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Greenpeon: Does seem like perpetual motion. And although these particular ones eat food too, making the whole thing less physically impossible, it seems like it's just a lifestyle choice and others don't. At first I was thinking maybe they supplement with livestock blood? But no sign of that yet.
I'm willing to set it aside though. It's part of the premise, and I'm usually willing to pretend I buy even a very outlandish premise as long as things work fairly plausibly from there. I kind of like the story so far. Although when it comes to nonhumans in a post-human-apocalypse world, it's hard to beat
Donyatsu.