Scientists use robot crab ‘Wavy Dave’ to study fiddler crab mating

By Elizabeth Hunter

A robot crab called "Wavy Dave" has helped scientists learn about crab mating behaviors.

Male fiddler crabs have one oversized claw, which they use to attract females by standing outside their burrow and waving.

In the new study, led by the Centre for Research in Animal Behaviour (CRAB) at the University of Exeter, a robot crab was set up to wave a claw on a mudflat with male fiddler crabs.

When the robot crab - named "Wavy Dave" - was waving, real males waved for longer in response, and they were less likely to retreat into their burrows, especially when the robot had a small claw.

The study suggests male crabs notice the behavior of rivals and can adjust their behavior in response.

Dr. Joe Wilde said, “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.

“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 behavior to compete in a dynamic environment, investing more in signalling when it is likely to be most profitable.”

Female fiddler crabs prefer males with a larger claw and those that wave their claw quickly.

In the study, carried out in southern Portugal, males waved for longer when the robot was waving, but they did not wave faster.

Commenting on this, Dr. Wilde said the crabs might interpret a waving rival as a signal that a female is around, but wait to see the female themselves before committing their full effort.

Thousands of crabs live on the mudflats where the study was conducted.

For each test, the robot crab was set up 30cm from a real male’s burrow, with two cameras recording.

The results show that males were less likely to compete when a rival had a larger claw, possibly because they expected to lose or were fearful of being attacked.

However, some crabs took issue with the robotic intruder.

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

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

Dr. Wilde said Wavy Dave began as a “pipedream” during the COVID-19 lockdown.

At the time, Dr. Wilde was learning about 3D printing, and he happened to see that someone had created 3D scans of fiddler crabs and made them freely available.

He found a 3D printer to make a model, and taught himself enough robotics to make a crab that waved its claw. He then developed an app to control the crab via Bluetooth signals.

Dr. Wilde’s work was funded by the Natural Environment Research Council GW4+ Doctoral Training Partnership.

The paper, published in the journal Proceedings of the Royal Society B, is entitled: “Biomimetic robots reveal flexible adjustment of sexual signalling in a wild invertebrate.”