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In my fantasy world, I have created a sentient species descended from an arboreal felid (it evolved to eat squrriels and small monkeys). They are my elves. The one problem is that I want them to be herbivores. So I would like someone to help me figure out why a carnivore descendant would do this. This world has natural magic, which sort of radiates from magical sources such as mana springs and arcanacrystals) so feel free to use that in your answer.

You could start with the panda pattern. Giant pandas are carnivores which have adapted an all vegetarian diet. Your elves are also pandalike fallen carnivores.

https://en.wikipedia.org/wiki/Giant_panda

Despite its taxonomic classification as a carnivoran, the giant
panda's diet is primarily herbivorous, consisting almost exclusively
of bamboo. However, the giant panda still has the digestive system of
a carnivore, as well as carnivore-specific genes, and thus derives
little energy and little protein from consumption of bamboo. Its
ability to digest cellulose is ascribed to the microbes in its gut.
Pandas are born with sterile intestines and require bacteria obtained
from their mother's feces to digest vegetation. The giant panda is a
"highly specialized" animal with "unique adaptations", and has lived
in bamboo forests for millions of years. The average giant panda eats
as much as 9 to 14 kg (20 to 30 lb) of bamboo shoots a day to
compensate for the limited energy content of its diet. Ingestion of
such a large quantity of material is possible because of the rapid
passage of large amounts of indigestible plant material through the
short, straight digestive tract. It is also noted, however, that such
rapid passage of digesta limits the potential of microbial digestion
in the gastrointestinal tract, limiting alternative forms of
digestion. Given this voluminous diet, the giant panda defecates up to
40 times a day.[56] The limited energy input imposed on it by its diet
has affected the panda's behavior. The giant panda tends to limit its
social interactions and avoids steeply sloping terrain to limit its
energy expenditures.

Your elves would eat all the time and be large, sluggish, antisocial and productive of large quantities of splintery feces. That is pretty much elves, I think.

Pandas just stumbled into the bamboo forest and started eating. The bamboo probably does not get much from the deal. I think if your elves are intelligent it offers another more conventionally elvish property. These elves are gardeners in the manner of the acacia tree ants.

https://news.nationalgeographic.com/news/2013/11/131106-ants-tree-acacia-food-mutualism/

Trees Trap Ants Into Sweet Servitude
A sip of nectar enslaves ants to the trees they guard.

The elves grow and protect the trees, and in return the trees give them leaves and sugar. Like the ants, the elves are totally addicted to the leaves and sugar from this particular tree and so must carry supplies with if they travel. Fortunately the sugar and leaves ferment very well, providing concentrated calories and thus even though the elves are large, sluggish and antisocial the elf liquor helps them overcome this with wild dancing and parties.

The drink is also addictive to nonelves. If humans partake, they might find themselves dancing at an elf party until the drink runs out. That takes a long time.

The Carnivore Energy Trap

The problem with being a Carnivore is that meat is energy dense, and more readily digested.

Whether they hunt small or large prey, it means that they have to hunt fewer times, which is a great cost saving in time and energy. They can have smaller more streamlined digestive systems which takes less space, weighs less, and consumes less energy. These cost savings allow for a host of other adaptations that make them better hunters.

These adaptations also make existing on other food much harder. The streamlined gut reduces the nutrition extractable from less easily digested foods. Hunting adaptations cost energy and offer little benefit when browsing for vegetation. In short, trying to become a herbivore again, requires sacrificing on what is easy.

Ways out of the trap

Survival.

Those that are alive, are those that found a way to survive where others didn't. Congratulations your felids were the ones that didn't die.

They did it by changing their behaviours, and adapting. The first of the omnivours identified a relatively nutritious plant/plant parts. This could have been fruit. They gained access because they were more than scary enough to displace any other herbivore. Eventually the range of nutritious plants broadened as they adapted to eat harder food stuffs.

Intelligence.

At some point your felids became intelligent. If they were moderately intelligent earlier in their omnivore days it would be better, but any point would do well - except already being 100% herbivores. The intelligence required is not counting, language, or anything terribly complicated. I'm talking about the intelligence required to work with fire, and specifically how to cook with it.

It would have had a similar effect for them as it did for us humans. Suddenly inedible foods are now safe, and nutritious.

As to why fire did not herald a new age of predatory behaviour is going to be a tough sell though. Fire opens a lot of door-ways including weapons, and improved hunting tactics.

Either have all the mega-fauna be extinct, or make it so that mega-fauna are quite powerful. I'm thinking dragon class here. Very difficult to kill, very likely to wipe out entire clans, almost impervious to sharp weapons, ignorant of fire, and foul tasting (even when cooked).

As for other smaller prey, make them scarce, or make them exceptionally hard to hunt even with advanced tools and fire.

You will have a few still hunting but the art will slowly fade from the culture as more and more individuals support themselves by gathering and eventually farming.

At the same time have fire open up even better tasting, and nutritious plants.

The change from carnivorous to herbivorous is not likely to happen quickly. For example, it would be unlikely for intelligent carnivores to just choose to go vegetarian as they wouldn't have an even approximately appropriate digestive system. You would end up with a bunch of sickly, lethargic and grumpy Elves.

What could happen would be a gradual evolutionary change from carnivore to herbivore - more likely if they were already partially omnivorous. This could be due to environmental pressures. For example, repeated boom/bust in squirrel population means that only the more omnivorous felids survive periods of starvation. Then either some environmental disaster totally devastates the squirrel (and monkey) population so felids go totally vegetarian, or now social/religious/ethical drives could push them in that direction.

As a real-world example, consider bears - polar bears are almost totally carnivore; pandas are totally herbivore; but they both descend from the same ancestral proto-bear.

But you are probably needing many thousands of years (possibly up to millions) for this to happen.

Farming and direct choice

It is a lot easier to farm plants than it is to farm animals, and easier to store them as well. You don't have to feed the dried apples you have in your storehouse over the winter. Cattle still have to be fed, which means you're growing and storing the food anyway.

Instead of evolving this way, your elves deliberately chose this path. While they may have been carnivores or partial omnivores, they steered their culture towards farming - And while obligate carnivores require meat, your elves were able to harness the nature magic to change their whole physiology to accept and thrive on plants - And to have those plants produce more and better nutrients. This is why elvish travel bread is so good for its size.

Some cats are mostly vegetarian. Dried cat food is 70% grain. With a bit of meat flavouring they like it enough.

Meat diets requires far more land per person than vegetarian. So resource limitations would put pressure to do more with less. A push factor.

Any civilisation forms after they can generate food surpluses, freeing up labour for other things, like war, religion, building wagons. These are pull factors for eating vegetarian diets.

And an event could precipitate the change, a volcano blocking the sun for a year, forced them to conserve food by adding plant matter. Afterwards, no one got hurt and it was cheaper. Poorer families kept eating veggies with their meat.

The Elves evolved from arboreal felids to herbivores due to a combination of their intelligence, dwindling food supply, gut bacteria, and culture. The primeval elf was a highly intelligent tool user that hunted the other arboreal creatures to near extinction. Their use of pack tactics, tools (spears, bow/arrow, etc), and high intelligence allowed them to continue constantly increasing in number past what their food supply could sustain by becoming more and more efficient at hunting the remaining animals. When the tree-dwelling animals were hunted to near extinction they moved to consume land-based animals which they also drove to near extinction.

When the great famine finally came it was vicious, the Elves would kill and eat anything that moved and would devour every portion, skin organs, everything. They had taken to the practice of eating any animals they caught immediately, raw and whole to prevent the food from being stolen by fellow elves. This was the beginning of the turn around for the elven race. By eating the stomachs and intestines of herbivores raw they incorporated the gut bacteria of their victims with their own and were able to a limited extent able to start effectively digesting the abundant fruits in the area.

Over time the elves evolved to be able to handle a fully plant-based diet via natural selection while the animal populations in the areas they lived in recovered. The elves never forgot this part of their legacy which they carried on via oral tradition for hundreds of years which shaped their culture into one that strives to this day to live in a sustainable equilibrium with nature.