Sunday, 24 June 2012

Are Other Animals Ever Right-handed?

Humans normally display a preference for using one hand, rather than the other, for performing complex tasks, such as writing. It's most commonly the right hand, although exactly how common left-handedness is is debatable - largely due to insistence on the use of the right hand in some cultures, regardless of individual preference. While a small number of people are ambidextrous, this seems to be something that's learned, rather than something that you're born with.

Do other animals also display a similar preference? For most, that's a difficult, if not impossible, question to answer. A right-handed horse, for example, would prefer to use it's right front hoof to do... what exactly? When an animal has no manual dexterity to start with, we can't really say which side is the more dextrous. Studies looking at handedness in other animals have therefore focussed on those which are capable, to at least some degree, of grasping, and of performing tasks that are least somewhat analogous to how we use our hands.

The obvious example here is primates. Primates have hands, not paws, and, while there are certainly variations in the anatomy of their arms, and how, and for what, they use them, there is a clear parallel to what humans are doing. Unsurprisingly, since we ourselves are primates. And, yes, handedness has been seen in a great many primates, including chimpanzees and other apes, macaques, marmosets, lemurs, among others. It's not something that's unique to us.

I should also add that, while primates have been the main focus for such studies, both for their physical similarity to us and for what light they might be able to shed on our own evolution of the trait, there have been studies on other animals, too. Handedness has also been found in rats, bats, and cats, and also in wallabies. It's not even restricted to mammals; it has also been seen in parrots, and. perhaps most bizarrely, in toads.

By looking at these various animals, we can see that a sort of pattern emerges. The more complex and difficult the task, the more likely animals are to resort to using a specific hand (or equivalent appendage) to perform it. That no other animal does anything quite so complicated as writing or sophisticated tool use may explain why the phenomenon is so strong in our own species.

There is also a suggestion that the more bipedal a species is, the more likely it is to show a strong handedness preference. Again, that obviously fits with us, and the explanation may well have to do with the complexities of maintaining a proper balance in such a situation. Reaching out with your right hand to pick something off a supermarket shelf may not seem the height of agile gymnastics for us, but there are rather more complex muscular adjustments going on in such a stance than there are for a cat swatting with out its paw while standing on the other three legs.

What about something part way, anatomically speaking, between a cat and a primate? The primates are one of the larger orders of mammal, and include quite an array of different creatures. Closest to humans, we have the two families of apes, and then anything from four to six families of monkey, depending on how you want to divide them up. Together with the tarsiers of South East Asia, these form the "higher" primates, distinguished by an enhanced sense of vision, larger brains, and more complex facial expressions, among other features. The remaining, "lower" primates - which have, in reality, been evolving for just as long as all the others - include animals such as lemurs. Despite their differences, animals like lemurs are still clearly primates, living in trees and often swinging between branches and so forth, and having fore-paws that can, quite reasonably, be described as "hands".

So what's the next step back? What are the closest living relatives of the primates as a whole? That's somewhat in dispute, because the answer lies with one or the other of a couple of groups, neither of which have been particularly well studied. The general agreement these days seems to be that it's the colugos, although that isn't completely certain. Either way, one fairly close group of relatives are the tree shrews.

Simians    Tarsiers
   ^          ^
   |          |
   |          |        Lemurs, etc.
   ------------             ^
        |                   |        Colugos
        |                   |           ^
        ---------------------           |        
                  |                     |      Tree-shrews
               Primates                 |           ^
                  |                     |           |
                  -----------------------           |
                             |                      |
                             |                      |
The alternative hypothesis, incidentally, is not that the tree-shrews are the closest relatives to primates, but that they and the colugos are equally close. In other words, they had a single common ancestor that lived some time after the origin of the primates.

Tree shrews are so similar to primates, that they were once thought to actually be primates - albeit more "primitive" even than lemurs. As their name suggests, they are small, tree-dwelling animals that somewhat like large shrews, although not at all related (the next branch out in the primate family tree is that of the gnawing mammals; shrews are on an entirely separate line). They move about on all fours, and have grasping paws, suitable for climbing, but still strongly clawed and not at all hand-like.

We don't really know that much about them. Being relatively inconspicuous creatures living in the jungles of South East Asia, it's probably only because of their relationship to primates that anyone much beyond specialists has heard of them at all. Nonetheless, Marine Joly, of the Institute of Zoology in Germany, and co-workers, have recently published a study on their handedness. How would small, four-legged creatures stack up against their relatives, the primates?

In the first part of the study, they placed northern tree shrews (Tupaia belangeri) in an enclosure, with some food, in the form of mealworms. Which paw would they use to pick up the food?

Neither: they used their mouths.

But that's actually quite useful. Most primates pick up things with their paws before eating them, which mammals in general don't tend to do (although, obviously, there are plenty of exceptions). This shows that tree shrews are basically quadrupedal, even in comparison to primates - which, humans aside, are hardly pure bipeds themselves. In their normal life, they probably don't use their front paws for much other than climbing and walking. So, would an animal that normally sees no need to even pick things up show any signs of handedness when using its front paws is the only option?

To find out, the experimenters set up a small transparent box, containing mealworms, and attached it to the side of the tree shrews' cage. The box could only be reached through a small hole, to narrow to poke a snout through, but sufficient for a hand. At first, they placed a platform beneath the hole so that the tree shrews could easily stand on three legs while reaching out with the other one. In subsequent variations of the test, they moved the platform lower, forcing the animals to sit up on their hind quarters - a more awkward position for them - or so low that they had to stand up on their hind legs and stretch. They also tried it without the platform at all, compelling the tree shrews to climb up the side of the cage and hold on to the mesh as they reached out with a free paw.

The idea of these four situations was to see whether the tree shrews would show a stronger preference for one particular hand when they needed to concentrate more on keeping balance - as theory says they should. One thing that became apparent is that, if you're tree shrew, it's the sitting that's the hardest one to do. Compared with that, clinging to the side of the cage actually isn't that difficult; it's the sort of thing they're used to.

You can see the results of one trial in a video here. While some of the animals didn't really care which hand they used, most of them used the same one most of the time, and half of them only ever used a particular hand. Clearly that's not as strong an effect as in humans, or some other primates, but it shows that, even in tree shrews, most of them have a clear preference. It's worth noting, though, that, of those that did show a preference, half were right-handed and half left-handed. So they have a preference at the individual level, but not at the level of the species, as we do (and some other animals, too).

However, the posture they had to stand in didn't make any difference. If a tree shrew was left-handed, for example, it was still left-handed no matter how easy the task, or how little balancing was required. That's not what we'd predict, and its generally not what we see in monkeys. However, similar results have been shown with grey mouse lemurs (Microcebus murinus), an unusually small primate that, like tree shrews, doesn't use its hands much.

It would seem that, as mammals evolve to use their hands more, handedness become more important, more likely to be fixed in one particular direction across the species, and they also become more adapted to taking account of their overall bodily position when choosing which hand to use. It's perhaps not so much something hard-wired into evolutionary paths, but a product of how the animal has to live, and the sorts of things it commonly has to do.

[Picture by JJ Harrison, from wikimedia commons. Cladogram adapted from Janečka, et al. 2007]

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