A symmetrical face is often thought a sign of beauty, but symmetry may be a disadvantage when it comes to the brain. "Left-brained" or "right-brained" fish are more adept at handling multiple activities than fish with no hemispheric preference, according to a new study. Their ability to multitask could help explain why vertebrate brains evolved to function asymmetrically.
Scientists used to think that only humans had lateralized, or asymmetric, brains. We generally use the left side of our brain to interpret language and the right to appreciate music, for example. Recently, though, researchers have come to believe that lateralized brains are universal among vertebrates. Some think the condition may allow animals to focus on multiple stimuli at once, with each hemisphere dealing with particular cues.
Psychologists Marco Dadda and Angelo Bisazza of the University of Padova in Italy decided to test the theory in fish. They first assembled groups of the minnow Girardinus falcatus that had been bred to be either lateralized or nonlateralized. A right-lateralized fish tends to look at a companion out of its left eye and vice versa, because hemispheres process vision on the opposite side of the body. A nonlateralized fish does not favor either eye. When the researchers placed each type of fish in a glass tank and gave it shrimp to eat, they found that lateralized and nonlateralized fish took about the same time to catch 10 shrimp. In addition, both groups caught the shrimp at equal rates on both sides of their bodies, indicating that they weren't favoring either eye while scoping out their prey.
The team then repeated the experiment with a twist. This time, the fish had more to worry about than just catching dinner: The researchers placed a sunfish--a minnow predator--into a neighboring tank. With a predator on their minds, the nonlateralized minnows took nearly twice as long to catch 10 shrimp as the lateralized fish, the researchers reported online 19 June in Animal Behaviour. That's probably because both eyes of the nonlateralized fish were forced to share prey-catching and predator-monitoring duties. Left-brained fish, on the other hand, used their right eye to monitor the sunfish 70% of the time, and right-brained fish used their left eye 70% of the time. The fish used the side that wasn't busy looking at the predator to catch most of the shrimp.
Although the experiments don't directly test whether the minnows use one part of the brain to catch shrimp and another to monitor the sunfish, says Dadda, the fact that each eye seems to favor one activity strongly suggests that fish are devoting different brain hemispheres to attend to the different tasks.
"These task divisions between the two hemispheres are very practical," says evolutionary biologist Ruth Byrne of the University of Vienna in Austria. But she notes that the asymmetry can only go so far before it becomes disadvantageous. "It's not practical for one eye to do one hundred percent of the work and the other none at all," she says. If that were the case, a left-brained minnow dutifully checking for predators with its right eye could be thoroughly surprised by a sunfish sneaking up on its left side.