When an experiment was carried out on monkeys with electrodes attached to their heads, researchers got a surprise. The mirrors had been placed inside the cages to stimulate the animals' brains. But the scientists found that the monkeys often wound up staring at their images, leading them to surmise that something else was at play.
Their results, published in the journal PLoS One, recounts how the head implants prompted the monkeys to examine themselves before the mirror and, in fact, demonstrate a form of self-awareness.
For instance, one monkey would smell, lick and then look at his fingers while grooming in front of the mirror, an indication the animal had established a connection that the area he saw in the mirror being groomed belonged to him.
Although they occasionally groomed the area around the implant in the absence of the mirror, their gaze was not fixed in any particular location. When grooming was guided by mirror viewing, the monkeys always turned to face it and looked into it. Furthermore, there were no attempts to touch or groom the image in the mirror, which would have suggested that the monkey saw the reflection as another animal.The behavioral clincher may have come when they observed no social responses when the monkeys looked at themselves and began grooming themselves in front of the mirror. Carl Zimmer of Discover has more about this here - including a video clip that's worth a watch.