Why do snakes shed their skin?

Do you ever wish you could just crawl out of your own skin? Snakes are some of the few creatures on Earth that actually can. Dozens of times throughout its life, a snake slithers out of its old skin in a process called "ecdysis," leaving behind papery sheds delicately imprinted with the unique pattern of its scales.

It's not unusual to shed skin; humans do it, too. "But unlike us, whose skin sheds off in little flakes, snakes produce a whole new layer of skin, and the old layer of skin falls off in one big slough," said Jason Dallas, a postdoctoral researcher who studies bacterial-fungal interactions in snakes and amphibians at Middle Tennessee State University.

But why do snakes shed their skin, and why does it happen in one go?

Snake skin is composed of two main layers. The inner, softer layer — called the dermis — contains the pigments that give snakes their intricate colors and patterns. The dermis is covered in the epidermis, a tougher layer of semitransparent keratin scales.

The outer layer of human skin also contains keratin, but the structure of our skin allows it to flake off in individual scales, which are almost invisible to the naked eye. In snakes, the exterior keratinous layer forms a unit, which creates a sheath-like covering that's bound to the snake's more delicate dermis below.

Related: What are the world's deadliest snakes?

The trade-off of this hardy external layer is that it doesn't expand; it remains relatively inflexible and constricts the snake as it grows. This is one of the primary reasons snakes shed their skin: to create more room. Snakes grow more quickly when they are younger, so they also shed more in youth.

In fact, snakes' first shed happens "within a few days of hatching or being born," Daniel Kane, senior reptile keeper at the London Zoo, told Live Science in an email. By adulthood, snakes typically shed around three or four times a year, Kane said.

When a snake is almost ready to shed, it starts to develop a second fresh keratin layer atop the dermis and below the original keratin sheath. It also secretes a fluid to help loosen the old layer from the new one. As this process unfolds, the snake typically finds a rough spot to rub its head against. This creates a slit in the old skin, through which the snake can then start to slither out.

With the help of continued rubbing and muscle spasms, the snake eventually loosens its entire skin and escapes the wrinkled relic with a set of glossy new scales. A fresh shed may be up to 20% longer than the original snake, as this recently lubricated skin is "quite stretchy before it dries out and becomes brittle," Kane explained.

Once the skin is shed, the snake discards it and slithers away. Although snakes don't typically eat their skins, some other reptiles do, such as lizards and geckos, to regain lost nutrients.

Shedding is associated with big life events, such as a snake's emergence from a long hibernation, or reproduction. "They typically will shed right before they either lay their eggs or give live birth, and then they may shed again after that period," Dallas told Live Science. "So it typically is synced up with life history events in adults."

Interestingly, snakes also use shedding to deal with unwanted parasites, infections and disease, Dallas said. One common example is snake fungal disease, which poses a significant threat to snakes worldwide.

"The pathogen is able to colonize deeper tissues as well as internal organs," Donald Walker, an assistant professor of biology at Middle Tennessee State University and principal investigator of the lab where Dallas works, told Live Science. "It's thought to be able to get the eyes, and maybe even the brain, of the animal. … But it starts by colonizing the skin and feeding on keratin and lipids in snake scales." Shedding could be lifesaving in cases like this. Yet the ability to shed skin also incurs a cost. Not only is it an energy-intensive process, but the trailing skin may hinder the snake's hunting prowess by disturbing its vision. Snake eyes appear clear, but they're covered by a single, bubble-like, keratin scale.

"The transparent scale, known as the 'spectacle,' protects the snake's eyes from day-to-day scratches and abrasions as it moves headfirst through the world," Kane said. "When the snake is in the process of sloughing off its skin, vision is impaired by the cloudiness caused by the secretions used to separate the old from the new layers."

A less-mobile, sight-impaired shedding snake is typically more vulnerable to predators, too. So ideally, a snake should not be shedding too frequently.

Disease might complicate snakes' natural shedding schedules and turn this survival tactic against them — particularly in species such as the eastern massasauga rattlesnake (Sistrurus catenatus) in the United States, Dallas said.

"These snakes are very susceptible to the fungus that can obscure their eyes and their mouth, but also increased shedding rates can increase their risk of predation," Dallas said. "And this has caused relatively large population declines in some populations of the species."

Overwhelmingly, however, a snake's shedding ability is vital to its survival. In the wild, discarded skins can also help scientists and conservationists protect these reptiles, by offering telltale genetic fingerprints and other clues that enable them to detect threats to snake health, study population numbers and species biodiversity.

"Snake skin often contains a faint version of the pattern of the snake and the specific number and arrangement of scales, meaning it's often possible to identify a species based on a shed skin," Kane said.

Of all animals, snakes produce the most instantly recognizable shed specimens in the wild. But they share their shedding ability with all other reptiles, which, unlike their serpentine cousins, mostly release their skins in fragments. There are some exceptions, however. For example, lizards from the Abronia genus are small, dragon-like creatures that wriggle out of their skins and leave behind a perfectly intact replica, legs and all.

Snake quiz: How much do you know about the slithering reptiles?