Mantis Shrimps Avoid Deadly Fights by Pummeling Each Other
Many animals use threatening signals to steer clear of costly bouts, but for these boxing crustaceans, the fights are the signals.
Few animals wield more formidable weapons than mantis shrimps. These crustaceans are named after the folded arms they hold beneath their bodies, which end in either a stabbing spear or a smashing club. When unfurled, the tips of these arms can move at up to 51 miles per hour—underwater, no less—and reach accelerations over 10,000 times that of gravity. Mantis shrimp punches, the fastest in the animal kingdom, can shatter snail shells and aquarium glass alike.
So what happens when mantis shrimps smash each other, as often happens when they fight over territory?
Based on the behavior of other animals, you’d expect dueling mantis shrimps to go easy on each other. Fighting carries risks of injuries, infections, and outright death, so well-armed animals are better off avoiding conflict when they can. They typically do this through signals that indicate strength, health, or stamina, from formidable pairs of horns to vivid patches of colour. If rivals can use these signals to size each other up, and predict the winner of their duel beforehand, they can avoid costly blows. As Douglas Emlen from the University of Montana writes in his book Animal Weapons, “In one of nature’s more amusing paradoxes, the most extreme weapons are also the least likely to be deployed in pitched battle.”
To see if mantis shrimps would follow this model, Patrick Green and Sheila Patek from Duke University studied a small smasher species that lives in the seagrass flats of Panama. There, males compete with each other over burrows in which to live, eat, and mate. Green and Patek collected several of these animals and had them smash a force sensor to measure how hard they could hit. They then put the animals in artificial burrows made from PVC piping, introduced challengers into their tanks, and watched the ensuing confrontations.
When two mantis shrimps square off, they flare their arms to the side, in a threatening display called the meral spread. Green and Patek predicted that most conflicts should end there. The meral spreads should show off the size of each combatant’s clubs, the size of the clubs should correlate with how forcefully they can strike, and the strength of those blows should be the deciding factor in any battle. With all that information readily available, the animals shouldn’t ever need to escalate to actual blows.
These predictions were wrong on every count.
In reality, the mantis shrimps almost always laid into each other, whether they used meral spreads or not. What’s more, the spreads turned out to be terrible signals because the dimensions of the club don’t accurately predict how hard its owner can hit. And besides, strike force doesn’t correlate with victory. Instead, it’s the number of blows that matter. The winners aren’t the ones that hit the hardest, but those that keep on hitting.
These odd results made more sense to Green and Patek when they thought about the details of the fights. Most take place in burrows, which are dark and cramped. Even though mantis shrimps have famously extraordinary eyes, it might be hard for them to visually judge an opponent’s quality. In that environment, the easiest way of gauging an opponent’s mettle is to punch them.
At first, in ritualistic fashion, they deliver sequential blows to each other’s tail-plate, or telson. These structures are heavily armored, with many microscopic bumps and ridges that excel at dissipating energy. They’re like shields with punching bags strapped to them, and dueling mantis shrimps use them to accordingly, coiling them in front of their bodies to absorb the incoming blows.
Green thinks that this “telson sparring” plays the same role that he originally ascribed to the meral spreads: they tell each combatant about its rival. “The cool thing about the telson is that its ability to dissipate energy scales with body size,” he says. “If they strike a rival’s telson and feeling the energy being dissipated, they could potentially get an indication of its size.” In other words, they’re checking to see if their opponent can take a hit. Or, if their opponent can take lots of hits.“Winners strike a greater number of times,” says Green, “so telson sparring could be used to communicate information about an individual’s energetic persistence or motivation.”
In this way, the mantis shrimps are behaving a bit like deer or antelope that lock horns and push against one another. They’re testing strength and stamina—they’re just doing it through the unconventional means of punching the shit out of each other. They don’t use signals to avoid coming to blows, because the blows are the signals.
These ritualized super-punches may help the animals to avoid even more costly escalations. Indeed, Green saw a few instances where a winner would strike a fleeing loser in the abdomen rather than the telson, puncturing its shell and inflicting severe injuries.
“I love the idea that the fights themselves may be the signals that matter,” says Doug Emlen, who thinks that many other animal fights may work in the same way. This would explain why many exaggerated weapons, like the dramatic horns of rhinoceros beetles, look like reliable signals of strength, but don’t appear to be visual ones. “Most are used in the dark, and there is no evidence that either males or females look at the weapons,” he says.
But Emlen adds that the mantis shrimps might still be using their meral spreads to avoid coming to blows. Green and Patek pitted equally sized individuals against each other; if the match-ups had been more random, the displays would have allowed larger males to deter smaller ones without needing to spar. Such was the case in a study of wild caribou: Although researchers documented more than 11,000 confrontations between males, they found that only 1,700 progressed to physical contact, and just 6 escalated into actual fights. In most cases, the smaller male called off the altercations.
“Most of us in the field ignore these brief, non-start interactions and only score the outcomes of things that actually look to us like proper contests,” says Emlen. “This bias means we may be missing the lions-share of the story. Lots of ‘fights’ never really turn into fights.”