Baby monkeys smile in their sleep

Baby monkeys smile in their sleep

For the first time, baby Japanese macaques have been seen spontaneously smiling

When a newborn baby appears to smile for the first time it is perhaps one of the most rewarding experiences a new parent has.

Of course, the baby is probably not smiling in response to a social trigger. An apparent "smile" in the earliest days might simply be an unintentional change in facial expressions.

In 1959 a researcher called Peter Wolff called these "spontaneous neonatal smiles". They involve, he wrote, a "slow, gentle, sideward, and upward pull of the mouth, without rhythmical mouthing movements or contraction of other facial muscles".

In fact, this type of spontaneous smile often happens during sleep. However, the cause of these neonatal smiles is unclear.

We now know that humans are not alone in this seemingly innate reaction. A decade ago it was discovered that chimpanzee babies show similar spontaneous smiles.

Chimps are one of our closest living relatives in the animal kingdom today. As we share a common ancestor with them, which lived between six and 13 million years ago, the finding suggests that smiling goes back to at least that far.

A team led by Fumito Kawakami of Kyoto University's Primate Research Institute in Japan, has now discovered that spontaneous smiles go back even further.

"Infant Japanese macaques are super-cute and their facial configuration is almost the same as humans," says Kawakami. "So when I observed this smiling in an infant macaque, I simply thought 'oh this is the same as humans'."

After noticing some smiles while performing health checks on the macaques, he wanted to uncover whether it was more common.

To do so, his team monitored seven newborn Japanese macaques. They all showed spontaneous smiles at least once, within an hour of observation. In all cases they were in a state of REM (rapid eye movement) sleep.

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