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I'm creating a species of beings that are able to eat part of themselves when they are starving.

In a sense, Earth animals do this already by putting on fat. When there is a shortage of food, they can burn fat and keep going.

However, I'm looking for a plausible reason that my creatures cannot lay down internal fat. This means that, in times of need, they have to chew off a part of their external body. Luckily, when food becomes available again they can regrow the missing parts.

Question

What prevents these creatures from evolving the equivalent of fat?


EDIT

This shouldn't affect the existing answer by @DKNguyen. I thought it might be useful to describe the general body-plan of these creatures.

They are carnivores. Superficially they have a similarity to centaurs, i.e four legs and two arms. The body has the flexibility of a leopard's and the legs have claws instead of hooves. The hands have two opposable "thumbs" and the digits also have claws. The head is reminiscent of that of a Rugops https://en.wikipedia.org/wiki/Rugops. The tail is lizard-like but not so heavy as that of a bipedal dinosaur because it is not needed for balance in the same way. The tail is useful in making tight turns when chasing prey.

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    $\begingroup$ Octupi do this already. Often it's due to boredom or stress. They regenerate as well. $\endgroup$ – DWKraus Jul 6 at 5:04
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    $\begingroup$ @DWKraus: don't forget dim-witted snakes ;D $\endgroup$ – user6760 Jul 6 at 5:31
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    $\begingroup$ @user6760 Ouroboros Uber Alles! $\endgroup$ – DWKraus Jul 6 at 5:35
  • $\begingroup$ @DWKraus ♫"Ewigkeit und Endlose Kreise, für die zeitliche Kausalschleife"♪ $\endgroup$ – Chronocidal Jul 6 at 9:39
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    $\begingroup$ all animals including humans can digest body tissue when they starve, it is called autophagy, and will even include digesting muscle tissue, and it is far more efficient than taking a bite out of yourself. Organisms Start with fat because it evolved as a storage medium, but it is by no means limited to fat. $\endgroup$ – John Jul 6 at 14:08
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Animals are still able to re-absorb organs and muscle for energy. It doesn't need to be fat. I don't think your real problem is the existence of fat. I think it's why an animal would need to go through the roundabout and injurous method of eating itself rather than just re-absorbing some muscle, organs, or fat. Even if you explained away fat, the animal will still always have muscle and organs. This is a very persistent problem. For example:

  1. The animal has an exoskeleton so can't put on fat because it would be too tight a squeeze. Not to mention how it would get through its own exoskeleton if it were to eat itself.
  2. There is a persistent toxin that was introduced to the environment that is stored in fat cells and over many generations, slowly poisoning and debilitating individuals that were able to develop fat leaving descendants with a decreasing ability to form fat reserves.

It's still always a problem why the animal can't re-absorb its own organs and muscle even if fat wasn't there.

One way around this is that the animal isn't actually one organism, but a very tightly knit colony of symbiotic, highly-specialized organisms. If the colony needs to consume part of itself to keep itself alive, it needs to get the part of it to be consumed back to the digestive system where the energy and nutrients can be re-distributed. The reason being is that, due to specialization, the organisms responsible for the digestive functions are the only ones in the colony with the capability to breakdown matter into nutrients and distribute it to the rest.

You might be interested to know that when a lizard sheds its own tail, it might come back to eat the tail since the tail is full of fat. I don't know what you are trying to do in your story, but perhaps this circumstantial behaviour is sufficient for your needs rather than the much more difficult to explain unconditional self-cannibalism.

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    $\begingroup$ You could even play off that last idea a bit. Maybe the creatures used to have a natural predator that caused them to 'drop their tails' (or equivalent) in confrontations. That predator has been wiped out hundreds/thousands of years ago, but the evolutionary instinct to drop a part of their body under certain conditions remains. For whatever reason it happens a lot, so they have a behaviour/habit/ritual of preserving the body part and eating it again for energy and to aid in the regrowth process. $\endgroup$ – Kayndarr Jul 6 at 5:22
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    $\begingroup$ Adding to @Kayndarr - if the part of the body the creature sheds is also something that consumes a lot of energy (lots of muscles in it?), you can sort argue for it being a viable strategy since you get a lot of food at once, and your energy consumption goes down. Perhaps this species has an absurdly muscular tail created by sexual selection, which isn't really used for anything other than attracting partners. In times of scarcity, the tail is shed and consumed, and in times of plenty, it's regrown (triggered by surplus calories?) and you can go around attracting mates again. $\endgroup$ – gustafc Jul 6 at 9:12
  • $\begingroup$ This is all looking good. I've edited my question to add some anatomical details. $\endgroup$ – chasly - supports Monica Jul 6 at 11:00
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It's not just yourself you're trying to feed:

I think the fat thing isn't the issue. What you really need is a reason why a species would eat itself when it was starving, and that this is the normal for the species instead of fat. You can certainly have an animal that eats itself, and one that regenerates. Octupi do this already, although it's not usually for food reasons - but it could be. https://io9.gizmodo.com/there-may-be-a-disease-that-makes-an-octopus-eat-its-ow-1694165746

Maybe there are body parts that are useful when food is abundant (like extra arms to hold tree branches while pursuing prey in trees, but in a famine the trees are bare). The loss of these limbs reduces caloric needs and provides food. It might even make your creature more nimble to pursue prey on the ground instead of up in trees.

If there were social reasons for operating this way, it would totally make sense. You can't feed your children and mates your fat easily (well, there are ways like milk, but...) but you can have a successful hunter rip off a limb (or specialized food/bodypart) even when dragging a dead animal home isn't practical. This wouldn't even need to be a famine thing, but just a way to transport calories. You could even incorporate fat into this - the fatty growths on the back of hunters are MEANT to be eaten.

If the alpha member of a pack is hungry, the underlings can offer a piece up for the benefit of the tribe/pack, or as a way to show status in the tribe. Or maybe a hunter's status is passed along through consumption - you absorb hormones from eating the hunter that make you want to follow it. The more you get chewed on, the more popular you are with the tribe, the ladies, whatever.

This could also be part of some sort of symbiosis. Predators don't kill their prey, they gnaw off the growths on their back and let them go. Predators then protect the prey animals from more lethal predators. Organisms throughout your ecosystem may incorporate this as a behavior for various reasons. Are there bigger predators than your critters? Are they genetically related to chosen prey animals? Lots of room to expand on this if you want to.

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Frame Change: Sexual Selection

Other answers say fat is not the problem. Most body parts can be re-absorbed if necessary. So what we really need is a reason for the animal to eat parts of itself rather than just reabsorb. I propose it is sexual selection.

The peacock's huge useless tail says "Wow I must be strong to be able to survive despite such a ridiculous tail". The females mate with him and future generations have even more ridiculous tails

Your animal is similar. Instead of shooting itself in the foot with a huge tail, it shoots itself in the foot by gnawing off a portion of itself.

Boys: "Feast your eyes ladies $-$ I chewed off my own arm!".

Girls: "Wow he must be strong to survive despite his stupidity. Let's have his babies!"

This in turn makes the practice more widespread in their children.

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    $\begingroup$ How did this start? By biting his nails? (+1 though) $\endgroup$ – chasly - supports Monica Jul 6 at 15:50
  • $\begingroup$ @chaslyfromUK The buzzword you are looking for is irreducible complexity. $\endgroup$ – Daron Jul 6 at 16:08
  • $\begingroup$ @chaslyfromUK youtube.com/watch?v=fTIJA7rUBkQ $\endgroup$ – Daron Jul 6 at 16:12
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Since the mechanism of packing muscle and fat it times of abundance and auto-consuming fat and muscle in times of starvation comes from the randomness of food resources, you'll need a species that has evolved since million years while never having any issue to find food. They live in permanent abundance of food and never had to worry about it.

The species hasn't developed the mechanism of storing fat.

However, this species is highly predated and wouldn't survive for long without limbs. It has evolved to be able to regrow limbs.

So now the setup has changed and they no longer have access of the abundant food they used to. For the first time of their history as a species, they know hunger.

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The ability to break down the fat (and other material) in one's own body, just as anything physiological, is regulated by genes. Your creatures simply don't have the genes to release fat from tissues in order to burn it.

Your creatures may go around this by detaching limbs or other body parts during periods of starvation. This has the added benefit that the part will no longer be consuming calories either, so the creature might live on less food. Detachable limbs might be as easy to come off as the tails of some lizards, and might regenerate either in a similar way, or the way that insects do (regrowing when molting).

This also made me think of spiders, which usually eat their own webs to regain some energy. Sure, the web is not a bodily part of the spider, but it took energy and material to build and not getting a portion of that back would be a waste.

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  • $\begingroup$ healing the injury will consume a huge amount of calories. $\endgroup$ – John Jul 6 at 14:09
  • $\begingroup$ @John yes, but in the long run it's less expensive than a constantly calorie-consuming limg. $\endgroup$ – The Square-Cube Law Jul 6 at 14:11
  • $\begingroup$ Only if the limb does not contribute significantly to its life, in which case why does it have it in the first place? $\endgroup$ – John Jul 6 at 14:17
  • $\begingroup$ @John redundancy - also we could live quite well with just one arm, but having two makes things easier. $\endgroup$ – The Square-Cube Law Jul 6 at 14:18
  • $\begingroup$ "can live", and "can survive and gather food without a significant loss in effectiveness" are very different things. the improved ability to gather resources drastically exceeds the offset cost of the limb. $\endgroup$ – John Jul 6 at 14:21
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It is possible the food or environment of the animals contains toxins that can be safely handled by the digestive system but are harmful when released in the body (like some snakes can eat venomous other snakes safely but are still at risk when bitten). if these toxins accumulate in the animals body absorbing fat (or other parts of the body like muscle of organs) can release the toxin directly in to the blood and be dangerous while removing a pound of flesh and eating it will allow "recycling" tissue against starvation while preventing poisoning. this can also be a defensive tactic making the animal less appealing to predators.

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I propose a wrinkle on @Daron's answer (which I've upvoted), which I think is on the right track by remarking on body parts which are costly in terms of nutrients but of lower general function. The animals produce some large appendage, useful only for mating presentations, like a peacock's tail, which cannot be easily re-absorbed (no vasculature?), but which can be shed or removed and which, for some reason, contains valuable nutrients; perhaps not an energy source but something like rare minerals for coloration or calcium for rigidity. Could be a tail, or a mane, or a rack of antlers or a horn.

In theory things like limbs, tentacles and sense organs might be far more efficiently re-absorbed than re-eaten, but maybe the animals just haven't evolved re-absorption or, more likely, re-absorption is not specific. Starving humans, for example, will begin to re-process protein until the mechanism of protein processing is itself destroyed, beyond which further nutrition is impossible. So maybe it'd be better to eat your own leg before you get to that point.

Hard part may be to explain why they don't just gang up and pick another individual to eat.

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  • $\begingroup$ gang up and pick another individual to eat They are one hunters like many carnivores. The behaviour occurs when they run out of prey.. $\endgroup$ – chasly - supports Monica Jul 6 at 17:00

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