Snakes: Facts about one of the most iconic creatures is animal hiss-tory

Snakes are exclusively carnivorous, meaning that they only eat other animals — and in some cases, eggs. Their diets range widely. Larger snakes like pythons and anacondas can swallow enormous prey, including deer and alligators. In rare cases, pythons have eaten humans.

Snakes use a variety of techniques to capture their prey. Many are ambush predators, lying in wait to strike.

Some use venom, a toxic type of saliva injected using sharp fangs. The venom can affect various parts of the prey’s body, subduing or killing it so that it can be swallowed. Others kill by constriction, wrapping their bodies around their prey and squeezing until it is unconscious or dead.

Some species use their tails to entice prey — moving the end mimics the action of a worm or insect. The spider-tailed horned viper has a spider-shaped appendage to attract birds.

Tiny blind snakes and thread snakes simply slither through colonies of ants and termites, gobbling up their eggs, larvae and pupae. And a number of species only eat bird eggs, swallowing them whole, then using structures on their spines to crush them so they can be digested.

While popular myth suggests that snakes can unhinge their jaws in order to swallow objects larger than the width of their body, they actually have a specialized system of bones and ligaments in their jaws that allows their mouths to expand.

Most snakes reproduce sexually. A male and a female mate — the male inserts one of his two penises (called hemipenes) into the female's cloaca (an orifice for waste and reproduction), using sperm to fertilize her eggs. Some species mate individually while others engage in "mating balls", where large numbers of individuals come together to mate at the same time.

Some species are able to reproduce without mating in a phenomenon known as parthenogenesis, or "virgin births", although this is rare..

Some snake species lay eggs, while others give birth to live young. Species that give birth to live young produce eggs as well, but the eggs hatch inside the mother.

Most egg-laying species abandon their eggs after they are laid, leaving the young snakes to fend for themselves. However some species, such as pythons and cobras, incubate the eggs and guard them until they hatch. Snakes that incubate their eggs or give birth to live young may guard their offspring for a brief period, usually until they shed their skins for the first time.

The shells of snake eggs are soft and rubbery, unlike those of birds. The snakes incubating inside use special "egg teeth" to cut through the shell when they are ready to emerge. Once they hatch, they shed these teeth and are ready to hunt on their own.

Not all snake species produce venom. Those that are venomous produce a form of toxic saliva from glands in their jaws. It is injected using fangs in either the front or rear of the jaw. Some species have fangs that function like hypodermic needles, injecting the venom directly. Most, however, have grooves in their teeth that channel the venom into the wounds created by the fangs.

Snake venom is thought to have evolved mostly as a means of capturing prey, though it is also a useful defense mechanism. Snake venom functions in three main ways — damaging cells, disrupting blood clotting and impairing the nervous system.

While it is widely believed that venomous snakes have vertical, slit-like pupils and non-venomous snakes have round pupils, there are plenty of exceptions and the shape of the pupil is not a reliable indicator as to the danger posed by a given snake. In fact, most snake pupils appear round in low light due to their dilation.

Up to 140,000 people die from snake bites every year, and many more suffer permanent injuries, such as the amputation of limbs. Antivenom treatments have been developed for many snake species. These are produced by injecting small amounts of the snakes’ venom into animals and then harvesting and purifying the antibodies that are produced.

While snakebites can be deadly, snake venom has also led to important developments in the treatment of diseases such as cancer and Alzheimer’s disease.

Snakes evolved from lizards. It is unclear whether their lizard ancestors burrowed underground or preferred aquatic environments. In any case, they certainly had legs.

Scientists speculate that the loss of limbs may have been beneficial to early snakes if they evolved underground and needed to navigate narrow burrows, or if they evolved in the water — a body shape without limbs may have been more streamlined.

The first clearly identifiable snake fossils date to the Middle Jurassic (174.1 to 163.5 million years ago). Though they are poorly preserved, it is believed that they likely had hindlimbs.

Primitive snakes fossils dating to the Early Cretaceous (145 to 100.5 million years ago) feature tiny limbs as well, indicating that legs persisted for quite some time during the early evolution of snakes.

These remnant limbs may have still served a purpose. The back legs could have helped male snakes to grasp their partners during mating. In fact, some modern snakes have pelvic spurs — small protrusions that evolved from legs and help males to stimulate and grasp females during mating. They even retain portions of the pelvis, to which the hind legs would have attached. However, more "advanced" snakes have lost these features entirely.