If you're equipped with the most fearsome punch on the planet, you'd better make sure that you're also well protected with the most resilient armour; territorial mantis shrimp are not averse to hammering on each other's tails to establish who's boss. ‘When mantis shrimp fight over protective burrows in rock, they exchange high-force blows from spring-powered hammer-like limbs onto each other's coiled, armoured tail plates, or telsons. We call this behaviour “telson sparring”’, says Patrick Green from University of California (UC), Santa Barbara, USA. But how much hammer force can their armour-plated tails withstand? Back in 2010, Jennifer Taylor and Sheila Patek, then at UC Berkeley, USA, discovered that the shrimps’ telsons could absorb 69% of the energy rained down in individual blows. But Green wondered whether there was more to the crustaceans’ resilience. ‘Instead of laying their telsons on the ground, competitors raise their telsons in front of their bodies in a “telson coil” behaviour’, explains Green. Might the shrimps’ curled posture allow them to absorb even more energy while resisting the most ballistic assaults? Green decided to find out.
Filming the feisty crustaceans with high-speed cameras, Green recorded the crustaceans’ combat in fine detail as they hammered it out over territory. Then he and a dedicated team of research assistants, including Kacey Rhinehart (UC Santa Barbara), painstakingly tracked the assailant's hammer limb trajectories and the curl of the target's body to record the telson's recoil each time it absorbed a blow. Calculating the amount of energy that the crustacean's curled body dissipated as its opponent's ballistic impact landed, Green realised that the resilient shrimp could absorb 20% more energy than when the static telson was laid on the ground. And the body's recoil provided even more protection from higher speed impacts, in addition to heavier blows from larger and weightier hammer limbs.
‘Overall, these findings show we need to study both an animal's structures – like armour – and their behaviours, to understand how they resist impacts’, says Green. In other words, having tough armour is only part of the story; it's how mantis shrimp use it that matters.
