How Being a “Lefty” Can Save Your Life – a Lesson from Snakes and Snails

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.  

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​Righty-tighty, lefty-loosey
 
Left-sided snails coexist with Pareatid snakes, a species that also has asymmetry – in this case, in its dental records. These snakes can have up to 6 more teeth on the right side of their jaw than on the left. Hoso’s team found that this unique lateral bias is a specialized feature allowing snakes to prey on “righty” clockwise coiling snails. Lacking the jaw strength to crush the shell, the snakes feed by alternatively pushing their mouth parts in and out to pull the soft-bodied snails out of their shells. 

​However, when faced with “lefty” counterclockwise coiling snails, the snakes drop the snail because the shell gets in the way of their grasp. When the researchers fed snails to the snakes, they found that the snakes were able to consume nearly all the “righty” snails, but ate only 12.5% of the lefty snails. The “lefty” snails went on to live another day, producing even more snails with built-in antipredator defense. 

A snail-eating snake (Pareas iwasakii) enjoying a Satsuma snail  (CC license wikipedia)

Asymmetric skull of a snail-eating snake 
​(CC license wikipedia)

​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.
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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.  

Lauren Koenig

​Lauren is the Editor-in-Chief of the Scientista Foundation and a graduate student at Michigan State University studying electric fish. Since obtaining her B.A. in biology from Vanderbilt University, she worked on several wildlife research projects throughout North and South America. She is excited to work with Scientista to increase accessibility to research and expand outreach efforts geared for women in STEM. When she is not dealing with unrequited love from her research animals, Lauren enjoys wildlife photography, kayaking, and dance.