By Lauren Koenig
While left-handedness in humans is now generally accepted as a natural trait, it has taken thousands of years to overturn cultural discrimination against “lefties.” In the Western World, most bias today is limited to the uncomfortable design of tools like scissors and musical instruments, mass produced for the 90% “righty” majority.
Yet, body asymmetry is far from unnatural - researchers have uncovered a slew of skew throughout the animal kingdom. All primates appear to have some kind of preference, while a majority of kangaroos and about half of mice are left-handed. Even animals without hands appear to have a side preference.
For some of these species, the difference between being a “righty” or a “lefty” is the difference between life or death. A team led by Masakai Hoso at the University of Tokyo traced the origin of left-sided dominance in Satsuma snails, an animal that lacks both hands and feet, and how it’s linked to predatory snakes.
Satsuma snails have shells that mostly coil towards the right, but in one region in Japan, there are an abundance of shells that coil towards the left. Hoso’s team dug deep into the snail genome and came up with a single gene that codes for the direction of shell coiling. But what determines whether a snail is born with the left or right-sided gene has more to do with its parents’ ability to survive a snake attack.
Two sides to the story
Even though Hoso’s team had a good idea of where this tale ends, they hoped to trace it back to its beginning: where did the "lefty" gene come from if most snails are "righties"? Further research into the snails’ distant past indicates that a random mutation must have arisen that twisted shells counterclockwise. This area of DNA must have been prone to change because this mutation happened at least six times. If one wobbly gene leaned towards the left frequently enough, then enough "lefty" snails existed simultaneously to find one another and propagate the line.
Modern day snails are more firmly entrenched on either side of the bias line because their shape prevents them from mating with one another. No matter how the snails contort, the location of the shell in proximity to snail genitalia prevents "righty" and "lefty" snails from configuring their bodies for successful reproduction. From the perspective of most scientists, this means that the snails are now two different species.
Can snakes get a grip?
If snails can harness their genetic armor to adapt to predators, it may be just a matter of time (albeit on a long, evolutionary time scale) before snakes respond in kind. It all depends on the snakes’ own genetic material, as well as whether they rely on the snails as their main meal.
Snakes may never get their chance though, as decreased habitat, deforestation and a changing climate have placed these animals on the endangered species list. As the quest to understand the relationship between genetics, handedness and body asymmetry in our own species continues, looking to other species may be key to understanding all sides of this story.
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