Common marmosets, Callithrix jacchus, are vocal little monkeys from South America with a hearing range similar to that of humans. And like us, it appears these New World primates can perceive pitch.
The findings, published in Proceedings of the National Academy of Sciences this week, suggest that certain aspects of pitch perception emerged early in the evolution of primates. Our species have been separated by at least 40 million years.
Being able to perceive the sound quality known as pitch means distinguishing between high and low notes. For humans, it’s crucial for processing speech and making music. But are the underlying mechanisms of pitch perception unique to just us? Recent studies with two Old World macaque monkeys and the common marmoset – which is located between macaques and non-primate mammals on the evolutionary tree – indicate that we’re not the only ones with pitch perception abilities.
Marmosets have a rich vocal repertoire: phee, twitter, egg, moan, and squeal calls, just to name a few. And the nerves in a brain region called the primary auditory cortex respond to pitch-evoking sounds in a similar way to our brains.
To see if primates perceive pitch the way we do, a Johns Hopkins team led by Xindong Song gave hearing tests to four male adult common marmosets trained to lick a feeding tube near its mouth for a food reward after hearing a change in pitch. During testing, the marmosets were seated in the center of a sound isolation chamber lined with thick acoustic absorption foam; sounds were played through a speaker powered by an amplifier mounted 40 centimeters (15 inches) in front of the monkey.
The marmosets exhibited all three main features of human pitch perception. First, people are better at distinguishing differences in pitch at low rather than high frequencies. Next, we pick up on subtle changes in the spread between pitches at low frequencies. For example, we notice if a series of tones increases by just 100 hertz. And finally, at high frequencies, people's abilities to perceive pitch differences among simultaneous tones is related to how sensitive they are to the rhythm of sound waves.
Together, these findings suggest that human-like pitch perception mechanisms originated relatively early in primate evolution – perhaps as early as (or even earlier than) 40 million years ago when New World and Old World primates (like us) split. As study author Xiaoqin Wang of Johns Hopkins explains in a statement: "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."
