Fish apparently fear their own mirror images, according to research to be published in an upcoming issue of the journal Biology Letters.
Julie Desjardins and Russell Fernald of Stanford University answer the question of how fish perceive their own mirror images by comparing the behavioural, hormonal and brain activity when male cichlids (Astatotilapia burtoni) fight other males and their own reflections.
Previous observations and experiments have suggested that fishes are not capable of self-recognition and view their own mirror images as another (usually intruding) individual.
The authors used size-matched dominant males to carry out the experiment. The males were placed in tanks that had been separated by both a transparent and a removable opaque barrier. The opaque barrier was removed to reveal another male fish, a mirror, or an empty chamber (the control) in three different treatments.
The experiment was carried out in three groups of eight fish each, with the fish exposed to the condition for 20 minutes.
The authors recorded the number of bites, rams and side body displays, and the duration that this aggressive behaviour was exhibited.
They then drew blood samples and measured levels of plasma testosterone and 11-ketotestosterone (these are hormones associated with aggression).
The fish were then euthanised and the authors measured brain activity in four regions: the fish equivalent of the amygdala (controlling fear response), the fish equivalent of the hippocampus (controlling spatial learning), pre-optic area (controlling reproduction) and cerebellum (controlling attention).
Brain activity was measured by testing for the expression of two immediate early genes (c-fos and egr-1).
The authors found that while the behaviour and blood testosterone/11-ketotestosterone levels were not significantly different whether or not the male cichlid sparred with another male or with its own reflection, fishes that fought their own reflections had significantly more activity in the amygdala than in other parts of the brain compared to the other two treatments.
This implied that the fish faced an element of fear when sparring with their own reflections.
The authors hypothesise that the fishes fear their reflections because it is a completely novel stimulus in which the mirror image does not react in familiar ways.
According to lead author Julie Desjardins, "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. 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."
For more information, see the paper: Desjardins, JK and RD Fernald (2010) What do fish make of mirror images? Biology Letters doi:10.1098/rsbl.2010.0247