Crotalinae

Crotalinae
Timber rattlesnake, Crotalus horridus
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Reptilia Order: Squamata Suborder: Serpentes Family: Viperidae Subfamily: Crotalinae Oppel, 1811

Crotalinae, whose members are commonly known as pit vipers or pitvipers, is a subfamily of venomous vipers (family Viperidae) characterized by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head. One of four families of Viperidae (ITIS 2004; Nilson and Gutberlet 2004), Crotalinae includes such well-known vipers as rattlesnakes and moccasins, among others. Members of this subfamily technically are known as crotalines.

There more than 150 species of crotalines, distributed in both the Americas (North, Central, and South America) and in East Asia and Central Asia (Nilson and Gutberlet 2004). There also is one species, Gloydius halys, whose range extends into eastern Europe, north of the Caspian Sea (Nilson and Gutberlet 2004). Crotalines are the only viperids found in the New World.

Overview and description

As characteristic of members of the snake family Viperidae, commonly known as vipers, crotalines have a generally triangular head that is distinct from the neck and have a single pair of long, hollow, venom-injecting fangs that can be folded back against the roof of the mouth, tip inward, when the mouth is closed. The hollow fangs can be used to inject venom, like a hypodermic needle, from glands located towards the rear of the upper jaws.

Crotalinae is one of four subfamilies of Viperidae, the others being Viperinae, Causinae, and Azemiopinae (Nilson and Gutverlet 2004; ITIS 2004). The Viperidae comprise about 12 genera and 70 species of true or pitless vipers, such as the puff adders. Azemiopinae has a single genus and species (Azemiops feae) and Causinae has a single genus, Causus, with six species.

Depending upon the taxonomic scheme, Crotalinae includes from 18 general (ITIS 2004) to 22 genera (Nilson and Gutberlet 2004), and from 151 species (ITIS 2004) to 174 species (Nilson and Gutberlet 2004). There is a greater number of genera and species in the New World, where they can be found from southern Canada southward through Mexico and Central America to southern South America (McDiarmid et al. 1999). In the Old World, they are found from eastern Europe eastward through Asia to Japan, Taiwan, Indonesia, peninsular India, and Sri Lanka (McDiarmid et al. 1999).

Crotalines range in size from the diminutive hump-nosed viper, Hypnale hypnale, that grows to an average of only 30 to 45 centimeters ( to the bushmaster, Lachesis muta; a species that is known to reach a maximum of 3.65 m in length — the longest viperid in the world.

This is a versatile group, with members found in habitats ranging from parched desert (e.g., the sidewinder, Crotalus cerastes) to rainforests (e.g., the bushmaster, Lachesis muta). They may be either arboreal or terrestrial, and one species is even semi-aquatic: the cottonmouth, Agkistrodon piscivorus. The altitude record is held jointly by Crotalus triseriatus in Mexico and Gloydius strauchi in China, both of which have been found above the treeline at over 4,000 m elevation.

Pit vipers are unique in that they have a pit organ. Some also have a rattle. And among vipers, these snakes are also unique in that they have a specialized muscle, called the muscularis pterigoidius glandulae, between the venom gland the head of the ectopterygoid. Contraction of this muscle, together with that of the m. compressor glandulae, forces venom out of the gland.

Pit organ

The pit organ is clearly visible between the eye and the nostril of this New Mexico ridge-nosed rattlesnake, Crotalus willardi obscurus.

What makes the crotalines unique is that they all share a common characteristic: a deep pit, or fossa, in the loreal area between the eye and the nostril on either side of the head. These pits are sensitive to infrared radiation, in effect giving the snakes a sixth sense that helps them to find and perhaps even judge the size of the small warm-blooded prey on which they feed.

The pit organ is complex in structure and is similar to, but much more highly evolved than the thermoreceptive labial pits found in boas and pythons. It is deep and located in a maxillary cavity. The membrane is like an eardrum that divides the pit into two sections of unequal size, with the larger of the two facing forwards and exposed to the environment. The two sections are connected via a narrow tube, or duct, that can be opened or closed by a group of surrounding muscles. By controlling this tube the snake can balance the air pressure on either side of the membrane. The membrane has many nerve endings packed with mitochondria. Succinic dehydrogenase, lactic dehydrogenase, adenosine triphosphate, monoamine oxidase, generalized esterases and acetylcholine esterase have also been found in it. When prey comes into range, infrared radiation falling onto the membrane allows the snake to determine its direction. Having one of these organs on either side of the head produces a stereo effect that indicates distance as well as direction. Experiments have shown that, when deprived of their senses of sight and smell, these snakes can strike accurately at moving objects that are less than 0.2°C warmer than the background. It would seem as though the pit organs work like a primitive pair of eyes, although it is not known whether the snake experiences this sense as a visual image or in some other fashion. Regardless, it is clear that these organs are of great value to a predator that hunts at night.

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, for example the copperhead, Agkistrodon contortrix, or 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.

Genera

Genus	Authority	Species	Subsp.*	Common name	Geographic range
Agkistrodon	Palisot de Beauvois, 1799	3	9	Moccasins	North America from the northeastern and central USA southward through peninsular Florida and southwestern Texas. In Central America on the Atlantic versant from Tamaulipas and Nuevo León southward to the Yucatan Peninsula, Belize and Guatemala. Along the Pacific coastal plane and lower foothills from Sonora south through Guatemala, El Salvador, Honduras and Nicaragua to northwestern Costa Rica.
Atropoides	Werman, 1992	3	2	Jumping pitvipers	The mountains of eastern Mexico southeastward on the Atlantic versant and lowlands though Central America to central Panama. On the Pacific versant, they occur in isolated populations in east-central and southern Mexico, Guatemala, El Salvador, Costa Rica and Panama.
Bothriechis	Peters, 1859	7	0	Palm-pitvipers	Southern Mexico (southeastern Oaxaca and the northern highlands of Chiapas), through Central America to northern South America (Colombia, western Venezuela, Ecuador and northern Peru.
Bothriopsis	Peters, 1861	7	2	Forest-pitvipers	Eastern Panama and most of northern South America, including the Pacific lowlands of Colombia and Ecuador, the Andes Mountains from Venezuela and Colombia to Bolivia, the Amazon Basin and the Atlantic forests of Brazil.
Bothrops	Wagler, 1824	32	11	Lanceheads	Northeastern Mexico (Tamaulipas) southward through Central and South America to Argentina; Saint Lucia and Martinique in the Lesser Antilles; Ilha da Queimada Grande off the coast of Brazil.
Calloselasma	Cope, 1860	1	0	Malayan pitviper	Southeast Asia from Thailand to northern Malaysia and Java, Indonesia.
Cerrophidion	Campbell & Lamar, 1992	3	0	Montane pitvipers	Southern Mexico (highlands of Guerrero and southeastern Oaxaca), southward through the highlands of Central America (Guatemala, El Salvador, Honduras, northern Nicaragua, Costa Rica) to western Panama.
CrotalusT	Linnaeus, 1758	27	42	Rattlesnakes	The Americas, from southern Canada to northern Argentina.
Deinagkistrodon	Gloyd, 1979	1	0	Hundred-pace pitviper	Southeast Asia.
Gloydius	Hoge & Romano-Hoge, 1981	9	9		Russia, east of the Ural Mountains through Siberia, Iran, the Himalayas from Pakistan, India, Nepal and China, Korea, Japan and the Ryukyu Islands.
Hypnale	Fitzinger, 1843	3	0	Hump-nosed pit vipers	Sri Lanka and India.
Lachesis	Daudin, 1803	3	1	Bushmasters	Central and South America.
Ophryacus	Cope, 1887	2	0	Mexican horned pitvipers	Mexico.
Ovophis	Burger, 1981	3	4	Mountain pit vipers	Nepal and Seven Sisters (Assam) eastward through Myanmar, Cambodia, Thailand, Laos, Vietnam, West Malaysia, Taiwan, Japan (Okinawa) and Indonesia (Sumatra and Borneo).
Porthidium	Cope, 1871	7	3	Hognose pit vipers	Mexico (Colima, Oaxaca and Chiapas on the Pacific side, the Yucatan Peninsula on the Atlantic side) southward through Central America to northern South America (Ecuador in the Pacific lowlands, northern Venezuela in the Atlantic lowlands).
Sistrurus	Garman, 1883	3	6	Ground rattlesnakes	Southeastern Canada, eastern and northwestern USA, isolated populations in northern and central Mexico.
Trimeresurus	Lacépède, 1804	35	12	Asian lanceheads	Southest Asia from India to southern China and Japan, and the Malay Archipelago to Timor.
Tropidolaemus	Wagler, 1830	2	0	Temple vipers	Southern India and Southeast Asia.

*) Not including the nominate subspecies (typical form).
^T) Type genus.

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.

See also

• List of crotaline species and subspecies
• Crotalinae by common name
• Crotalinae by taxonomic synonyms
• Snakebite

References

ISBN links support NWE through referral fees

• Integrated Taxonomic Information System (ITIS). 2004. Viperidae Oppel, 1811 ITIS Taxonomic Serial No.: 174294. Retrieved July 26, 2008.

• McDiarmid, R. W., J. A. Campbell, and T. Touré. 1999. Snake species of the World: A taxonomic and geographic reference, vol. 1. Herpetologists' League. ISBN 1893777014.

• Nilson, G., and R. L. Gutberlet. 2004. Viperidae. In B. Grzimek, D. G. Kleiman, V. Geist, and M. C. McDade, eds., Grzimek's Animal Life Encyclopedia. Detroit: Thomson-Gale. ISBN 0787657883.

pitvipers.

Further reading

• 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.

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