Fish are scared of their own reflection — a new finding that suggests their brains are more sophisticated than originally thought, say biologists.

Stanford University researchers compared the brain activity of male African cichlid fish during and after encounters with either a mirror or other another male cichlid. The fish are territorial and usually react to another male by fighting with a series of alternating movements triggered by the other fish's tactics.

While scientists have long known that the fish will fight with their own reflection, up till now they didn't know what was going on in their brains.

Post-doctoral researcher Julie Desjardins suspects the fish fighting their own reflections got scared because their enemy in the mirror didn't act as they would expect.

"In normal fights, they bite at each other, one after the other, and will do all kinds of movements and posturing, but it is always slightly off or even alternating in timing," Desjardins said in a release. "But when you are fighting with a mirror, your opponent is perfectly in time. So the subject fish really is not seeing any sort of reciprocal response from their opponent."

She and biology professor Russell Fernald described in last week's Biology Letters how they arranged 20-minute sparring sessions for their fish and the brain analysis that followed.

First they placed a clear wall across the middle of the tank to keep combatants apart when two fish were pitted against each other, so there was never any actual fish-to-fish contact. Whether their foe was real or a reflection, the fish always tried to fight. Moreover, their fighting moves were similar whether they were fighting a real opponent or their reflection.

Afterward, the researchers performed postmortems on the fish. They found the levels of testosterone and another hormone associated with aggression circulating in the cichlids' bloodstreams were comparable regardless of whether the foe was a reflection or real.

But the biologists uncovered a significant difference while dissecting a part of the brain called the amygdala, which is associated with fear in all vertebrates. They found evidence of substantially more activity in that region in the mirror-fighting fish than in those tussling with real foes.

Desjardins said she suspects the fish were afraid when confronted by an opponent whose behaviour was off-kilter.

Although higher vertebrates such as humans have very elaborate amygdalas in comparison to fish, there is still a part of the human amygdala that is analogous to the fish brain and performs similar functions.

The discovery suggests other lower vertebrates such as frogs, lizards and birds may also be able detect differences more subtle than we thought.

"I think it certainly indicates that there is more going on cognitively than people have long assumed in most lower vertebrates," said Desjardins.