Origins of handedness reconsidered thanks to left-handed kangaroos

Most humans are right handed. Some are left-handed, and fewer still are ambidextrous. In each case, we can’t fully explain why this is true, partially thanks to the multitude of influences we see in humans as they develop. Is handedness genetic? Is it coming from mimicry of your parents? Are epigenetic factors at play? To help simplify things, we’ve also studied animals for signs of handedness in order to try to isolate causes or function of only being able to scribble with your off hand while being capable of calligraphy with the other. A recent look at marsupials has found a number of new handed species, while simultaneously casting doubt on some of our assumptions about how handedness works.

Why would we start a search for handedness with marsupials? We didn’t, actually. Handedness has already been found in many primates, including chimpanzees, macaques, and lemurs. Less obvious animals like rats, bats, cats and parrots have also shown a preference for one hand/paw over the other. So this recent look at wallabies and kangaroos isn’t completely trailblazing, but the comparison between four closely related species did reveal an interesting pattern.

South paws from the southern hemisphere

Red kangaroos, eastern gray kangaroos and red-necked wallabies were all left-handed. Tree kangaroos, on the other hand (sorry), showed no preference. This fits the model that handedness is connected to a need for grasping dexterity, and tree kangaroos are quadrapedal and thus are using their front limbs more like legs, not hands. They don’t need the finer motor skills needed for by a browsing wallaby to pick leaves to eat.

The big question raised in this study is that handedness was believed to be tied to the connections between the right and left hemispheres of our brains. However, marsupials lack such a connection, and so either handedness has evolved more than once, or it stems from some other neurological process. Considering how far back marsupials branched off from other mammalian evolution, and that it’s found in non-mammals, it may be a very old solution to developing fine motor skills.

My wife asked: What about squirrels? They use their front paws like hands. The model presented by the kangaroos and wallabies would suggest that squirrels should be handed, although at this point we couldn’t find any research on the matter.

Source: Kangaroos are lefties by Sarah Zielinski, Wild Things