Coevolution: Monkeys Distinguish Ripened Fruit Based On Odor, Researchers Say

Spider monkeys rely on their sense of smell in order to determine when fruit has ripened to perfection, a new study revealed. Researchers from Linköping University presented five monkeys from the Mexican rainforest ten combinations of flavors that resembled the two tropical tree species' fruit in varying periods of ripeness. This allowed researchers to see if monkeys could actually sense when fruits had reached their optimal ripeness, based solely on odor.  

"Distinguishing between the ripe and the completely unripe odor -- they learned this in one day. And when the mixtures were made more similar to each other, the monkeys could still tell the difference quickly," Matthias Laska, a professor of zoology at Linköping University, explained in a news release. 

Their study, recently published in Scientific Reports, suggests that monkeys have a better sense of smell than we often give them credit for. 

This is similar to how humans have learned to harvest crops at the appropriate time or base their diets on foods they know they like. Essentially, the spider monkeys have learned to associate the odor of ripe fruit with fruits that taste sweet and delicious. For their study, researchers were able to simulate this same reward system by offering the monkeys a treat when selecting the correct (ripened) mixture. 

This is an example of coevolution, where the fruit-eating monkeys and the tropical fruit trees have adapted to each other's needs: the plants want their fruit to be eaten only when the seeds are ready to be dispersed and the monkeys get more nutritional benefits from ripened fruit that contains energy-rich sugars. 

"This adaptation goes back some fifty thousand years, when the first primates appeared. Initially they ate mostly insects, before eventually trying fruit and vegetables. Some fruit didn't want to be eaten, so they developed toxic substances. Others acquired better and better odors that signaled energy-rich sugar and nutrients," Laska explained. 

This ability has also been observed in tropical bats, whereas birds are rely more heavily on their vision and are attracted mainly to pretty colors. 

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