Dr. Michael David Gumert with macaque monkeys (Credit: Hiram College)

Though it has been observed that primates trade grooming for food sharing or infant care, this is the first time this kind of exchange was observed between male and female primates in a sexual context. From the human point of view, it might seem trivial that such actions lead to sex between mammals, but the study's author suggests a more favorable interpretation. The macaques' exchange of services illustrates a simple system of cooperation that allows for successful mating. The basic premise, says Gumert, is called the Biological Market Theory, which follows the elementary principles of supply and demand. 

In the case of the voluntary sex life of long-tailed macaques, it means that the price that one group is willing to pay for a commodity that the other group has depends on the scarcity or abundance of that commodity on the market. Scientists assume female macaques use grooming, too, to try to maintain social relationships to benefit their offspring, or as a way to distract or appease males from becoming overly aggressive – a behavior that is quite common after a sexual encounter. In fact, when female macaques groomed males, their services decreased.  

Macaque monkey (Credit: Blaise Droz)

That inquiry is at the heart of primate studies similar to Gumert's. While scientists' knowledge of the long-tailed macaques' social world is growing, this recent study claims that figuring out how this market concept can be applied to the social settings of other animals, including humans, will be its main value. 

TFOT has covered a robotic lemur currently under development by NASA, and MIT's "HolyBot", which can actually walk on water. 

Gumert's original article can be found on sciencedirect (subscription required), and more information on this research project can be found in the Time/CNN special report.

Top image: Macaque monkeys in Tibet (Credit: Noel Rowe).