The [macaque] males use grooming as a form of currency to buy sex from the female, the study found. ... Left to her own desires, a female macaque mates about 1.5 times an hour, but that rate jumps to 3.5 times an hour when the male partner first provided grooming services, the study found.What scientists call "currency," the rest of us call "foreplay."
Seriously, the problem here is that personal services are not currency. I don't know what would qualify as prostitution in a barter economy, but I think at minimum the female would have to receive, in exchange for sex, something she could exchange for something else instead of using it herself. Unless the male macaques were handing out day-spa gift certificates, this doesn't qualify.
UPDATE: This, however, does qualify as monkey prostitution:
Something else happened during that chaotic scene, something that convinced [Keith] Chen of the monkeys' true grasp of money. Perhaps the most distinguishing characteristic of money, after all, is its fungibility, the fact that it can be used to buy not just food but anything. ... What he witnessed was probably the first observed exchange of money for sex in the history of monkeykind. (Further proof that the monkeys truly understood money: the monkey who was paid for sex immediately traded the token in for a grape.)