Few sea creatures are as straight up ornery as the mantis shrimp, a kind of lobster-looking crustacean that is not in fact a shrimp, but a close relative called a stomatopod. It’s equipped with two astoundingly powerful hammers, which it uses to smash hard-shelled prey like clams and crabs—with a punch so fast that it briefly heats the surrounding water to the temperature of the surface of the sun. The mantis also uses them to fight off predators. “They have a hammer, and everything in the world looks like a nail,” a biologist once told me.

That is, everything up to and including the mantis shrimp’s rivals. This is a strange creature, that much is clear, but things get truly bizarre when it comes to its ritualistic duels over territory. When two stomatopods argue over land rights, they settle things by … smashing each other on the bum. And new research, published today in Royal Society Biology Letters, further unravels the mystery that is the mantis shrimp housing crisis.

The mantis shrimp’s home is a burrow, but the burrow market can be white-hot. So rivals are forced to fight each other for the choicest pads. But here’s the thing with fighting in the animal kingdom: You don’t want to do it. If at all possible, it’s best to instead nonviolently signal your displeasure with a rival, and indeed the mantis shrimp will try that by waving its arms, a maneuver known as a meral spread (named after the merus, the main bit of the hammer arm), perhaps to telegraph its size and aggressiveness. Its weapons are, after all, deadly.

Except that if mantis shrimp rivals are similarly sized, the display almost never works. “What we found is that they give the meral display in about half the contests, but even after they give the display they still might escalate to striking,” says Duke University’s Patrick Green, a behavioral ecologist who led the study. “There was only one contest that we saw out of 34 that didn’t escalate to striking, and that was resolved by just the meral spread.”

This is what a mantis shrimp fist fight looks like. This is what a mantis shrimp fist fight looks like. Roy Caldwell

This is where things get interesting. Instead of punching its sparring partner right in the face, the mantis shrimp will curl its tail forward, allowing its opponent to strike a special plate on their tail called a telson. This acts like a punching bag, dissipating some 70 percent of the impact energy. Back and forth, back and forth, until one of them gives up and scoots away.

Green reckons this isn’t about how hard a combatant can punch, but who’s able to strike a greater number of times. But why would the mantis shrimp develop such a ritualized battle? Well, chances are this is all about communicating information.

“When you think about these animals in their natural habitat, they’re in these burrows where it’s potentially hard to see a competitor’s body size,” Green says. “All you might be able to see looking into a burrow is the competitor’s tail. In a way you could be striking a competitor’s tail to get an idea of how big it is in the first place.”

As the battle escalates, still more info flies back and forth. “So if sparring is a signal that’s communicating information,” Green adds, “they might actually be communicating their aggressive persistence, or maybe their endurance, their ability to just continue striking during the contest.”

This sort of battling is not unheard of elsewhere in the animal kingdom, especially among males competing for the affection of females. Male moose, for instance, lock horns not so much to injure each other, but to communicate who’s boss with sheer strength. Still other species, though, go straight to the maiming: A prime example is the male mustache toad, which shanks his rivals with wickedly sharp spines along his upper lip. The mantis shrimp, then, seems to strike a middle ground, trying its hand first at waving at its rival with the meral spread before almost inevitably escalating to violence.

So let this be a lesson: Never drop in on a mantis shrimp unannounced. You do, after all, look a lot like a nail.

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