Video: Animal 'handedness'
HANDICRAFTS were never my strong point at school. For each project I attempted, I'd struggle with tools and techniques that didn't suit a left-hander like me, which often made me wonder why humans are wired to prefer using one side of the body over the other. Apart from a few wrist aches, though, my handedness hasn't been too much of a burden. Contrast this with the bad luck of a toad that fails to jump away from a snake approaching from its right, just because its right eye is much worse at spotting the danger than its left. Clearly, such asymmetry can have fatal consequences.
All the more perplexing, then, that creatures across the animal kingdom - including mammals, birds, fish and invertebrates - prefer to use one paw, eye or even antenna for certain tasks, even though they may then be let down in crucial situations by their weaker side.
The cause of this trait, called lateralisation, is fairly simple: one side of the brain, which generally controls the opposite side of the body, is more dominant than the other when processing certain tasks. Why would animal brains ever have evolved a characteristic that seems to put them in harm's way? Armed with a spate of ingenious cognitive tests, a group of animal psychologists think they've finally found the answer, in the shape of some previously overlooked benefits to a lopsided brain-body connection.
Not before time. Up until the not-too-distant past, it had been broadly assumed that handedness was a uniquely human trait that evolved as a by-product of our amazing capacity for language. "This unique skill depends predominantly on the left hemisphere, so everybody thought language and lateralisation were tied up," explains Richard Andrew of the University of Sussex, UK.
This notion rapidly fell apart as researchers starting spotting evidence of lateralisation in all sorts of animals. Back in the 1970s, Lesley Rogers, now at the University of New England in Armidale, New South Wales, Australia, was studying memory and learning in chicks. She had been injecting cycloheximide into the chicks' brains to stop them learning how to spot grains of food among distracting pebbles, but found the chemical only worked when applied to the left hemisphere. That strongly suggested that the right side of a chick's brain played little or no role in learning such behaviours - compelling evidence that the different sides of the animal's brain perform different tasks (Pharmacology, Biochemistry and Behavior, vol 10, p 679). "Injecting it on the right side had absolutely no effect. And that was the initial discovery of lateralisation in the chick, at a time when everybody thought it was unique in humans," she says.
Similar evidence appeared in songbirds and rats around the same time, and since then, researchers have built up an impressive catalogue of animal lateralisation. Sometimes it's as simple as a preference for a single paw or foot - primates, cats and even parrots fall into this category. In other cases, lateralisation appears in more general patterns of behaviour.
The left side of most vertebrate brains seems to process and control feeding, for example. Since the left hemisphere processes input from the right side of the body, that means animals as diverse as fish, toads and birds are more likely to attack prey or food items if they view them with their right eye. Even humpback whales prefer to use the right side of their jaws to scrape up sand eels from the ocean floor. Some more exotic recent examples of animal lateralisation include elephants with marked preferences for which direction they swing their trunk for feeding or sand spraying, and honeybees whose right antenna is more sensitive to odours.
Animals as diverse as fish, toads and birds are more likely to attack prey viewed with their right eyeThere are no hard-and-fast rules, however. Many fish, for example, consistently turn in the same direction when faced with a predator, apparently so that they can use a specific eye and brain hemisphere to deal with the situation, but a study of 16 different species found that the preferred direction varied between species, no matter how closely related they were (Laterality, vol 5, p 269). Similarly, parrots can be left-footed, right-footed or ambidextrous. The side preference can even differ within a species according to gender: tomcats tend to fish tuna out of a jar with their left paw, while females prefer their right paw.
Despite such diversity, we can't rule out the possibility that lateralisation was passed down from a single common ancestor. Lateralisation is caused by the way the brain is organised, with certain regions predisposed to handle certain aspects of cognition. Generally, only one side of the brain will contain the region that handles a given aspect of cognition. So a preference for a particular side therefore depends on which of these regions are typically involved in the task. Since there might be multiple ways of performing a task, each using different regions of the brain, a preference for one side may just be a symptom of the chosen cognitive strategy. "Different individuals or species may be using different cognitive approaches to deal with similar problems and this affects which side of the brain has the upper hand," says Giorgio Vallortigara at the University of Trento in Italy. In that case, the brain organisation underlying lateralisation may still have arisen in early ancestors, even if specific side preferences have shifted over the years.
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Evolution Of Optimal Strategy
Thu Apr 29 16:43:45 BST 2010 by Eric Kvaalen
"The side preference can even differ within a species according to gender: tomcats tend to fish tuna out of a jar with their left paw, while females prefer their right paw."
I assume you used the word gender (instead of sex) because a cat can be masculine, feminine, or neutered?
More seriously, the game theory explanation sounds good, but is there an mechanism by which this optimal strategy (optimal for the species as a whole) would come about? It might be necessary to assume that being odd (left-handed in our case) is NOT genetic.
Link With Autism Article?
Mon May 03 18:55:47 BST 2010 by Coran Sloss
I'm wondering if this connects to your autism article in any way. If handedness is to do with the sides of the brain being assigned to the sides of the body and autistic people are often found to be using both sides of the brain for a single task, surely this would suggest that autistic people would be less strongly handed than "normal" people?
Another Hypothesis
Thu May 06 00:11:54 BST 2010 by D L Jewett
http://www.abratech.comAnother GOOD hypothesis for lateralization says that the left hemisphere specializes in temporal pattern, while the right specializes in simultaneity. One way to remember this is that a piano player often plays chords with the left hand, and melody with the right. The evidence was compiled by Robert Miller in his book "Axonal Conduction Time and Human Cerebral Laterality" 1996: Harwood academic publishers.
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In the light of our interest in laterality as part of the graded motor imagery (GMI) programme, this article looks at lateralisation in animals. What is the purpose of favouring one side as surely it would be preferable and useful to be equally skilled with the use of right and left. However, on the basis that we need to conserve energy and prioritize our needs, perhaps lateralising benefits us through the most appropriate allocation of resources to achieve the most whilst expending the least.
Clearly we are not the only lateralisers with many animal examples being described in the article with the suggestion that perhaps we have descended from a similar source. It is argued that with a particular task there will be a region of the brain involved that will vary between individuals depending upon how they choose to approach the problem. Lateralisation is reported to be underpinned by the brain's organisation and therefore depending upon the strategy chosen by the individual (which brain part is used) will determine which side of the brain is activated.
Applying the concept of laterality in the clinic is part of the GMI programme that is an approach grounded in science and demonstrating good evidence. Through the recent work of Lorimer Moseley we now see a role for recognising left and right as part of a treatment sequence for pain, i.e. the concept of laterality. Further understanding of the normal role of lateralisation will clearly help us to optimise this approach.
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