Monkeys known as marmosets can use auditory cues, as humans do, to distinguish between high and low pitches, Johns Hopkins researchers reveal in a new study. This sheds light on the evolution of vocal communication and song.

"Pitch perception is essential to our [human] ability to communicate and make music," Dr. Xiaoqin Wang, one of the study researchers and biomedical engineering professor at Johns Hopkins University School of Medicine, explained in a news release. "But until now, we didn't think any animal species, including monkeys, perceived it the way we do. Now we know that marmosets, and likely other primate ancestors, do."

Marmosets are small, social monkeys native to South America. Similar to other primates, these animals rely on vocalizations and expressions to communicate with one another.  Researchers began studying marmoset pitch perception nearly 20 years ago, and while the scientists have since identified the region in the animals' brains that processes pitch, until now they lacked behavioral evidence.

Humans have three specialized features of pitch perception.  For starters, people are better at distinguishing pitch differences at low frequencies than high, and they can pick up on subtle differences. Furthermore, a person's ability to perceive pitch differences among tones played simultaneously is based on how sensitive that person is to rhythm.

The team spent years creating behavioral tests and specialized equipment, designed to monitor subtle changes in the monkeys' neural activity and their response to differences in pitch. Using a series of hearing tests, and training a group of marmosets to lick a waterspout only after hearing a change in pitch, researchers finally discovered the animals share all three features with humans. This suggests pitch perception evolved in primates much earlier than previously thought, before it was inherited by modern humans.

"In addition to the evolutionary implications of this discovery, I'm looking forward to what we will be able to learn about human pitch perception now that we have a primate relative we can study behaviorally and physiologically," Wang added in the university's release. "Now we can explore questions about what goes wrong in people who are tone deaf and whether perfect pitch is an inherited or learned trait."

Their findings were recently published in the Proceedings of the National Academy of Sciences.

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