Warring Hermit Crabs Show "Evolution of Fighting" Isn't All About Strength

When hermit crabs do battle, they rap aggressively on each other’s shells, relentlessly beating on each other until one admits defeat and is ousted from its shell for good. It’s brutal, aggressive, and, as one Plymouth University researcher hopes, the perfect model to discover how fighting — among all animals — came to be.

Fighting, as anyone who’s ever seen Planet Earth knows, usually happens between individuals who are trying to gain something from the other, whether that’s food, mates, or territory. Mark Briffa, Ph.D., a professor of animal behavior, tells Inverse he’s studying hermit crab fights to find out what factors affect the outcomes of those fights. In other words: Is there a way to predict who’s going to win?

"“I think that will tell us a little about what’s going on in these hermit crabs but more generally as well."

“I think that will tell us a little about what’s going on in these hermit crabs but more generally as well,” Briffa says. “I think it will tell us a little about the evolution of fighting in ways that we can extrapolate out to perhaps other species of animals.” Most recently, his work has shown that winning a fight depends on the ability to balance two factors: strength and skill.

Strength Versus Skill

Briffa believes that skill is an undervalued aspect of animal fighting with respect to strength. Even among human fighters, we instinctively associate big, muscular individuals with a better chance of winning. But skill and technique are important factors too, as he’s finding with hermit crabs.

When hermit crabs fight, they’re really squabbling over the shell that they can call their home. As crabs get bigger, they go in search of bigger shells, and the most convenient place to find a larger shell is on another crab. To earn its new shell, the challenger relentlessly beats its own, smaller shell against the larger shell of the hapless defender.

What Skill Represents

Size is always going to be important, but Briffa is also finding that vigor is an crucial factor in hermit crab victories. The ability to repeatedly and rapidly slam the opponent puts the odds in a crab’s favor, as does the ability to increase the rate of attack over the course of the fight.

This strategy, Briffa explained in a 2017 paper, is related to endurance — that is, how long each crab can last, while continuing that vigorous rate of attack. In turn, endurance is related to the ability to be efficient. If you’re really smart about how you attack, you should be able to keep it up for longer.

“In this example efficiency means how far they pull back the shell each time they hit the defender,” says Briffa. “I kind of thought maybe initially that the attackers that pull their shell back further were going to be more likely to win, but actually it was the other way around. The ones that pulled their shell back less far with each hit were more likely to win.”

Accuracy and Precision

Studying efficiency to understand who wins a fight is just a starting point. Over the next three years, Briffa will use his funding from the UK Research and Innovation’s Biotechnology and Biological Sciences Research Council to collect data on accuracy (how often a crab lands a blow) and precision (where the most successful crabs hit their opponents) in addition to skill and strength. All of these factors, he says, combine to form successful fighting technique.

"“Once we get that data can we analyze it in a way that might be meaningful."

To do this, he’s taking high-speed video of each fight and creating 3D animated models to study exactly how crabs target their opponent’s bodies. Doing so will not only give us a complete picture of the hermit crab fighter but, more importantly, also show that other kinds of fights can be analyzed, variable by variable, to show the best way to win. He says that this could have applications to elite sports, where athletes can improve their own technique based on these same variables.

“In terms of applying it to sports science, what we’re doing in hermit crabs is very much proof of concept,” he says. “The data that we’re gathering are very different, but once we get that data we can analyze it in a way that might be meaningful.”