Fascinating article in Slate on the subject of whether other species masturbate (they do, most enthusiastically). It's a topic I covered in Vol. I of The Truth About Sex.
But my book is more of a "least you need to know" about all the important issues and theories in human sexuality, whereas this piece goes in depth into the subject of animal behaviors, citing a range of studies and sources.
Despite this bestiary of autoeroticism, scientists have spent relatively little time on the question of why animals might have evolved to masturbate. At first glance, the behavior would seem to be maladaptive. First, there's all the energy that's wasted on the production of spilled seed—macaques, for example, are thought to devote between 1 percent and 6 percent of their daily metabolism to the production of ejaculate. Second, it distracts the animal from the more important work of finding food and evading predators, let alone mating. According to the literature on horses, a masturbating stallion sometimes takes on "a trance-like, glazed-eye appearance." What could be more inviting to a hungry bear?
Also a topic I hammer in my book, is how science writers go wrong when they assume that sex is for reproduction. If they stood back and examined the scientific research on the benefits of human masturbation, perhaps the picture would be clearer to them.
There's a reason the masturbatory impulse is instinctual in most forms of animal life: masturbation is healthy. Orgasms extend longevity. If you think of an orgasm as a kind of full-body wellness exercise --which, on an organic and chemical level it is -- then you can easily understand why animals masturbate. It easily explains why all species try to have orgasms as often as possible.
If you persist in believing the religious ideology that we were all put on the earth to procreate well, then, you end up treading into some of the muddy fuzzy places that detracts from Slate's otherwise excellent piece.
Get The Truth About Sex. Really.