"In primate societies, grooming is the underlying fabric of it all," said Dr Michael Gumert, a primatologist at the Nanyang Technological University in Singapore.

"It's a sign of friendship and family, and it's also something that can be exchanged for sexual services."

Dr Gumert spent 20 months observing 50 long-tailed macaques at a reserve in Central Kalimantan, Indonesia.

He observed that after a male grooms a female, the probability that she would have sex with him was three times greater.

Males would spend more time grooming a female if there were fewer females in the area.

"And when the female supply is higher, the male spends less time on grooming," said Dr Gumert.

"The mating actually becomes cheaper depending on the market."

The findings are a major contribution to the emerging theory of 'biological markets', which tries to explain the interaction of organisms in ways in which an exchange of commodities or services can be observed.

Dr Peter Hammerstein first proposed the concept of biological markets in 1994, along with Dr Ronald Noe.

"There is a very well-known mix of economic and mating markets in the human species itself," said Dr Noe, a primatologist at the University of Louis-Pasteur in Strasbourg.

"There are many examples of rich old men getting young attractive ladies."

Dr Hammerstein said that Gumert's findings indicate that the primates can alter their behaviour to match 'different market conditions.'

"It is not a rare phenomenon in nature that males have to make some 'mating effort' in order to get a female's permission to mate," said Dr Hammerstein, a professor at the Institute for Theoretical Biology at Humboldt University in Berlin.

"The interesting result of Dr Gumert's research on macaque mating is that the mating market seems to have an influence on the amount of this fee."