Crotalinae
Encyclopedia
The Crotalinae, commonly known as "pit vipers" or crotaline snakes, are a subfamily of venomous
viper
s found in Asia
and the Americas
. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head. Currently, 18 genera
and 151 species
are recognized: 7 genera and 54 species in the Old World
, against a greater diversity of 11 genera and 97 species in the New World
. These are also the only viperids
found in the Americas. The groups of snakes represented here include rattlesnake
s, lanceheads
and Asian pitvipers
. The type genus
for this subfamily is Crotalus
, of which the type species
is the timber rattlesnake, C. horridus
.
These snakes range in size from the diminutive hump-nosed viper, Hypnale hypnale
, that grows to an average only 30–45 cm (11.8–17.7 in), to the bushmaster, Lachesis muta
; a species that is known to reach a maximum of 3.65 metres (12 ft) in length—the longest viperid in the world.
What makes this group 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 loreal pit
s are the external openings to a pair of extremely sensitive infrared detecting organs, which in effect give 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 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 esterase
s 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.
Among vipers, these snakes are also unique in that they have a specialized muscle, called the muscularis pterigoidius glandulae, between the venom gland and the head of the ectopterygoid. Contraction of this muscle, together with that of the m. compressor glandulae, forces venom out of the gland.
from eastern Europe
eastward through Asia
to Japan
, Taiwan
, Indonesia
, peninsular India
and Sri Lanka
. In the Americas
, they range from southern Canada
southward to Central America
to southern South America
.
ranging from parched desert
(e.g., the sidewinder, Crotalus cerastes
) to rainforest
s (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.
, 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 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.
; 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.
: 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
.
Venomous snake
"Poisonous snake" redirects here. For true poisonous snakes, see Rhabdophis.Venomous snakes are snakes which have venom glands and specialized teeth for the injection of venom...
viper
Viperidae
The Viperidae are a family of venomous snakes found all over the world, except in Antarctica, Australia, Ireland, Madagascar, Hawaii, various other isolated islands, and above the Arctic Circle. All have relatively long, hinged fangs that permit deep penetration and injection of venom. Four...
s found in Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
and the Americas
Americas
The Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...
. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head. Currently, 18 genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
and 151 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
are recognized: 7 genera and 54 species in the Old World
Old World
The Old World consists of those parts of the world known to classical antiquity and the European Middle Ages. It is used in the context of, and contrast with, the "New World" ....
, against a greater diversity of 11 genera and 97 species in the New World
New World
The New World is one of the names used for the Western Hemisphere, specifically America and sometimes Oceania . The term originated in the late 15th century, when America had been recently discovered by European explorers, expanding the geographical horizon of the people of the European middle...
. These are also the only viperids
Viperidae
The Viperidae are a family of venomous snakes found all over the world, except in Antarctica, Australia, Ireland, Madagascar, Hawaii, various other isolated islands, and above the Arctic Circle. All have relatively long, hinged fangs that permit deep penetration and injection of venom. Four...
found in the Americas. The groups of snakes represented here include rattlesnake
Rattlesnake
Rattlesnakes are a group of venomous snakes of the genera Crotalus and Sistrurus of the subfamily Crotalinae . There are 32 known species of rattlesnake, with between 65-70 subspecies, all native to the Americas, ranging from southern Alberta and southern British Columbia in Canada to Central...
s, lanceheads
Bothrops
Bothrops is a genus of venomous pitvipers found in Central and South America. The generic name is derived from the Greek words bothros and ops that mean "pit" and "eye" or "face"; an allusion to the heat-sensitive loreal pit organs. Members of this genus are responsible for more human deaths in the...
and Asian pitvipers
Trimeresurus
Trimeresurus is a genus of venomous pitvipers found in Asia from Pakistan, through India, China, throughout Southeast Asia and the Pacific Islands. Currently 35 species are recognized...
. The type genus
Type genus
In biological classification, a type genus is a representative genus, as with regard to a biological family. The term and concept is used much more often and much more formally in zoology than it is in botany, and the definition is dependent on the nomenclatural Code that applies:* In zoological...
for this subfamily is Crotalus
Crotalus
Crotalus is a genus of venomous pit vipers found only in the Americas from southern Canada to northern Argentina. The name is derived from the Greek word krotalon, which means "rattle" or "castanet", and refers to the rattle on the end of the tail which makes this group so distinctive...
, of which the type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
is the timber rattlesnake, C. horridus
Crotalus horridus
Crotalus horridus, the timber rattlesnake, is a species of venomous pitviper found in the eastern United States. This is the only rattlesnake species in most of the populous northeastern United States. No subspecies are currently recognized....
.
Description
Hypnale hypnale
Hypnale hypnale is a venomous pitviper species found in India and Sri Lanka. No subspecies are currently recognized.-Description: Grows to an average of 30-45 cm in length. The Armed Forces Pest Management Board states 0.4-0.6 m in length....
, that grows to an average only 30–45 cm (11.8–17.7 in), to the bushmaster, Lachesis muta
Lachesis muta
Lachesis muta is a venomous pitviper species found in South America. Two subspecies are currently recognized, including the nominate subspecies described here.-Description:...
; a species that is known to reach a maximum of 3.65 metres (12 ft) in length—the longest viperid in the world.
What makes this group 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 loreal pit
Loreal pit
The Loreal pit is the deep depression, or fossa, in the loreal area on either side of the head in crotaline snakes . It is located behind the nostril and in front of the eye, but below the line that runs between the centers of each. It is the external opening to an extremely sensitive infrared...
s are the external openings to a pair of extremely sensitive infrared detecting organs, which in effect give 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
Infrared sensing in snakes
The ability to sense infrared thermal radiation evolved independently in several different families of snakes. Essentially, it allows these animals to “see” radiant heat at wavelengths between 5 and 30 μm to a degree of accuracy such that a blind rattlesnake can target vulnerable body parts of the...
is complex in structure and is similar to 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
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
, monoamine oxidase
Monoamine oxidase
L-Monoamine oxidases are a family of enzymes that catalyze the oxidation of monoamines. They are found bound to the outer membrane of mitochondria in most cell types in the body. The enzyme was originally discovered by Mary Bernheim in the liver and was named tyramine oxidase...
, generalized esterase
Esterase
An esterase is a hydrolase enzyme that splits esters into an acid and an alcohol in a chemical reaction with water called hydrolysis.A wide range of different esterases exist that differ in their substrate specificity, their protein structure, and their biological function.- EC classification/list...
s 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.
Among vipers, these snakes are also unique in that they have a specialized muscle, called the muscularis pterigoidius glandulae, between the venom gland and the head of the ectopterygoid. Contraction of this muscle, together with that of the m. compressor glandulae, forces venom out of the gland.
Geographic range
This family of snakes is found in the Old WorldOld World
The Old World consists of those parts of the world known to classical antiquity and the European Middle Ages. It is used in the context of, and contrast with, the "New World" ....
from eastern Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
eastward through Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
to Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, Taiwan
Taiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...
, Indonesia
Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...
, peninsular India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
and Sri Lanka
Sri Lanka
Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka is a country off the southern coast of the Indian subcontinent. Known until 1972 as Ceylon , Sri Lanka is an island surrounded by the Indian Ocean, the Gulf of Mannar and the Palk Strait, and lies in the vicinity of India and the...
. In the Americas
Americas
The Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...
, they range from southern Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
southward to Central America
Central America
Central America is the central geographic region of the Americas. It is the southernmost, isthmian portion of the North American continent, which connects with South America on the southeast. When considered part of the unified continental model, it is considered a subcontinent...
to southern South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
.
Habitat
This is a versatile group, with members found in habitatsHabitat (ecology)
A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...
ranging from parched desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
(e.g., the sidewinder, Crotalus cerastes
Crotalus cerastes
Crotalus cerastes is a venomous pitviper species found in the desert regions of the southwestern United States and northwestern Mexico. Three subspecies are currently recognized, including the nominate subspecies described here.-Description:...
) to rainforest
Rainforest
Rainforests are forests characterized by high rainfall, with definitions based on a minimum normal annual rainfall of 1750-2000 mm...
s (e.g., the bushmaster, Lachesis muta
Lachesis muta
Lachesis muta is a venomous pitviper species found in South America. Two subspecies are currently recognized, including the nominate subspecies described here.-Description:...
). They may be either arboreal or terrestrial
Terrestrial animal
Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...
, and one species is even semi-aquatic: the cottonmouth, Agkistrodon piscivorus
Agkistrodon piscivorus
Agkistrodon piscivorus is a venomous snake, a species of pit viper, found in the southeastern United States. Adults are large and capable of delivering a painful and potentially fatal bite. When antagonized they will stand their ground by coiling their bodies and displaying their fangs...
. The altitude record is held jointly by Crotalus triseriatus
Crotalus triseriatus
Crotalus triseriatus is a venomous pitviper species found in Mexico. Two subspecies are currently recognized, including the nominate subspecies described here.-Description:...
in Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
and Gloydius strauchi
Gloydius strauchi
Gloydius strauchi is a venomous pitviper species found in western China. A small snake with a pattern of four longitudinal stripes, although some older specimens may be a uniform black. Distinguished from G. monticola by its higher midbody dorsal scale count...
in China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, both of which have been found above the treeline at over 4,000 m elevation.
Behavior
Although a few species are highly active by day, such as Trimeresurus trigonocephalusTrimeresurus trigonocephalus
Trimeresurus trigonocephalus is a venomous pitviper species found in Sri Lanka. No subspecies are currently recognized.-Description:A sexually dimorphic species, at a maximum of 70 cm in length the males are considerably smaller than females, which can grow to 130 cm. In addition, male tends to...
, a bright green pit viper endemic
Endemic (ecology)
Endemism is the ecological state of being unique to a defined geographic location, such as an island, nation or other defined zone, or habitat type; organisms that are indigenous to a place are not endemic to it if they are also found elsewhere. For example, all species of lemur are endemic to the...
to Sri Lanka
Sri Lanka
Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka is a country off the southern coast of the Indian subcontinent. Known until 1972 as Ceylon , Sri Lanka is an island surrounded by the Indian Ocean, the Gulf of Mannar and the Palk Strait, and lies in the vicinity of India and the...
, 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
Gloydius shedaoensis
Gloydius shedaoensis is a venomous pitviper species found only on Shedao Island in China. Although very small, this island is home to an extraordinarily large population of these snakes. No subspecies are currently recognized.-Description:...
of China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, 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
Hibernation
Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...
), 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
Agkistrodon contortrix
Agkistrodon contortrix is a species of venomous snake found in North America, a member of the Crotalinae subfamily. The more common name for the species is "copperhead". The behavior of Agkistrodon contortrix may lead to accidental encounters with humans...
, or the Mojave rattlesnake, Crotalus scutulatus
Crotalus scutulatus
Crotalus scutulatus is a venomous pitviper species found in the deserts of the southwestern United States and central Mexico. It is perhaps best known for its potent neurotoxic venom...
.
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 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 ovoviviparousOvoviviparity
Ovoviviparity, ovovivipary, or ovivipary, is a mode of reproduction in animals in which embryos develop inside eggs that are retained within the mother's body until they are ready to hatch...
; that is, females give birth to live young. Among the oviparous (egg-laying) pit vipers are Lachesis
Lachesis (genus)
Lachesis is a genus of venomous pitvipers found in remote forested areas of Central and South America. The generic name refers to one of the Three Fates in Greek mythology who determined the length of the thread of life...
, Calloselasma
Calloselasma
Calloselasma is a monotypic genus created for a venomous pitviper species, C. rhodostoma, which is found in Southeast Asia from Thailand to northern Malaysia and on the island of Java. No subspecies are currently recognized.-Description:...
, and some Trimeresurus
Trimeresurus
Trimeresurus is a genus of venomous pitvipers found in Asia from Pakistan, through India, China, throughout Southeast Asia and the Pacific Islands. Currently 35 species are recognized...
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
Bothrops atrox
Bothrops atrox is a venomous pitviper species found in the tropical lowlands of northern South America east of the Andes. No subspecies are currently recognized.-Description:...
: 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 familyFamily (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
: the Crotalidae. Today, however, the monophyly
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
of the viperines
Viperinae
The Viperinae, or viperines, are a subfamily of venomous vipers found in Europe, Asia and Africa. They are distinguished by their lack of the heat-sensing pit organs that characterize their sister group, the Crotalinae. Currently, 12 genera and 66 species are recognized...
and the crotalines as a whole is undisputed, which is why they are treated here as a subfamily of the Viperidae
Viperidae
The Viperidae are a family of venomous snakes found all over the world, except in Antarctica, Australia, Ireland, Madagascar, Hawaii, various other isolated islands, and above the Arctic Circle. All have relatively long, hinged fangs that permit deep penetration and injection of venom. Four...
.