Did you know that some animals are pretty much designed to be able to re-enact the plot of Gerald's Game? You can't un-know it now. Learn how "fragile skin" allows animals to flay themselves alive.
For some animals, life is a perpetual torture porn movie. The only way they can stay alive is to do horrible things to themselves. They drop limbs and tails, but that isn't always enough. Sometimes, "enough" means ripping off one's skin like a candy bar wrapper. Scientists first noticed the "fragile skin" phenomenon when studying geckos. More accurately, they noticed it when attempting to study geckos. They couldn't capture an intact specimen. No matter how carefully they captured the gecko, huge patches of skin from its back, belly, and head, came off. The four-clawed gecko became so notorious for it that it earned the name Gehyra mutilata.
This adaptation isn't confined to geckos, or even to reptiles. Many species of mice can shed the skin from their tails, leaving people who grab them by the tail with nothing but a layer of dead skin and a sick feeling that will never entirely go away. African spiny mice can shed nearly every part of their skin, including - brace yourself - the skin over their ear holes. The leftover skin contracts very rapidly, to minimize the exposure of tissue, and the mouse regrows it all, right down to hair follicles and sweat glands.
The exact mechanical properties of fragile skin haven't been uncovered in mice, but scientists do know how geckos manage to shed their skin so easily. Skin consists of the epidermis, which is primarily a barrier against the outer world, and the dermis, which is composed of fat tissue and connective tissue, and holds the blood vessels, sweat glands, and hair follicles. The dermis itself is made up of two layers, a loose and spongy outer layer and a denser inner layer appropriately called the stratum compactum. We have only one layer to our stratum compactum - and more fool us. The gecko has two layers. The outer layer is incredibly fragile. It's only 1/50th as tough as the inner layer. The inner layer is almost like a second layer of skin, and is anchored very firmly to the body. The two layers slide over each other regularly.
Once the animal is grabbed the epidermis and the outer stratum compactum rip away, leaving the inner layer exposed. The patch of skin ripped off is minimized by weak points along the outer layer of skin. These act like lines of perforation in paper, letting patches of the skin get ripped away, while leaving the rest. The gecko's skin is basically lightly held on in patches, waiting for the day something tries to take a bite out of it. Then the real horror begins.