Are dolphins right-handed or left-handed?

Humans do not act symmetrically. Most of us prefer, and are better at, using one hand rather than the other; balancing on one leg rather than the other; and for those of us who spin (gymnasts, dancers or divers, for example), spinning in one direction rather than the other.

Brains also do not function symmetrically. A version of this idea has long lived in pop psychology, where people are sometimes characterized as being either left-brained (analytical) or right-brained (creative). And although the pop-psych version of this may rest on questionable data, the underlying idea of asymmetrical brain function (what scientists call lateralization) is well-established. For example, in humans, language is typically processed in the left hemisphere, while spatial information is processed in the right.

Because each side of the brain controls a different side of the body, studying asymmetrical behaviours can provide us with information about asymmetrical brain function. And if we study this in animals, it may give us insights into brain evolution.

Handedness without hands

The type of lateralization most familiar to people is undoubtedly handedness. This has been studied in animals by looking at things like which hand monkeys use to grab something, which paw dogs use to knock food out of a container, and so on. But what do you do when the animal you're studying doesn't have hands (or paws)? How do you study lateralization in an animal like a dolphin?

It turns out that behavioural asymmetries come in various types, not just limb biases like handedness and footedness, but also sensory asymmetries, in which we do better on different types of tasks depending on which eye (or visual field) we use; and turning biases, where we prefer turning in one direction rather than the other.

Because different types of biases may come from different underlying causes, studying many different behaviour types, across many different animals, can provide us with a fuller understanding of brain lateralization and its evolution.

A new spin on spinning

This is where it gets tricky. When comparing across animals, we have to take account of the fact that body plans and typical ways of moving may be different. For example, if the animal walks upright (like humans and birds) the long axis of its body is vertical, but if it walks on all fours, the long axis of its body is horizontal. This means that “turning” can involve very different types of movements. For an animal on all fours, turning involves crunching the long axis of its body to one side or the other. For an animal on two legs, turning involves spinning around the long axis of its body, which is kept straight. And for an animal like a dolphin who locomotes in three-dimensional space, either type of turning is possible.

When we set out to study lateralization in dolphins, we were careful to separate these two different types of turning, but we ran into another problem when our researchers kept disagreeing about what counts as a spin “to the right” (or left). After a lot of discussion (and sometimes argument), we realized that we had stumbled upon a weird quirk of human perception. Apparently, humans interpret the direction of spinning in opposite ways depending on the orientation of the animal.

To get a feel for this, try the following: First, stand up and spin “right.” Then lie down face-down on the floor and roll “right.” If you are like most people, in the upright case your right shoulder moved toward your back, while in the horizontal case your right shoulder moved toward your belly. That is, you made the exact opposite rotation. (And in case you’re wondering, no, you can't get around this by describing spins as clockwise/counterclockwise instead of right/left. You get the same results if you substitute “clockwise” for “right” in the examples above.)

Before this, almost all scientific studies of lateralization of turning or spinning motions had studied a single species in a single orientation, like a human turning (upright) or a whale breaching (horizontal)—so the issue had never come up. However, this meant that published research studies had in fact been using opposite coding systems for different animals, depending on their orientation. A spinning turn in which the animal’s right side moved towards its front was typically coded as left/ counterclockwise in studies of humans and walking birds, but as right/clockwise in studies of dolphins and whales. But of course, if we want to look at turning lateralization across different species, we all need to agree on the direction of a turn. This meant we needed a new coding system.

Remember that thing from Physics class?

The system we came up with was actually inspired by the “right-hand rule” of electromagnetism that many of us learned in high-school or college physics. According to that rule, if you point your right thumb in the direction in which an electrical current flows through a wire, the curve of your fingers shows you the direction of the magnetic field flowing around that wire. We adopted the general outline of this schematic model to create the Right-Fingered Spin (RiFS) versus Left-Fingered Spin (LeFS) coding system. In this system, when a coder’s outstretched thumb is oriented along the animal’s long axis, pointed toward its head, the curled fingers of the relevant hand describe the direction of rotation. This allowed us to quickly and unambiguously code spinning/turning behaviours no matter the animal’s orientation or direction of movement.

So, what did we find?

Some previous scientific papers had claimed that dolphins show strong rightward behavioural asymmetries, similar to human right-handedness, and therefore had a left-hemisphere specialization for action. But since “right” didn't always mean the same thing in the earlier coding systems, it wasn’t clear if this claim was really true. To test it, we examined different types of behavioural asymmetries in a group of 26 dolphins, such as “Which direction do they swim around a lagoon?,” “Which side of their body do they touch things with?” and “Which direction do they spin if they dive up and to the side?” By making sure to separate out the different types of motion, and using the unambiguous RiFS/LeFS coding system, we found that—contrary to previous claims—dolphins do not have a general rightward asymmetry after all.

People often think that scientific progress happens when we learn something new that we didn't know before. But another kind of scientific progress happens when we realize that there is a problem with the way we’ve been looking at things all along. In those cases, figuring out a different way of looking can lead to seeing things more clearly. And as science fiction writer Isaac Asimov once pointed out, “The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka!’ but ‘That’s funny...’”