The fact that snakes and lizards are closely related is reflected by the placement of the two groups within the same taxonomic categorythe reptilian order Squamata. The precise origin of snakes, however, remains uncertain. Snakes and lizards could have evolved as separate lineages of an ancient group, the Lepidosauria, or they could have evolved from a common lepidosaurian ancestor. It is most likely that snakes actually evolved from lizards, the earliest representatives of which are known from the fossil record to have existed some 230 million years ago during the Paleozoic/Mesozoic transition. The earliest unambiguous snake fossil appeared in the early Cretaceous, about 135 million years ago. Diversification of the snake group had occurred by the late Cretaceous/Paleocene transition (70-65 million years ago), and by the Miocene (22.5 million years ago) the dominant modern families, Colubridae, Elapidae and Viperidae, were present. Most biologists believe that snake ancestors were adapted to a secretive or semi-burrowing existence, since their descendants possess a number of characteristics suggestive of such an existencenamely, body elongation, modification of internal organs (including extreme elongation and/or loss of paired structure), loss of limbs and extreme reduction or loss of supporting shoulder and hip girdles, replacement of movable eyelids with a transparent cap over the eye, rearrangement and increased complexity of jaw and head muscles, and complete separation of the right and left halves of the lower jaw so that each half can be moved independently. Some recent fossil evidence suggests that the lizard ancestors of snakes may have been aquatic forms, probably related to the mosasaursa group of giant marine lizards that became extinct at the same time as the dinosaurs. Indeed, many of the features listed above could just as readily be aquatic adaptations. Whatever the original line of descent, the ability to capture prey through constriction and the use of venom were later developments. Today snakes, along with lizards, are the most numerous reptiles on earth; they can be found on all continents except Antarctica and they inhabit many oceans.
Despite the ready availability of information about almost any subject, venomous snakes and snakebite remain mysterious to most people. More than one recent survey has revealed that people are more afraid of snakes than they are of almost any other group of organisms; this probably reflects a relationship extending far back into human prehistory. Given the abundance and diversity of snakes in Texas and the increasing frequency with which Texans and visitors from elsewhere are exploring the outdoors, it is essential that accurate knowledge be available about the identification, distribution and biological characteristics of the state's venomous reptiles, as well as about the relative risks of snakebite, how to prevent it, and how to treat it should it occur.
The first major study of the incidence of venomous snakebite in Texas was reported in 1927 by Afranio do Amaral, then director of the Antivenom Institute of America. During the twelve-month period from July 1926 to June 1927, a total of 150 cases of snakebite envenomation (venom injection) were known to have occurred in Texas, 28 of them fatal. A subsequent survey by R. H. Hutchison showed that 163 cases of snakebite were reported in 1928, 9 of which resulted in death. John Werler of the Houston Zoo recorded 1,318 snakebites in Texas during the five-year period from 1949 to 1953; 18 of these were fatal.
Henry M. Parrish reported on 559 snakebite cases seen by Texas physicians in 1958 and 1959. Sufficient information was available about 461 of these cases to extract information. Rattlesnakes were involved in 47% of the 461 cases, copperheads were involved in 22%, cottonmouths in 7% and coral snakes in 1%; a full 23% of the cases involved unidentified venomous snakes. Bites were less frequent (or perhaps just less frequently reported) in the sparsely populated western third of the state, and were particularly high around larger cities (Austin, Beaumont, El Paso, Dallas/Fort Worth, Houston, Port Arthur and San Antonio). Ninety-seven percent of all snakebites reported in the survey occurred between April and November, when snakes are most active and when people are more likely to be outdoors. The largest proportion of bite victims (46%) were under 20 years of age, highlighting the importance of supervising young children closely when they are in areas where venomous snakes are known to occur, and of instructing older children and teenagers in matters of snakebite prevention.
Most recently, T G. Glass reported on 175 snakebite cases he personally treated in the San Antonio area from 1966 to 1975. The victims ranged in age from 18 months to 85 years, with more than twice as many males as females being bitten (121 versus 54). The snakes inflicting the bites were Western Diamondback Rattlesnakes (Crotalus atrox; 135, or 77%), Copperheads (Agkistrodon contortrix; 30, or 17%), Cottonmouths (Agkistrodon piscivorus; 5, or 3%) and Coral Snakes (Micrurus fulvius; 5, or 3%). Of those, 101 bites (75%) from Western Diamondbacks, 19 bites (63%) from Copperheads and 3 bites (60%) from Cottonmouths were treated surgically on the basis of the appearance of local symptoms such as swelling or tenderness. This level of surgical intervention is considered unnecessary by most medical personnel today. The removal of dead or infected tissue revealed obvious signs of intramuscular injection of venom in 69 (56%) of the cases. Bites occurred most often on the leg (33%), 28% of bites were on the foot or ankle, 20% were on the finger, and 15% were on the hand or arm. Fourteen patients developed blood-clotting problems, 16 were treated with antivenom, 13 developed serum sickness, and one 85-year-old woman died from complete defibrination (see glossary) of the plasma and a cerebral hemorrhage.
In general surveys of more than 1,300 snakebite cases from southern states during the 1960s and 1970s by L. H. S. Van Mierop and Henry Parrish and colleagues, 25% of the cases were "dry bites," meaning that fang marks were present, but there was little if any pain because little or no venom was injected, and no medical treatment was necessary. Mild envenomation with slight swelling and pain occurred in 39% of the cases, and required minimal medical treatment and usually no administration of antivenom. Moderate envenomation accompanied by pain, swelling, nausea and other symptoms of shock occurred in 22% of the cases, and required medical attention and the administration of antivenom. Finally, severe envenomation with heightened symptoms, including unconsciousness in some cases, occurred in 14% of the cases, which required hospitalization and treatment with high levels of antivenom.
Snakebites are relatively rare compared with accidents resulting from other outdoor-related activities, as shown by comparative data from the Bureau of Vital Statistics, Texas Department of Health (see p. 10). One aim of the 1978 edition of this book was to reduce the incidence of snakebite by educating people about the nature of venomous snakes and how to be better prepared to deal with such a contingency during outdoor activities. The subsequent threefold reduction in the yearly average rate of death from snakebite in Texas (2.7 versus 1.0), especially when compared with increases in death rates from automobiles (3,511 versus 3,698), boating accidents (57 versus 67) and firearm accidents (10 versus 171), may be attributed to the success of such educational efforts as well as to advances in medical treatment. Again, however, I urge every Texan to obtain a thorough working knowledge of the correct first-aid treatment for snakebite, so that proper action may be taken should it become necessary. Preventing a bite from happening is at least as important, and a necessary step toward attaining this goal is gathering knowledge about the habits, distribution and behavior of venomous snakes and how to identify such snakes. A further step is to understand the evolutionary adaptations of venomous snakes and their behavior patterns and, in a larger sense, to reintegrate humankind with the natural world. This book is intended as a small contribution toward meeting that goal.
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Precautions at Home
Statistics show that a large percentage of bites occur near the home. Although a few of these are inflicted upon small children playing in their own backyards, many are what physicians refer to as "illegitimate bites," resulting from people taking unnecessary or foolish risks with venomous snakes. Every year zoos, animal control officers and other wildlife agency personnel receive calls from distressed homeowners who have discovered rattlesnakes or copperheads beneath their house, in the garage or under trash piles. They ask for assistance in removing the snake and preventing a recurrence. Unfortunately, such incidents will only become more frequent as human populations grow, cities expand into snake habitats and greater numbers of people want to live closer to "natural" or "pristine" areas. Venomous snakes are a fact of life in Texas; encounters can be managed and minimized but not eliminated. Unfortunately, none of the commercially available "snake-proofing" devices, chemical or mechanical, have ever been demonstrated to be completely reliable.
Snakes, including the venomous kinds, occur around a home for two reasons: food and shelter. Snakes are found in or underneath objects either because those objects also attract their prey, such as rodents, or because the snakes are escaping inhospitable weather conditions. The attractions of a dwelling to a venomous snake can be minimized by moving the objects that attract rodents and other prey items, such as trash dumps, brush and woodpiles, and by constructing barns and livestock sheds as far away as possible from your home. Anticipate that overturned boats, trailers, tarps and similar objects may provide temporary shelter for a snake moving through the area. Snakes are adept at getting through seemingly impenetrable tiny openings, and this should be kept in mind when attempting to close off a basement, a detached garage, or a shed. Keep such areas as neat and tidy as possible, and remember that snakes seek out such areas for peace and quiet and so are likely to be tucked away somewhere instead of being obvious in the middle of the floor.
Charles M. Bogert, late curator of herpetology at the American Museum of Natural History, once suggested the use of a quarter-inch-mesh wire fence to keep snakes off residential property. This yard-high snakeproof fence is placed around the house in much the same manner as an ordinary picket fence, except that the bottom must be set about six inches down into the ground to prevent snakes from forcing their way beneath it. In addition, all gates must be provided with close-fitting sills on the bottoms and sides to ensure a completely tight enclosure. Experiments were done on fences of this kind to determine their effectiveness and to seek possible improvements in their construction. Copperheads and small rattlesnakes could not get over the vertically straight fence, but a six-foot rattlesnake used in the experiment was able to climb over, with the result that one important change was made: When the same fence was tilted outward at a 30-degree angle, not even the largest snake was able to reach the top. Although such fences are expensive and difficult to keep in good repair, they may be desirable under extreme circumstances. Electrical fences have been tried in certain situations, such as the invasion of the Pacific island of Guam by the Brown Tree Snake (Boiga irregularis), with some success. These fences are at least 2 feet (60 cm) high, with vegetation cleared away an equal distance from both sides. They are usually constructed of vinyl netting or similar material and are most effective if several electrical strands are embedded in each fence.