| Rattlesnake | |
|---|---|
| Crotalus cerastes | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Suborder: | Serpentes |
| Family: | Viperidae |
| Subfamily: | Crotalinae |
| Genera | |
Rattlesnakes are a group of venomous snakes of the genera Crotalus and Sistrurus[1] of the subfamily Crotalinae ("pit vipers"). There are 32 known species of rattlesnake, with between 65-70 subspecies,[2] all native to the Americas, ranging from southern Alberta and southern British Columbia in Canada to Central Argentina.
Rattlesnakes are predators who live in a wide array of habitats, hunting small animals such as birds and rodents. They kill their prey with a venomous bite, rather than by constricting. All rattlesnakes possess a set of fangs with which they inject large quantities of hemotoxic venom. The venom travels through the bloodstream, destroying tissue and causing swelling, internal bleeding, and intense pain. Some species, such as the Mojave Rattlesnake, additionally possess a neurotoxic component in their venom that causes paralysis and other nervous symptoms.
The threat of envenomation, advertised with the shaking of the rattle, deters many predators. However, rattlesnakes fall prey to hawks, weasels, king snakes, and a variety of other species. Rattlesnakes are heavily preyed upon as neonates, while they are still weak and mentally immature. Very large numbers of rattlesnakes are killed by humans. Rattlesnake populations in many areas are severely threatened by habitat destruction, poaching, and extermination campaigns.
Rattlesnake bites are the leading cause of snakebite injuries in North America, and cause approximately 82% of fatalities. However, rattlesnakes rarely bite unless provoked or threatened; and if treated promptly, the bites are rarely fatal.
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Rattlesnakes receive their name from the rattle located at the end of their tails which, when shaken,it makes a loud rattling noise that deters predators or serves as a warning to passersby. The scientific name Crotalus derives from the Greek, κρόταλον, meaning "castanet".[3] The name Sistrurus is the Latinized form of the Greek word for "tail rattler" (Σείστρουρος, Seistrouros) and shares its root with the ancient Egyptian musical instrument, the sistrum, a type of rattle.[4]
Rattlesnakes are native to the Americas, living in diverse habitats from Southwestern Canada to Central Argentina. The large majority of species live in the American Southwest and Mexico. Four species may be found east of the Mississippi river, and two in South America. In the United States, the state with the most forms of rattlesnakes is Arizona, with seventeen species and subspecies.[5][6]
Rattlesnakes are found in almost every type of habitat that is capable of supporting terrestrial ectothermic vertebrates; but individual species can have extremely specific habitat requirements, only able to live within certain plant associations in a narrow range of altitudes. Most species live near open, rocky areas. Rocks offer them cover from predators, plentiful prey (e.g. rodents, lizard, insects, etc. that live amidst the rocks), and open basking areas. However, rattlesnakes can also be found in a wide variety of other habitats including prairies, marshes, deserts, and forests.[7] Rattlesnakes prefer a temperature range between 80 and 90°F (26-32°C), but can survive temperatures below freezing, recovering from brief exposure to temperatures as low as 4°F, and surviving for several days in temperatures as low as 37°F.[8]
Evidence has been collected to show the most probable ancestral area of rattlesnakes is the Sierra Madre Occidental region in Mexico. This same study found the most probable vegetation or habitat of the ancestral area to be pine-oak forests.[6]
Rattlesnakes consume mice, rats, small birds and other small animals[9], playing an important ecological role by limiting the size of rodent populations, which prevents crop damage and stabilizes ecosystems.[10]
Rattlesnakes lie in wait for their prey, or hunt for it in holes.[11] The prey are killed quickly with a venomous bite as opposed to constricting. If the bitten prey moves away before dying, the rattlesnake can follow it by its scent.[12][13] When it locates the fallen prey, it checks for signs of life by prodding with its snout, flicking its tongue, and using its sense of smell. Once the prey has become incapacitated, the rattlesnake locates its head by odors emitted from the mouth. The prey is then ingested head-first, which allows wings and limbs to fold at the joints in a manner which minimizes the girth of the meal.[14] The gastric fluids of rattlesnakes are extremely powerful, allowing for the digestion of bone as well as flesh. Optimal digestion occurs when the snake maintains a body temperature of between 80 and 85°F (25-29°C). If the prey is small, the rattlesnake will often continue hunting. If it was an adequate meal, the snake will find a warm, safe location in which to coil up and rest until the prey is digested.[15]
Rattlesnakes are believed to require at least their own body weight in water annually in order to remain hydrated. The method in which they drink depends on the water source. In larger bodies of water (streams, ponds, etc.) they will submerge their head and ingest water by opening and closing their jaw, which sucks in water. If drinking dew, or drinking from small puddles, they will sip the liquid either by capillary action or by flattening and flooding the lower jaw.[16]
Newborn rattlesnakes are heavily preyed upon by a variety of species including ravens, crows, roadrunners, raccoons, opossums, skunks, coyotes and weasels, whipsnakes, kingsnakes, and racers. Neonates of the smaller Crotaline species are frequently killed and eaten by small predatory birds such as jays, kingfishers, and shrikes. Some species of ants in the genus Formica are known to prey upon neonates, and it is very likely that Solenopsis invicta (fire ants) do as well. On occasion, hungry adult rattlesnakes will cannibalize neonates. The small proportion (often as few as 20%) of rattlesnakes that make it to their second year are heavily preyed upon by a variety of larger predators including coyotes, eagles, hawks, owls, feral pigs, badgers, indigo snakes, and kingsnakes.[17]
The common kingsnake (Lampropeltis getula), a constrictor, is immune to the venom of rattlesnakes and other vipers, and rattlesnakes form part of its natural diet. Rattlesnakes sense kingsnakes' presence by their odor.[18] When they realize a kingsnake is nearby, they begin enacting a set of defensive postures known as "body bridging". Unlike its normal erect and coiled defensive-striking posture, the rattlesnake keeps its head low to the ground in an attempt to prevent the kingsnake from gaining a hold of it (the head being the first part of the rattlesnake that is ingested). The rattlesnake jerks its body about, while bridging its back upwards, forming an elevated coil which faces the kingsnake. The elevated coil is used to strike the attacker, and is also used to shield the head from the kingsnake.[19]
Like all pit vipers, rattlesnakes have two organs that can sense radiation: their eyes, and a set of heat-sensing "pits" on their face that enable them to locate prey and strike towards it, based on the prey's thermal radiation signature. These pits have a relatively short effective range of approximately 1 ft, but nevertheless give the rattlesnake a distinctive advantage in hunting for warm-blooded creatures at night.[20][21]
Rattlesnake eyes, which contain a large number of rod cells, are well adapted to nocturnal use.[22][23] However rattlesnakes are not exclusively nocturnal, and their vision is more acute during daylight conditions.[23] Rattlesnakes also possess cone cells, which means that they are capable of some form of color vision. The rattlesnake eye lacks a fovea making it impossible for them to see sharply defined images. Instead, they mostly rely on the perception of movement.[22] Rattlesnake eyes are capable of horizontal rotation, but they do not appear to move their eyeballs to follow moving objects.[23]
Rattlesnakes have an exceptionally keen sense of smell. They can sense olfactory stimuli both through their nostrils, and by flicking their tongue, which carries scent-bearing particles to the Jacobson's organ in the roof of their mouth.[24][25]
Like all snakes, rattlesnakes do not have external ear openings, and the structures of their middle ear is not as highly specialized as those of other vertebrates, such as mammals. Thus their sense of sound is not very effective. However, they are capable of sensing vibrations in the ground, passed via the skeleton to the auditory nerve.[22]
Rattlesnake fangs are connected by venom ducts to large poison glands near the outer edge of the upper jaw, towards the rear of the head. When the rattlesnake bites, muscles on the sides of the venom glands contract, which squeezes the venom through the ducts and into the fangs. When the fangs are not in use, they remain folded against the palate.[26][27]
Rattlesnakes are born with fully functioning fangs and venom, and are capable of killing prey at birth.[17][28] Adult rattlesnakes shed their fangs every 6–10 weeks. At least 3 pairs of replacement fangs lie behind the functional pair.[29]
Most species of rattlesnakes have hemotoxic venom, destroying tissue, causing necrosis and coagulopathy (disrupted blood clotting).[30] In the U.S., some varieties of the Mojave rattlesnake (C. scutulatus) have a presynaptic neurotoxic venom component known as Mojave Type A toxin, which can cause severe paralysis.[30][31][32] C. scutulatus is widely regarded as producing one of the most toxic snake venoms in the Americas, based on LD50 studies in laboratory mice.[33]
Rattlesnake venom is a mixture of 5-15 enyzmes, various metal ions, biogenic amines, lipids, free amino acids, proteins, and polypeptides. It contains components designed to immobilize and disable the prey, as well as digestive enzymes which break down tissue to prepare for later ingestion.[25][29] The venom is very stable, and retains its toxicity for many years in storage.[25]
Older snakes possess more potent venom, and larger snakes are frequently capable of storing larger volumes of it.[34]
According to Rubio, "The most widely accepted hypothesis for the evolution of the rattle is that the rattle is a warning device for predatory animals that might be a threat to the rattlesnake. It produces a signal to drive them away."[35]
The rattle is composed of a series of hollow, interlocked segments made of keratin, which are created by modifying the scales that cover the tip of the tail. The contraction of special "shaker" muscles in the tail causes these segments to vibrate against one another, making the rattling noise (which is amplified because the segments are hollow).[1][36][37] The muscles that cause the rattle to shake are some of the fastest known, firing 50 times per second on average, sustained for up to three hours.[38]
Each time the snake sheds its skin, a new rattle segment is added. They may shed their skins several times a year depending on food supply and growth rates. Rattlesnakes travel with their rattles held up to protect them from damage, but in spite of this precaution, their day-to-day activities in the wild still cause them to regularly break off end segments. Because of this, there is no way to determine the age of a rattlesnake based on the number of rattles on its tail.[1][37][38]
One of the differentiating features of males and females is that males have a thicker and longer tail. The tails of males also taper gradually from the body, whereas female tails narrow sharply at the vent.[39]
Rattlesnake skin is composed of a set of overlapping scales which cover the entire body, providing protection from a variety of threats including dehydration and physical trauma.[40] The skin of snakes is highly sensitive to contact, tension, and pressure; and they are capable of feeling pain.[41]
An important function of the skin is the sensation of changes in air temperature, which can guide the snakes towards warm basking/shelter locations.[42] All snakes are ectotherms. In order to maintain a stable body temperature, they exchange heat with their external environment. Snakes often move into open, sunny areas to absorb heat from the sun and warmed earth - a behavior known as basking. Nerves in the skin regulate the flow of blood into the veins near the surface. Rubio states that "The skin's acceptance of radiant energy, its ability to determine the temperature, and the snake's ability to move toward or away from one temperature gradient to another are among the most important behavioral actions in its daily life."[41]
The skin of rattlesnakes is intricately patterned in a manner that camouflages them from their predators.[42][43] Rattlesnakes do not generally have bright or showy colors (reds, yellows, blues etc.), instead relying on subtle earth tones that resemble the surrounding environment.[44]
There are creases in the epidermal tissue which connects the scales of rattlesnakes. When ingesting large prey, these creases can unfold, allowing the skin to expand to envelop a much greater volume. It appears that the skin is tightly stretching to accommodate the meal, but in reality, the skin is simply smoothing out from its creased state and is not under very high tension.[45]
Most rattlesnake species mate during the summer or fall, while some species mate only in the spring, or during both the spring and fall.[46]
Females secrete small amounts of sex pheromones, which leave a trail that males follow by using their tongue and Jacobson's organ as a guide.[46] Once a receptive female has been located, the male will often spend several days following her around (a behavior that is not common outside of the mating season), frequently touching and rubbing her in an attempt to stimulate her.[47][48]
The males of some species, such as timber rattlesnakes (C. horridus), will fight each other during the mating season, in competition over females. These fights known as "combat dances" consist of the two males intertwining the anterior portion of their bodies, often with the head and neck held vertically. The larger males usually end up driving the smaller males away.[49][50]
Although many kinds of snakes and other reptiles are oviparous (lay eggs), rattlesnakes are viviparous (give birth to live young).[51] The female produces the ova ("eggs") in her ovaries, after which they pass through the body cavity and into one of her two oviducts. The ova are arranged in a continuous chain in a coiled section of the oviduct, known as the "tuba".[47] Male rattlesnakes have sexual organs known as hemipenes, located in the base of the tail. The hemipenis is retracted inside of the body when mating is not occurring. Unlike a penis, where semen travels through an internal seminal duct, the semen in a hemipenis travels via a groove into the female's oviduct. Females can store semen for months in internal recesses known as spermathecae, which permits them to mate during the fall, but not fertilize the ova until the following spring.[47]
Rattlesnakes generally take several years to mature, and females usually reproduce only once every three years.[52]
Some rattlesnake species hibernate in the colder winter months. They often gather together for hibernation in very large numbers (sometimes over 1,000 snakes), huddling together inside of underground "rattlesnake dens" or hibernacula.[53][54] Rattlesnakes will regularly share their winter burrows with a wide variety of other species (such as turtles, small mammals, invertebrates, and other types of snakes).[53]
Rattlesnakes often return to the same den, year after year, sometimes traveling several miles to get there. It is not known exactly how the rattlesnakes find their way back to the dens each year, but it has been hypothesized that they use a combination of pheromone trails and visual cues (e.g. topography, celestial navigation, and solar orientation).[55]
Species with long periods of hibernation tend to have much lower reproductive rates than those with shorter hibernation periods, or those that don't hibernate at all. Female timber rattlesnakes in high peaks in the Appalachian Mountains of New England reproduce every three years on average; the lance-headed rattlesnake (C. polystictus), native to the warm climate of Mexico, reproduces annually.[56]
Like most other snakes, rattlesnakes aestivate during very hot or dry periods, which is why they are very rarely seen during the hottest and driest months of summer.[57]
Rattlesnakes tend to avoid developed areas, preferring undisturbed, natural habitats. Rapid habitat destruction by humans, mass killings during events like rattlesnake round-ups, and deliberate extermination campaigns all pose severe threats to rattlesnake populations in many areas. Several species of rattlesnakes, such as the timber rattler, massasauga, and canebrake rattlesnake are listed as threatened or endangered in many U.S. states.[58]
A very large number of rattlesnakes die from being run over by cars.[17]
In more heavily populated and trafficked areas, there have been increasing reports of rattlesnakes that do not rattle. This phenomenon is commonly attributed to selective pressure by humans, who often kill the snakes when they are discovered.[59] Non-rattling snakes are more likely to go unnoticed, and therefore survive to reproduce offspring who, like themselves, are less likely to rattle.
Rattlesnakes are the leading cause of snakebite injuries in North America, and are a significant cause in Central and South America.[32][60]
Rattlesnakes tend to avoid wide open spaces where they cannot hide from predators, and will generally avoid humans if they are aware of their approach.[61] Rattlesnakes rarely bite unless they feel threatened or provoked. A large majority of victims (~ 72%[62]) are males, often young and intoxicated. Approximately half of bites occur in cases where the victim saw the snake, yet made no effort to move away.[25]
Hikers and campers should avoid contact with rattlesnakes by remaining observant and not approaching the animals. Hikers are advised to be particularly careful when negotiating fallen logs or boulders and when near rocky outcroppings and ledges where rattlesnakes may be hiding or sunning themselves - however, snakes will occasionally sun themselves in the middle of a trail, so such areas are not the only places where they are encountered. When encountering a rattlesnake on a trail, hikers are advised to keep their distance and allow the snake room to retreat.
Caution is advised even when snakes are believed to be dead; rattlesnake heads can see, flick the tongue, and inflict venomous bites for up to an hour after being severed from the body.[63][64]
Rattlesnake bites are rarely fatal to humans, if treated promptly.[30] Between 7,000 and 8,000 people are estimated to have been bitten by venomous snakes in the United States each year, and about five of those die.[65] The most important factor in determining survival following a severe envenomation is the amount of time elapsed between the bite and treatment. Most deaths occur between 6 and 48 hours after the bite. However, if antivenom treatment is given within 1–2 hours of the bite, the probability of recovery is greater than 99%.[66]
When a bite occurs, the amount of venom injected is under voluntary control by the snake. The amount released is dependent on a variety of factors including the condition of the snake (e.g. having long, healthy fangs and a full venom sack) and its temperament (an angry, hungry snake that has just been stepped on vs. a satiated snake that was merely surprised by walking near it).[67] Approximately 20% of bites result in no envenomation at all. A lack of burning pain and edema 1 cm away from the fang marks after 1 hour, suggests that either no or minimal envenomation occurred. A lack of edema or erythema in the area of the bite after 6–8 hours indicates a lack of envenomation for most rattlesnake bites.[68]
Common symptoms include swelling, severe pain, weakness, anxiety, nausea and vomiting, hemorrhaging, perspiration, and eventually heart failure.[67][69] Local pain following envenomation is often intense, increasing with the ensuing edema.[67] In general, children experience more severe symptoms because they receive a larger amount of venom per kilogram of body mass.[68]
Data on the effectiveness of first aid techniques for rattlesnake bites is limited.[70] However, general recommendations for first aid in the field are as follows:
In no case should tourniquets be used, nor should any incisions or suction be applied to the wound.[70][72]
Crotaline antivenom (or "antivenin") is commonly used to treat the effects of local and systemic pit viper envenomations.[73] The first step in the production of crotaline antivenom is collecting ("milking") the venom of a live rattlesnake - usually from the Western Diamondback (Crotalus atrox), Eastern Diamondback (Crotalus adamanteus), South American rattlesnake (Crotalus durissis terrificus), or fer-de-lance (Bothrops atrox). The extracted venom is then diluted and injected into horses, goats, or sheep, whose immune systems produce antibodies that protect from the toxic effects of the venom. These antibodies accumulate in the blood, which is then extracted and centrifuged to separate the red blood cells. The resulting serum is purified into a lyophilized powder, which is packaged for distribution and later use by human patients.[74][75]
Because antivenom is derived from animal antibodies, people generally display an allergic response during infusion, known as serum sickness.[76][77]
In the United States, more than 15,000 domesticated animals are bitten by snakes each year. Rattlesnake envenomations account for 80% of the deadly incidents.[78]
Dogs are most commonly bitten on the front legs and head. Horses generally receive bites on the muzzle, and cattle on their tongues and muzzles. If a domesticated animal is bitten, the hair around the bite should be removed, so that the wound can be clearly seen. The crotaline Fab antivenom has been shown to be effective in the treatment of canine rattlesnake bites.[34]
Aztec paintings, Central American temples, and the great burial mounds in the southeastern U.S. are frequently adorned with depictions of rattlesnakes, often within the symbols and emblems of the most powerful deities.[79] The Feathered Serpent deity of the preclassic Olmec was depicted as having the combined features of the quetzal bird and rattlesnake.[80] The Ancient Maya considered the rattlesnake to be a "vision serpent" that acted as a conduit to the "otherworld".[81]
Members of some Christian sects in the southern United States are regularly bitten while participating in "snake handling" rituals. Snake handling is when people hold venomous snakes, unprotected, as part of a religious service inspired by a literal interpretation of the Bible verse Mark 16:17-18 which reads "In my name ... they will pick up snakes with their hands".[25][82]
Journalist Alistair Cooke claims that rattlesnake tastes "just like chicken, only tougher."[83] Others have compared the flavor to a wide range of other meats, including veal, frog, tortoise, quail, fish, rabbit, and even canned tuna.[84]