In his first test in 1970, Gordon Gallup put two male and two female chimpanzees alone in a room. After two days, he put a mirror in each room. First, the chimpanzees thought their reflections were rivals. They showed aggression to make it clear who was the strongest (for hierarchy purposes). Gradually, they changed their behavior. The chimpanzees sat in front of the mirror and cleaned their fur in places they could only see in the mirror. They even pulled faces.