A recent study conducted in Indonesia shows that male primates "paid" for sexual access to females in the form of pre-sex rituals, and that the success of these rituals was reduced as the number of available females went up. Although the conclusions are not decisive, it is suggested that sex is a "currency" in the monkeys' biological market.
In a recent study of macaque monkeys in Indonesia, researchers found that males performed several services in order to encourage females to agree to sexual intercourse. The article, "Payment for Sex in a Macaque Mating Market," was published in the December issue of Animal Behavior and is based on the work of a group of scientists led by Michael David Gumert from Hiram College in Ohio. It describes the behavior of about 50 long-tailed macaques in Kalimantan Tengah, Indonesia. After observing the monkeys for about 20 months, the scientists concluded that after the males performed their grooming ritual, sexual activity more than doubled: from an average of 1.5 times an hour to 3.5 times an hour. The study also showed that the amount of time that males spent grooming hinged on the number of females available at the time. For instance, as the number of females increased, the males spent less time picking nits off the females.