I edited a conclusion in, maybe we can agree on it.

Incidentally, because I'm puzzled by the "perpetual motion" aspect too, another lever the body can pull is to reduce or increase energy that goes to the immune system and the brain. (But in searching for the Hadza I found https://news.ycombinator.com/item?id=47042766 "Thinking hard burns almost no calories but destroys your next workout", so it's debatable.)