Robot crab reveals how males compete in claw-waving contest

A robot crab has helped scientists understand how male fiddler crabs compete over females.

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

In the new study, a robot crab – nicknamed Wavy Dave – waved its claw on a mudflat teeming with male fiddler crabs.

When the robot 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 – led by the Centre for Research in Animal Behaviour (CRAB) at the University of Exeter – suggests male crabs notice the behaviour of rivals and can adjust their behaviour in response.

“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,” said 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 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.

If a female enters a male’s burrow, the male fertilises her eggs. Once the eggs have hatched, the larvae float away to sea.

Males also retreat into their burrow at intervals for a variety of other possible reasons – including avoiding predators and fights with rivals, resting and wetting their gills.

‘Wavy Dave’ under attack

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 realised 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.”

Lockdown ‘pipedream’

Dr Wilde said Wavy Dave began as a “pipedream” during the Covid 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.”

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