Bison theory, appearance, fatalities, coiling and biting

Rattlesnakes are a group of distinctly American snakes that evolved on the plains of North America (O'shea, 2005)1. Rattlesnakes are found across most of North and South America with the exception of Panama, Ecuador, Chile, and some smaller remote island of the region.

A characteristic of all but one of the 32 known rattlesnake species is the jointed rattles at the tips of their tails. Rattlesnakes are deaf and cannot hear their own rattle, a trait that has sparked two main theories on why they have a rattle at all. The most recognized theory is the bison theory.

The Bison Theory

This theory states that the rattle evolved to reduce the risk of being stepped on and crushed by bisons, and secondly, as a means of warning any bisons to stay away.

Another, but less accepted, theory is that the rattle evolved as a specialization of the enlargement of the tail present in most snakes due to the shedding of skin. Later, this evolved into whatever purposes the tail of rattlesnakes has in today's species.

Timber rattlesnake

The problem with the latter theory is that it does not give a true explanation of the purpose of the rattle, it merely explains how the rattle could have evolved.


                                                                        








Bites and fatalities

Rattlesnake bites are common, and although fatalities are relatively rare (on average 0.96 fatalities per year), amputations and other medical complications are common consequences of rattlesnake bites. Above is a figure showing the number of rattlesnake bites in a period from 1983 to 2007 and the number of fatalities from rattlesnake bites. The data are from a study by Walter et al. 2009 2. The article explains that there is a decline in the number of «major outcomes», such as amputations of limbs. The number of deaths is constant while the number of reported bites is increasing.

Bites and fatalities from 1983 to 2007

Although the graph shows that there are people dying from rattlesnake bites each year, even more people die from bee stings.

Rattlesnake coiling posture

Rattlesnakes have two coiling postures: one for resting and one for striking. When they rest, they lie flat like a pancake. However, if the snakes are in a striking position their bodies become raised in a vertical spiral, allowing them to strike as fast as the eye can see. Their range is from about ½ to ¾ of their length, but do note that it's possible for the rattlesnake to strike from any position.

Strategy after bite

After biting a mouse for example, a rattlesnake will typically release the mouse in order not to risk getting hurt. The rattlesnake then engages in a series of stereotypical post-bite behaviors3. The envenomated mouse tries to flee, but soon succumbs to the venom. In the meantime, the snake has begun to follow the prey's trail. When the prey is found it is swallowed. The success of individual rattlesnakes depends on their ability to locate their prey following envenomation.

Pictures: The archetypal rattlesnake in the USA is the Western rattlesnake.

Rattlesnakes and Man

Rattlesnakes normally depart or flee when approached by humans. They are shy and they do not seek or want contact with humans. In fact, the last thing they would like to do is strike at an animal too big to swallow! If a rattlesnake is approached by a human and cannot find shelter, it will usually give a warning rattle.

Rattlesnake Life-cycle

Rattlesnakes are viparious, meaning the embryo develops inside the female and not in an egg outside the body. Usually they give birth in June, and by September the young snakes have reached a length of 12 inches. By studying their movement, using radio sensors, it has been shown that they migrate to and from their dens in spring and fall, and that their activity range is not more than two kilometers from their den4.

Rattlesnake species

Sidewinder rattlesnake

There are three subspecies of this relatively small, sand colored desert rattlesnake that received its name from its peculiar way of moving. The sidewinder rattlesnake moves sideways and in a manner completely different from that of other rattlesnakes. First, it lifts the forepart of its body to one side, and then places it on the sand. Then, it brings its rear end over leaving J or S shaped markings in the sand. The advantage of this type of locomotion is that it minimizes contact time with the hot sand. The sidewinder rattlesnake can be found in the Mojave and Sonoran deserts (Arizona and California). The snake is also known as the American Sidewinder, the Mojave Desert Sidewinder, and the Horned Rattlesnake due to horn like scales covering its eyes. It is often seen basking on rocks in the hot deserts of Arizona.

Santa Catalina Rattlesnake

This rattlesnake is indigenous to Santa Catalina in the Gulf of California, but it can also be found in the Southern part of the Baja California peninsula. It is a rather short rattlesnake, reaching an average length of two feet. The tail of this rattlesnake is degenerated, and its buttons fall off after each shedding. It is hypothesized that the missing rattle is an adaption to capturing birds; however when capturing a bird, a rattling rattle could be a disadvantage. Also, there are no heavy-footed animals to drive an evolution of a rattle on the island of Santa Catalina.

Western Rattlesnake

The Western Rattlesnake is of course a rather dangerous snake although fatalities from Western Rattlesnake bites have never been reported. The Western Rattlesnake is a heavy-bodied 3½ feet long snake. It has many small scales on its head, and relatively large and keeled scales on the remainder of its body. It is found throughout most of western USA.

Mojave Rattlesnake

The Latin name of the Mojave Rattlesnake is Crotalus Scutulatus. Scutulatus means something like having a shield shaped patch, and indeed this characterizes the Mojave Rattlesnake. They have, with some imagination, patches that resemble shields on their backs. The largest snakes of this specie can reach a length of four feet, but most Mojava Rattlesnakes reaches a length of three feet only. It is found in the Mojava Desert (California, Arizona, Utah, and Nevada) and in some parts of Western Texas. It has a reputation of being quite aggressive5, although this perception probably has to do with the locals fear towards scuts.

Red Diamond rattlesnake

Crotalus ruber; ruber means red. The red diamond rattlesnake is found in Southern California and Baja California, Mexico. It prefers rocky habitats with bushes rather than grassland. It is one of the least venomous rattlesnakes.

Western Diamondback rattlesnake

A description of the western diamondback rattlesnake can be found here.

Ridge Nosed Rattlesnake

This snake can be found in South Western US and Mexico only. They have a ridge running tip of the snout to the eye. They reach a length of two feet and they prefer woodland and forest on rocky slopes.

Eastern Diamondback rattlesnake

A description of this snake is also found at the diamondback rattlesnake site.

The archetypal rattlesnake in the USA is the Western rattlesnake

Red Diamond rattlesnake

Red diamond rattlesnakes are most commonly found on the ground, but occasionally they are found in trees and brush. Sometimes they are even found basking on the top of cactuses. This large, pink-colored snake has diamonds down the length of its back and is closely related to the Western Rattlesnake. It has diamonds down the length of its back. Red diamondback rattlesnakes are relatively large (4-5 feet). Their background color may vary quite a bit, but in general it is red. Its tail is grey and white. Males engage in ritual combat as a part of mating.

Bites from Pygmy Rattlesnakes are more common than bites from more audible rattlesnakes. When you can't hear them you won't be able to escape before they bite you

Pygmy Rattlesnake or Pigmy Rattlesnake

This snake is found in Eastern and Central USA. Its rattle is difficult to hear, making bites from this rattlesnake more common that bites from more audible rattlesnakes. It is a small snake and it rarely reaches a size of more than 2½ feet. Plenty of bites are reported from this snake, but fatalities remains to be recorded. They are found in the variety of habitats, but most often the habitat is close to water.

Timber Rattlesnakes

Timber rattlesnakes Crotalus horridus are found in the Eastern USA. They are the most prominent from from southern Minnesota to Texas, as well as all the states in between and those states east of Texas and Minnesota. Prior to a bite they will usually rattle and faint for a time, giving the victim time to try and avoid getting bitten by these venemous snakes.

Its venom is quite toxic compared to other rattlesnake species. Timber rattlesnakes have a relatively low tolerance for human disturbances6 and they will migrate to somewhere else if people continously disturbing them at their basking spots and near their dens.

The Timber Rattlesnake is often confused with the Eastern Diamondback Rattlesnake.

Timber rattlesnake prey

In a study from 19497 it was found that the only type of prey found in 19 timber rattlesnake specimens from Mountain Lake in Virginia, were mammals. In this small population most mammals mice, which suggests that mice are abundant in the area studied.

Size and appearance

Male timber rattlesnakes are larger than females. This holds true for adult snakes and newborn timber rattlesnakes8. Adult individuals can reach a length of 5 feet (1½ meter). With respect to color they fall into two groups - black background color and yellow background color.

Rattlesnake round-ups

Originally arranged as an excuse to drink and party, Rattlesnake round-ups have now become one of the most common forms of organized animal cruelty in the United States.

Killing and rattlesnake roundups

Organized killing of rattlesnakes still take place in Texas, Georgia and other mid-western states in so-called rattlesnake roundups.

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References

1. O'Shea, M. Venomous Snakes of the World 2005
2. Walter, F.G. Stolz, U. Shirazi, F. & Mcnally, J. Epidemiology of severe and fatal rattlesnake bites published in the American Association of Poison Control Centers Annual Reports. Clinical Toxicology 47, pp. 663-669 (2009)
3. Kardong, K.V. The predatory strike of the rattlesnake: When things go amiss. Copeia 3, pp. 816-820 (1986)
4. Landreth, H.F. Orientation and behavior of the Rattlesnake, Crotalus Atrox Copeia 1 pp. 26-31 (1973).
5. Berger, C. Venomous Snakes 2007.
6. Furman, J. Timber rattlesnakes in Vermont and New York: biology, history, and the fate of an endangered species (2007)
7. Smyth, T. Notes on the Timber Rattlesnake at Mountain Lake, Virginia Copeia 1 pp. 78 (1949)
8. Stewart, M.M, Larson, Gary E. (!) & Matthews T.H. Morphological Variation in a Litter of Timber Rattlesnakes Copeia 4 pp. 66-67 (1960)

Acknowledgements

Thanks to Terry, John and Don for allowing me to use their photos.

Web resources

Rattlesnake Pestnote (pdf)
Rattlesnakes (Arizona)
Rattlesnakes (Pennsylvania)


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