Being right- or left-handed is a familiar fact about yourself you likely don’t think about much on a day-to-day basis. However, your handedness affects how you interact with the world.
For many people, it determines how they brush their teeth, use tools, play sports, write, eat and much more. Simply try to enjoy a bowl of soup or sign your name with your nondominant hand to appreciate the impact a hand preference can have on your daily life.
Interestingly, this behavioral asymmetry is not unique to humans. Preferences for using the right or left hand, paw or eye exist in most species. For example, many primate species have individual left- or right-hand preferences for manual tasks. Similarly, different bird species have varying eye preferences for distinct visual tasks. Even the largest animal alive, the blue whale, shows a preference for the direction of its rolls during feeding. This inherent and often-overlooked feature of behavioral asymmetry is a widespread phenomenon in the animal kingdom.
The universality of behavioral asymmetry suggests that having an assigned hand, eye or other preference is beneficial. But depending on one hand for so many important tasks means that a single injury could be devastating. This paradox poses an important question: Why would having handedness be better for survival than not?
Insights from fish ‘handedness’
To address this question, scientists have tried to understand the genetics of handedness. While large-scale genetic studies in humans have identified dozens of genes associated with handedness, researchers also found that genetics alone only partially accounts for whether someone is left- or right-handed. This means behavioral asymmetry like handedness is likely the product of complex interactions between genetics, development and the environment.
For the past six years, my research lab has been interested in understanding behavioral asymmetry and how such behaviors get encoded in the brain. We primarily use larval zebrafish to explore the neural basis of behavioral asymmetry. These animals have transparent bodies and rapidly develop into adults in just a few days, making them ideal models to study. Additionally, the genetics and brain structure of zebrafish are highly similar to those of humans.
Fish have a form of handedness called motor asymmetry, which involves sustained periods of turning in the same direction. I had previously found that when light was cut off, larval zebrafish start circling in a leftward or rightward direction, sometimes for up to a minute or more. The fish would continue to preferentially turn in that same direction over the course of hours, days and even weeks, looking for a light source. This meant that vision drove their motor asymmetry.
