In areas where there are fewer females, males are forced to groom their partners for up to twice as long before they are able to have sex, the research found.

Sexual activity among a 50-strong group of long-tailed macaques in Kalimantan Tengah, Indonesia, increased after bouts of male-to-female grooming, according to findings published in Animal Behaviour journal and reported in New Scientist.

On average, females had sex 1.5 times an hour, but it jumped to 3.5 times an hour immediately after the female monkey was groomed by their male partner.

The females were also less likely to look elsewhere for sex after being groomed.

The monkey version of the "world's oldest profession" is also a rare example of market forces acting in nature, with the availability of females affecting the "price" of mating.

Michael Gumert of Nanyang Technological University, Singapore, said unlike examples of "reciprocal altruism" - in which one organism provides a service to another in return for getting something back at a later date - the value of sex fluctuated like any other economic commodity.