One of my co-workers, Matthew Campbell walks around with a plastic bucket in the bottom of which he has cut a small hole. The chimpanzees at the Yerkes Primate Center (at Emory University, in Atlanta) have learned to press an eye to the hole to watch an iPod held up at the other end of the bucket. This way, we know exactly who is watching.
The purpose of this "peep show" is to measure yawn contagion, a peculiar phenomenon linked to empathy. For example, humans prone to yawn contagion also have the most empathy. And children with empathy deficits, such as those with autism, lack yawn contagion altogether.
Watching videos of yawning apes, our chimps yawn like crazy, but only if they personally know the ape in the video. Videos of stranger chimps have no effect. This suggests that it is not just a matter of seeing a mouth open and close: identification with the videotaped individual is part of it. The same role of familiarity is, by the way, also known of human studies. In an Italian field study, it was found that people yawn when a friend or family member yawns next to them, but not if it is a yawning stranger.