PREVIEW

It might look like something from a sci-fi movie, but this tiny robot crab – nicknamed 'Wavy Dave' – has given scientists an extraordinary insight into how male fiddler crabs fight for love.

Male fiddler crabs are instantly recognisable thanks to their one oversized claw, which they wave in a comical display to lure in females while standing proudly outside their burrow. But now, a new study has revealed that these crustacean Casanovas are keeping a close eye on the competition – and a robot has helped prove it.

Researchers from the University of Exeter’s Centre for Research in Animal Behaviour (CRAB) unleashed Wavy Dave on a mudflat swarming with real fiddler crabs – and the results were fascinating.

When the robot waved its claw, the real males responded by waving for longer and were far less likely to scuttle back into their burrows. Surprisingly, the effect was strongest when the robot’s claw was small, suggesting that the crabs felt emboldened by weaker-looking rivals.

“We know many animals adjust their sexual displays if rivals are nearby, but less is known about how they react to the actual displays themselves,” explained Dr Joe Wilde, now at BioSS.

“If you own a shop and your rivals start selling things really cheaply, you might have to change how you run your business.

“The same might be true for males signalling to attract females – and our study suggests males do indeed respond to competition.

“Our findings reveal the subtle ways in which these crabs adjust their behaviour to compete in a dynamic environment, investing more in signalling when it is likely to be most profitable.”

Female fiddler crabs, the study confirmed, have a soft spot for males with a bigger claw – and those that wave it quickly. But while the males waved for longer when Wavy Dave joined the party, they didn’t wave faster.

According to Dr Wilde, this could mean the crabs interpret a waving rival as a sign that a female is nearby – but they hold back on giving their all until they see her for themselves.

The high-stakes mating game is simple: if a female enters a male’s burrow, he fertilises her eggs, which will eventually hatch into larvae that drift out to sea. But males don’t just hang around the entrance to show off – they also dive back into their burrows to avoid predators, rest, wet their gills or dodge fights with other males.

The mudflat where the research took place, in southern Portugal, is home to thousands of fiddler crabs – and not all were impressed with their robotic visitor.

For each trial, Wavy Dave was placed 30cm from a real male’s burrow, with cameras rolling.

The team found that crabs were far less likely to challenge rivals with a bigger claw, likely because they didn’t want to pick a fight they couldn’t win. But some couldn’t resist a scrap with the strange mechanical newcomer.

“The females realised he was a bit odd, and some of the males tried to fight him,” laughed Dr Wilde.

“One male broke Wavy Dave by pulling off his claw. We had to abandon that trial and reboot the robot.”

Wavy Dave’s story began during the Covid lockdown, when Dr Wilde found himself tinkering with 3D printers.

“Wavy Dave” started as a “pipedream,” he recalled. After discovering free 3D scans of fiddler crabs online, he printed a model, taught himself some basic robotics, and even built an app to control the crab via Bluetooth.