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.

Size Matters

One way to group male snow crabs is to use their relative claw size. Male opies can be small-clawed adolescents or large-clawed adults. Their claw size distinction is based on the height of their claw, or chela, compared to the carapace width.

It seems pretty straight forward, but the difference in claw size represents where each male snow crab is in his life cycle. Crustaceans molt/shed their hard exoskeleton in order to grow. Once they shimmy out of their old exoskeleton, they're really soft and vulnerable. But because they're soft, they're able to swell their bodies by taking in lots of water so that once their new shell hardens they're larger.

Snow crabs have a final, or terminal, molt. This means that after the terminal molt, they will no longer grow. When the males terminally molt, their claws become disproportionately larger compared to their bodies' growth. Why the larger claws? It's thought that larger claws (and larger muscles within them) give terminally-molted males a physical advantage when competing for females.

Not all male snow crabs terminally molt at the same time; some may skip a molt for a year or "choose" to stay adolescent for one more molt (it's not really well understood). Because of this, adult males come in a wide range of carapace widths. In order to determine which males are adolescent versus adult, I have to use a logarithmic discriminate function, which I won't get into too much, but I think it sounds impressive. It goes something like this: