Crafty crayfish males trick bigger adversaries

February 27th, 2009 - 5:37 pm ICT by IANS  

Sydney, Feb 27 (IANS) When two crayfish come face to face, they size each other up in a ritualistic display, which can quickly escalate from careful tapping of their opponent’s enlarged front claws or chelae to a full-blown fight.
Studying these fascinating animals, ecologist Robbie Wilson of the University of Queensland discovered that crayfish decide whether to flee or fight based on the size of their adversary’s chelae, and that victorious males always have larger and stronger claws.

But to his dismay, he found that some males with weaker claws cheat; they defeat stronger foes despite having a weaker - albeit larger - claw. “Theory does not predict such dishonesty,” Wilson said.

Deceptive signals of weapon strength should not exist, as opponents would quickly stop taking notice of an unreliable cue. Wilson wondered how the crafty males get away.

Teaming up with Candice Bywater, Wilson first took a closer look at the relationship between claw size and strength in female crayfish, to compare them to the males.

“We knew that male signalling is unreliable, so we expected to see more variability in weapon strength in males than in females,” Wilson explained. To measure crayfish claw strength, Frank Seebacher of the University of Sydney helped Wilson design a custom-made apparatus consisting of two thin parallel beams with external force transducers.

Luckily, it wasn’t too tricky to entice the animals to clamp down on the contraption. Crayfish are “very enthusiastic about biting”, said Wilson. “We just had to direct their claws to the device and they’d bite.” The team found that, for a given claw size, males had huge variation in claw strength compared with females.

This makes it hard for males to size each other up using claw size alone, which allows males to cheat when it comes to advertising their strength.

To understand just how serious the cheating was, muscle physiologist Rob James from Coventry University in Britain measured how much force crayfish muscles produce.

He found that the muscles of female crayfish are actually more powerful than those of males. “So males are not only cheating by creating large chelae with little muscle inside, but the muscle they do put in there is actually weaker,” Wilson concluded.

These findings were published in Friday’s issue of The Journal of Experimental Biology.

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