At England’s Cocester Zoo, the mandrills have been seen covering their eyes. Why? Scientists aren’t exactly sure, but one idea is that covering the eyes means that a mandrill wants to be left alone. A new study says that this behavior has been observed in the Colcester Zoo mandrills for at least ten years, and probably started with one mandrill. All of the other mandrills copied the behavior, and now all of the zoo’s mandrills cover their eyes.

This is especially interesting to scientists because humans didn’t teach this behavior to the mandrills. Study authors say that the mandrills’ behavior could be considered cultural, because of natural way the gesture developed.

Read more about the study on National Geographic News Watch.

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Photograph courtesy Mark Laidre, University of California, Berkeley