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.”