Monkeys Whisper, Study Says—But Why?

Cotton-top tamarin photo
A cotton-top tamarin. Photograph by Joel Sartore, National Geographic

By Jennifer S. Holland

Here’s a little secret: Humans aren’t the only animals who know how to speak softly.

Gophers sometimes whisper. There are bats that do it to avoid detection by moth prey. A certain fish does it to initiate sex (whispering sweet nothings into her mate’s ear canal?). (See “‘Whispering’ Bat Evolved to Trick Prey.”)

Now we can add a non-human primate to the list of low talkers. Scientists Rachel Morrison and Diana Reiss of the City University of New York recently reported their discovery that cotton-top tamarins (Saguinus oedipus) can choose to keep their voices down when a predator or otherwise unwelcome visitor is nearby.

It was a serendipitous finding. The biologists initially were studying the loud calls and “mobbing behavior” that tamarins often belt out in response to seeing people they fear.

A particular zoo worker whom the monkeys had “mobbed” previously would enter their enclosure, and the scientists would record the animals’ reactions. But time and again, instead of lunging and making ear-splitting noise as expected, the tamarins seemed to go silent when they saw their nemesis.

It turned out, though, that they weren’t completely holding their tongues. Instead the monkeys were doing what the scientists call “low amplitude signaling,” which in this case means very soft chirps, too soft for humans to hear. The scientists described it as a more cautious type of alarm, as if the animals weren’t sure what this potential threat held in store for them and were “discussing” the proper response.

If it weren’t for the ability to create and analyze spectrograms—visual representations of sound—this whispered communication between the monkeys would have remained a secret, according to the study, published recently in the journal Zoo Biology.

Of course, being able to whisper makes perfect sense for cooperative animals like tamarins (and humans); while loud alarm calls are appropriate in many situations, other times the animals need to avoid detection or thwart eavesdroppers while still getting the word out.

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