Often during rapid eye movement (REM) sleep, human infants exhibit a spontaneous smiling effect on their faces. This involuntary response is common and decreases with age, but it may not be limited to human beings.
Scientists from the Kyoto University in Japan have reportedly observed the same behavior in infant Japanese Macaques.
In a study, which has been published in detail in the journal Primates, seven newborn Japanese Macaques were observed during sleep cycles and every single one of them exhibited the odd smiling behavior at least once.
During observations averaging around 44 minutes each, they were studied both while sleeping and while awake, but drowsy. The sleeping behavior was observed twice, and in total, the seven Macaques smiles 58 times.
Image Credit: Kawakami, F., Tomonaga, M. & Suzuki, J. Primates (2016)
The small details in the smiles, which seemed to focus primarily on the left side of the face, fall in-line with that of human infants. The major difference, however, is that the Japanese Macaques exhibited the behavior in much shorter durations than infant humans would.
"Spontaneous macaque smiles are more like short, lop-sided spasms compared to those of human infants. There were two significant similarities; they both happened between irregular REM sleep, and they show more lop-sided smiles compared to symmetrical, full smiles," said the study’s lead author Fumito Kawakami. "A major difference, though, is that the smiles were much shorter."
Chimpanzees, like Japanese Macaques and humans, also exhibit this behavior when they’re infants and are sleeping. It’s believed that this may be a way to help develop the muscles that are behind the facial skin that help make expressions possible.
Although the smiling isn’t as heavily expressed as it is in humans, it’s still smiling nevertheless.
Source: Kyoto University