Humans live in a right-handed world. Estimates vary, but some 75 to 90 percent of us use our right hands as our “write hands.” If our species has dominant sides, it begs the question: How common is this trait across the animal kingdom? Are our near-cousins, like chimpanzees, right-handed or left-handed? And what about animals like whales that don’t even have hands? If an octopus had to high-five, which tentacle would it use?
For many years, scientists thought that handedness was unique to humans. But according to a growing body of research, many animals do have a preferred hand, limb, or even tentacle, and it likely starts in the brain.
“As soon as you have two sides of the brain, they start task-dividing,” says Ruth Byrne, a biologist who’s studied handedness in octopuses.
The $5 word that describes this phenomenon is biological chirality. Essentially, it describes a type of asymmetry that can be expressed either physically — say, one hand being a little bigger than the other — or a behavioral tendency to favor the use of one side over the other.
This can be expressed on an individual and a population level. And researchers have found evidence of favoring one side over the other in a whole host of other life-forms. Most other primates tend to have a specific preference, and scientists even wrote an entire book on the handedness of plants that shows some plants have preferred directions of stem coiling or tendril reaching. Like the majority of humans, walruses often prefer their right flipper. Even slime molds — brainless, single-celled organisms — may turn right more often than left.
Runs in the Family
The question of why humans are right-handed is still up for some debate, but some researchers believe we may have inherited our right-handed bias from our evolutionary ancestors. The left side of the human brain, commonly tasked with things like science, logic, language, controls the right side of our bodies, just as it does with our primate ancestors. Some researchers believe the development of the left side of the brain for speech and language might have also led to our right-handed bias among humans, though this isn’t completely understood.
Great apes, our closest primate relatives, are mostly right-handed, says William Hopkins, director and chairman of the Keeling Center for Comparative Medicine and Research at the University of Texas. When you measure behaviors like which hand they use to throw, chimpanzees and other great apes show a 65 to 70 percent bias for the right hand.
“They do show what we would call a population-level bias, but there’s a couple of caveats to that,” Hopkins said.
Not all studies agree on the proportion of lefties or righties in a particular species. Part of the reason is researchers don’t always compare apples to apples. Humans might skew right-handed more often when it comes to writing, for example, but we often hold the paper with our left hand. If we were to climb a fruit tree, would we hold onto the trunk more often with our left or our right hand when reaching for fruit? Sometimes it’s hard to make direct comparisons between species that don’t necessarily spend the bulk of their time in the same types of activities.
What Explains Southpaws?
Some researchers believe that deep in our evolutionary past, primates used to be mostly left-handed, but evolution or changes in behavior led to a switch over time. As a result, some of our more distant ancestors like lemurs tend to be also left-handed, says Stephanie Poindexter, a biological anthropologist at the State University of New York at Buffalo. But she says there are many gaps in our knowledge about these primates. Research she conducted on the slow loris, another distant primate relative of ours, revealed them to skew right-handed when reaching for food.
“It could be an exception, but the thing that we concluded is that we need to rethink what we know about handedness among these older primates,” Poindexter said.
Hopkins says the prevalence of left-handedness among lemurs and other more distant human ancestors could be related to them being arboreal. In trees, primates had to hold onto tree trunks or branches with their hind limbs and one hand while reaching for food with their other arms. Individual preferences developed and more successful individuals may have been favored through natural selection until population-level biases developed.
Best of Eight
Even animals under the sea show preferences, according to a growing number of studies. One recent study found that the biggest animals in the world either go clockwise or counterclockwise depending on their feeding strategy: The researchers found that blue whales typically turned right in their rotations while conducting smaller rolls at deeper depths while feeding, but went left on larger rolls at shallower depths.
Meanwhile, despite having a lot of limbs to choose from, octopuses still seem to play favorites when it comes to tentacle use, but this seems to be determined on an individual level. Byrne and her colleagues studied common octopuses in tanks and found that they often preferred one eye over the other when peering out of the tanks, presumably wondering themselves why humans preferred one arm over the other while taking pictures. Further research revealed that their favorite eye typically accompanied a favorite tentacle — if they used their left eye more often, then they also favored their two front left tentacles.
“We realized it’s all about eye-arm coordination,” she says. But they didn’t notice any type of overall population trend in octopuses.
Byrne says that the development of a bias in a species likely occurred early when brains evolved to have two sides. While both sides were equally capable of dealing with tasks, the work is smarter rather than harder when tasks are divided between the hemispheres.
In terms of why a species develops a preference, Hopkins believes it has to do with some particular task that favors one side over the other.
When primate species began to come down from the trees more often, the importance between left and right may have shifted. For example, the arm holding the tree may have been more important than the arm grabbing food, as a fall could be fatal. But down on the ground, the holding arm could have just been used to prop a primate’s weight up while conducting more important tasks of tool use with the other.
“You do get some shifting from right-handedness to left-handedness,” Hopkins says.
Whatever it was that drove this shift, the preference became more pronounced in humans. Hopkins says that while biology or evolution likely partly drove it, culture also probably played a role when it came to humans skewing so overwhelmingly right. He points out certain cultural biases in many countries that make it tough for lefties — parts of Africa and the Middle East have taboos against touching communal food or shaking hands with the fingers on your left.
But he says there is still a lot we don’t know about the topic in general.
Like Hopkins, Poindexter agrees that the way you measure handedness often depends on what a creature is doing, and how important it is for them. “The task matters,” she says. “There isn’t a really clear pattern.”