During the mirror test, the researchers placed a mark on the fish in a location that could only be seen in a mirror reflection. Initially, the fish reacted aggressively and repeatedly tried to bite their reflections. But over the next few days, they stopped biting and started “behaving weirdly” in front of the mirror, swimming upside down, for instance, or doing repeated bursts of acceleration past the mirror.

According to the authors, the fish were “contingency testing” – doing strange things to see whether the reflection did the same as a way of figuring out the function of the mirror. They were also observed attempting to remove the marks by scraping their body on hard surfaces after viewing themselves in the mirror. These activities were not seen when the fish were given marks without a mirror present or when they interacted with marked fish across a clear divider.