Nature is cruel. There is no getting round that. Some creatures inflict all manner of horrible torments on others in order to feed and reproduce, or even just to play.

Many species across the animal kingdom are cannibals, eating members of their own species for sustenance or dominance.

But there is one behaviour that is even more extreme than simple cannibalism. It sounds like something Hannibal Lecter would come up with, rather than something evolution would favour. Some animals will, on occasion, eat parts of their own bodies. This weird behaviour is known as "autocannibalism".

We asked our readers if they had ever heard about animals eating themselves.

It is better to lose a limb than to lose your life

"I don't think any animal would deliberately try to consume itself for sustenance.... kind of defeats the self-preservation part [of] desperate eating," says Selina Tick Konkin. "Lots of animals will however chew through their own legs or tail to free themselves from a trap."

This is true, and distressingly well-documented. Dogs, bears and apes are frequently found to have chewed through skin, muscle, tendons and bone, in order to remove a foot or leg that has been caught in a trap.

One unfortunate tiger in Tesso Nilo National Park in central Sumatra was photographed in 2007 with a front paw missing. Apparently it had chewed it off to escape a snare trap.

However, none of these are cases of autocannibalism. The animals involved did not eat their body parts, but merely removed them.

Extreme as these acts are, there is a clear logic to them. After all, it is better to lose a limb than to lose your life. However, other animals eat parts of their bodies for – seemingly – far less sensible reasons.

William T. Terrell told us via Facebook about one such example: "The sea squirt eats its own brain as part of its life cycle."

Some animals will, on occasion, eat parts of their own bodies

Sea squirts, also known as tunicates, are simple animals that live in the ocean. They begin life as swimming larvae, which look like tiny tadpoles. Each larva attaches itself to a rock or other surface, after which point it never moves again.

Much like land-based caterpillars, sea-squirts undergo a metamorphosis and become unrecognisably different. The adults look like small, lumpy bags and survive by filtering food from the surrounding water.

Their different lifestyles mean the larval and adult sea squirts are quite different internally.

"The larva's body has a basic layout, with a very simple nerve chord that runs along its back, a bit like the spines of more complex animals," says John Bishop of the Marine Biological Association. "At the front of this nerve chord is a ganglion or 'brain vesicle', and organs to sense light and gravity, which help the sea-squirt find a site to call home."

Could some snakes really be so dim-witted they mistake themselves for dinner?

Most of that disappears once the sea squirt reaches adulthood. "Once attached, the juvenile adult no longer needs the sense organs, nerve chord or even its tail, so it reabsorbs them," says Bishop. "The brain vesicle is transformed into a cerebral ganglion, which only helps the stationary adult to feed."

So the sea squirt's behaviour is not as gory as it sounds. It does not so much "eat" or "digest" its simple brain. Instead, it recycles it to make other, more useful organs.

However, there are other animals that are famous for eating their own tails.

"Snakes who eat other snakes (like King and Rat) will confuse their own tail for another snake and will end up eating itself," writes John Allen Gordon-Levitt Gerlach. "There is a greek word for it called ouroboros and it symbolizes the eternal cycle of life and death."

Another user, whose comment has since been removed from Facebook, reported that his snake had eaten over half of its own body and suffocated as a result.

While snakes do try to eat themselves from time to time, it does not seem to be intentional

Could some snakes really be so dim-witted they mistake themselves for dinner?

"Most snakes use heat-sense to find their prey, so their own tail is unlikely to capture their attention," says Sally South from the South Australian Museum in Adelaide. "But a few snakes also do 'caudal luring' (fast tail wagging) to attract prey. Some even do it just when they're excited. Snakes have small brains and are more reactive than proactive, so this movement could catch their eye and make them think 'prey'."

Similarly, James B. Murphy of the Smithsonian Museum of Natural History in Washington, DC once "had a king snake that was shedding [its skin] and began eating its shed while still on the body near the tail. It continued to swallow its own body tail-first until I intervened."

Murphy has a suggestion for why the snake did it. "Odour from prey on its body, especially with constricting snakes, could confuse the snake that it is swallowing prey," he says.

In other words, while snakes do try to eat themselves from time to time, it does not seem to be intentional.

What about an animal closer to ourselves? In particular, do mammals ever practise self-cannibalism?

"Animals consume portions of themselves quite often for different reasons… including mother animals (like cats) consuming afterbirth," Charity Young comments on Facebook.

Coyle found no solid evidence that eating the placenta has health benefits

This behaviour is quite common. "Most mammalian placental species consume their placenta and amniotic fluid during labour and delivery," says Cynthia W. Coyle of Northwestern University Feinberg School of Medicine in Chicago, Illinois. "There are several theories as to why this may be advantageous for animals, but most are not supported across species."

Mark Kristal of the University of Buffalo in New York has suggested that placentophagy (eating the placenta) might act as pain relief for animals that have just given birth.

Kristal's study mostly discusses rodents, but it does also highlight a more complex mammal that occasionally engages in placentophagy. As Mark A Rhodie points out on Facebook, "Some women [and men] eat their placenta".

In a 2015 study, Coyle and her team investigated human placentophagy and found that it is fairly rare. It is also a relatively modern phenomenon, even though alternative medical practitioners and health gurus promote it as natural and traditional.

Human placenta-eaters are a rare example of an animal eating part of itself on purpose

"We did not find any documented historical accounts across cultures of human mothers doing it," Coyle says. "The practice appears to be a relatively recent phenomenon, primarily in Western cultures."

She also found no solid evidence that eating the placenta has health benefits for humans.

"Advocates for placentophagy often cite animal studies as support for human benefits," says Coyle. "But these benefits, including prevention of postpartum depression and increased lactation and energy, have not been studied or supported in animal research. This raises the question as to whether in humans there is, at least in part, a placebo effect."

If Coyle is right, human placenta-eaters are a rare example of an animal eating part of itself on purpose – but unlike the leopard caught in a trap, they may not have a terribly good reason to do it.

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