The Viperidae (vipers), are a family of venomous snakes found all over the world, except in Antarctica, Australia, New Zealand, Ireland, Madagascar, Hawaii, various other isolated islands, and north of the Arctic Circle. All have relatively long, hinged fangs that permit deep penetration and injection of venom. Four subfamilies are currently recognized. They are also known as viperids.
All viperids have a pair of relatively long solenoglyphous (hollow) fangs that are used to inject venom from glands located towards the rear of the upper jaws. Each of the two fangs is at the front of the mouth on a short maxillary bone that can rotate back and forth. When not in use, the fangs fold back against the roof of the mouth and are enclosed in a membranous sheath. The left and right fangs can be rotated together or independently. During a strike, the mouth can open nearly 180° and the maxilla rotates forward, erecting the fangs as late as possible so as the fangs do not become damaged. The jaws close on impact and powerful muscles that surround the venom glands contract to inject the venom as the fangs penetrate. This action is very fast; in defensive strikes it will be more a stab than a bite. Viperids use this mechanism primarily for immobilization and digestion of prey. Secondarily it is used for self-defense, though in cases with non-prey items such as humans they may give a dry bite (not inject any venom).
Almost all vipers have keeled scales, a stocky build with a short tail, and, due to the location of the venom glands, a triangular-shaped head distinct from the neck. The great majority have vertically elliptical, or slit-shaped, pupils that can open wide to cover most of the eye or close almost completely, which helps them to see in a wide range of light levels. Typically, vipers are nocturnal and ambush their prey.
Viperid snakes are found in the Americas, Africa and Eurasia. In the Americas, they are native from southern Canada, through the United States, Mexico, Central America and into South America. The Adder branch of The Viperidae family contains the only venomous snake found in the United Kingdom. Wild viperids are not found in Australia.
Viperid venoms typically contain an abundance of protein-degrading enzymes, called proteases, that produce symptoms such as pain, strong local swelling and necrosis, blood loss from cardiovascular damage complicated by coagulopathy, and disruption of the blood clotting system. Death is usually caused by collapse in blood pressure. This is in contrast to elapid venoms that generally contain neurotoxins that disable muscle contraction and cause paralysis. Death from elapid bites usually results from asphyxiation because the diaphragm can no longer contract. However, this rule does not always apply: some elapid bites include proteolytic symptoms typical of viperid bites, while some viperid bites produce neurotoxic symptoms.
Proteolytic venom is also dual-purpose: firstly, it is used for defense and to immobilize prey, as with neurotoxic venoms; secondly, many of the venom's enzymes have a digestive function, breaking down molecules in prey items, such as lipids, nucleic acids, and proteins. This is an important adaptation, as many vipers have inefficient digestive systems.
Due to the nature of proteolytic venom, a viperid bite is often a very painful experience and should always be taken seriously, even though it may not necessarily prove fatal. Even with prompt and proper treatment, a bite can still result in a permanent scar, and in the worst cases, the affected limb may even have to be amputated. A victim's fate is impossible to predict as this depends on many factors, including (but not limited to) the species and size of the snake involved, how much venom was injected (if any), and the size and condition of the patient before being bitten. Viper bite victims may also be allergic to the venom and/or the antivenom.