Primatologists have uncovered a mating market amonsgt monkeys where long-tail macaques trade "grooming" for sex. Male macaques groom the females in exchange for more frequent mating and increased loyalty. As the number of potential male companions rises and females become relatively scarce, the price for sex rises and the amount of time spent grooming the females increases, the so-called "biological market theory".
But sex isn't the only commodity that grooming is traded for, females macaques will groom new mothers for the chance to hold her offspring, and rare skills can improve a monkey's pattern of grooming. In South Africa, a low-ranking vervet monkey in the wild was trained to open a box of apples, this rare skill landed her a pattern of grooming similar to a dominant animal. And when the reseracher increased the competition by training another low-ranking money to perform the same skill, a duoploy emerged with an equal (but lower than in monopoly) distribution of grooms being received by the two.