A reader writes:
I'd posit that the reason humans "shake hands with the unemployed" more frequently than similar species is that our mating and child rearing is a whole lot more complex than for most animals. Humans produce few offspring and there's a much greater time investment in each child because young humans are unusually vulnerable for several years after birth.
Due to the complexity of mating, humans end up with a lot of ... time on our hands. We have to have the strong sex drive or we'd never procreate but at the same time we need a way to release the tension that comes about from the extended courtship process. If that wasn't the case, we'd make more rash decisions during the courtship process and end up with children that aren't being raised properly.
Thanks to you, I now refer to it as my "portable marvel."
I hesitate to link to this, but this chimp seems to have as much time on his hands as many humans. A reader writes:
If the impulse to masturbate is purely an instinctual stimulus-response action in animals then experimentation of this sort would be very unlikely. The fact that the chimp uses the frog in this way shows that it had actually thought about what it was wanting to do. Whether or not other animals do this would be an interesting question.