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The Crotalinae are a subfamily of venomous vipers commonly known as pit vipers. These snakes are distinguished by their heat-sensing pit organs located between the eye and the nostril on either side of the head. 175 species are currently recognized: 69 in Southeast Asia and 106 in the New World. Also called crotalines, they also are the only viperids found in the New World.
Description
Like all viperids, pit vipers all have a pair of relatively long, solenoglyphous (hollow) fangs that are used to inject mainly proteotoxic venom. The head has an obvious triangular shape and eyes have elliptical pupils. For further information, see Viperidae.
Pit vipers are named after their specialized thermoreceptors: heat-sensitive organs located on either side of the head that look like small pits. These pits contain membranes sensitive to infrared radiation and allows the snakes to locate their prey based on temperature differences with their environment. To a pit viper, rodents and birds that are only fractionally warmer than the background stand out even in complete darkness. Like a primitive pair of eyes, these pits even give them depth perception, allowing them to strike accurately under such conditions. Since the crotalines, like other viperids, are nocturnal, ambush predators, this adaptation serves them particularly well. In an example of parallel evolution, only the boids have developed similar heat-sensitive organs.
Crotalines range in size from small, such as the eyelash viper, Bothriechis schlegelii, with a maximum of 50 cm (20 inches), to the bushmaster, Lachesis muta, that grows to an imposing 3.5 metres (11.5 feet) and is the longest viper in the world.
Habitat and behavior
Although a few species are highly active by day, such as Trimeresurus trigonocephalus, a bright green pit viper endemic to Sri Lanka, most are nocturnal, preferring to avoid scorching daytime temperatures and to hunt when their favored prey are also active. The snakes' heat-sensitive pits are also thought to aid in locating cooler areas in which to rest.
As ambush predators, crotalines will typically wait patiently somewhere for unsuspecting prey to wander by. At least one species, the arboreal Gloydius shedaoensis of China, is known to select a specific ambush site and return to it every year in time for the spring migration of birds. Studies have indicated that these snakes learn to improve their strike accuracy over time*.
Many temperate species (e.g. most rattlesnakes) will congregate in sheltered areas or dens to overwinter (see hibernation), the snakes benefitting from the combined heat. In cool temperatures and while pregnant vipers also bask on sunny ledges. Some species do not mass together in this way, such as the copperheads, Agkistrodon contortrix, and the Mojave rattlesnake, Crotalus scutulatus.
Like most snakes, crotalines keep to themselves and will strike only if cornered or threatened. Smaller snakes are less likely to stand their ground than are larger specimens. Pollution and the destruction of rainforests has caused many viper populations to decline. Humans also threaten vipers, as many vipers are hunted for their skins or killed by cars when they wander onto roads.
Reproduction
With few exceptions, crotalines are ovoviviparous; that is, females give birth to live young. Among the oviparous (egg-laying) pit vipers are Lachesis, Calloselasma, and some Trimeresurus species. It is believed that all egg-laying crotalines guard their eggs.
Brood sizes range from two for very small species, to as many as 86 for the fer-de-lance, Bothrops atrox: a species among the most prolific of all live-bearing snakes. Many young crotalines have brightly coloured tails that contrast dramatically with the rest of their bodies. Used in a behavior known as caudal luring, the young snakes make worm-like movements with their tails to lure unsuspecting prey within striking distance.
Taxonomy
In the past, the pit vipers were usually classed as a separate family: the Crotalidae. Today, however, the monophyly of the viperines and the crotalines as a whole is undisputed, which is why they are treated here as a subfamily of the Viperidae.
Genera
Closely related groups
Besides the pit vipers (Crotalinae), there are three other viperid subfamilies:
- The Azemiopinae, represented by a single species, the Fea's viper (Azemiops feae).
- The Causinae, or night adders.
- The Viperinae, or true/pitless vipers.
See also
References
- Gumprecht, A. & F. Tillack (2004) A proposal for a replacement name of the snake genus Ermia Zhang, 1993. Russian Journal of Herpetology 11: 73-76.
- Wright & Wright (1957), Handbook of Snakes Volume II, Comstock Publishing Associates, Seventh Printing 1985.
External links
- ITIS Report - Crotalinae Oppel, 1811
- Animal Diversity Web - Family Viperidae
- EMBL Reptile Database - Viperidae
- University of Wales - Viperidae - Crotalinae - 2004 Publications
Snakes | Vipers | Pit vipers
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"Crotalinae"