SICB Lessons: You can’t trust a fiddler crab

can you believe this guy?

Fiddler crabs lie! Well, that’s not exactly true. But male two-toned fiddler crabs Uca vomeris can be misleading when it comes to their displays of strength. Males will wave their large chela to show off their guns and attract mates, but some males’ large claws have been broken off and regenerated. A regenerated claw, while being the same length, is often skinnier and weaker, as seen in U. pugilator from South Carolina:

a normal U. pugilator claw on the left compared to a regenerated claw on the right:

the regenerated claw is skinnier and missing the teeth and tubercles

(v = manus length, iii = dactyl length, iv = propodus length)

Candice Bywater of the University of Queensland monitored behavior of male U. vomeris and measured their claw strength and metabolic rates to compare regenerated vs. original claw action and strength. While regenerated claws were, in fact, weaker, that didn’t stop the males from putting their claws in the air like they just don’t care. Males with regenerated claws were basically saying, "Hey ladies, I can protect you from that other dude", even if they can't. Bywater calls this type of deceit ‘unreliable signaling’ and presented her work at the 2011 SICB Symposium.

SQUEEEEEEZE!
Uca vomeris in Candice's lab

She has also seen unreliable signaling in Australian freshwater crayfish Cherax dispar males. Dominance is determined by claw size for both males and females, but while females have to back up their size with strength, male claw strength is not as important for determining social standing. Two males of similar claw size may not have similar claw strength, therefore, when they display their claws before a battle, they may be dishonest about their true strength.

fight! fight! fight!

For discussion’s sake, the congener C. destructor showed male dominance to be dependent on both size and strength while females only needed bigger, not stronger, chelae. Go figure.

I wonder what this one is trying to signal to us

What's that you say?

Bywater, C., M. J. Angilletta, and R. S. Wilson. 2008. Weapon size is a reliable predictor of weapon strength and social dominance in females of the slender crayfish. Functional Ecology 22: 311-316.

McLain, D. K., L.D. McBrayer, A.E. Pratt, and S. Moore. 2010. Performance capacity of fiddler crab males with regenerated versus original claws and success by claw type in territorial contests. Ethology Ecology & Evolution 22: 37–49.