# How might a carnivorous species subsist on a cycle of cannibalism

Let's say a carnivorous species loses all available food sources, and they begin to eat one another to survive. Let us also say that prion diseases are not an issue for this particular predator. Assuming there were enough of these creatures when this practice began that they could survive through a few breeding cycles and adapt to this way of life, could such a system actually be sustainable? Or is it pure fantasy?

If possible, what kind of special physical traits, population density, behavior, etc. would the creatures need to have in order to prevent their own extinction?

• The system is not sustainable without lots and lots of energy entering the cycle from somewhere. – user535733 Feb 25 '18 at 14:57
• i have feeling that the food sources would need to be taken away gradually forcing ever greater dependence on cannibalism this would make your situation more likely than a sudden change, also at least some small amount of energy would need to be added some how (probably a great deal if we're talking "predator" in the traditional fictional sense, big scary monster with lots of muscle and giant teeth) but this could be achieved a number of different ways, perhaps symbiotic photosynthetic organisms (i.e. koalas and their green fur) – Ummdustry Feb 25 '18 at 17:59
• Do they hunt each other, or just recycle their dead? – WGroleau Feb 25 '18 at 23:57
• As many questions of this site, the numbers have already been explored on xkcd. – vsz Feb 26 '18 at 7:13
• @vsz xkcd should be the FAQ for this site :P – Muuski Feb 26 '18 at 15:33

I'm going to say this would be impossible.

If all the species eats is other members of the same species, where does it get its energy to reproduce from? Your species would have to have a huge number of offspring to provide food for itself but you would have no way to give the parents the energy to produce those children.

You're basically asking for the biological equivalent of a perpetual motion machine.

• If it was capable of photosynthesis to the extent of being able to sustain others with its meat, it would certainly be able to sustain itself - i.e., it would have no need of being an obligatory cannibal. It might still do so to scavenge some necessary nutrients that might be insufficiently concentrated in its normal diet (there would have to be such a diet - if only dirt and lichens. Photosynthesis and atmosphere only give you CHON, and biology as we know it requires several other elements - phosphorus for example). – LSerni Feb 25 '18 at 16:18
• Or for that matter where does the whole population get the energy to move from, as that energy is effectively lost from the system – Richard Tingle Feb 25 '18 at 17:31
• @LSerni maybe its one of those things if you can eat your competition then you have more energy to breed eggs. Like you know mosquitoes. They dont actually need blood, but getting it makes then able to do so much more larvae that its worth it. – joojaa Feb 25 '18 at 18:42
• What if energy comes from one or more other sources, but other necessary nutrients are insufficient concentration? It would still be diminishing, but it might last longer. – WGroleau Feb 25 '18 at 23:55
• I think you could be hand-wavy about being able to get some energy through photosynthesis and heat. I'm not sure if there is any animal that gets nutrients by eating the feces of its own kind, since feces theoretically contains the stuff the unusable waste of the body, but this could be another potential source of food. – David Baucum Feb 27 '18 at 15:36

What if this is a creature that doesn't become 100% carnivorous until it reaches adulthood?

This means that the younger of the population can sustain themselves by other means but once they mature they shift to a meat-based diet.

The adults could breed, allow the young to fatten themselves up and then go in for the kill, allowing only a small percentage of the younger generation to mature.

• allowing only a small percentage of the younger generation to mature => or maybe they don't really allow anything, but like with our own herbivores some eventually manage to escape their predators. – Matthieu M. Feb 25 '18 at 16:13
• For a somewhat similar real-world species, there's the African bullfrog en.wikipedia.org/wiki/African_bullfrog which is known to eat its own young. Of course it eats other things, too. – jamesqf Feb 26 '18 at 0:02
• Matthieu M. I didn't mean 'allowing' in the active sense, would be more of a passive process. – plasticroyal Feb 26 '18 at 11:33
• Larry Niven, Jerry Pournelle, and Steven Barnes used exactly this premise in The Legacy of Heorot – Paul Sinclair Feb 26 '18 at 17:43
• they got the idea form a type of frog that eat their own tadpoles the tadpoles eat algae and in certain places that is basically the entire food chain. – John Feb 28 '18 at 1:57

could such a system actually be sustainable?

As laid out in the original post, not for any reasonable length of time. One of them would need to eat lots of its fellows during its lifetime in order to grow and survive, which means that each generation would be, say, one hundredth the size of the previous one. In a very short time, they would dwindle to nothing.

It is necessary to continuously supply energy to the cycle; otherwise, each generation only contains a fixed quantity of nutrients and energy (locked in their meat, expressed as calories), and dissipation will inevitably drive it to zero.

One could do this with @plasticroyal's workaround, having the young subsist on something else (obligate herbivores if at all possible) and most of them serving as food for the adults. This is the lifecycle of Leo Frankowski's Mitchegai:

...a species whose biology has made them inherently evil. The carnivorous adults lay and abandon vast numbers of eggs, some of which grow into vegetarian juveniles, which are the adults' only food supply.

The above would be certainly sustainable, as long as there were enough juveniles, and the adults would then eat "only their own" (nothing prevents them from also eating each other, as long as it's not their only source of sustenance).

For the same reasons, an organism cannot subsist on something it grows from itself.

• Thanks for the edit, I can see it now that I know but... that notch in the "n" totally fooled me. – Ville Niemi Feb 25 '18 at 17:56

There are such species of desert frogs. Some of the tadpoles are carnivores, feeding on the rest of their siblings who are herbivores. They are in a hurry to grow and mate before the water dries up.

Herbivores are able to feed on whatever organic matter is in the puddle, carnivores can grow faster, and the species as a whole can survive until the next rain.

Sometimes nature is stranger than our wildest imagination.

• Bear in mind, these frogs don't eat their siblings exclusively, and the initial energy for the system comes from the eggs from which they were born. The OP is talking about a multi-generation cannibal system where their diet is exclusively each other. This example, while interesting, doesn't effectively answer the question. – jdunlop Feb 28 '18 at 7:18
• This is a multi-generational cannibal system. And yes, substantial amount of energy comes from outside (other organic parts, plants and other insects, etc). For a closed system, law of entropy applies and number of individuals would decrease each generation, leading to extinction. – Peter M. Feb 28 '18 at 13:59

Pure fantasy or not, Niven, Pournelle, and Barnes' The Legacy of Heorot uses the same biological structure:

However, the colonists make a disturbing discovery: the grendels and the aquatic samlon are actually the same species. Their life cycle is similar to that of terrestrial frogs – the herbivorous samlon are in fact the juvenile form of the carnivorous grendels. Like certain species of frogs, they change gender over the course of their lifetimes. The juvenile samlon are male. The adult grendels are female. Interaction is unnecessary as the grendels continually lay their unfertilized eggs in the water for the samlon to fertilize. And like many species of frogs, they are cannibalistic – if no other prey is present, they will eat their own young.

If I recall correctly, the juveniles ate algae or other plant matter, and the adults ate (some of) the juveniles. The concept was based on some terrestrial frog species, but I'm having trouble finding good references for the specifics.

• [I enjoyed the books, but...] While there are aspects of cannibalism by the adult grendels, this lacks the most important part of the OP's question, which is: can the species exist by eating only it's own species. As has been explained in another answer, it's not physically possible for such to be sustained without other significant energy input into the system (as does exist with the grendels. The process for converting food into flesh is very inefficient, even more so for carnivores. – Makyen Feb 25 '18 at 17:38
• Sure, can’t escape entropy. It’s sustainable if there’s a ton of external energy applied, but that’s the case with more diverse ecosystems as well. – Mike Renfro Feb 25 '18 at 17:43

Reptiles can be flexible as regards body size. You can make your scheme happen for a while by having the carnivores gradually shrink with the generations.

I have laid out a schematic. Imagine dragons. They are huge. But some cataclysm happens and their prey is gone. They sleep, ready to wake if the scent of prey comes, but it never comes.

Every generation they mate, and after mating the female eats the male. Each female has two eggs. Once they are able, they eat their mother. Then they go back to sleep.

I have made an excel file. The dragons start at weight 1000. Each time a dragon is eaten there is a loss of 100 with the rest of the weight going into the next generation; each time eggs are laid there is a loss of 100 and the weight of the eggs (100 each); these are sacrifices to the hot god of Entropy who prevents perpetual motion machines.

The adult dragons get smaller and smaller with time although the young are always the same:100. By generation 6 the females are still only the weight of new offspring after eating their mates.

In generation 7 there is one female who has eaten the 4 others. She can lay two eggs and all three remaining dragons are the same size. The daughter eats her mother, mates with her brother and eats him but still weighs only 100. She is generation 8, the end of the line.

The larger the dragons are to start with the longer this can go on because there is more accumulated caloric treasure to spread over the generations. My loss of 100 with each exchange (eating a male, laying eggs, young eating the mother) is arbitrary - you can make the losses to entropy as large or as small as you want. You could make losses to entropy be a percentage rather than a flat 100 which would be reasonable and this cycle could go on substantially longer.

Dwarf crocodiles are small because their growth is stunted by a lack of food.

• A fixed 100 is exceedingly unreasonable, both for being fixed and how small it is. The biological efficiency of your dragons is ... magical. – Yakk Feb 26 '18 at 15:23

Continuing where plasticroyal and LSerni left off...

The juveniles must not only have a separate food source, they must be protected from the adults while growing.

I think the best model would be the amphibians. An amphibian terrestrial carnivore would have the right properties for this kind of life cycle. I think such beasts used to exist before reptiles, birds, and mammals evolved.

First, amphibians are ectothermic. This means your predator will use much less energy while it is waiting for food to appear. This is a very good thing for your scenario.

Second, most dry land amphibians must lay their eggs in a body of water. The eggs cannot survive on dry land. And the first stage of their life cycle is aquatic and typically eats different food in a different environment than the adults do. A predator evolved to hunt on dry land might be starving while watching a lake filled with life edible by the larvae.

This separation is enforced by the tadpoles having gills and the adults having lungs. Eyes might also differ.

So once the tadpoles fatten up eating aquatic life, they will undergo a metamorphosis to adults with legs and lungs, rise to dry land. They will probably eat others of same gender while looking for mates. You could evolve this to harem style social grouping if you wish. Although unless it is a pack predator pair bonding seems more likely.

After finding a mate, the female will lay eggs in a suitable body of water and the cycle will repeat.

• * I think such beasts used to exist before reptiles, birds, and mammals evolved* Could you give a reference to such a thing, please. – StephenG Feb 25 '18 at 17:16
• It is mentioned in the Wikipedia article on amphibians. The amphibians were the first tetrapods to conquest the dry land (mostly swamps and such though), so for a time they were the top predators. Still most eating insects, I guess. But also other amphibians and early reptiles. For those not interested in reading Wikipedia, this was 360 to 345 million years ago. – Ville Niemi Feb 25 '18 at 17:26
• A harem of that sort might be a rather violent place. – WGroleau Feb 25 '18 at 23:49

There's theoretical biology papers that work on exactly these type of questions, so best use Google Scholar. There's enough papers on the evolution of cannibalism, and most examples are from fishes. I'm working below from 15y old knowledge; consume with pinch of salt. A Google Scholar search on "cannibalism fish theoretical model" produces heaps of accessible papers from 2002--2006.

What I remember from these things is that there's no problem being cannibalistic, but there is a problem preferentially eating your own kids over others'. So if you have a tendency of eating babies close to where you left yours, you'll statistically eat more of your own than of others, and your genes won't spread [meaning: your exclusively cannibalist species cannot evolve; only by creation it might exist]. But if your species' babies and juveniles have another (abundant) food source, then there may be enough of them to feed your adults AND let them lay abundant (small!) eggs; so this system can both evolve and sustain itself (hunt for juveniles on their feeding ground; you'll mostly eat others' babies).

Generally there's a 10x biomass decrease per trophic layer (think "100kg of grass can sustain 10kg of rabbits which sustains 1kg of fox"). So the numbers must be right, to get 10kg of snack-sized juveniles allows 1kg of adult of which 100---200gr becomes babies/eggs. See Wikipedia (for "trophic level") and that link scrolls to the "10% rule, which is exactly the factor 10.

Notice (see the "dragons with 2 babies" model in another answer) these things only work with "many small offspring" species, not "few large offspring". Any intense parenting requirement fails this!

Summary: (a) it's possible to make a stable population dynamic model with purely cannibalistic adults (b) that it must have many small children exploiting another source (maybe algae, else other creatures) that's too fiddly for adults (or unreachable: think of a net/gauze only the little ones can pass). The exact efficiency (8%? 10%? 12?) per trophic layer, you can look up if relevant.

• @user3445853 your gender does not matter here. Why would it, really? And this is not a classroom, it is an Q&A site. If you don't have an answer, post a comment. If you do have an answer, post all relevant bits, simple as that. – Mołot Feb 27 '18 at 12:09

It's trivially possible by arguing semantics.

Digesting food can at best convert 100% of energy. Even if that was possible (it isn't) the total amount of energy available to a species would shrink anytime a member moved, until there's no energy left and the entire species is dead. Related: a random page on the internet claims a dog needs to be fed 15-20% of their body weight a week, so a dog would have to eat 10 other dogs a year.

This problem can be solved if the species has ways to acquire energy (and mass) that are not called "food". The cheesy way out is if they acquire most of their energy by "drink" instead of "food". An alternative approach is to make them capable of harvesting completely different energy sources, e.g. by means of photosynthesis. In all cases you can let the "non-food" energy be insufficient, so some cannibalism is still required.

This strikes me as a possible scenario where the carnivorous species lives in symbiosis with another species and the carnivore is a host.

For example, the microflora may evolve rapidly to supply additional inputs, such as proteins, by direct synthesis. EDIT to add: remember most of our energy and nutrients is currently provided via our enzymes and gut microflora already.

• Not enough surface area on typical carnivore for that to work out. – Mołot Feb 26 '18 at 10:18
• @Molot. OP is not interested in typical sizes. Also, carnivores have intestines and derive pretty much all their nutrients through enzymes and microflora. We could propose a situation where CO2 and heat levels are so high in this fictional world that the carnivore becomes little more than a substrate for microflora. – Sentinel Feb 26 '18 at 10:32
• While not the most correct, this is definitely the most thought-provoking answer. Now I'm imagining a crocodile with a bed of moss and flowers growing from its back :-) – Shanenopolis Feb 26 '18 at 15:33
• @Shanenopolis - Thanks, though I do actually think that this could work. Something else in the environment would need to be present that provides enough energy for the carnivore to continue on very limited food supply, and given that the carnivore cannot rapidly evolve, the only other thing that can is its gut microflora. – Sentinel Feb 26 '18 at 16:21

Though it may not be the focus of your story and I apologize for failing the in depth details needed but the question is reminiscent of the movie Soylent Green.

In the movie, the premise is that humanity has ravaged the ecosystem to the point where the majority of humanity subsists of a semi-mysterious source of food known as Soylent Green rationed out to the populace. The entire movie revolves around the world's dependence on this food source and a quest to discover it's true origins. It is a 39 year old movie and if you aren't aware of the meme "SOYLENT GREEN IS PEOPLE!" Sorry if I spoiled it for you.

How does this pertain to your civilization, you may ask? Well, if they are a cannibalistic culture, measures would need to be put in place to:

• Increase supply
• control demand

Increasing supply could be through any means. As the movie attempts, persuasion for euthanasia will increase supply. It may also help to introduce a blood sport such as gladiatorial battles or the promotion of small scale conflicts for entertainment, think football with swords and shields. Another idea, to take from a different movie "Logan's Run" is to set an age in which people are carted off under some other motive to be turned into food.

controlling demand has many ways of achieving this goal. Most are rationing, either in how many a source can dispense to how many a source receives to dispense. Perhaps limit the number of population by capping off at a certain age.

Again, I apologize for any leaps being made but I suppose this answer only pertains to if your species has advanced to such a level as to have a society.

• +1 Well spotted. In fact the exact match in a fictional case study. – KalleMP Feb 27 '18 at 14:39

There are species of frogs that eat their young. There are crocodiles that eat their young. Of course that's not ALL they eat, and there would have to be at least some other forms of food for them (other creatures?).

In your fantastic setting you could look at some of these critters that exist IRL and extrapolate. Larry Niven, Jerry Pournelle, and Steven Barnes wrote Legacy of Heorot about just such a scenario. Good book too.

Edit:

Maybe this is a solution(?), though it is most unpalatable (lol). Neanderthals and Homo Sapiens were different enough DNA-wise that one could argue it wasn't necessarily cannibalism if one ate the other. Maybe your creature splits into sub-species so they're not actually eating "each other"?

Long term survival of the species by means of cannibalism is simply not possible. Without any other mechanism to put energy in the system it is simply not sustainable. The stronger will eat the weaker and then the last remaining will starve to death. Although a vast population spread over a large area could take quite a while before complete extinction.

Consider that even on Earth, with its countless species of life, there is no such thing as a self-sustainable ecosystem. For the vast majority of ecosystems on the planet, save for deep sea creatures, 90% of the energy actually comes from the outside - namely that is solar radiation, a.k.a sunlight. A simplified foodchain energy pyramid puts every step of the pyramid at an order of magnitude less energy content. Meaning that plant matter constitutes only 10% of all incoming solar radiation energy, primary consumers only 10% of plant matter, and so on.

Now, eliminate every other food source save for cannibalism. It will probably be worse than 10x decrease, for the sake of simplicity assume that a predator reaches maturity in 2 years, and all predators are cannibalized at that age, it would take on average consuming 1 member of the species monthly for 2 years, or 24 in total for the system to produce another "ready to eat" adult animal.

Now, if you have a 24 fold decrease for every iteration, it is quite obvious that sustainability is entirely out of question, and you have an example population size, it is trivial to calculate how many generations it will take until extinction. Let's assume a rather numerous population of 1 million:

gen 0 - 1 000 000
gen 1 -    41 666
gen 2 -     1 736
gen 3 -        72
gen 4 -         3


In just 4 generations, or 8 short years that population will be reduced to only 3 individuals, which will have quite the dilemma - breed or eat, presuming you still have members of both genders, and of with the prospect of no more food whatsoever.

In reality it will be even worse, much, much worse than the theoretical worse, because younglings will have a really hard time taking down adults, and parents will have higher energy expenditures and less chance overwhelming another adult to feed themselves and their offspring until it is large enough to hunt. It will be far more likely that adults consume the young, because of the energy disadvantage their parents are put into. There will be a lot less additions to the population than the theoretical number and extremely slim chance than any newborn reaches maturity. In practice, it will be nothing short of a miracle if the population is not completely extinct in 5 years or less.

If a plausible scenario is desired, then there must be another food source, one that is of very low efficiency, requiring a lot of effort to obtain food with very low nutritional value.

This also offers additional plot directions, as the population will gradually diverge into a cannibal group that seeks the high stakes, risking their lives to obtain high protein meat, and a group that is more like herbivores, although they can go for insects or microorganisms or some symbiotic relation with other species of life that can still provide nourishment. Depending on the environment setting, both groups could develop either collective or individualistic behavior and grow more and more distinct.

The divergence can be either a conscious choice or a product of evolution. A subset of the species might not be able to extract nutritional value from a source other than meat.

Another possibility is that younger individuals, being smaller and more nimble and agile, might be able to catch food that older, larger and slower individuals might not. But that's not very sustainable IMO.

It could also be the reverse, as individuals age, they might acquire traits and skills to obtain food from alternative sources. For example growing external physical features with the properties of a net that can trap large enough quantities of insects or microorganisms (like a whale does with plankton) to be a viable source of food for a long term survival. The older an individual grows, the more calories it can obtain, allowing it to grow in size significantly, to the point of becoming a high value target for a pack of younger individuals that will be required to take it down. With age individuals might also develop glands that produce chemicals that attract more of their foodstuff.

That sounds fairly sustainable, the young will initially be breast-fed up to a point, after which they will be forced to prey on one another, as they wouldn't really pack enough force to take down an adult, in a process of natural selection that will allow only the strongest to reach physical size large enough, so that they can take down adults by forming packs. And only the most successful members of each pack would be able to reach adulthood when they can attain self-reliance by developing capabilities to access another food source, and over time become prey to the subsequent generations.

Back around 1970, Professor Pat Doyle wrote a paper for the in-house journal of the Mathematics Department at Michigan State University. I assume it is difficult to find, but the main idea in the paper is that if you set the prey variables to be the same as the predator variables in a standard model of predation, the resulting system had a stable solution. In other words, yes, it is theoretically possible, under those assumptions, and the population will stabilize at some level.

If I remember, the title of the paper was something like An in-eating cannibalistic society could be stable.

• I am sure this model doesn't build around the prey and the predators having the same food source. Stable environments require sustainable cycles. If all you have is a single cannibalistic predator there is no mechanism to put back energy into the system, and eventually it will completely run out of it. The major input of energy into the ecosystem is sunlight, and plants are the only species of life that can pass it up the food chain to herbivores which serve as food to carnivores. And I presume that cannibalistic carnivores are not able to photosynthesize or digest cellulose ;) – dtech Feb 27 '18 at 0:38

I think this question might have different answers based on whether you consider the species as a single unit, or as a collection of groups. Does the loss of food affect everyone equally, or some more and some less? Is this a small, closed ecology they are in, or is there diversity? And is social dominance a part of this equation?

Based on those questions, here's a suggestion that relies on tribalism or grouping, plus ideas of social dominance.

Imagine the species organized by tribes, units, families, or some kind of grouping. And their normal primary food source is based on subsistence agriculture, where they focus on growing enough food to feed themselves and their entire families. The output is mostly for local requirements with little or no surplus trade.

Now imagine some kind of change or event has happened causing a partial to full collapse of food sources. Group A has a complete collapse of their farming ecology, creating famine for them. Group B has had partial collapse, but has just enough food to get by.

Group A attacks Group B, eating Group B's meager food supply, and then shifts to eating Group B when the food is gone. Strengthened by this big influx of food energy, Group A starts to think perhaps this cannibalism thing is a lot easier than farming. And they continue doing it to other weak groups by becoming the dominant, cannibalistic driven group.

This cycle could be kept up for a while, but eventually it might falter. Group A perhaps has to travel farther and farther perhaps, or other groups flee or move, or crop conditions improve and other groups become strong again. Perhaps Group A purposely keeps other groups in a weak position of barely subsiding on what they create for themselves, thus making them easy to "harvest" for food.

There are a number of variables in this splendidly oxymoronic and gastronomic evolutionary construct, most of which have been investigated in the well-observed comments above.

The difficulty with cannibalism is that it leads, eventually, to degenerative brain diseases such as the spongifom encephalitis strains.

A pandemic would, thus, lead to pandemonium; et ergo extinction.

• this sounds more like a comment... – L.Dutch Feb 27 '18 at 6:10
• Welcome to Worldbuilding.SE! We appreciate your participation and look forward to it. Please note that this doesn't answer the OP's question. You're cetainly correct aobut encephalitis, but what genetic or gastronimcal traits might overcome that? Most of the fun of this site is coming up with creative ways to bend the rules of the real world to create an imaginative fiction. – JBH Feb 27 '18 at 7:45
• @HottBlakk, Please read the question again. Prion diseases are specifically excluded from this scenario in the second sentence. – Shanenopolis Feb 27 '18 at 18:49
• Apologies. Heh! – Hott Blakk Feb 28 '18 at 4:30

Good News

While a species that fulfills nutritional needs solely through cannibalism might be slightly far-fetched sounding, it might not be as crazy as it sounds.

For example, there's a species of squid known as the Humboldt Squid that regularly with attack and eat other members of its shoal when food is running low. This can be due to the victim having an injury or many other things, but it still happens. Furthermore, these are a large species of squid, nearly 5 feet long on average, perhaps even bigger.