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Kangaroos are left-handed, which means humans aren't all that unique

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They're more 'handed' than many primates, even great apes

Andrey Giljov, National Geographic Society

Here's an interesting bit of information for your next trivia night: by and large, kangaroos are left-handed. This might seem like really weird news, but the study that backs this up may help scientists get a better handle on the evolution of mammals — in addition to dispelling the notion that true "handedness," the tendency to favor one limb over another, is uniquely human trait.

True handedness isn't a uniquely human trait

"We found a pronounced degree of handedness in an animal group only distantly related to humans," says Yegor Malashichev, a zoologist at Saint Petersburg State University and a co-author of the study published in Current Biology today. And the degree of handedness was "comparable to that in our species."

No too long ago, researchers thought of handedness as a trait only humans possessed. That notion has changed somewhat over the last few decades, as scientists demonstrated that hand preferences and other behavioral lateralization relating to brain asymmetry are "much more widespread spread in the animal kingdom than previously thought," Malashichev says. These preferences have even been found in bees, worms, and frogs that walk instead of jump. But, for the most part, the degree of handedness that we've seen in non-primates hasn't been as strong as what we see in humans, and it tends to be related to specific behaviors and tasks, instead of a general way of approaching the world, as is the case with humans.

"More lateralized, or handed, than many of primates and even Great Apes."

Now, "this last bastion has fallen," Malashichev says. Researchers have found a group with the same pattern of handedness as the one seen in humans. "So, we can say that kangaroos possess not just some population level preferences in certain actions, but possess real true handedness, like humans have," he says. "In this respect, they appear to be more lateralized, or handed, than many of primates and even Great Apes."

As you might have guess, Malashichev and his colleagues spent a lot of time observing kangaroos before they reached this conclusion. They observed seven species of marsupials living in the wild. Those species included red-necked wallabies, Goodfellow's tree kangaroo, the eastern grey kangaroo, and the red kangaroo. The scientists watched them as they groomed themselves, grabbed food with their paws, and leaned on their forearms while feeding on grass. And throughout this period, they collected data relating to hand preference for a set of tasks.

A red-necked wallaby, manipulating food with one forelimb. Credit: Andrey Giljov and the National Geographic Society

Eventually, the scientists concluded that the majority of wild kangaroos are lefties. They like to use their left forelimbs for grooming their noses, picking leaves, and bending tree branches, for instance. This preference was most apparent in two species, the eastern grey kangaroo and the red kangaroo. Red-necked wallabies, however, prefer to use their left hand for tasks that require fine manipulations, and their right hand for task in which strength is key. Overall, the researchers found a lot less evidence for handedness in species that spend a lot of time in trees.

Credit: Giljov, et al (2015)

Humans and kangaroos aren't all that similar, but when you think about it, they do have one very important thing in common: both species spend a lot of time walking around on two legs. As a result, Malashichev and his colleagues suggest that moving on two legs is a triggering factor in the emergence of handedness. Animals that rely on four legs, in contrast, only show a "subtle degree of handedness," Malashichev says.

T. rex had really small hands, so it might not have shown a preference

That said, walking on two legs probably isn't the only condition for developing "true handedness." Some dinosaurs, like our pal the T. rex, also walked around on two, gigantic and terrifying limbs. But its "hands" were very small, proportionally speaking, which means they might not have used them for fine manipulations or grooming, Malashichev says. (Remember, many dinosaurs had feathers, so some bipedal species may have groomed themselves.)

True handedness really wasn't expected in kangaroos, largely because their brains are different from other mammals. They don't have the same neural circuits that bridge the right and left hemispheres of the brain. But this result muddles that idea a bit, and researchers may have to study the kangaroo brain further. The researchers who conducted this study now plan to look at other marsupials, such as koalas. They would also like to focus on other animals that occasionally get around on two legs, like meerkats.

Marsupials hold a pretty weird place among mammals, so studying their brain function may yield some interesting information about our shared evolutionary history. But for now, this is yet another study that shows that humans might not deserve that pedestal we like to put ourselves on. We're really not that special, and that's just as fascinating.