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“[Fantasy humanoid monsters] also tend to be described as stupid and animal-ish, which implies less energy is spent on the brain.”

That depends on the depiction. If they’re being shown as intelligent animals (equivalent to forest apes or elephants), then yes, they’re utilizing significantly less energy on the brain. However, if they’re being described as slightly stupider and more brutish than humans but are still using language, wearing clothes and armor and weapons (even if stolen), and having stratified societies with chiefs and warrior classes, then we’re probably looking at an issue a lot closer to the aforementioned Neanderthals – which did eat a primarily meat diet, still had to cook their food, and still required double the energy requirements of a modern human of roughly the same size.

A middle point would probably be the last hominid ancestor that we know for sure could not cook, H. habilis, which only produced and utilized relatively simple, unspecialized stone tools. Something of that intelligence level might be able to figure out simple weapons if you gave them to it, but I rather doubt they’d know what to do with the armor.

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“What reason is that? The fossil record has plenty, though of course it is possible that many, like grizzlies were omnivores. My understanding is that the presence of large predators is only limited by the presense of large prey.”

The fossil record contains many separate species of large mammalian predator, but except for unusual preservational situations with predator traps such as La Brea Tar Pits they tend to be much rarer finds.

The reason for this is twofold. One, warm-bloodedness is an extremely expensive way to live. We don’t tend to think about it that way because it’s such a common adaptation in most large terrestrial animals, but as an example, a large dog and a hypothetical reptile of equivalent size would have radically different energy needs. The dog would need approximately ten times the calories of the reptile, because it burns the extra 90% simply to maintain its internal body temperature. In terms of total species and biomass, being cold-blooded is more efficient and more common, although the evolution of large, warm-blooded terrestrial animals in the late Permian and early Triassic likely forced most reptiles out of this ecological niche.

Secondly, because a warm-blooded predator requires so much food, they tend to hold very large territories. The sizes of these fluctuate based on prey density and the overall productiveness of the environment, i.e. a lion pride in semidesert scrub will have a much larger territory than one in rich savannah. The territory size will also depend on the size of the predator and, as such, its total energy needs.

While the fossil record compounds this rarity due to the unlikeliness of preservation for any terrestrial animal, modern predatory mammals probably give us our best snapshot of large warm-blooded predator density. In extremely productive environments with an abundance of large prey they can be very common (i.e. African savannah), while in areas where large prey is less abundant their territories either expand significantly (tigers living in seasonal forests), the predators get physically smaller (mountain lions, wolves), or their diet becomes more diverse (bears).

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“So while getting rid of unneeded mass is one way to re-route energy to the brain, i think it is wrong to assume that smaller is a better foundation for intelligence.”

It’s not smaller so much as lighter. Humans have less mass in our bones and muscles than would be expected for a primate of equivalent size. Neanderthals represent a more robust (heavy-boned and heavy-muscled) example of humans, and as noted in the article they required a significant amount more calories. While increasing gracility would be limited by biomechanical constraints – i.e., how much bone and muscle you need to hold up a body and move it in any given planetary gravity – it would be a potential option for rerouting energy to increasing brain size.

A couple comments:

“Fantasy monsters and evil humanoids in stories tend to be described as larger and stronger than humans (sometimes quite significantly so)…”

They also tend to be described as stupid and animal-ish, which implies less energy is spent on the brain.

“…there’s a very good reason why large mammalian predators, whatever their intelligence level, are rare.”

What reason is that? The fossil record has plenty, though of course it is possible that many, like grizzlies were omnivores. My understanding is that the presence of large predators is only limited by the presense of large prey.

“On the science fiction front, it presents limitations that should be kept in mind for any sapient alien. To build a large brain, either body mass has to give somewhere (muscle, bone, guts) or the caloric intake needs to increase to keep pace with the higher energy costs.”

As i understand things, the amount of brain mass needed for sapiency is probably fixed. So while us humans devote 20% of our energy to running our brain, a sapient elephant-oid, or whale-oid of similar intelligence would need less of their total energy. Yes, their brains would need to be bigger than ours to manage their larger bodies, but the percentage of their brain/energy needed for sapience would be small or even trivial in a whale-oid’s case.

So while getting rid of unneeded mass is one way to re-route energy to the brain, i think it is wrong to assume that smaller is a better foundation for intelligence. If i’m right that the amount of neurons needed for sapience is roughly fixed, then a sapient brain hobbit would use a larger percentage of its energy than humans do. A small sapient would need to have an extremely efficient eating strategy. And at some point of smallness would be too small to have any energy left over for anything but the brain.

In the case of giant warm-blooded sapients, no matter how they ate, they could still support far fewer individuals per acre. Which would presumably make forging a civilization harder — which IMHO is an important point. As individuals, large sapient bauplans, would probably be more efficient (assuming it is not too big to be supported by the local food supply. But that efficiency advantage might be erased by the multiplying power of civilization that smaller sapients can more easily leverage.