They are often said to have a three-second memory, but the brain power of fish has been considerably underestimated, according to scientists who found some fish can recognise themselves in the mirror.
Their findings suggest that a small, otherwise unremarkable species called the cleaner wrasse has joined an elite handful of others to have passed the so-called mirror test, which has been used for decades as a gold standard measure of animal intelligence.
Passing the test is widely viewed as an indication of self-awareness and until now the only animals to have crossed this threshold are great apes, bottlenose dolphins, killer whales, Eurasian magpies and a single Asian elephant. Now the select club may have an unlikely new member.
“These fish are fascinating in their breadth of cognitive abilities – and underappreciated,” said Alex Jordan, an evolutionary biologist at the Max Planck Institute for Ornithology in Germany and the study’s senior author. He and colleagues are calling for an overhaul of the traditional hierarchy of animal intelligence, saying that despite their reputation for being “basically vacant”, fish perform exceptionally well on certain tasks.
The prospect of fish rising up the ranks of animal cognition has not been universally welcomed though, and the paper proved so controversial it took the authors five years to get it published. “Some areas of the academic community seem fairly intent on fish not joining the pantheon of smart things because then their own animals lose their special place in the world,” said Jordan.
The cleaner wrasse (Labroides dimidiatus) is about 10cm long with a stripe, and lives in coral reefs. Previous research has revealed these fish have complex social lives, forming allegiances and enemies, making logical inferences about whether they will beat other fish in fights and showing a capacity for deception. The fish live in mutually beneficial partnerships with larger client fish from whom they feed on dead skin and parasites.
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.