The Web of Life (98) Snakes

Snakes

There are over 3,400 species of snake in 500 genera in over 20 families. Snakes live on every continent except Antarctica and in the sea. Unlike lizards, snakes have made their way to many oceanic islands.

Snakes range greatly in size: from 10 cm burrowing thread snakes to 7+ meter pythons and boas.

Specialization is extensive. Arboreal snakes are long and thin. Many vipers, boas and pythons are heavy-set and relatively short. A diverse range between the ends of that spectrum exist. Female snakes are typically larger than males, though sexual dimorphism is not generally pronounced in snakes.

Anatomy

For all their diversity, snakes are a clearly recognizable group. They have a flexible body with no limbs. Their eyes have no eyelids. Snake eyes are protected by the same keratin sheet that covers the body.

Snakes differ from other reptiles by what they lack: sternum (breastbone) and forelimbs, along with shoulder girdle. There are other anatomical anomalies.

Internally, snakes are mostly a digestive tract. The stomach accounts for more than 1/3rd of body length in some snakes.

Like lizards and birds, snakes have no urinary bladder. Nitrogenous wasted is passed as semi-solid uric acid; not urea, as with mammals.

Most snakes have a single lung. In the snakes with 2 lungs, the left lung is invariably smaller: from 85% to as little as 1%.

A snake heart has 3 chambers. The right atria receives blood from the lung(s); the left from the rest of the body. Both atria pass their blood to the single ventricle for recirculation. Lacking a diaphragm, a snake’s heart can move around to accommodate ingestion of large prey.

The snake skull is an impressively flexible construction. With few exceptions, most skull bones are movably connected to each other, and only loosely attached to the braincase. This allows a snake to stretch and distort much of its head in various directions.

Snake jaws are constructed to create a cavernous opening. Snakes easily swallow something wider than they are.

Snake teeth are hinged on flexible ligaments, enabling them to lock their teeth in a backward-pointing position when prey is being swallowed, thus preventing dinner from struggling free.

Otherwise, snake dentition varies. Most snakes have 2 rows of teeth on the upper jaw, and one on the lower jaw(s). Some snakes’ lower jaws are not fused into a single jaw, so that there is a gap between the 2 lower jaws.

A snake doesn’t suffocate while swallowing a huge meal because the end of its windpipe is reinforced with cartilage rings. Further, the windpipe can be extended forward, along the floor of its mouth, so that breathing can continue unimpeded.

Because they have no breastbone, the ends of ribs can widely separate to let large prey pass into the stomach.

Poisonous snakes have serious fangs: either grooved, to guide venom delivery from glands on the top of the head; or hollow, like hypodermic needles, through which venom flows.

Skin

Snakes, like all reptiles, are covered in scales, which protect from abrasion and dehydration. The outermost layer is a continuous sheet of keratin, which is a tough fibrous protein.

Snakeskin, which is very flexible and elastic, is water impermeable. Hence, snakes cannot absorb moisture through their skins. But about 2/3rds of their water loss is through the skin, with the rest by excretion. Snake scales are no better at retaining water than other skin types.

Snakes grow throughout their entire lives. So, to accommodate growth and facilitate repair, snakes periodically shed their skin; a process termed ecdysis.

Snakes are the only vertebrate that completely molt their entire skin. Snakes molt 1 to 3 times a year, depending upon species. A snake typically does not eat for a couple weeks prior to molting.