Name: Fiddler Crabs' Giant Claws
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Nova

Fiddler crabs' large claws look intimidating, but is deadly combat their main purpose?

Why Fiddler Crabs Have Giant Claws

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<div class="rich-text">Fiddler crabs' large claws look intimidating, but is deadly combat their main purpose?
</div>

<div class="rich-text">Fiddler crabs' large claws look intimidating, but is deadly combat their main purpose?</div>

Fiddler Crabs' Giant Claws

Nature

<div class="rich-text"><h3>
 Fiddler Crabs' Giant Claws
</h3>

 Published May 30, 2018



 Narrator:
 
 Fiddler crab claws are proportionally bigger than any other known animal weapon on Earth.



 Brook Swanson:
 
 There’s about 103 different species of fiddler crabs. They live all over the world in the tropics, and they eat algae off the surface of the mud.



 Narrator:
 
 Brook Swanson studies the costs and benefits of this surprisingly large claw.



 Swanson:
 
 So, females have two little claws, and they can actually eat twice as fast as the males. The males only have these giant claws and can’t use their weapon claw to eat.



 Narrator:
 
 The huge claw comes at a great cost. Males must eat extra food to fuel their muscles, but they are only able to gather it half as fast as the females.



 Doug Emlen:
 
 All right, so he’s bitten me three times already. Can you show me what these guys can do?



 Swanson:
 
 We can use this force meter to measure how strong their claws are.



 Emlen:
 
 This just measures how hard they squeeze?



 Swanson:
 
 Exactly. We put their claw right there, they squeeze. That’s about 20 newtons, so the crab is producing about 20 newtons with its claw, five pounds of force—so, like having a bag of sugar on a pin pushing on you.



 Emlen:
 
 So, it’s not just five pounds, it’s five pounds concentrated on a very sharp point.



 Swanson:
 
 Exactly.



 Emlen:
 
 That’s why it hurt so much.



 Narrator:
 
 So, is this strong enough to pierce another crab’s shell? A machine in the lab can measure that, using the shell of a dead crab.



 Swanson:
 
 That could take about five newtons of force.



 Emlen:
 
 Five newtons, so way less than the squeezing force.



 Narrator:
 
 The big claw’s pinching strength is four times more than what is needed to pierce another crab’s shell.



 Emlen:
 
 So, this is technically a lethal weapon?



 Swanson:
 
 Exactly.



 Narrator:
 
 Crab claws are powerful enough to kill a rival. But there’s a paradox.



 Swanson:
 
 So, they’re plenty strong enough to kill each other, but when we keep them in the lab, we hardly ever see them fighting. And I’ve never seen one kill another one in the lab. And when you study these in the field, it’s very rare to see them fighting there, as well.



 Narrator:
 
 Male fiddler crabs sometimes duel with their weapons on the beach, but they spend much more time using their giant claws in another way.



 Swanson:
 
 So, here we have two male fiddler crabs, and their body size is about, about the same. But if you look at their claws—the claw’s twice as big as the other one.



 Emlen:
 
 Wow. That is so obvious.



 Swanson:
 
 What they spend most of their time doing is not fighting with these claws, but waving them in the air. So, they walk around on the sand, and they wave the claw, and they’re signaling to the other crabs how big and how strong they are.


 So, what makes the claw a good signal is that it’s hypervariable. There’s a lot of variation between crabs, and so, if you’re looking at crabs by their claws, you can easily tell the difference. You can easily tell that this crab is bigger and stronger and a better fighter. And that’s what makes the claw a good signal.



 Emlen:
 
 This is awesome. You’d think that the species with the really big weapons would use them to fight all the time, and yet what we see is the reverse, that the species with the biggest weapons are actually the most peaceful.
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