But laughter, I suspect, may be rarer by far than mercy.

Curious why you suspect this. Is it particularly mammalian in some respect? I confess I could be naive, but it seems to me that any sufficiently intelligent being/agent would be just as likely as we humans are to have humor. I suppose that raises the question of how likely it is, and are we just incredibly lucky to have inherited such a trait. Still, it's such a core aspect of so much of our species -- even more than mercy, I think! -- that I'm curious why you think that.