When fruit is scarce, these monkeys hunt animals

Same with bears. For instance, the grizzly (brown) bears in Denali National Park who are farther away from salmon breeding areas (coastal areas) tend to eat a lot more berries than their coastal counterparts.

A wild animal documentary that I saw mentioned that stags would eat their own bloodied velvet hangings that were scratched off on branches ("velvet" is the living covering of the antlers at the beginning of the growing season).

It claimed "biologists don't know why the deer, which are herbivores, eat the bloody remains." Which is stupid, of course.

The answer is because deer can't read, so they don't know they are "herbivores".

When cows have phosphorus deficiency, they occasionally eat small birds, rodents, insects, or bones

Is this just based on that one video or is there something more to this

Same here, first time I had balut in Vietnam it unlocked some kind of primal level in me I didn’t know I had.

Horses are clearly and very much into the whole cowboy thing, they LIKE chasing down cows and catching them, some do it all by themseves and catch cows by grabbing there tails and hauling them around that way. Horses are panoptic motherfuckers and are startlingly good stalkers, which I learned living in close quarters with a horse needing to chase them away while doing stuff in the padock, but have them reapear, it took very precise sneeky peeking to catch them moving towards me, as they only move when I am not looking, which led me to the only viable deterance, which is to pretend that they are invisible/of zero importance which un nerves them, but so much as one eyelash of a look at them is taken as an invitation to partisipate, fuckers. Horses also work well with dogs, and for arabians in Arabia, dates and dried fish were used as feed for both. note, live with an arab/percheron cross far enough out that there is a wide representation of wild critters, and most everything is on most everythings menue, depending. And one of my more radicle, radicle friends would say, "we will eat the vegans first, they are slow and weak", refering to what to do durring an opocaliptic event, and until they could get out to my place in the country.

A biologist with the surname “Wild.” That woman was meant to be in this field.

beat me to it

Everything an animal does isn't a selective advantage, sometimes it's just not a disadvantage.

Ground squirrels, and the name is key. I haven't researched, but I know that often such animals eat small insects. And worms. Insects are meat, too.

So if there is a vole population explosion, and voles live underground, and are hungry, a ground squirrels den is going to get invaded, with the voles trying to steal food. And the ground squirrel will of course attack and kill intruders. Especially if they have young in that underground nest.

So it's not a far stretch for them to get used to the taste. I think they prefer nuts, because they can store nuts, but on that front they also eat berries and really anything they find, so storage isn't a diet limiter. And after killing a vole, why waste it?

that we are animals?

Eating animal is easy, mostly proteins and fat that are also in plants. The main difference I can think of now is that animals have glycogen instead of starch. Glycogen is more branchy than starch, but IIRC it's not so difficult to digest and there is not to much of it anyway.

The animals adapted to eat plants probably have a gut that is too long, so it will be less efficient, and I guess they have to remember to eat their vegetables to avoid problems.

The reverse is harder. An animal adapted to eat mostly animals have a short gut and will not extract all the nutrients from the plants. Also, plants have cellulose that is super hard to digest, so some animals have special parts of the digestive tube to let the food wait and be partially digested by friendly bacteria. (Yes it's gross, unless you think it's almost like free beer.)

Would they spend the energy if there wasn't any gain to it?

I don't feel like the article is lacking for that. It's evolution at work. There could be monkeys eating animals that can properly digest them due to some mutation or unique gut biomes. Actually eating the animals is an important first step. Monkeys that usually eat fruits are hunting and eating animals when food is scarce, that's an article worthy observation.