Archerfish (Toxotes jaculatrix) are famous for an unusual talent: They can shoot down above-water insects by squirting a powerful stream of water from their mouths. This ability has puzzled scientists, though, because light refraction at the water surface distorts a prey's apparent size and location. A new study suggests that sharpshooting is something the fish learn over time, and the findings may provide clues to how animals adapt when their senses don't tell the truth.
Animals must be able to size up their prey to hunt and navigate effectively. But the apparent size also depends on the distance from the viewer, so animals have to develop ways of keeping things in perspective. For aquatic animals that hunt terrestrial prey, the refraction of light when it strikes water further complicates the matter. Yet 25-centimeter-long archerfish, which dwell in river estuaries and mangroves, manage to hit their targets while not spitting at prey that are too big to swallow. How do they do it?
Setting up a carnival booth of sorts, a team of researchers led by Stefan Schuster of the University of Freiburg in Germany constructed a target made of eight different-sized black disks that could be placed at various heights above the water surface. Inexperienced fish, which received a dead fly reward regardless of which disk they hit, often shot at disks too big to swallow. But, over 4 to 8 weeks, the researchers trained some fish to hit a specific disk regardless of its height, they report in the 7 September issue of Current Biology. This showed that the fish could learn to gauge a target's absolute size. When some of these fish were retrained to recognize another disk, they nailed it even when it was placed at heights not included in the retraining. Thus, says study co-author Samuel Rossel, archerfish can learn and apply the underlying laws of perspective. “This explains how archerfish optimally select prey,” he says. “Otherwise, they would be shooting at objects that they couldn't handle.”“These fish are much cleverer than we thought,” says Michael Land, a neuroscientist at the University of Sussex, U.K. He points out that the final experiment in particular shows that the fish have learned how to do a complex calculation rather than simply memorizing a set of relationships. “That's a hard job,” he says.