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Both the prey (plural) and predator (singular) are intelligent and sapient.

The predators are semi-solitary - they meet up occasionally to conduct rudimentary trade (think bartering), find mates and raise a family. They'll teach their children hunting skills and tool use, but they spend the rest of their lives alone. They don't mate for life and don't maintain strong ties to family. They don't have stationary homes and wander around instead, creating and discarding simple shelters as they go. They can hand-sew bags or preserve meat using salt, which gives you an idea of their technology. They haven't learnt to make weapons because they already have extremely powerful claws and jaws.

Note: prey is plural here. There are multiple prey species which are all intelligent and sapient. I will give an overview of their abilities here. Their tool use, like the predators, is fairly basic.

Among their abilities are hand-sewn clothes and bags, simple food preservation using honey or salt, and the creation of items such as boxes or simple beds out of wood and cloth. The prey (plural) usually live in stationary homes, typically rough log cabin-like structures made out of branches, or systems of burrows or caves. Their homes are scattered around, not organised into villages and towns. They aren't semi-solitary like the predator, but they don't have much of a social structure either. They usually live in small family units which share one house. Different family units may meet up to find mates or conduct barter/trade.

The predators are obligate carnivores, so they can't eat plant matter to stave off hunger. The predators can preserve meat using salt to avoid killing for longer.

Let's say, between the two societies, they come up with this system:

  • Each week, a few of the prey species die natural deaths.
  • The bodies of these dead members are left in a clearing, which the predators know about.
  • The predators collect the dead bodies and eat them.

No killing is involved, and the predators and prey can maintain friendly relationships because they don't live in fear of one another.

In your answer, if possible, please include population sizes and prey mortality rates that would make this system feasible. It doesn't need to be hard science; just a rough estimate.

Edit: The comments made me realise that I'm probably going to have to go back to the drawing board on this one. However, I'm leaving the question up for posterity's sake.

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    $\begingroup$ If you're looking for population estimates we're going to need way more info. Are your predators 20 meters tall, and your prey 1m? How about the meat/fat content of the prey? How fast do they gestate / reproduce? Is the prey the only food source for the predators? I don't see any reason why this couldn't happen, so long as both species are sentient enough, but precise numbers could vary wildly. A human could feast off one cow for a long time, but if we had to eat squirrels we would have to kill a lot of squirrels. $\endgroup$ – cegfault Nov 17 at 14:33
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    $\begingroup$ I'd suggest editing your question with more details, as it's probably too broad right now. The population size could be one predator to one thousand prey (large predator, small prey), or vice versa, one prey to one thousand predators (ant-like predator, large animal prey) $\endgroup$ – cegfault Nov 17 at 14:35
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    $\begingroup$ Why would the carnivores agree to this? During a mild and bountiful summer, fewer herbivores are likely to die - won't that result in famine for the carnoivores? Why would one side, under a great stress, keep the agreement? Seems like this only works if the herbivores have already won the hunting campaign and are subjugating for their own purposes the few surviving carnivores. "Honey, we get to live in the Herbivore's moat and eat the corpses of their prisoners and be their slaves." $\endgroup$ – user535733 Nov 17 at 15:54
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    $\begingroup$ I find it hard to believe that a species that can only survive on one other specific species and both happen to be intelligent - how did they end up like this? That's the part where your world will stop being believable. I think that's the true question. If you just want a simple math problem to be answered, you should perhaps check out how much callories they need and how much there is in meat. That's googled in like 20 minutes $\endgroup$ – Raditz_35 Nov 17 at 15:55
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    $\begingroup$ Is the prey species the predator species's ONLY source of food, or can the predators go off and hunt someone other than the prey species? VTC for being too broad: you've got too many variables at play here. You mention sports, clearly weapons and warcraft are factors at play. Please narrow your question to one single aspect. I'd like to answer, but there are too many possibilities to offer a succinct and direct answer. $\endgroup$ – elemtilas Nov 17 at 15:59
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I'm not sure it could work as described. The main problem is that unless the prey species is involved in large scale warfare or something else resulting in massive death of healthy individuals, the vast majority of deaths are going to come from some combination of disease, malnutrition, and old age. Unfortunately for the predators, bodies from any of these categories are low in nutrition at best and possibly deadly at worst. I've done some very basic math below to try and get a grip on what relative population sizes would have to be.

According to Gurven and Kaplan at UCSB (https://gurven.anth.ucsb.edu/sites/secure.lsit.ucsb.edu.anth.d7_gurven/files/sitefiles/papers/GurvenKaplan2007pdr.pdf), in hunter/gatherer societies all violent and accidental deaths account for almost 19% of total mortality. Infectious disease causes around 70%. If we make the optimistic assumptions that all violent/accidental deaths and half of deaths from disease are edible, that means that about 54% of prey deaths will result in the predators gaining food. Assuming that the prey species has a similar death rate to human hunter/gatherers, using the data for the Hadza hunter/gatherers gives a number of 125 deaths for 706 individuals over a period of 10 years. This leads to around 1.8% of the population dying every year, meaning that a little more than 0.9% of prey individuals are eaten a year. If both predator and prey are roughly human sized, then the predators will need somewhere in the neighborhood of 2200 calories a day, plus or minus a few hundred depending on their size and amount of exercise. According to James Cole (https://www.nature.com/articles/srep44707?utm_source=commission_junction&utm_medium=affiliate), the average human male contains around 143,000 calories. However, once you remove bones, teeth, and places likely to be infected (brains, lungs, and gastrointestinal tract), you are left with an upper limit of almost 113,000 calories. This is 51 days of energy for the predator, so each predator will need to eat about 7 prey a year. Because of this the ratio of dead:alive prey is 7/778 per predator, meaning that there must be a minimum of 785 prey for every predator. A stable breeding population of a few hundred predators will require hundreds of thousands of prey. You can tweak the math but it won't change the fact that the predators are going to be outnumbered and/or outweighed by hundreds to one, to the point that the predators are so insignificant compared to the prey that the relationship between them is trivial.

However, if you like this idea, there are ways to make it work. The first few that come to my mind are:

The predators could have a technological advantage over the prey. If there was no technology gap, the prey could wipe the predators out with virtually no effort and eliminate the potential threat they posed. Even if they have ethical concerns about eating unwilling prey, they probably have no problem defending themselves. However, you would have to hit the sweet spot with advanced enough tech to defend against any threat, but not advanced enough to grow meat from stem cells. This could work, but would probably lead to a much colder relationship between the species than you described.

Another way would be if the prey species was constantly involved in warfare (or some other activity with a high death count). Increasing the prey mortality rate would help even the numbers gap, but there would still me many more prey organisms. This would change the relationship up a bit. To the prey, the predators could simply be an efficient way of disposing the bodies. This would provide some building blocks for the culture of both species. Who are the prey fighting, and why? Are the predators trying to keep the conflicts going to ensure they have a food supply? If they are, what measures are they taking?

For a less gory and to me much more interesting approach, make it so that the predators largely subsist on other species, but eating the dead of the prey has some religious/cultural meaning for both species. The relationship was formed for some reason in the past, and continues to be practiced, perhaps to reaffirm the friendship between the two species. I think a relationship like this would lead to the best relationship between the species, but the predators would still be predators. If you wanted them to be entirely opposed to eating live meat, this could be the eventual development of a situation like the first one I described, after the predators have gained technology to grow meat or otherwise overcome being an obligate carnivore.

There are of course many other ways that the relationship could work, I just felt like sharing some of the best ideas I could come up with.

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No, unless... the predators limit their own population in order to match the natural death rate of the prey. As soon as there are more predators than there are naturally dead prey, that means either a predator is going hungry or a prey is going to be hunted.

How likely is it that the predator is going to go hungry? How likely is it that the predator society will "rebalance" their numbers by eliminating one of their own? Some would say not bloody likely at all.

Let's factor in environment and go to the logical extreme. Let's say the ecosystem is at max capacity for prey, i.e., the prey are at max possible population for that ecosystem. The predator population can grow to consume the number of naturally dying prey. Then add one more predator. Now the predator population is feeling some stress. How will it react to that stress?

There are many ways and many reasons that species limit their own numbers. A new alpha lion notoriously will kill off the cubs of the preceding alpha. Many species, say crocodiles, will indiscriminately eat the young of its own species. And of course there's always straight up cannibalism and of course, war. Chimps go to war. Ants go to war--even between colonies of the same species.

While none of these actions are specifically undertaken with the goal of controlling the population of one's own species, maybe this predator prefers to do one of these things before choosing to kill prey. (Which also would need some 'splainin.)

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    $\begingroup$ There’s an interesting power inversion implied by the question and this answer. Predators normally control prey. If prey is providing meat to predators, they have the upper hand financially. Interesting to hear how that happened and how bitter the predators are about it. After all, if not enough prey die in a given week, they’ll be able to make the predators bid for what is available. (I tried to resist saying “charge an arm and a leg”. :-) ) $\endgroup$ – SRM - Reinstate Monica Nov 17 at 22:51
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    $\begingroup$ So situation I can think of there this could work is the typical undead vs living situation so the predators are only a few and have to artificially reproduce (turning someone else into a zombie/vampire etc) so keeping their numbers low is pretty easy and because they are so few they can't risk a war with the numerous prey. $\endgroup$ – ChoTimberwolf Nov 18 at 6:39
  • $\begingroup$ Once the predators start hunting, won't the prey decrease in numbers, further increasing the stress until predator or both populations crash? Isn't what OP describes similar to domestication which seems to work pretty well for the Alpha predator on this planet? The big difference I see is that "natural death" probably means old, not the best/biggest posible. $\endgroup$ – kutschkem Nov 18 at 8:30
  • $\begingroup$ The other alternative is that the prey population is artificially increased to meet demand (as we do with pretty much every domestic meat source), but there’s a pretty impressive lag time on new meat being harvested if you wait for them to die naturally. $\endgroup$ – Joe Bloggs Nov 18 at 9:47
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    $\begingroup$ The predator population could be naturally slow growing and the predators small (so each additional predator does not need much more meat). Might not be workable forever but certainly a few centuries if the predators start off from a low base. Main issue is disease/old age but you could imagine the predators having systems like vultures to deal with that. $\endgroup$ – Christy Nov 18 at 11:09
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If a non-sentient predator gets hungry, it will kill someone, so in that case this arrangement couldn't be stable. But if the predators are capable of long-term planning, then I don't see how this would make any difference to the basic ecology. It's just like the human transition from hunter-gatherers to farmers: you still need the same ratio of food to mouths, but organising the process lets you spend much less energy and have a much more stable food supply.

If the predators mostly don't have to worry about hunting, they can spend their time doing science or art or working in a bank; they're not going to throw that away just because they're temporarily hungry one day. And perhaps more to the point, they're not going to tolerate each other behaving like that. The rate of natural deaths might fluctuate, but on average, as long as the prey population is big enough, then there will be a steady supply of corpses.

If the predator population gets too large, then some of them will starve. But reverting to killing won't make that problem better; in fact, it will rapidly make it worse, in addition to breaking the beneficial long-term arrangement. There are lots of different ways this situation could play out (see: most of human history), and some of them would involve violence and collapse, but some of them wouldn't. In general, though, the consensual arrangement is the most efficient for both predators and prey.

Regarding exact numbers, it would depend on the relative sizes, how much food a single predator needs, etc. But the consensual aspect makes no difference; a kilo of meat from natural causes is the same as a kilo of victim meat. Actually, the predators would need a little less food because they're saving energy on hunting.

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    $\begingroup$ For a given ecosystem productivity level, you get more meat per input of plant nutrition from killing younger adults; that's why real-life factory farming doesn't let animals grow old before slaughtering. Also, many natural causes (disease) leave corpses emaciated. $\endgroup$ – Peter Cordes Nov 18 at 4:03
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I almost feel it would make more sense for the prey to offer the predators sacrifices rather than naturally dead individuals. This is because I feel that the predators wouldn’t get enough food from natural deaths. Maybe the prey species has an annual raffle. Whoever “wins” the raffle gets murdered, then offered to the predators.

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The closest thing would be carrion birds, which subsist off of dead animals. But they generally don't go after living things, so they wouldn't really be a threat to your prey. But if you're looking for a steady population that only subsists off of naturally-dying members of its prey, this might give you some ideas.

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Evolution gives predators that can kill live prey a distinct advantage. They can take control of their own reproduction by means of taking prey rather than waiting for prey to die.

With a larger subset of food, these predators can pass on more of their genes.

Think about it this way, if you had a fast growing population of prey, and you were a scavenger, you'd have to wait for them to get old and die to get a large portion of food. This puts a restriction on your offspring for a particular time until the prey ages.

Given a predator was able to take live animals from this growing population, they would have a bigger group to feast on than those who would have to wait for the eventual death of the initially booming population.

With more food while the group is growing, and more the same food when the prey dies of old age and other causes, they predator that is not strictly a predator takes the advantage in total food.

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It could work, but only with birth control - for both predators AND prey

Prey need predators to survive.

This might seem counter-intuitive, but the birth rates and growth rate of most prey animals evolved to function in an environment where they are being killed off on a regular basis. Consider that humans - a species that takes over a decade to reach maturity, and can produce maybe 15-20 children in a lifetime (and even that's a stretch) would start to push overpopulation of the planet within a few centuries if they aren't being limited by other factors. Now replace that with rabbits, which can reach maturity in a year and produce nearly one thousand offspring over the course of its life. Even large herbivores like cows reach maturity much faster than humans, which can create significant population booms if nothing is killing them off.

The takeaway: If predators stop killing prey, the world will very quickly fill up with herbivores. Plants will be wiped out and then mass starvation will kill the society.

The only way for a predator-prey civilization to work is if they all learn to control their own birth rates very early on. Herbivores will be limited by the population of available plants, while carnivores will be limited by the expected birth and natural death rates of herbivores. (Herbivores must greatly outnumber carnivores, naturally). They will have to be very careful not to exceed the allowed limits, or they will face society-wide famine, likely leading to both reverting back to their primal state in order to survive.

Enforcing birth rate laws will be one of the main roles of the government in this society. These laws will likely change over time as well, depending on circumstance - new farming technology, like vertical farming, will allow for larger populations.

This setup will likely be harder on herbivores than carnivores. Many predators, particularly apex predators, already have natural birth control instincts modulated by their available food sources. Herbivores usually lack these instincts, since in the wild they can depend on predators to keep their numbers at a manageable level. It may also have interesting effects on the development of medical technology - since reducing the rate of natural deaths can create temporary food shortages for carnivores.

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What if predators are of one sex and prey of another. Predators know they cannot afford to kill off whole prey because it would mean extincion. Self-sex predatory would be not possible due to toxic blood or hormones for instance. (Did you know human blood is actually toxic to other humans?)

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