> daily ejaculation results in healthier sperm and greater fertility. Causality is rarely simple.

Sure, but I think here you are being overly adaptionist. Put another way, if we lived in a world where daily ejaculation did not improve sperm quality, would we still expect animals to masturbate? I would say yes.

It's just occurred to be that was in fact answered by the comedian, Jasper Carrott:

Q: Why do dogs lick their bollocks? A: Because they can.