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

By Lauren Koenig

Left-handedness has long been considered at best, an insult, and at worst, a crime. One of the few skills attributed to "lefties" was the ability to conjure the devil. Only within the past few decades has cultural discrimination against "lefties" been largely overturned in the Western World. Most bias 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. 

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 genetic origin of left-sided dominance in Satsuma snails, an animal that lacks both hands and feet, and how it protects them against predatory snakes.  

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​Righty-tighty, lefty-loosey
 
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. 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 the snakes drop left coiling snails because the shell gets in the way of their grasp. When the researchers fed snails to the snakes, the snakes consumed nearly all of 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 be 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.