QUANTA: A little blue-and-black fish swims up to a mirror. It maneuvers its body vertically to reflect its belly, along with a brown mark that researchers have placed on its throat. The fish then pivots and dives to strike its throat against the sandy bottom of its tank with a glancing blow. Then it returns to the mirror. Depending on which scientists you ask, this moment represents either a revolution or a red herring.
Alex Jordan, an evolutionary biologist at the Max Planck Institute for Ornithology in Germany, thinks this fish — a cleaner wrasse — has just passed a classic test of self-recognition. Scientists have long thought that being able to recognize oneself in a mirror reveals some sort of self-awareness, and perhaps an awareness of others’ perspectives, too. For almost 50 years, they have been using mirrors to test animals for that capacity. After letting an animal get familiar with a mirror, they put a mark someplace on the animal’s body that it can see only in its reflection. If the animal looks in the mirror and then touches or examines the mark on its body, it passes the test.
Humans don’t usually reach this milestone until we’re toddlers. Very few other species ever pass the test; those that do are mostly or entirely big-brained mammals such as chimpanzees. And yet as reported in a study that appeared on bioRxiv.org earlier this year and that is due for imminent publication in PLOS Biology, Jordan and his co-authors observed this seemingly self-aware behavior in a tiny fish.
Jordan’s findings have consequently inspired strong feelings in the field. “There are researchers who, it seems, do not want fish to be included in this secret club,” he said. “Because then that means that the [primates] are not so special anymore.” Read more.