"The reactions are more instinctive," said Dr. Julie Dejardins, a psychologist at Stanford University. She just published study of male African cichlid fish and found evidence that when defending their territories, they can have fear. This discovery raises the possibility that other, lower vertebrates such as frogs, lizards, and birds may also be able to detect more subtle nuances than we imagined.
Dr. Dejardins used the African cichlid because the species is extremely social and territorial. Males fight for dominance and the ability to breed with females. They continually test each other by facing off at borders, posturing, and occasionally nibbling at each other. Dejardins altered those environments by putting mirrors in the tanks.
Males saw their reflections, assumed they had competition, and faced off against them -- not for seconds as they normally would, but for minutes at a time. When their mirror images appeared to mock every move, the males became stressed, feisty, exhausted, and fearful.
"I think this stimulus is so far outside their realm of experience that it results in this somewhat emotional response," said Desjardins.
This research is significant because, at their core structural levels, fish brains and human brains are very similar.
When the researchers performed postmortems on the fish, they found levels of testosterone and other hormones associated with aggression were comparable with others in a control group.
When dissecting another part of the brain called the amygdala, which associates with fear, researchers found evidence of more activity in the mirror fighting fish, than those who tussled with real foes.