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This is because humans have a voice print: our vocal tract leaves a unique signature in the tone of our voice across everything that we say.

“You can push air through your vocal tract, create these tones and rhythms, and it’s like giving the baby an analgesic,” Dr. Mehr said.

Over a period of about 20 months, 128 disk-shaped electrodes placed on top of Pancho’s sensorimotor cortex recorded electrical activity in brain regions involved in speech processing and vocal tract control as he attempted to speak words aloud.

There are around 50 phonemes in English, and each is created by a stereotyped movement of the vocal tract, tongue and lips.

Elizabeth St. Clair, a biological anthropologist at Johns Hopkins University who studies the evolution of the primate vocal tract, says she was surprised by the rhythmic similarities between indri and human songs.