predation, and a captive conservation program at the Butantan Institute exists. The genome researchers say a planned comparison of wild and captive genomes could show how genetically healthy the captive group is. We have no confirmed population trend beyond that 2014 study.

Categories are reviewed and can change, and the copy GBIF distributes can trail the IUCN website by some months, which is why we print the date we checked. GBIF’s copy does not give the year of the IUCN assessment, so we do not state one.