The art of seduction
A male fiddler crab waves his large claw territorially.
Image: anthonyjhall/iStockphoto

It’s 35 degrees Celsius and I’m sitting in a beach chair on the baking mangrove mudflats of Darwin. Sweat streams down my back, but I hardly notice: I am mesmerised by the miniature world unfolding centimetres from my feet.

It’s like watching a small-scale nightclub. Hundreds of male fiddler crabs, one massive yellow claw resting nonchalantly by their side, take their positions on the dance floor.

Suddenly, a pulsing yellow patch of movement erupts. A female, on the prowl for Mr Right, has wandered by and every male in the vicinity is feverishly striving to attract her attention, waving his oversized claw in a beckoning arc.

“It’s a really sophisticated behaviour and amazing to watch,” says ANU PhD student Sophia Callander, who has been studying these fascinating creatures over the past three years. “The males get increasingly excited, falling over rocks and into holes in their eagerness to attract the female.”
Once the female has chosen a male, he leads her to the entrance of his burrow. If she’s pleased with his home maintenance skills, she’ll allow him to mate with her. If not, she’ll continue on her way.

Females can be pretty fussy, inspecting the burrows of up to 100 males before finally deciding on the future father of their offspring. Callander lists the qualities they’re searching for in a man.

“We know that females prefer larger claws and faster waving. They also prefer leaders: males who wave slightly before the other members of the courtship group.”

As we watch, the female moves off into another area and the recently exuberant patch of waving dies down. Another visitor approaches the group; a male crab, recently evicted, is on the hunt for a new burrow. He begins to pick fights, interlocking his large claw with other males’, attempting to flip them out of their burrow and take it for himself. Just when it looks like the invader might prevail, another crab enters the fray and sends the bully running for cover.

“This is one of the most fascinating fiddler crab behaviours,” Callander whispers to me. “Males form defence coalitions: a male will step in to protect his neighbours from invading males trying to steal territory.”

These coalitions are more likely to occur when both the neighbour and the invader are smaller than the helping crab, adds Callander.

“Basically, large males will protect their smaller neighbours, but only when they’re sure they will win.”

At first sight, it might seem the burlier males are soft-hearted, taking pity on their weaker compatriots and lending a helping hand. But, as we all know, there is no such thing as a free lunch and Callander suspected there might be an ulterior motive to this seemingly selfless behaviour.

She set out to investigate with the help of RoboCrab, a male crab look-a-like consisting of a plaster replica claw attached to a moveable metal arm.
Custom-made by Stephen Sims, a retired manufacturer of electronic Christmas decorations (such as dancing Santas), RoboCrab was commissioned by Callander’s supervisor Associate Professor Pat Backwell four years ago.

The female crabs are none the wiser, acting toward RoboCrab as they would a real male.

“These crabs have relatively low resolving power in their vision. But they make up for it by being extremely sensitive to movement and they have a strong preference for yellow objects, so RoboCrab works really well,” says Callander.

Callander used a set of RoboCrabs with different-sized claws to set up a simple experiment to test whether having smaller-clawed neighbours gives males an advantage in the dating game. She presented female crabs with one of two options: a male flanked by smaller-clawed males, or the same male flanked by larger-clawed males.

The females were much more likely to approach the male when he was surrounded by smaller-clawed wingmen.

“This result suggests that by protecting and retaining their smaller neighbours, males are gaining a mating benefit – making themselves look comparatively more attractive to females,” says Callander.

So, guys, you heard it here first. Next time you go clubbing, invite your least attractive mates.

Editor's Note: A story original featured in the Autumn 2012 issue of the ANU Reporter. This article is under copyright; permission must be sought from the Australian National University to reproduce it.