Fiddler crab claws divert predator attacks from their bodies

Male fiddler crabs are bizarre-looking creatures. While most crabs have two equally sized claws, male fiddler crabs possess one normal-sized claw and one massively enlarged claw. They wave around their giant claw to attract females and ward off other males, in a manner reminiscent of an (admittedly rather graceless) violinist, which gives them their common name. The two-toned fiddler crab, Gelasimus vomeris, is a fiddler crab that is – unsurprisingly – two-toned: its body is typically blue or black and its enlarged claw is orange. Interestingly, crabs can self-amputate their claws, just like some lizards can drop their tails. Lizards do this to escape predators. The tails, being brightly coloured, encourage predators to attack the tail rather than the lizard's head, leaving the predator gripping only part of its planned meal, while the rest of the lizard scurries off to safety. This observation in lizards led researchers at the University of Queensland, led by Diogo Jackson Aquino Silva, to ask whether fiddler crab claws, being both conspicuous and capable of self-amputation, might function in much the same distracting way.The researchers tested this by making robotic models of fiddler crabs, which they presented to Australian brush-turkeys, Alectura lathami, in parks and urban spaces in Brisbane, Australia. Although brush-turkeys do not naturally eat fiddler crabs, they forage on the ground like other birds that do hunt crabs. The researchers observed how brush-turkeys attacked robotic models that had conspicuous orange claws and compared this with how they attacked models that had claws that were black like the crab's body.When the claw of the robotic crab was orange and conspicuous, brush-turkeys tended to attack the claw at least 50% of the time. However, when the claw was black, and therefore less conspicuous, they tended to attack the body 96% of the time. In other words, the conspicuous claw diverted predator attention from the body to the claw. For a real crab, this would mean that it could self-amputate that claw and – hopefully for it – hurry away to safety before the predator realised that half of its dinner was escaping.However, the conspicuous claw of male fiddler crabs may well be a double-edged sword. By waving it around to attract females or to deter other males, a male fiddler crab is very obvious to predators. But because the claw diverts predator attacks from the rest of the crab's body, it gives the crab an opportunity to escape. So, if predators are indeed attracted by a crab's claw-waving display, the ability of the claw to divert attacks might mean the difference between life and death.Yet, unlike lizard tails, which at least provide some food for a predator, there is no evidence that self-amputated crab claws are eaten by predatory wading birds such as ibises. This might explain why other research has found that ibises prefer to attack female crabs, or males without a claw, rather than males with claws. So, the conspicuous claw might even help the crabs – after a few attacks where predatory birds are left with nothing more than a beak-full of inedible claw, they might learn that attacking crabs with enlarged claws is fruitless. This would leave male fiddler crabs safe to wave their claws around like an awkward violinist, while their predators stalk off to hunt some other prey that lacks diversionary tactics.