Look left, look right, charge!

Most animals favour the left or right paw, and even elephants prefer one side of their trunk. Are they more human than we think?

Research into left and right-handedness in animals has revealed that individual members of a surprisingly diverse range of species prefer to use one side of their body over the other. The largest land mammal can now be added to the list of handedness in nature following a study showing that elephants exhibit a definite preference for using one side of their trunks to the other. Some elephants twist their trunks to the left, others twist them to the right.

Research into left and right-handedness in animals has revealed that individual members of a surprisingly diverse range of species prefer to use one side of their body over the other. The largest land mammal can now be added to the list of handedness in nature following a study showing that elephants exhibit a definite preference for using one side of their trunks to the other. Some elephants twist their trunks to the left, others twist them to the right.

It probably wouldn't come as any surprise to Indian mahouts that elephants are left or right "trunked". The elephant's trunk is such a good substitute for hands that the nimble "finger" at its end is a useful source of income. Riding high in the saddle, mahouts gather coins collected by the extended snout of their sacred charge as they frisk passers-by.

Scientists' research over many years has shown that animals as diverse as cats, mice, birds and newts show preferences for one side or the other. Cats, for example, will use their preferred paw to dab at a moving toy, and a right-clawed parrot will manipulate food with its right foot while it eats. The reported pawedness of toads and the eyedness of lizards suggests that almost anything with a backbone displays some degree of asymmetry in its preferences.

Hands, paws and claws come in pairs, so the predominance of one over the other is not always easy to explain. The elephant's trunk, though, is a single organ. The latest study, by zoologist Franziska Martin, on Asian elephants shows that each elephant exhibited a definite preference for using its trunk with a twist to the left or the right, indicating that muscles and nerves on that side are used in preference to the other. The results, which were presented at the December meeting of the Association for the Study of Animal Behaviour, follow only one other known example in nature where animals show "handedness" in a solitary organ. Spider monkeys, it seems, are left or right tailed. They use their tail to grasp branches and food, so it too functions very accurately, as would be expected from a limb that could determine the difference between life and death when you're 50ft up in a tree.

Universally, the magnitude of the preference for one side over the other depends on the dexterity required. A task like writing will always be done with the preferred side. Carrying a shopping bag, however, does not require fine manipulation, so can be done almost as well with either arm. This distinction was also apparent in elephants - the manipulation of food was most often performed with a bend towards the preferred side, whereas carrying food to the mouth was performed more evenly.

What may be uniquely human, though, is the extreme predominance of right-handedness. About 90 per cent of us are right-handed, but preference is evenly divided 50:50 in elephants or spider monkeys. A majority of either left or right handers within a species is more common in higher primates, and our closest relatives, the chimps, tend to be mainly right-handed. Strangely, though, primates living in the wild do not show a species-level bias for one side. The skew reported in captive animals is possibly due to the greater dexterity required by the artificial tasks researchers give them.

Contrasts between the handedness of humans and other animals have fuelled the theory that human language evolved as a result of right-handedness. Even in primates, the right hand is preferred for tool use, which demands fine manipulation. Right-handed tool use among our predecessors is thought to have encouraged specialisation and dominance of the left hemisphere of the cerebellum (as it is the opposite side of the brain that controls a given limb). Human language is associated with circuitry that is almost always located in the left hemisphere.

Tool use is a crucial ingredient for this theory to work. The argument is that people who live in pre-literate cultures, like some tribes in parts of Africa and South America, only display distinct handedness when they use precision tools. This raises the possibility that the extreme of human right-handedness is not a biological reality but actually the result of the type of tasks scientists use to measure the trait. The skilled tool-use that is usually measured comprises only a small proportion of our manual activities. This bias in measurement may mask the fact that our degree of handedness is not so different from that in apes - rather, the difference may lie in the greater amount of tool-use humans engage in.

One of the puzzles about handedness is why should an animal be an expert with one side of their body instead of both? Evolutionary biologists would argue that energy is always at a premium in nature, and if an animal can perform a task by specialising with one side, it will effectively "undercut" more extravagant animals who use both sides. The energy for the neural and muscular development required for mastery of highly dextrous tasks is significant - imagine having to learn how to write with both hands. Some people, of course, are ambidextrous, and can perform tasks equally well with both sides. These generalists are found in other species too, but seem to be less successful. A study of wild chimpanzees fishing for termites showed that chimps who probed termite mounds with a preferred hand gathered more food than those who changed hands. Handedness, therefore, seems to confer an advantage.

Presumably, "trunkedness" and "tailedness" are similarly advantageous. So what about other organs? Most people are right-legged, right-eyed and right-eared. Leggedness is revealed by which foot is used to kick a ball, or step out. Eyedness shows up when one eye must be selected to look through a telescope or take a photograph. And a preferred ear will be directed towards a quiet sound such as a softly ticking clock.

All this lopsidedness in our apparently symmetrical animal kingdom could get a bit depressing when one considers that symmetry is universally regarded as highly appealing. The ability to remain symmetrical in an asymmetric world is even subconsciously favoured by humans; during experiments using photos of the opposite sex, the most symmetrical subjects are always chosen as the most desirable.

Without asymmetry, though, animals might be less capable of interacting with the environment. Its fundamental value is highlighted by the fact that we share it with animals as primitive as reptiles. Handedness, whether you are an elephant or a lizard, appears to confer added dexterity. For zookeepers in the elephant wing it means you are more likely to lose the contents of your pockets. But for an elephant, handedness is another way of getting the best out of what you're born with, a feature that may once have made all the difference between survival and premature death.

Andrea Lord is a researcher in the zoology department at the University of Oxford

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