Sweet dreams? Baby Japanese macaques smile more while they sleep than any other species – far more than human babies or infant chimpanzees.

Baby humans and chimps were the only species known to smile in their sleep. Now Fumito Kawakami at Kyoto University in Japan, and his colleagues are looking for others.

The researchers filmed seven snoozing Japanese macaques aged between 4 and 21 days and recorded how often they smiled during rapid eye movement (REM) sleep, which is associated with dream states, and non-REM sleep.

The baby macaques smiled 41 times an hour in REM sleep, and not at all in non-REM sleep. In comparison, human babies smile twice per hour, while dozing infant chimpanzees smile just once in 5 hours in REM sleep.


“We are not sure what is triggering this smiling in their brains,” says Kawakami.

Spontaneous or social smiling?

The types of smiling seen in newborns are different to those of adults, says Kawakami.

Before about 2 months of age, newborns often smile as they sleep, but it is just a spontaneous lifting of one or both corners of the mouth. After this age, spontaneous smiles are gradually replaced by ‘social smiles’– voluntary facial expressions that adult people and primates use to communicate.

Spontaneous smiles were once believed to help foster parental love, but ultrasound images showing these expressions in unborn human fetuses undermined that idea, says Kawakami.

Spontaneous smiles probably help develop facial muscles for social smiling later on, he says. Another possibility is that the infants grin in response to their dreams, but this is difficult to establish, Kawakami says.

The researchers are now going to look for spontaneous smiles in newborns of other primate and perhaps even non-primate species to better understand their function.

Ben Gee of Melbourne Zoo in Australia, where orangutans and baboons are raised, says he has not seen any smilers. “We recently hand-raised a young baboon, so we had a good chance of seeing that behaviour, but we didn’t,” he says.

Journal reference: Primates, DOI: 10.1007/s10329-016-0558-7

Read more: Sleep: What it is and what it’s for