Study: Monkeys 'pay' for sex by grooming
Here is an interesting quote from the article:
And as with other commodities, the value of sex is affected by supply and demand factors: A male would spend more time grooming a female if there were fewer females in the vicinity.
"And when the female supply is higher, the male spends less time on grooming ... The mating actually becomes cheaper depending on the market," Gumert said.