A Japanese monkey munching on a persimmon.

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Social Learning in Macaque Troops

Let's say you're dining with friends at a pizzeria, and one person orders a pie with anchovies. You've never tried anchovies, but you turn up your nose at the odorous fish. When the pizza comes, your friends devour the anchovy slices. Observing their positive reactions, you decide to live a little and take a bite. The next time you eat pizza, you take yours with anchovies as well.

A similar type of behavioral learning through social interaction happened in a group of Japanese monkeys on the island of Koshima in the 1950s. Researchers were studying the macaques and to lure them into the open to better observe their behavior, the researchers gave them sweet potatoes.

Usually, the monkeys would brush excess dirt off the potatoes with their hands before eating them because the grit hurt their teeth. But one day in September 1953, a female the researchers named Imo took the rudimentary potato cleaning a step further by washing hers in a freshwater stream. Five years later, six out of nine members of Imo's family practiced the same potato-washing habit [source: Carpenter]. By 1965, not only had the washing habit been passed to new generations of the macaques, but they also had started rinsing with saltwater instead of freshwater. The monkeys would repeatedly dip their potatoes in the water, then take a bite, implying that they enjoyed the salty flavor. For that reason, researchers suspected that the purpose behind the saltwater habit had evolved from merely cleaning the potatoes to actually seasoning them.

Lead researcher Masao Kawai published the findings in 1965 in the journal "Primates," under the title "Newly-acquired pre-cultural behavior of the natural troop of Japanese monkeys on Koshima islet." Notice there's nothing in that heading about food. That's because the monkeys' acquisition of a taste for salt was merely a byproduct of what had unfolded. Rather, the social learning in macaque troops garnered attention because it implied that monkeys are capable of creating basic cultural practices.

Taking a closer look at how the saltwater-dipping custom caught on throughout the macaque family uncovers other clues at the development of the novel habits. Read on to learn the details of how the potato seasoning spread in the macaque troop.