Reptiles
Boomslang
Dispholidus typus · also called tree snake, Afrikaans tree snake
The boomslang is a slim, wide-eyed African tree snake that spent a long time miscategorised as harmless. Its venom is delivered by fangs at the back of the jaw and acts slowly, and that combination cost one of the twentieth century’s most famous snake scientists his life.
Overview
Boomslangs live throughout sub-Saharan Africa, in savanna, bushveld, lowland forest, and dry scrub, wherever there are trees and bushes. They are almost entirely arboreal and diurnal, gliding slowly and deliberately through the branches, and they are shy — a boomslang’s usual response to a person is to freeze, hoping not to be seen, or to move away. Its slender build, keeled scales, and highly variable colour (bright green, brown, grey, or black) make it easy to overlook among foliage.
Built to hunt by sight
The boomslang has the largest eyes, relative to head size, of any African snake, giving it sharp, forward-facing, binocular vision — an adaptation for spotting still, camouflaged prey in the canopy. Its main food is chameleons and other tree lizards, along with tree frogs, nestling birds, and eggs.
The venom, and Karl Schmidt
Because the boomslang’s fangs are set at the rear of the mouth, it was once assumed to be dangerous only if it managed to get a good chewing grip, and its venom to be weak. In fact its haemotoxic venom is highly potent, progressively destroying the blood’s ability to clot until the victim bleeds uncontrollably, internally and externally.
The danger was proven in 1957, when the American herpetologist Karl Patterson Schmidt was bitten on the thumb by a juvenile boomslang, judged the dose too small to matter, refused antivenom, and recorded his worsening symptoms in a diary until he died the next day. His case established that rear-fanged colubrid snakes could kill.
Boomslang venom is also slow to show effects — a bitten person may feel fine for many hours — so every bite is treated as serious and monitored in hospital, with the specific antivenom given if clotting deteriorates.
Defence and reproduction
Threatened, a boomslang inflates its neck to over double its width, exposing bright skin between the scales, and rears into an S before striking. Females lay clutches of around 8 to 25 eggs in tree hollows or leaf litter; the hatchlings are already venomous and take small lizards.
Status
The boomslang is Least Concern, common, and widespread. It is beneficial as a controller of pest lizards and rodents, and because it is retiring and rarely bites, it poses little real risk to people who leave it alone. Localised pressures are habitat clearance and killing on sight by people who fear it.
Boomslang: common questions
How dangerous is a boomslang bite?
Potentially deadly, but bites on humans are very rare because the snake is shy, arboreal, and reluctant to bite — most people are bitten only while handling one. The venom is a potent haemotoxin that gradually shuts down the blood's ability to clot, leading to bleeding from the nose, gums, wound, and internally, which can be fatal without the specific antivenom. Crucially, symptoms are often mild or absent for the first several hours to a day, which has led victims to underestimate a bite. Anyone bitten needs to be monitored in hospital regardless of how they feel.
How did a boomslang kill the herpetologist Karl Schmidt?
In 1957 the American herpetologist Karl Patterson Schmidt was bitten on the thumb by a juvenile boomslang while examining it at the Field Museum in Chicago. He assumed a small rear-fanged snake could not deliver a serious dose and declined antivenom, and — being a scientist — kept a detailed diary of his symptoms. Over the next 24 hours he developed nausea, fever, and bleeding from the mouth and other sites, and he died the following afternoon of internal haemorrhage and respiratory failure. His account became a landmark case demonstrating that "harmless" rear-fanged snakes could be lethal.
Why does the boomslang puff up its neck?
It is a warning display. A threatened boomslang inflates the front part of its body, especially the neck, to more than twice its normal thickness, which stretches the skin apart and reveals vivid colour — often black, blue, or orange — between the scales, and it holds its head and forebody up in a striking S. This makes the slim snake look far bigger and more dangerous, and if the threat continues it will strike, sometimes repeatedly, with its jaws opening to a wide angle to bring the rear fangs into play.
What does a boomslang eat?
It is mainly a hunter of chameleons and other tree lizards, which it locates with its large, forward-facing eyes as it moves slowly and precisely through the branches. It also takes tree frogs, small birds, and birds' eggs — raiding nests — and occasionally small mammals. Chameleons, which rely on staying still and camouflaged, are especially vulnerable to a predator with such good vision that hunts by sight in the canopy.