Figure 1. Prairie rattlesnake, Crotalus viridis viridis
Rattlesnakes are distinctly American serpents. They all have a jointed rattle at the tip of the tail, except for one rare species on an island off the Mexican coast. This chapter concerns the genus Crotalus, of the pit viper family Crotalidae, suborder Serpents. Since snakes evolved from lizards, both groups make up the order Squamata.
This article describes the characteristics of the common species of rattle-snakes that belong to the genus Crotalus. These include the eastern diamondback, (C. adamanteus); the western diamond (back) rattlesnake, (C. atrox); the red diamond rattlesnake, (C. ruber); the Mohave rattlesnake, (C. scutulatus); the sidewinder, (C. ceraster); timber rattlesnake, (C. horridus); three subspecies of the western rattlesnake, (C. viridis): the prairie rattlesnake (C. v. viridis); the Great Basin rattlesnake (C. v. lutosus); and the Pacific rattlesnake (C. v. oreganus).
There are 15 species of rattlesnakes in the United States and 25 in Mexico. Other front-fanged poisonous snakes of the Crotalidae family, which are not included in this discussion, are the massasauga and pigmy rattlesnakes, both of the genus Sistrurus. Also not included are two snakes that do not have rattles, hence are not called rattlesnakes: the water moccasin or cottonmouth, and the copperhead, both of the genus Agkistrodon. Two other genera of poisonous snakes in North America are coral snakes (Micrurus and Micruroides) of the family Elapidae.
Rattlesnakes are usually identified by their warning rattle — a hiss or buzz —made by the rattles at the tip of their tails. A rattlesnake is born with a button, or rattler, and acquires a new rattle section each time it molts. Rattlesnakes also are distinguished by having rather flattened, triangular heads. The heads of all Crotalus rattlesnakes are about twice as wide as their necks. Only pit vipers possess this head con-figuration; coral snakes do not.
Rattlesnakes belong to the pit viper family Crotalidae, so named because all possess visible loreal pits, or lateral heat sensory organs, between eye and nostril on each side of the head (Fig. 2). These heat sensory pits are not present in true vipers, which do not occur in the Western Hemisphere. The facial pits enable rattlesnakes to seek out and strike, even in darkness, warm objects such as small animal prey, as well as larger animals that could be a threat. The vertically elliptical eye pupils, or “cat eyes,” are also a characteristic of rattlesnakes (Fig. 2). Identifying a dead rattler whose rattles are missing can be done by looking at the snake’s scales on the underside in the short region between the vent and the tip of the tail. If the scales are divided down the center, the snake is harmless. The scales on rattlesnakes are not divided.
Rattlesnakes come in a great variety of colors, depending on the species and stage of molt. Most rattlers are various shades of brown, tan, yellow, gray, black, chalky white, dull red, and olive green. Many have diamond, chevron, or blotched markings on their backs and sides.
When a rattlesnake strikes its prey or enemy, the paired fangs unfold from the roof of its mouth. Prior to the completion of the forward strike motion, the fangs become fully erect at the outer tip of the upper jaw. The erectile fangs are hollow and work like hypodermic needles to inject a modified saliva, the venom, into the prey. Rattlesnakes can regulate the amount of venom they inject when they strike.
Mature fangs generally are shed several times a season. They may become embedded in the prey and may even be swallowed with the prey. When one mature fang in a pair is lost, it will soon be replaced by another functional mature fang. A series of developing fangs are located directly behind one another in the same sheath at the roof and outer tip of the mouth (Fig. 3). If a newly replaced fang is artificially removed, it may require weeks or longer before another replacement will be fully effective. One fang can function, however, while the other in the pair is being replaced. Fangs that get stuck in a person’s boot are not very dangerous; they cannot contain much venom since they serve only as a hollow needle. The external opening of the hollow fang is a groove on the outside of the fang, set slightly back from the tip to prevent it from becoming plugged by tissue from the prey (Fig. 3).
Rattlesnakes cannot spit venom, but the impact of a strike against an object can squeeze the venom gland, located in the roof of the mouth, and venom may be squirted. This can happen when a rattler strikes the end of a stick pointed at it, or the wire mesh of a snake trap. The venom is released involuntarily if sufficient pressure is exerted, as occurs when venom is artificially “milked” from live snakes. Such venom is dangerous only if it gets into an open wound. Always wear protective clothing when handling rattlesnakes.
Female rattlesnakes are ovoviviparous. That is, they produce eggs that are retained, grow, and hatch internally. The young of most species of rattlesnakes are 6 to 8 inches (15 to 20 cm) when born. They are born with a single rattle or button, fangs, and venom. They can strike within minutes, but being so small, they are not very dangerous. Average broods consist of 5 to 12 young, but sometimes twice as many may be produced.
The breeding season lasts about 2 months in the spring when the snakes emerge from hibernation. Sperm is thought to survive in the female as long as a year. During summer, pregnant females usually do not feed, so few are ever captured that contain eggs about to hatch. The young are born in the fall. Most rattlesnakes are mature in 3 years, but may require more time in northerly areas. Rattle-snakes may not produce young every year.
The sex of a rattlesnake is not easy to determine. Even though the tail of the rattlesnake (the distance between the vent and the rattles) is quite short, it is much longer in males than in females of the same size. The paired hemipenises of male snakes are not visible except during mating, when one of these paired hollow organs is turned inside out and extruded from the cloaca. If both are extruded artificially, they appear like two forked, stumpy legs.
Snakes never close their eyes, since they have no eyelids. They are deaf, but can detect vibrations. They have a good sense of smell and vision, and their forked tongues transport microscopic particles from the environment to sensory cells in pits at the roof of the mouth. A rattlesnake uses these pits to track prey it has struck and to gather information about its environment.
Snakes have a large number of ribs and vertebrae with ball-and-socket joints. Each rib is joined to one of the scales on the snake’s underside. The snake accomplishes its smooth flowing glide by hooking the ground with its scales, which are then given a backward push from the ribs. Rattlesnakes often look much larger when seen live than after they have been killed. This happens because their right lung extends almost the full length of the tubular body, and when the snakes inhale they can appear much fatter and more threatening. The expulsion of the air can produce a hiss.
Rattlesnakes, like other snakes, periodically shed their skin. When the new skin underneath is formed, the snake rubs its snout against a stone, twig, or rough surface until a hole is worn through. After it works its head free, the snake contracts its muscles rhythmically, pushing, pulling, and rubbing, until it can crawl out of the old skin, which peels off like an inverted stocking. Each molt produces a new rattle. Some rattles usually break off from older snakes. Even if no rattles have been lost, they do not indicate exact age because several rattles may be produced in one season.
Even though the optimum temperature for rattlesnakes is around 77oto 89o F (25o to 32o C), the greatest period of activity is spring, when they come out of hibernation and are seeking food. If lizards are active, be alert for rattlesnakes. The activity period for rattlers can vary from about 10 months or so in warm southern regions to perhaps less than 5 months in the north and at high elevations. Depending upon availability of good, dry denning sites below the frost line, rattlesnakes may hibernate alone or in small numbers. However, sometimes they den in large groups of several hundred in abandoned prairie dog burrows or rock caverns, where they lie torpid in groups or “balls.” All dens must be deep enough so the temperature is not affected by occasional warm days. If not, the snakes might emerge too early in spring only to become sluggish and vulnerable should the weather again turn cold. Since snakes are cold-blooded animals and their body temperature is altered by air temperature, refrigeration makes them sluggish and easy to handle for displaying.
Rattlesnakes usually see humans before humans see them, or they detect soil vibrations made by walking. They coil for protection, but they can strike only from a third to a half of their body length. Rattlers rely on surprise to strike prey. Once a prey has been struck, but not killed, it is unlikely that it will be struck again. Experienced rodents and dogs can evade rattlesnake strikes.
Rattlesnakes may appear quite aggressive if exposed to warm sunshine. Since they have no effective cooling mechanism, they may die from heat stroke if kept in the sun on a hot day much longer than 15 or 20 minutes.
If a rattlesnake has just been killed by cutting off its head, it can still bare its fangs and bite. The heat sensory pits will still be functioning, and the warmth of a hand will activate the striking reflex. The head cannot strike, but it can bite and inflict venom. The reflex no longer exists after a few minutes, or as long as an hour or more if it is cools, as rigor mortis sets in.
Rattlesnakes occur only in North and South America and range from sea level to perhaps 11,000 feet (over 3,000 m) in California and 14,000 feet (4,000 m) in Mexico, although they are not abundant at the higher elevations. They are found throughout the Great Plains region and most of the United States, from deserts to dense forests and from sea level to fairly high mountains. They need good cover so they can retreat from the sun. Rattlers are common in rough terrain and wherever rodents are abundant.
Young or small species of rodents comprise the bulk of the food supply for most rattlesnakes. Larger rattlers may capture and consume squirrels, prairie dogs, wood rats, cottontails, and young jackrabbits. Occasionally, even small carnivores like weasels and skunks are taken. Ground-nesting birds and bird eggs can also make up an appreciable amount of the diet of some rattlers. Lizards are frequently taken by rattlers, especially in the Southwest. The smaller species of rattlesnakes and young rattlesnakes regularly feed on lizards and amphibians.
Rattlesnakes consume about 40% of their own body weight each year. Many prey are killed but not eaten by rattlesnakes because they are too large or cannot be tracked after being struck. One male rattler captured in the field had consumed 123% of its weight, but young rattlers frequently die due to lack of food. Domestically raised rattlesnakes will survive when fed only once a year, but in the field, snakes usually feed more than once, depending on the size of prey consumed. A snake may kill several prey, one after another, and of different species. When rodents and rabbits are struck, the prey is immediately released. The snake then uses its tongue to track the prey to where it has died.
Digestion is quite slow and usually no bones remain in the feces, called “scats.” Hair, feathers, and sometimes teeth, however, can usually be identified in scats. Rattlesnakes use very little energy except when active, and they probably are active for less than 10% of their lives. They are not very active unless food is scarce. They store much fat in their bodies, which can last them for long periods.
Walter E. Howard. Professor Emeritus of Wildlife Biology and Vertebrate Ecology. Department of Wildlife, Fisheries and Conservation Biology. University of California Davis, California 95616