Dominance, Handedness, and Ambidexterity

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Biology 202

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Dominance, Handedness, and Ambidexterity

Tamara Tomasic

Why do we have a dominant and non-dominant side? We have always known that we are not perfectly symmetrical, but why should our asymmetry translate into preference for one side of our bodies? Thinking in an evolutionary/selective type of way, why are there still two (or three or more) different types of dominance? Wouldn't it have made sense that, since an overwhelming majority (90-95%) of the population is right-handed and consequently the world suited most for right-handed people, the left-handed preference would have been systematically unselected for? Or maybe it would have made the most sense for no preference to exist at all, and for the entire world to be ambidextrous?

It might seem like a good idea to be equally proficient in both hands-if one cannot be used for a particular task at any point in time the other can substitute as it can be equally well controlled. Interestingly, observations of human and animal behaviour have shown that ambidexterity is not as advantageous as it seems at first. In observed termite eating behaviour of chimpanzees, both right- and left-handed preference were seen, as well as ambidexterity. But those individuals that showed a strong preference for either one hand or the other ate a third more termites than those individuals that showed no preference and used both hands (1)Science and Nature: Animals, An article about handedness in apes. One explanation for this was that those who preferred only one hand used it more often, thus becoming more specialized and precise in their task. The ambidextrous individuals were not as proficient at catching termites, though their level of proficiency was equal in either hand.

Looking at handedness from a historical point of view, being right-handed was a distinct advantage when it came to defense. Because the heart is on the left side of the body, it would make sense to hold the shield on the left side, and thus on the left arm, to defend it. This means that the left arm was passive for most of a man's life, while the right arm was actively used. Could it be that handedness was determined through necessity? If this is the case, it means that we have evolved to have preference for one hand over the other. But because left-handedness is so rare, wouldn't the element of surprise be reason enough to keep left-handedness in the gene pool? And if this were the case, shouldn't dominance in hand preference shift between right and left? It would also make sense to be able to use both hands equally well in a battle-if one of your arms was injured, you could continue the fight with the other arm; this would be a very advantageous adaptation.

Despite this argument for ambidexterity, historical evidence points to the fact that right-handedness was the predominant (and favored) preference. Recent studies have shown that handedness can be linked to bone length, or rather, bone length can be used to determine handedness in individuals from other eras. By measuring the bone length of modern day British individuals with a known preference a correspondence was found: longer arm bones were present in the arm that was preferred (or bones were of equal length if both hands were preferred equally). The numbers obtained from living individuals were 82% right-handed, 3% equal preference, and 15% left-handed. When the skeletons of medieval English villagers were measured, surprisingly similar numbers were found, with 81% having longer bones in the right arm, 3% bones of equal length, and 16% longer bones in the left arm (2)New Finding on the Frequency of Right and Left-handedness in Mediaeval Britain, A study about handedness using modern day and mediaeval British subjects. This shows that since the middle ages, when literacy was at a low point and the left was associated with evil, natural handedness seems not to have drastically changed.

Also interesting is seeing whether humans and hominoids are the only ones with dominance and handedness. Looking at the fiddler crab, we see that handedness is present even within this organism. But while handedness exists, the ratios vary among the species: some species show a virtually 50:50 ratio or right to left, while in others the right predominates (3)Science Week: Zoology: on Fiddler Crabs, An article about handedness in Fiddler Crabs. Why this difference in dominance/preference? The fiddler crabs would seem to suggest that either the environment or the genetic isolation would have the determining effect on ratios of handedness within a population.

To this day, science has been unable to come up with a satisfactory explanation for handedness and dominance in general. Organisms with preferences for one side of the body over the other seem to function better, but why they would pick one side over the other has yet to be understood.

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