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It's Easy to Be Fearless When You Have a Good Shell

By Elizabeth Preston
Sep 4, 2015 7:57 PMNov 19, 2019 10:02 PM


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Aesop never penned a fable about a snail. If he had written about a certain freshwater mollusk, the moral might have been Boldness comes from a strong shellor maybe Careless snails get chomped. But because the snail and its variable shell are real, their lesson has more to do with the the weird workings of evolution. Individual Radix balthica snails can have differently shaped shells. They also have varying "personalities," at least as far as you can measure such a thing in a mollusk. To see whether their shells and personalities were linked, Johan Ahlgren, a researcher at Lund University in Sweden, gathered snail eggs from four nearby ponds. Back in the lab, Alberg waited for the eggs to hatch. He fed the young snails algae and lettuce until they were grown. Then he subjected the animals to a personality test. The trait that interested Ahlberg and his colleagues was "boldness." Measuring an animal's boldness or shyness is a common way to study personality in species all across the animal kingdom: Does it readily explore a new space? Does it stand frozen in a corner? In snails, shyness means staying inside your shell. So Ahlberg gently prodded the snails' shells with tweezers to frighten them ("The snails retracted after one poke," the scientists write). Then he counted the seconds until the animals stuck their heads back out. After repeating the experiment a week later, Ahlberg could sort the snails into two groups. Bold snails consistently emerged from their shells in under 10 seconds. Shy snails took 15 seconds or longer. Some especially timid individuals waited three or four minutes before daring to poke their antennae out. Next, Ahlberg scanned the shells of all the snails and analyzed their shapes. There were significant differences, he discovered, between the shells of bold and shy snails. Bold individuals had rounder shells with wider openings. Shy ones had more elongated shells with narrower openings.

Bold R. balthica snails have rounder shells, like the one on the right. Shells that are round with wide openings are harder for fish to crush and eat, the authors write. So bolder snails have safer shells. In other words, they can afford to be bold, because they have better protection from predators. Links between personality and body type have shown up in other animals. Bolder anole lizards have tails that break off more easily, for example. As in the snails, animals with a more fearless personality seem to balance that trait with a body that's safer from predators. Do the snails learn to be bold because they grow up with sturdier shells? Although this is possible, the authors write, in this experiment all the snails grew up in the lab, safe from predators. Their personalities emerged without any experience of fish trying to eat them. So it seems that the two traits—boldness and well-defended shells—are genetic, and have evolved alongside each other. The scientists call this "the ghost of predation past." The hungry fish that pursued R. balthica in its history have left their mark on its DNA. Today, even if they grow up in a safe space, snails only act fearless if their bodies can back them up. Or maybe they learned from Aesop: Preparedness for war is the best guarantee of peace.

Images: Top, illustration by Milo Winter for "The Tortoise and the Ducks" (Foolish curiosity and vanity often lead to misfortune) from Aesop's Fables; middle, Ahlgren et al. (2015); bottom, illustration for "The Wild Boar and the Fox" (Preparedness for war is the best guarantee of peace), same source as top.

Ahlgren J, Chapman BB, Nilsson PA, & Brönmark C (2015). Individual boldness is linked to protective shell shape in aquatic snails. Biology letters, 11 (4) PMID: 25904320

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