The study's lead author, Fumito Kawakami, noticed macaque newborns smiling during health exams, and that prompted a closer look at the behavior, culminating in a study just published in the journal Primates.
As can be seen in the video, the word "smiling" is used broadly, when compared with the everyday impression of what constitutes, say, a human smile.
"Spontaneous macaque smiles are more like short, lop-sided spasms compared to those of human infants," explained Kawakami.
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What's going on with the macaque smiling? The researchers say the smiles are more akin to grimaces than expressions of pleasure. The facial movements, they suggest, may be an aid to development of cheek muscles that will let them smile, laugh and, yes, grimace – just like people and monkeys.
Tomonaga suggested that infant chimps, humans and monkeys may not be the only spontaneous smilers: "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?"