Baby Macaques Smile In Their Sleep, Just Like Humans

Macaque smiles don't always mean the same thing as human smiles. Gabi Siebenhuehner/Shutterstock

Researchers in Japan have swapped their Bunsen burners and petri dishes for a much cuter piece of lab equipment, using baby macaques to investigate the evolutionary origins of smiling and laughter. By watching the facial movements of these lovable monkeys as they slept, the team discovered that, like human babies, newborn macaques display “spontaneous smiles”, which could indicate that grinning and beaming have much deeper evolutionary roots than we once thought.

Among humans, smiles are obviously used to convey feelings of joy and happiness, although previous research has shown that we actually have to “practice” this basic facial expression during the first few months of our lives. Newborn babies are known to regularly crack out spontaneous smiles in their sleep, but tend to grow out of the habit around the time that they begin to show “social smiles” during waking hours.

Scientists therefore believe that the purpose of these early unconscious smiles is to train the zygomaticus major muscle, which is responsible for drawing up the edges of the mouth in order to form smiles. Until recently, it had been assumed that spontaneous smiling while sleeping was unique to human infants, although a 2006 study found that baby chimpanzees also engage in this adorable nap-time behavior.

To discover whether the trait can also be observed in Old World monkeys, the researchers watched seven newborn Japanese macaques while they slept for a period of 44 minutes each. Their results – which have since been published in the journal Primates – show that all seven monkeys did spontaneously smile in their sleep, doing so an average of 8.29 times each.

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