Reptiles
Viper
Viperidae · also called vipers, adders
A viper carries the longest fangs of any snake, and it keeps them folded flat against the roof of its mouth like the blades of a pocket knife. When it strikes, a bone in the upper jaw rotates, the fangs swing down and lock forward, and the snake’s own momentum drives them in. The whole sequence takes less time than a blink.
Overview
Vipers are the family Viperidae, around 370 species, on every continent except Antarctica and Australia. All are venomous, most have a broad triangular head, vertical pupils, and a stout body, and all share the folding front fangs. The family splits into the pit vipers (rattlesnakes, copperheads, lanceheads, bushmaster) with heat-sensing pits, and the true vipers (adders, the Gaboon and puff adders, Russell’s viper) without them.
The strike
An ambushing viper lies coiled and waits. When prey walks into range it launches its head, erects the fangs, stabs, injects, and recoils in about 50 to 90 milliseconds. Large prey is usually released and then tracked by scent once the venom has done its work, which avoids injury from a struggling animal.
Venom that dissolves
Most viper venom is built from enzymes that destroy tissue, rupture blood cells, and wreck clotting. A serious bite means severe pain, spreading swelling and bruising, blistering, and sometimes permanent damage — a very different injury from the mainly nerve-blocking bite of a cobra.
Heat vision and northern living
Pit vipers read infrared through a membrane-lined pit between eye and nostril, building a thermal image accurate enough to strike in the dark. Many vipers give birth to live young, letting the female bask to warm the embryos, which is how the European adder lives inside the Arctic Circle.
Status
Vipers as a family are Not Evaluated. Many are common, but several island and mountain species with tiny ranges — the Aruba and Santa Catalina rattlesnakes, Europe’s meadow and Orsini’s vipers — are threatened by habitat loss, collection, and deliberate killing. Vipers also cause a large share of the world’s ~100,000 annual snakebite deaths.
Viper: common questions
What is the difference between a viper and other venomous snakes?
The key feature is the fangs. Vipers have a pair of very long fangs mounted on a short bone in the upper jaw that rotates, so the fangs fold back against the palate when the mouth is shut and swing forward to vertical when the snake strikes. This lets a viper carry fangs far longer than its closed mouth would otherwise allow, and drive them deep. Cobras, mambas, coral snakes, and sea snakes, the elapids, have shorter fangs fixed in an upright position at the front of the mouth. Vipers also tend to have broad triangular heads, vertical pupils, stout bodies, and mainly tissue-destroying venom, whereas elapid venom is usually more neurotoxic.
What does a viper bite do to the body?
Most viper venoms are cocktails of enzymes and proteins that attack tissue directly. Within minutes a serious bite causes intense pain, rapid swelling, and bruising that spreads up the limb, often followed by blistering and areas of dying tissue around the wound. Many viper venoms also interfere with blood clotting, either using it up so the victim bleeds from the gums, nose, and old wounds, or triggering clots, and some damage the kidneys or cause a dangerous drop in blood pressure. Antivenom, given early, neutralises circulating venom and is the main treatment, alongside care for the wound and support for the kidneys and circulation. Local tissue loss can still lead to lasting disability.
How do pit vipers sense heat?
Pit vipers have a pair of specialised organs, one on each side of the face in a pit between the eye and the nostril. Each pit is a small chamber with a thin membrane suspended across it, packed with heat-sensitive nerve endings and cooled from behind by air. Warmth radiating from a nearby animal warms the membrane unevenly, and the snake's brain combines the signal from both pits with its normal vision to build a crude thermal image, accurate enough to guide a strike in total darkness and to tell the snake where a mouse's head is. Some pythons and boas have simpler heat-sensing pits in the lip scales, but the pit viper organ is the most refined.
Which vipers are the most dangerous to humans?
In terms of total deaths, the saw-scaled vipers of Africa, the Middle East, and South Asia probably kill more people than any other snakes, because they are small, abundant in dry farmland, superbly camouflaged, and very quick to bite. Russell's viper is a leading cause of death and kidney failure across the Indian subcontinent. In the Americas, the lanceheads, including the fer-de-lance and the terciopelo, cause most serious bites. The Gaboon viper has the longest fangs and one of the largest venom yields of any snake, though it is placid and bites few people. Almost all fatal viper bites happen in rural tropical areas with limited access to antivenom.