Only a few animals are known to smile in their sleep: human babies, chimps, and now, monkeys. A new study from Kyoto University in Japan, though, finds that baby monkeys—Japanese macaques, to be specific—smile far more while snoozing than humans or chimps, according to New Scientist

In a study of seven newborn Japanese macaques, published in Primates, every test subject spontaneously smiled during REM sleep at least once. The macaques were between four and 21 days old. REM sleep is the stage at which humans can dream, so when babies smile in their sleep, it's possible that dreams affect the expression. But because we don’t know if macaques have dreams, we can’t know if they’re smiling at something specific or just grinning spontaneously. Babies and chimps tend to stop smiling during sleep only a few months after birth, replacing spontaneous smiles while unconscious with social smiles while awake in response to the world around them. Scientists still aren’t quite sure what function spontaneous sleep smiles serve. 

The monkeys smiled much more in this study than in the previous observations of human and chimp infants, though it’s hard to compare the data since the duration of sleep during which spontaneous smiles are recorded varies wildly. These macaques showed 58 spontaneous smiles during an hour and a half of REM sleep, while a previous study found that 10 infants showed 24 smiles during 10 hours of observation, and in another, three chimps showed 60 smiles over the course of 304 hours of REM sleep. Are they working out their smile muscles for grinning later on? Are they trying to make their parents love them? (Because it would definitely work on us.) Researchers aren’t completely sure. Regardless, it’s adorable, as you can see in the video.

Now that you’ve primed yourself for monkey cuteness, you should probably revisit Baby Monkey Riding on a Pig

[h/t New Scientist]

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