In a statement, lead author Fumito Kawakami explained that “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.”
Macaques are known to use two different types of smile in their social interactions. The first of these, known as relaxed open-mouthed displays (ROM), are similar to human smiles and convey feelings of happiness, while the silent bared-teeth display (SBT) is a sign of submission. Because of the importance of these social smiles in macaque daily life, the study authors believe that, like humans, baby macaques spontaneously smile in their sleep in order to train up their zygomaticus major muscles, which are so vital for social communication later in life.
Interestingly, in both humans and macaques, these spontaneous smiles occur during REM sleep, which is associated with dreaming. This raises a number of fascinating questions about the emotional content of our dreams as these smiles occur – though the answer to these questions may never be known.
Summing up the relevance of this study, study author Masaki Tomonaga explained that “we can infer that the origin of smiles goes back at least 30 million years, when Old World monkeys and our direct ancestors diverged.” Intriguingly, he also notes that “there are case reports about mice laughing when they get tickled and dogs displaying facial expressions of pleasure. It may be the case that many mammal infants display spontaneous smiles, in which case smiling would have an older evolutionary origin. Who knows…”
Like humans, chimps are excellent smilers. Eric Isselee/Shutterstock