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Crotalus is a of venomous pitvipers found only in the Americas from southern Canada to northern Argentina.^1] 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 that make this group so distinctive.^[3] 29 species are currently recognized.^[4]

Description

Members of this genus range in size from only 50-60 cm (C. intermedius, C. pricei), to over 150 cm (C. adamanteus, C. atrox).^[3] In general, adult males are slightly larger than females. Compared to most snakes they are heavy-bodied, although some African vipers are much thicker.^[5] Most forms are easily recognized by the characteristic rattle on the end of the tail, although a few island populations form exceptions to this rule: C. catalinensis has lost its rattle entirely, C. ruber lorenzoensis usually has no rattle, and both C. r. lucasensis and C. molossus estebanensis exhibit a tendency for rattle loss. The rattle may also be lacking in any species due to a congenital abnormality.^[3]

The rattle.

The rattle itself consists of a series of loosely interlocking hollow shells, each of which was at one point the scale covering the tip of the tail. In most other snakes, the tail tip, or terminal spine, is cone-shaped, hardly any thicker than the rest of the skin, and is shed along with it at each successive molt. In this case, however, the end-scale, or "button", is much thicker and shaped like a bulb with one or two annular constrictions that prevent it from falling off. Before each molt, a new button will have developed inside the last one and before the skin is shed, the tip of new button shrinks, thereby loosening the shell of the previous one. This process continues so that a succession of molts produces an appendage that consists of a number of interlocking segments that make an audible noise when vibrated. Since younger specimens may shed 3-4 times in a year, every time adding a new segment to the rattle, the number of segments bears no relation to the age of the snake. In theory, the rattle could become very long indeed, but in practice the older segments tend to wear out and fall off. How quickly this happens depends on the snake's environment, but end segments tend to break off after the rattle becomes about 6-7 segments long; it is uncommon to find specimens with as many as a dozen segments. In captive specimens, however, as many as 29 segments have been found.^[6][7]

Geographic Range

Found in the Americas from southern Canada to northern Argentina.^[1]

Behavior

None are considered aggressive. In fact, when threatened most will retreat quickly. However, most species will defend themselves readily when cornered.^[3]

A highly controversial issue has always been how far these snakes can strike. Obviously this depends on the size of the animal, but other factors may also play a role, such as the species, the position the body is in and the degree of excitement. Additionally, there is the question of definition: from which point on the snake should a strike be measured: from the front, the middle, or the back of the anchor coil on the ground? Even if the length of the specimen is known, once it strikes it is almost impossible to determine the limiting point reached by its head and the position of its body when the movement started. Therefore, it is not surprising that many conflicting statements can be found in the available literature about how far these snakes can strike. Estimates have been given that range from ? of the body length, to ½, to ?, and even the full length of the animal. Klauber (1997) considered that they rarely strike further than ½ of their body length, and almost never more than ¾, but that it is still not wise to trust such values if only because it is not possible to accurately judge the length of a coiled snake.^[5]

Feeding

The diet generally consists of vertebrates, although many invertebrate species have also been noted. Smaller species feed mainly on lizards, while larger species start by feeding on lizards as juveniles and then switch to preying mainly on mammals as adults. According to Klauber (1936, 1971, 1972), prey items more frequently taken include rabbits, ground squirrels, tree squirrels, chipmunks, prairie dogs, gophers, rats and mice, while those less frequently taken include birds, snakes and amphibians. Cannibalism has been reported in a number of different species. Individuals that feed on rodents will usually release their prey after a strike and there is evidence that these snakes can discriminate between trails left by prey that has and has not been envenomated.^[3]:506

Predators

For all species, the most significant threats come from people, but the y also face many natural enemies. These include other snakes, such as kingsnakes (Lampropeltis), coachwhips (Masticophis), indigo snakes (Drymarchon) and racers (Coluber); birds, such as hawks, eagles, owls, roadrunners and ravens; and mammals, such as coyotes, foxes, wildcats, badgers, skunks and pigs (Keegan, 1944; Klauber, 1927, 1936, 1971, 1972). Certain species of birds frequently prey on these snakes, but this is not without risk. Heckel et al. (1994) described two cases in which dead hawks were found near venomous snakes and had suffered hemorrhage and gangrenous necrosis. Their conclusion was that this was due to snakebite.^[3]:514

Reproduction

This genus is ovoviviparous, giving birth to live young.[5] The basic life cycle of many Nearctic species has been known for quite some time. Klauber (1936) describes how females at an age of 26 months undergo vitellogenesis as they enter their third hibernation, mate the following spring and give birth a number of months later in September or October.^[3]:516

There are, however, a number of variations to this basic cycle. In North America, this is due to the ability of the females of some species to store sperm in the oviduct for at least eight months, and the males (all species of which undergo spermatogenesis during the summer) to store sperm in the vas deferens for at least a year. Thus, species that store sperm for a shorter duration mate in the spring and store sperm in the vas deferens, while those that do so for a longer duration mate in the fall and store sperm in the oviduct over the winter, after which fertilization occurs the following spring.^[3]:516 In addition, species that occur further north, where it is colder during much of the year and the feeding and growing season is short, may reproduce only every other year or less. Those that are found in central and southern Mexico or the tropics have reproductive cycles that correspond mostly with the rainy season.^[3]:519

Venom