Our Window on Nature

. . . exploring the world around us

Left-Handed Plants And Right-Handed Snails

Filed under: Critters,Plants — Lowell Christie -- August 7, 2015 @ 2:31 pm

Fallen TreeFor the most part, left-handed people occupy the same status as unemployed brothers-in-law – they’re both insulted and ignored. It isn’t enough that automobiles and most tools and machines are built for the convenience of right-handed people, but even the dictionary perpetuates a variety of negative connotations associated with lefthandedness.

The American Heritage Dictionary includes in its definition of the word left-handed the description “awkward; maladroit, of doubtful sincerity; dubious” (as, for example, in a “left-handed compliment”). But while admittedly not the dominant pattern in nature, left-handedness does occur in a variety of living as well as nonliving forms (weather, for example.)

Thanks to the Coriolis force caused by the earth’s rotation, the overall circulation of wind and weather in the northern hemisphere is in a righthanded (clockwise) direction. Within that pattern, high-pressure cells rotate clockwise and lows rotate counterclockwise. Admittedly, the low-pressure cells bring the bad weather, but before you pounce on that as proof of left-handedness being bad news, consider that the pattern is reversed in the southern hemisphere. There the right-handed lows bring hurricanes, and left-handed highs bring balmy weather. Score: one-all.

Plants that have a helix, or screw twist, in their growth structure don’t simply twine at random. Bindweeds (such as morning glory) follow the more common pattern and always twine to the right as they grow; honeysuckles just as regularly twine to the left.

Another kind of twist found in plants isn’t readily apparent until death strips away the covering and exposes the bare wood of a tree. Although scientists haven’t agreed upon the reason for it, in some locations a certain percentage of trees twist in a spiral as they grow. The trunk twists, as do the branches; however, not all trees spiral in the same direction. Some turn to the right; others turn the opposite way for no apparent reason.

Right-handedness and left-handedness occur in the animal kingdom as well, most noticeably among the mollusks. Obviously, a snail or conch shell must turn in one direction or another, and in most species the spiral is right-handed. Still, in other species the shell always turns to the left. As if that weren’t strange enough, some species of mollusk are left-handed in one location but righthanded in another. However, even in the most rigidly handed mollusk societies, now and again a “sport” appears that – because such shells are exceedingly rare – is prized by shell collectors.

Working our way up the evolutionary ladder, we find that crabs and lobsters grow claws of differing sizes. This size difference is dramatic in the case of the fiddler crab, which uses its enormous left claw to attract a mate and the smaller right one to dig its burrow and to obtain its dinner.

We tend to think of fish as being marvelously uniform creatures, but those fish that spend most of their lives on the bottom of the sea have evolved a striking divergence. At birth, a flatfish is as symmetrical as its more active neighbors, but as it grows it settles to the bottom to spend the rest of its life lying on its side. The eye on the side that faces downward would become useless under these circumstances; so, during the flatfish’s “adolescence” one of its eyes migrates up and over the forehead to the other side. Most flatfish, including halibut, are right-eyed, but turbot are left-eyed. Sole are the oddballs; some types are right-eyed in cold water regions and left-eyed in warm tropical waters.

Among the larger sea creatures, the narwhal deserves special mention. This small whale has but two teeth in its upper jaw. In the male, one tooth grows into a long spearlike tusk that spirals counterclockwise (left) to the tip. A 12-foot narwhal may sport a tusk two-thirds as long as its body. Occasionally a narwhal develops two tusks, but they don’t spiral in opposite directions as one might expect; both tusks twist in the same direction.

Many mammals appear to be ambidextrous, comfortable using both hands (paws) for a variety of tasks; others consistently prefer one hand over the other. It appears, however, that in these cases handedness is not species specific. Within the same species – indeed within the same family lines – there may be approximately an even number of lefthanded or right-handed individuals.

That leaves man. When did handedness become an important issue? If we study Stone Age tools and pictures painted or chiseled on rock during that time, we learn that early man was about equally divided between left-handed and right-handed preference. By the Bronze Age righthandedness dominated. Depending upon whose study you cite, today left-handers total 15 to 30 percent of the population – a decided minority. But those of us who reach for the fork, the pen, and the softball with the left hand can take comfort in the fact that as far as fiddler crabs and narwhals are concerned, we’re in the majority.


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