This robo crab tests the desires of female fiddler crabs

Fiddler Crabs. Image: ANU

The mystery of why choosy female fiddler crabs visit many males during the mating season has been solved.

Australian research conducted in Darwin finds that the females aren’t hunting for the perfect match, they are searching for a safe haven, a hole in the mud, from birds and other predators.

The findings overturn previous theories and help understand the breeding habits of fiddler crabs, crucial to the ecological health of mangroves, salt marshes and muddy beaches.

“This behaviour of visiting and supposedly rejecting successive males has always been taken as a defining feature of female choosiness, but this study shows that things are not always what they seem,” says Professor Patricia Backwell, from the ANU Research School of Biology.

The researchers used a male robo crab in the study and the female crabs found this just as attractive as the real thing. Male fiddler crabs are known for having one claw considerably larger than the other.

“If a bird attacks, female fiddler crabs can move quickly and directly back to the last burrow it visited,” says Professor Backwell.

The robo crab setup. Image: ANU

“Having this map of burrow positions is essential if they are to survive a bird attack, and this is true for females who are looking for a mate and those who are looking for a burrow.”

The research team used male-mimicking robotic crabs — essentially mechanical waving claws — to test reactions.

Co-lead researcher Marianne Peso says the team noticed female fiddler crabs not seeking a mate visited successive males before settling in a new burrow in the same manner as mate-searching females.

“We watched displaced females move across the mudflat, testing mate preferences with male-mimicking robotic crabs, examining male reactions to the females and testing the females’ response to a simulated bird predator,” says Dr Peso.

“In all experiments, mate-searching and burrow-searching females behaved identically.

“They all visited courting males, they found the same robotic males attractive, males treated them in the same way as potential mates and all the females retreated to the last burrow they visited when swooped by the plastic bird.”

The study was published in the Royal Society Open Science journal.

Andrew Kahn, a PhD candidate at the ANU, explains some of his research:

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