> And I've always found the argument that "more intelligent/sentient creatures deserve more protection and rights" to be basically a post hoc defense against cannibalism.
It's not some veiled aversion to cannibalism, it's because humans have empathy for other humans, and our empathy for non-humans scales with how human-like we perceive those animals to be. If someone sees intelligence as a defining trait of humanity, then they're likely to empathize with animals that display great intelligence. And if you empathize with the animal, you're more likely to be sensitive to its perceived suffering.
Raw intelligence isn't the only thing that drives our empathy toward animals though. I'd argue that it isn't even the main thing.
We care much more about an animal's biological/genetic similarity to ourselves, which is why people are comfortable eating octopuses but not lemurs, even though octopuses are much better problem solvers and lemurs are relatively dumb.
We also care more about sociability / the animal's ability to communicate with humans. This is why people are more comfortable eating pigs instead of dogs. Pigs might be smarter, but dogs are much better at communicating with us, eager to please, etc.
This is completely cultural and has little to do with "genetic similarity". People have been eating monkeys since forever. Monkeys that have the most human-looking gazes.
It's not some veiled aversion to cannibalism, it's because humans have empathy for other humans, and our empathy for non-humans scales with how human-like we perceive those animals to be. If someone sees intelligence as a defining trait of humanity, then they're likely to empathize with animals that display great intelligence. And if you empathize with the animal, you're more likely to be sensitive to its perceived suffering.