14 March 2012

Crayfish with pumped-up claws are big fakers

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A crayfish's biggest, most impressive, claw is not always its strongest, which gives it a tactical advantage over opponents who can't tell whether to avoid the right hook or the left.
crayfish yabbie fight claws faker evolution

Two slender crayfish go head-to-head in battle. Clearly no one’s claws were sufficiently intimidating to avoid a physical fight. Credit: Anthony O'Toole

crayfish fakers claw evolution

SYDNEY: A crayfish’s biggest, most impressive, claw is not always its strongest, which gives it a tactical advantage over opponents who can’t tell whether to avoid the right hook or the left.

This finding, which unmasks male slender crayfish (Cherax dispar) and their pumped-up claws as a bunch of fakers, could help us understand more about lying and cheating within a species.

“When we look through the animal kingdom, it’s really difficult to come up with examples of lying and cheating when two individuals of the same species are communicating,” said Robbie Wilson, a lecturer in biological sciences at the University of Queensland and co-author of the study published in Biology Letters today.

“If you think about a friend who constantly lies and cheats, you stop listening to them and don’t take any notice. It stands to reason that’s the way it would occur in nature as well. We want to find out at what stage is dishonesty so common that other individuals stop listening to it, and what factors make lying really successful.”

Shadowboxing crayfish

Previous studies have shown that slender crayfish males, who are extremely aggressive, show off their limbs to rivals as signals of their fighting power. They check out each other’s claws, or chelae, by rubbing and tapping them, which gives them the option to flee before the standoff escalates into physical combat.

Males with larger claws usually intimidate their opponents into backing down without a fight. However, Wilson and his colleagues have found that the size of a crayfish’s claw does not always reflect its strength. Just because it’s a bigger claw, doesn’t make it a stronger one. This is referred to as cryptic asymmetry, or dishonest signaling.

“Earlier studies have shown that it’s the stronger one that wins a fight, but the one with the larger claws is not always the stronger individual at all. A lot of individuals don’t actually have much muscle to back it up. They’re trying to bluff their opponents into backing down,” said Wilson.

Like body builders who bulk up with protein supplements instead of strenuous exercise, male crayfish often produce a poorer quality of muscle than the females. This suggests that males exaggerate the size of their claws at the cost of weaker muscle. As long as disputes are settled before the crayfish go claw-to-claw, giving off strong signals with larger weapons is a sound strategy for dominance.

How to catch a cheater

The researchers compared the size and strength of the claws of 97 slender crayfish males by measuring the force with which they could clamp down on a pressure sensor. “An individual that’s strong one day is strong the next. So it’s easy for us to tell if an animal is one of the liars, or one of the honest ones,” said Wilson.

“I’ve done many experiments with these guys, and you get to know the honest ones,” he said. “You get nervous around the ones that have got large claws and can pack a punch. They’re only small, about 10 cm long, but if you get their claws between your fingers, tears will come to your eyes. Get to know Joe the muscle man, and you do not treat him lightly.”

The researchers kept track of the size and strength of each crayfish’s right and left claw. When they compared claws of the same size, they found a tremendous variation in strength. Size is a poor predictor of muscle, yet crayfish will still back down from a fight against a bigger guy. They fall for the deception.

What evolution doesn’t correct

“It’s really interesting when we find cases that seem to be evidence for dishonest signaling,” said Luke Holman, a postdoctoral researcher in dishonest signaling at the Australian National University in Canberra, who was not involved in the study.

“Generally the consensus is that dishonest signaling should be rare, because evolution should correct it. If you know that somebody’s going to lie to you all the time, the best strategy is to not listen to them, or to do the opposite of what they tell you. Evolution should be smart enough to pick that up,” said Holman. “If other crayfish tend to exaggerate how good they are at fighting, then they should stop using claw size in their decision about whether to fight.”

Researchers speculate that crayfish prefer not to take the risk that a big claw cannot deliver a big bite. Sturdy claws can inflict serious injuries by tearing off an opponent’s arm, antenna or rostrum – a flat horn between the eyes. “If somebody wants to fight you down the pub and they say, ‘I know kung fu,’ you might not believe them. You might think they’re bluffing. But it’s not worth the risk to check by fighting them, because you might get badly beaten up,” said Holman.

Thinking like a crayfish

The next step for the researchers is to examine the benefits of cryptic asymmetry to understand how crayfish use dishonest signaling to their advantage. Wilson speculates that dishonest signaling may add a tactical uncertainty to aggressive encounters, because opponents cannot predict which claw poses a greater threat.

“The most confusing it can be for an opponent is when they present two claws, but it’s the smaller claw that is actually the most ferocious and threatening weapon. So it gives them a bit more of an advantage, if push comes to shove,” he said.

The phenomenon may also be a result of the need to quickly replace a limb that has been lost in battle. “If we delve into thinking like a crayfish, if you lose one of your claws then you get the hell beat out of you until you grow another,” said Wilson. “Growing a big claw that’s also strong takes a lot more energy than just growing a big claw and then letting the muscle catch up slowly. If I was a crayfish and I’d lost my arm, I’d prefer to have a big claw with not much muscle than a little gimpy one where everyone can tell that I’m easy meat. But this is just speculation.”

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