Snakes in U.S. zoos

Snakes are limbless reptiles of the suborder Serpentes, descended from lizards. About 3,900 species live on every continent except Antarctica. Most are non-venomous and kill prey by constriction or simply swallow it; venomous species inject modified saliva through fangs. Flexible, hinged jaws let them swallow large prey whole.

Most widely held

What makes a snake

Snakes are elongated, limbless reptiles that evolved from lizards; genomic analysis, anatomy and the fossil record all support that origin. Their skeleton is mostly skull, ribs and a spine of 200 to 400 vertebrae. A hinged jaw joint and a stretchy ligament at the chin let them swallow food much larger in proportion to their size, then go a long time between meals.

The Burmese python shows how extreme that can be: after a large meal its whole digestive system rebuilds itself, and its heart's main pumping chamber grows by roughly 40 percent. Pythons and boas still carry traces of hind limbs, visible as small claw-like spurs beside the vent.

Constrictors and their lookalikes

Pythons and boas are the giants of the group. The reticulated python is the longest living snake, and the green anaconda the heaviest. Most pythons lay eggs and coil around the clutch, while the boa constrictor and green anaconda give birth to live young, which is unusual among reptiles.

Two tree-dwellers are the classic lookalike pair. The green tree python of New Guinea and northern Australia and the emerald tree boa of South America are both bright green and rest with coils looped over a branch and the head tucked in the middle. They are unrelated, and people confuse them when they are seen outside their native ranges.

Smaller colubrids round out the group. The western hognose snake flattens its neck, hisses and bluff-strikes when threatened, then often rolls over and plays dead. The eastern indigo snake eats other snakes and is immune to the venom of North American rattlesnakes.

Venom versus constriction

Most snakes are non-venomous. Venomous species, about a quarter of all snakes, use modified saliva delivered through hollow or grooved fangs, and their venom serves mainly to kill prey rather than for defense. Venom evolved more than once, so venomous snakes are not a single family. The king cobra, the longest venomous snake, hunts mainly other snakes.

Snakes find prey largely by smell, flicking a forked tongue to carry scent to the vomeronasal organ. Pit vipers add heat-sensing pits between the nostril and eye.

Conservation picture

Many snakes are widespread, but some are in trouble. The Burmese python is Vulnerable on the IUCN Red List after losses to habitat clearance and over-harvesting, even as it spreads as an invasive species in Florida. The ball python is Near Threatened, largely because of collection for the pet trade. The Jamaican boa, found only on Jamaica, has declined since the late 19th century because of introduced mongooses, persecution and habitat loss.

Common questions

  • What is the longest snake in the world?

    The reticulated python is the longest living snake, at about 6.95 m (22.8 ft). The green anaconda is shorter, at about 5.21 m (17.1 ft), but is considered the heaviest snake on Earth at 97.5 kg (215 lb).

  • Are most snakes venomous?

    No. Most snake species are non-venomous; around a quarter of all species are identified as venomous. Non-venomous snakes typically subdue prey by constriction or swallow it alive.

  • What is the difference between a python and a boa?

    Both are constrictors that keep tiny vestigial hind limbs. Most pythons lay eggs and coil around them until they hatch, while boas such as the boa constrictor, green anaconda and emerald tree boa give birth to live young.

  • How do snakes smell?

    Snakes collect scent particles with their forked tongue and pass them to the vomeronasal, or Jacobson's, organ in the mouth. The fork gives them directional information about where a smell is coming from.

  • Why do snakes shed their skin?

    Shedding, called ecdysis, replaces old, worn skin and can be timed to mating cycles. A snake sheds the whole outer layer in one piece, and may do so four or five times a year; its eyes turn silvery just before.

  • Do snakes have eyelids?

    No. A snake's eyes are covered by transparent, permanently closed scales called spectacles, or brille.

  • Where are there no snakes?

    Snakes are absent from Antarctica and from islands such as Ireland, Iceland, Greenland and New Zealand, apart from rare sea snake visits to New Zealand's northern waters.

Sources

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