It bears repeating that reptiles potentially dangerous to man account for only a small proportion of Nevada’s herpetofauna. Of the twenty-five snake species found here, only the six rattlesnakes pose any possible threat. Similarly, of twenty lizard species native to Nevada, only the Gila monster is venomous. The vast majority of reptiles found in Nevada (and elsewhere) are completely harmless!
RATTLESNAKES comprise only about 30 of the more than 2,500 species of snake known worldwide. Occurring naturally only in the western hemisphere, these serpents occupy a variety of habitats from southern Canada to central South America at elevations ranging from sea level to over 10,000 feet (0 to 3,050 meters). They are thought to have evolved on Mexico’s central plateau during late Miocene or early Pliocene time something on the order of five to ten million years ago and subsequently to have radiated into their presently more widespread habitats. Some 64 different rattlesnake varieties (species and subspecies) are generally recognized, of which about 30 occur in the United States. Arizona has the most diverse, stateside assemblage with 17. By comparison, Nevada’s six seem a rather scant array.
These snakes are born alive and may live as long as twenty years. Most species reach adulthood at about three years. They typically mate in spring after emerging from hibernation. Most young are born in late summer or early fall; brood sizes commonly average between four and twelve. Typical rattlesnake prey includes small mammals, birds and lizards. The snakes are, in turn, preyed upon by various mammals (coyotes, badgers, etc.) birds of prey and, of course, man.
Like other reptiles, rattlesnakes are “cold-blooded” or ectothermic, relying essentially on outside heat sources rather than an internal metabolism to maintain their body temperature. They typically function most effectively at temperatures between about 70 and 90 degrees Fahrenheit (21 to 32 degrees Celsius). In Nevada, rattlesnakes are most active during spring and fall. Evening, night, or morning hours are favored activity periods; mid-day is generally shunned. These snakes retreat underground to avoid temperature extremes. Overheating caused by exposure to conditions above about 95 Fahrenheit (35 Celsius) will prove fatal to most species in just a few minutes. At temperatures much below 60 Fahrenheit (16 Celsius) rattlesnakes become sluggish and less able to secure food or avoid predators than when they are operating in their preferred temperature range. When fall’s average daytime temperatures approach the snake’s lower limits they “den up,” either singly or in multiples, and remain underground until spring.
These snakes’ most characteristic feature, their rattle, probably developed as a means to alert the other animals sharing the snake’s habitat to the snake’s presence, and thus protect it from being stepped on. The American colonies’ motto “Don’t Tread On Me,” displayed on the rattlesnake-depicting Navy Jack and Gadsden flags during the American Revolution, accurately sums up the rattle’s purpose and, no doubt, the snakes’ fondest wish: “Don’t step on me.” It is a warning well heeded.
The rattle consists of interlocking segments of dry skin. A new segment is added each time a snake sheds -- about two or three times a year in a healthy, growing animal. Because multiple segments are annually added, and because the rattle is fragile and frequently broken, it is not possible to tell the age of a snake by counting its rattle segments. Another persistent and common myth is that rattlesnakes always rattle before they strike. Don’t depend on it! A rattlesnake may rattle if given the opportunity, but nothing requires it to do so. A snake’s attempts to warn may also prove ineffective. A rattle on a juvenile snake may be too small or a rattle on a larger snake may be too badly damaged to be clearly audible.
Rattlesnakes are “pit vipers,” so named because of the paired openings (loreal pits) located on the sides of their heads, between and below the eyes and nostrils. These pits are infra-red sensing devices that enable detection of differences in surface heat (such as the temperature difference between a mouse and its background) and thereby allow the snakes to sense, locate, and accurately strike prey under low-light conditions. The combined presence of loreal pits and hinged fangs (many venomous snakes have rigidly fixed fangs) makes rattlesnakes among the most evolutionarily advanced serpents on the globe.
All rattlesnakes are venomous. Their venom developed primarily to assist the snakes in catching prey; secondarily, it provides a defense. Because rattlesnakes are generally heavy-bodied, slow-moving serpents, incapable of either quickly chasing down their prey or killing it by constriction, they require another means to make themselves competitive predators. Their venom provides it. Produced like saliva and consisting mostly of a complex mix of enzymes, rattlesnake venom kills by chemically interrupting normal cellular processes in a bitten victim. Venom also facilitates tissue breakdown and thus aids in digestion of prey.
As a defensive response, snakebite is employed to discourage further contact by a would-be predator or a careless interloper. Rattlesnakes typically bite by striking from a coiled position. They can, however, simply turn and bite if stepped on or picked up. Under most circumstances the effective strike range is from one third to one half the snake’s length. Rattlesnakes cannot jump, but particularly savage strikes may jerk a snake’s entire body in the general direction of the strike and leave the impression that a jump has occurred.
A rattlesnake can control the amount of venom injected during a bite and will adjust the quantity delivered to meet a given situation. About 40% of initial, defensive bites are “dry,” with little or no venom being injected. The greater the threat perceived by the snake, however, the more likely a larger dose will be discharged. It bears pointing out that a rattlesnake must learn to control the injection; thus a young snake, inexperienced in subduing prey and defending itself, may be more likely to deliver a larger dose of venom than would a more experienced animal under identical circumstances. As a result, a bite from a young snake may produce disproportionately serious results (variables affecting snakebite severity include location of the bite, venom toxicity, and amount injected), and this has led to the common adage that small rattlesnakes are more “deadly” than large ones.
A rattlesnake’s venom delivery apparatus consists primarily of a pair of curved fangs that lie against the roof of the snake’s mouth when it is closed. But as the mouth is opened, the fangs rotate down about 90 degrees to a position from which they can stab an object being struck. The fangs are hollow like hypodermic needles and, during a bite, venom is injected through them from paired venom glands lying along the back of the snake’s head. These glands help give the rattlesnake’s head its characteristic triangular shape when viewed from above.
Although individual temperaments certainly do exist among rattlesnakes, most are generally timid and inoffensive creatures, seeking no contact with man and wanting only to be left undisturbed. But because their bites are potentially lethal to humans, these snakes are all too often simply destroyed on sight. And while the danger of snakebite is not to be taken lightly, the chance of being bitten is low and the probability that a bite will be fatal is even lower. Recent statistics show that about 8,000 bites are inflicted by all venomous snakes (rattlesnakes, water moccasins, copperheads, and coral snakes) in the United States each year, with 9 to 14 resulting fatalities. Approximately seven rattlesnake bites per year are reported from Nevada. Most snakebites occur to men between the ages of 20 and 40 years. A commonly cited cause is an unhealthy mixture of alcohol and testosterone. Most at risk from snakebite are young children and older adults particularly those with heart or respiratory ailments.
When snakebite does occur, the best course of action is to simply keep the victim quiet and deliver him to professional medical attention as quickly as possible. Use of tourniquets, ice, electric shock and the old “cut and suck” strategies are now generally discouraged. All too often, more harm is done by well-meaning but ill-experienced application of treatment than by the bite itself. Knowing what type of snake delivered the bite is key to rapid, effective treatment because venom toxicities differ greatly among the diverse array of rattlesnake species and subspecies. The best advice for dealing with snakebite is: Don’t get bitten. And of course, the easiest way to avoid a bite is to avoid the snakes. A little common sense in rattlesnake country, like watching where you put your feet and hands, is generally all it takes.
Alex L. Heindl
Curator of Herpetology
Marjorie Barrick Museum of Natural History
University of Nevada, Las Vegas
Las Vegas, Nevada 89154-4009Heindla@unlv.nevada.edu