Snakes are reptiles that have no limbs, voice, external ears, or eyelids, only one functional lung, and a long, slender body. About 2,900 snake species are known to exist, most living in the tropics. Their skin is covered with scales. They have good eyesight, and they continually taste the surrounding air with their tongues. Though they lack any voice, they are capable of hissing. Most live on the ground, but some are arboreal or aquatic, and some are burrowers. They move by muscular contraction, aided by elongated scales on their abdomen. They focus 70% of their mostly solitary existence on tracking, capturing, and digesting their living prey. The construction of their jaws and bodies enables them to swallow large prey whole. Because they are cold-blooded, a single meal can often sustain them for weeks. Mating and laying eggs or bearing live young are brief seasonal activities. About one-tenth of snake species are venomous; some can kill humans with their bite. Others kill their prey by constriction or simply ingesting. Species range from less than 5 in. to over 30 ft long. Snakes grow continuously throughout their lives, shedding their outgrown skin at each growth increment. They are found worldwide, but few species are found on islands or in regions with long winters.
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