Humans are fascinated by snakes, arguably
more so than any other kind of animal. This
fascination may be based on fear (some snakes are
undeniably dangerous), religious fervor (snakes
figure, for good or ill, in nearly all world mythologies
and religions), or curiosity (how does an animal
move without limbs?). Regardless, this fascination
has led to myths and an inclination to
attribute mystical powers or malevolent intentions
to these bizarre (by human standards) creatures.
Snakes are fascinating for purely scientific reasons
as well. Based on diversity and total numbers,
they are very successful. They have adapted
to amazingly varied lifestyles in spite (or maybe
because) of many unique features, the most obvious
of which is the combination of extreme body
elongation and lack of limbs. These, in turn, help
explain other features. For example, long, slender
bodies limit the size of the body cavity, requiring
that paired internal organs are offset (placed one
in front of the other) or that one organ is disproportionately
elongated and its counterpart reduced
or absent. Similarly, all snakes are obligate
carnivores (meat-eaters; that is, they eat other animals).
Plants are difficult to digest, and the simple,
straight digestive tracts that can be accommodated
in a snake's body are not long enough to
provide enough time in passage for vegetable
matter to be adequately broken down. However,
these characteristics that appear to define snakes
are shared with many other squamate reptiles,
whereas those that are unique to snakes (number
of body vertebrae; modification, reduction, or absence
of some skeletal features; location of the
ophthalmic nerve; eyes without ciliary bodies to
facilitate focusing) are comparable to differences
between other squamate reptiles.
Relationships Among Squamate Reptiles
For many years, squamate reptiles were divided into three suborders, Sauria (lizards), Serpentes (snakes), and Amphisbaenia (odd, mostly limbless burrowers), but recent studies of relationships indicate that snakes and amphisbaenians are cladistically nested within lizards; in other words, instead of being "cousins" of lizards, they are siblings. Consequently, the traditional suborders no longer reflect true relationships. They are retained for the sake of convenience by some authors, but only because they reflect clades within Squamata. As suborders, they have no formal taxonomic status. Several distinct groups of snakes are recognized. The Microstomata (small-mouthed snakes) include small burrowing forms with blunt heads and tails.Amongother snakes, two primitive families (uropeltids and aniliids) are distinct from macrostomatan (large-mouthed) forms, but many relationships within the latter remain unresolved.
Body Forms and Habitats
Evidence suggests that snakes evolved as burrowers, but snakes in many families have subsequently and successfully exploited various habitats. Fossorial snakes (burrowers) are small, have conical heads (sometimes with modified snouts that help them dig), reduced scales that often overlap and reinforce sutures between skull bones, tiny eyes, and mouths located underneath the snout to avoid ingesting soil while burrowing. Many have spines on their tails for use as anchors when using the head to dig. Arboreal snakes live in trees, and often are very slender, although the center of gravity may have shifted posteriorly so the rear of the body or tail can be used as an anchor while extending the head and anterior body over open spaces. To facilitate extension, many have laterally compressed bodies and enlarged scales along the back and belly; this allows them to form a structure much like an I-beam for support. Tails are long and may be prehensile (capable of grasping branches). Eyes are often large, as vision is more important than chemical cues when searching for prey in trees. Snakes that inhabit loose ground litter in prairies or forests are small and often have enlarged scales on their snouts, with which they root for food. Like burrowers, body scales are usually smooth to reduce friction and ease passage through tight spaces. Aquatic snakes often have eyes and nostrils on the tops of their heads, and nostrils may be equipped with valves. Many are heavy-bodied and laterally compressed to increase the surface area with which they push against water while swimming. Scales are generally rough to increase friction. Some sea snakes have fins supported by extensions of their vertebrae. Most aquatic snakes exhibit countershading (dark above, light underneath), reducing their visibility to both prey and potential predators.
Most snakes move by means of rhythmic waves of muscle contractions that cause the body to undulate laterally. Very heavy-bodied snakes often rely on rectilinear propulsion. Instead of muscles on the sides of the body working alternately, they work in concert, contracting and relaxing while drawing the body forward in a straight line. Enlarged belly scales or friction with the surrounding substrate (water in aquatic forms) prevents backsliding. Concertina locomotion and thrust creeping are used by burrowers. In the former, snakes first wedge the anterior part of the body and draw the posterior portion forward, then wedge the posterior region while extending the head and anterior body. Thrust creeping is very similar, except the spiked tail is used as a posterior anchor instead of a body loop. The most unusual form of snake locomotion is sidewinding, which is employed on loose or slick substrates. Loops of the body are elevated and thrown laterally to serve as the contact points while the intervening areas are raised. The advantage of this method is that force on the substrate is directly downward, which reduces slippage. Regardless of method, snakes are not very fast. The world's fastest snakes, large, slender mambas (Dendroaspis) can achieve top speeds on ideal substrates of about 12 kilometers per hour (about 7.5 miles per hour). Humans, in contrast, can sprint at speeds up to about thirty-five kilometers per hour (over twenty miles per hour).
Except for many arboreal and some actively foraging terrestrial forms, most snakes rely primarily on chemical cues to monitor their environment for food, prospective mates, or danger. Limbless forms are close to the ground and have a limited scope of vision (in fact, most are nearsighted). Snakes also lack external and middle ears, and are essentially deaf to airborne sounds, although they are very sensitive to vibrations of the substrate. Thus, the importance of smell and taste is not surprising. Both smell and taste are very well developed in almost all forms, and are further enhanced by vomeronasal systems. The moist, forked tongue collects chemical molecules by flicking in the air or licking a substrate, and delivers these to paired sensory pits in the roof of the mouth. The importance of chemical senses is reinforced by the role of pheromones in snake reproductive ecology and observations that many snakes will follow a convoluted scent trail rather than rely on vision to directly approach a mate or food. Specialized infrared (heat) receptors occur in several boid snakes and in pit vipers (Viperidae). In boids, receptors are in pits on the lips or snout, whereas pit vipers have a single pit between the nostril and eye on each side of the head. These sensors apparently are able to generate infrared images that are superimposed on visual images. Although obviously valuable to species that consume mammals or birds, which generate body heat, infrared imaging works equally well for species that eat frogs or salamanders. Amphibians have moist skins and lose evaporative heat to the environment, appearing as distinct "cold" spots in thermal images.
Killing and Consuming Prey
All snakes eat animal food. Some are know to scavenge or eat dead animals, but most hunt and kill live prey. Venom, often prey specific, is used by some to immobilize and kill especially large and dangerous prey. Constrictors use teeth to grab prey, then rapidly loop one or more coils of their body around it, tightening with each breath until the animal dies of stress-induced cardiac arrest and suffocation (constrictors do not crush their food). Afew speciesmayuse loops of their bodies to press prey against the substrate. Some venomous forms constrict or compress prey while envenomating it; others quickly inject venom and then follow the prey's scent trail, ingesting it only after it dies. Other snakes eat animals with limited defensive capabilities, and prey is swallowed alive. Snake teeth vary in number and size, but species that hunt prey that is hard to hold (slimy fish or snails that must be pulled from their shells) generally have the longest teeth. Bird-eaters often have long teeth to grasp the body of their prey and avoid a mouth full of feathers while the prospective meal escapes. Some snakes have upper jaw teeth of approximately equal size, rear-fanged species have enlarged posterior teeth that may be grooved, and others (cobras and their kin) have enlarged anterior fangs. Vipers and pit vipers have very large fangs on small maxillary bones that are rotated to erect the fang. Most snakes can consume food much larger in diameter than their bodies by using a kinetic skull, a protrusible glottis (opening to the lungs) that is used like a snorkel when the mouth and throat are filled with food, and an elastic body wall not enclosed by ribs. Using teeth as anchors, upper and lower jaws on alternating sides separately "walk" over the prey until it is ingested. Rhythmic muscular contractions of the body wall then move the meal through the digestive tract. The ability to consume large prey reduces the need for frequent meals, allowing snakes time to rest and conserve energy before eating again. Many snakes forage actively for food, rooting through debris or scanning their environment with one or more sensory systems. Some species, however, lie motionless, often well camouflaged, along a game trail, patiently waiting to ambush a meal. Because this sit-and-wait strategy does not require constant movement (other than the strike, which may be lightning quick), snakes that employ this method are frequently heavy-bodied and capable of eating the largest prey items relative to their body sizes.
Snakes in Need of Conservation Snakes, because of the fear and revulsion they engender in many people, are subject to greater persecution than almost any other kind of animal. They often are actively hunted and, in many parts of the world, quickly killed whenever encountered. Nevertheless, habitat destruction and alteration are responsible formore declining snake populations than any other single factor.Manyspecies are exploited as food, especially in parts of eastern Asia, and others are killed for body parts thought to have medicinal or aphrodisiac qualities. Introduced exotic species exact a toll.Manyisland populations of diurnally active terrestrial snakes have been extirpated by the mongoose, an effective predator introduced to control introduced rat populations. However, rats are nocturnal and mongooses active by day; consequently they have a much greater impact on ground-dwelling snakes than on rats. The leather industry is responsible for the deaths of hundreds of thousands of giant snakes each year. In addition, many wild snakes die each year as a consequence of being caught for the pet trade, much of it illegal. Many species become roadkills as they migrate. Only a few species are formally protected in at least some parts of their ranges, and many others may be nearing extinction in spite of increasingly frequent efforts to conserve them and their habitats.
Superfamilies: Typhlopoidea, Booidea, Colubroidea
Families: Fifteen recognized, including Typhlopidae (blind snakes or worm snakes); Leptotyphlopidae (slender blind snakes); Aniliidae; Xenopeltidae (sunbeam snakes); Uropeltidae (shieldtail snakes); Boidae (pythons, boas, and wood snakes); Acrochordidae (wart snakes); Colubridae, with subfamilies Dasypeltinae (egg-eating snakes), Pareinae, Dipsadinae, Xenodontinae, Homalopsinae, and Colubrinae; Viperidae (vipers, rattlesnakes, moccasins, and relatives), with subfamilies Viperinae (OldWorld vipers) and Crotalinae (pit vipers); Elapidae (cobras, mambas, coral snakes, and relatives); Hydrophiidae (sea snakes)
Genus and species: 450 genera, nearly three thousand species
Geographical location: Worldwide
Habitat: Terrestrial, aquatic, and marine habitats, except at very high latitudes
Gestational period: Some snakes lay eggs, whereas others bear live young; clutch sizes range from one or two (tiny threadsnakes and some sea snakes) to one hundred or more (large boas and pythons), and the incubation period varies according to temperature
Life span: Varies by species
Special anatomy: No legs
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