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Chances are that if you are reading this you are right-handed. If you are not, you’re one of roughly 10% of the population that is left-handed. Or you may identify as ambidextrous. That is, you use both hands with a similar frequency, dexterity, and precision. We call the regular use of one hand manual laterality. Laterality is usually the most evident during complex tasks like tool use. Typically, people that are right or left handed use the same hand across many tasks: writing, cutting with scissors, swinging a baseball bat. This 9:1 ratio of right-handed folks to left-handed ones is found in different groups of people all around the world.

But why are so many people either right or left-handed? And why are most of us right-handed?

Many scientists have proposed hypotheses for why handedness has evolved. Most of these are related to how human locomotion has changed and how our brains have changed over time. Key to understanding why humans exhibit handedness is learning when it evolved.

One of the ways we can do this is by looking at fossils of extinct humans, like Neanderthals. Neanderthals are considered a sister species to humans. That is, we shared a common ancestor approximately one million years ago. For those super curious that ancestor was probably Homo heidelbergensis. Neanderthals may have evolved before modern humans at roughly 240,000 years ago and became extinct by about 40,000 years ago. Careful analysis of teeth from different individuals suggests that most Neanderthals were right handed and a few were left handed. Well, at least for one task.

From this evidence, it appears that both Neanderthals and humans were right-handed or left-handed. We need to go back further in time. But using fossils here is hard because we only have a few fossils for most of the species that come before us and Neanderthals. That’s where primatologists (like me) and the comparative method come in!

Humans shared a common ancestor with chimpanzees and bonobos about six million years ago. In evolutionary biology, we often assume that if a trait is shared by similar species, this trait was also present in the common ancestor. By studying chimpanzees and bonobos (and other primates), we can test whether handedness evolved before or after six million years ago.

Many scientists have tried to answer this question and have look at hand use in these primates across many tasks. One of the challenges in answering this question may surprise you. Scientists use different definitions for important terms like handedness. And this creates a lot of confusion.

If chimpanzees and bonobos were like humans, we would expect most of them to use the same hand (right or left) across many different tasks. This is what myself and my colleagues consider handedness. You may have a situation where most chimpanzees use their left hand for one task but then most use their right hand for another. We call this task specialization. Or individuals use the same hand across tasks but there’s no majority. This is manual specialization. Finally, one individual may use one hand for one task and switch for another but there’s no consistency across individuals or the task. We call this hand preference.

The framework that my colleagues and I use (from Marchant and McGrew 1994)

While this is the framework we use, not all scientists who study hand use do. This leads to confusion, particularly when they do not even define their own use of these terms. It’s also important to point out that there is no consensus about how many individuals or how many tasks are necessary to determine which of these four conditions (or none) are present in a population. Usually, it’s best to have as many study subjects as possible and at least two tasks.

In our study, we wanted to see whether two tasks would elicit hand preference in our study group of bonobos. At this point, I’ve mentioned bonobos a few times so I should probably let you know what a bonobo is (if you don’t already!). Bonobos are one of two species of chimpanzee. I’m sure you’ve heard of chimpanzees, made famous in part by Jane Goodall. Humans are equally related to bonobos and chimpanzees but many folks have not heard of bonobos because they are rarer than chimpanzees, less well studied, and are only found in a handful of zoos (only seven in the United States).


A wild bonobo

We decided to look at hand use in bonobos during grooming and during tool use. Grooming is an interesting behavior because you can groom with either one hand, which is pretty easy to do. But grooming with two hands requires coordination between hands and is cognitively demanding. Tool use also requires a lot of brain power. We predicted our bonobos would exhibit hand preference for tool use but not grooming. In order to study tool use, a team of zoo staff and researchers created an artificial termite mound and fitted it with PVC tubes. These tubes could then be filled with mashed fruit and peanut butter. Then, the bonobos would use sticks they found in their enclosure to “fish” the treats out of the mound. This behavior is similar to termite fishing that wild chimpanzees do. Interestingly, wild bonobos do not fish for termites in this way and don’t use tools when feeding like chimpanzees. But our subjects are very much able and enjoy using their fake termite mound!

Unga using the artificial termite mound

I study bonobos that live at the Columbus Zoo and Aquarium. I’ve studied these apes for years and work closely with other researchers to better understand these individuals. We watched these animals use tools for 1,000 hours and watched them groom for 128 hours. Then, we determined how many times an individual was either right-handed, left-handed, or ambilateral (there was no preference) for each task: one-handed grooming, two-handed grooming, and tool use.

We determine whether or not an individual exhibits hand preference due to actual hand preference (rather than by random chance) by running a binomial test. This test is just like flipping a coin a bunch of times. We can consider using one hand “heads” and using the other “tails.” If one hand is used more than expected by flipping a coin, we consider this hand preference to be statistically significant.

Some other researchers in this field calculate a handedness index (HI) to determine laterality by subtracting the number of left hand responses from the number of right hand responses and dividing by the total of both. This metric can certainly tell you whether an individual use one hand or the other (positive values mean right-handed and negative values mean left-handed). However, this metric does not take the number of times an individual completed a task into account. This is why we report HI values but use the binomial test to determine significance of hand use.

Of the nine individuals we tested for one-handed grooming, only one exhibited statistically significant hand preference. We tested eight individuals for two-handed grooming and only one, a different individual than before, exhibited significant hand preference. So these apes don’t have hand preference for grooming whether they are using one or both hands.

However, all but one individual exhibited significant hand preference for tool use. Interestingly, the one who did not was the youngest individual to use the artificial termite mound.

Maiko consistently used his right hand during tool use

So, we can say that bonobos exhibit hand preference for this tool use task but not for grooming. There was also no manual or task specialization for these tasks and definitely no handedness. In the future, we would like to examine the hand use of these same animals for different tasks.

This study adds to a growing picture that living non-human primates, like bonobos, sometimes exhibit hand preference. However, it looks like handedness is something that human ancestors evolved sometime after we split off from chimpanzees six million years ago.

Brand CM, Marchant LF, Boose KJ, Rood TM, White FJ, Meinelt A. 2017. Laterality of grooming and tool use in a group of captive bonobos (Pan paniscus). Folia Primatologica. 88: 210-222.



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