I love the statistic that male Macaques spend 1-6% of their metabolism on production of ejaculate though.
He puts it down to dealing with being in an unnatural environment and makes parallels with people living in cities.
One of the recent results in human fertility is that 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.
We probably do live in such a world. The "sperm quality" issue probably doesn't come up with animals that spawn at a given time of year.
Note that "sperm quality improves with masturbation" is a partial explanation for why masturbation is pleasurable for those species who do.