In the wake of Spitzergate, many people have wondered whether prostitution really is the oldest profession. Well, not only is it older than prop comedy; it's older than man.
Some suggestive evidence comes from research published in Animal Behavior in December. Male macaques spend more time grooming females who are in heat, and the more time they spend, the more sex they get. Male monkeys also groom potential mates more diligently when fewer females are around, thus raising the prices. The researcher, Michael Gumert, describes grooming as a kind of currency.
One sees a more obvious monkey correlate of prostitution in an experiment at Yale that went awry. Capuchins were taught to value tokens and exchange them for food, but when left to their own devices, and before the experimenters could stop them, the cheeky monkeys also demonstrated their willingness to exchange tokens with each other for nookie.
So prostitution, or something like it, is very old. Does that mean we should legalize it? Maybe, but not necessarily. Consider the naturalistic fallacy ("what is natural is good"). Is what's good for monkeys good for humans? After all, some types of monkeys kill infants. And others happily live in barrels.
Looking at the tradeoffs animals (and humans) make in the mating economy at least gives reason to widen the spectrum of what counts at prostitution, making the current definition arbitrary and perhaps meaningless. As Cacilda Jethá and Christopher Ryan write, "Classic Darwinian theory posits that the female of most mammalian species trade sexual fidelity for various things the male can offer: protection, status, meat, and so on. So, if we accept the basic assumptions of sexual selection theory, aren't pretty much ALL female mammals prostitutes?"
Maybe, but somehow I don't think that excuse will work well if you accidentally call your girlfriend a ho.