It is quite unique amongst mammals to be able to quickly mirror the facial expressions of their own kind.
Up until 2017 we did not know that other animal species than humans could signal to one other using facial expressions. Then a study, made by researchers from Germany and England, showed us otherwise. They showed how orangutans mirror certain facial expressions shown by their own kind. Later on, other monkey species have been shown to mirror facial expressions too, it was found out first with the orangutans.
There are two different types of so-called ‘facial-mimicry,’ where facial expressions are mirrored by a fellow member of your species: One is voluntary, the other involuntary. In case of the involuntary type, the facial expression is copied as quickly as within one second. This is called ‘rapid involuntary mimicry’ and it has only been observed with humans and very few monkey species. the other, voluntary facial mimicry is known from more different mammal species – i.e. when they exercise threatening behavior or show teeth.
The facial expression mirrored by the orangutans in the study in question, was one where they open their mouths whilst playing. Even though we cannot ask the orangutans what they mean to signal with this facial expression, we can assume that they are probably having a good time, as they are engaged in fun and games. Maybe it can be interpreted as a big smile, or a good old monkey-grin. Under all circumstances, it bares witness to our close relation to the orangutans, when we share such a unique behavioral trait with them, as mirroring facial expressions.