Robotic crab named 'Wavy Dave' stirs up courtship drama with a real fiddler crab

On a summer tide-flat in southern Portugal, a 3D-printed intruder called Wavy Dave poked his single oversized claw above the mud and began to wave.

Wavy Dave is not a real fiddler crab; he is a Bluetooth-controlled replica designed by behavioral ecologist Joe Wilde during the COVID lockdown. But he was convincing enough to fool hundreds of wild male Afruca tangeri guarding nearby burrows.

With two cameras recording every gesture, Wilde and colleagues have now shown that males monitor a rival’s actual performance, not merely his presence.

They fine-tune both their signaling effort and their willingness to remain exposed when the cost-benefit balance looks favorable.

Crab claws, courtship, and competition

The findings, reported in a new study led by the Center for Research in Animal Behavior at the University of Exeter, add fresh detail to sexual selection theory.

The study also demonstrates how low-cost, biomimetic robots can pry apart the decision rules that are used by animals in crowded, competitive arenas.

Robotic models have been deployed with lizards, birds, and even fish, but the fiddler crab test is one of the first to run in the open, chaotic field. Dozens of live males, hundreds of meters of tunnels, overhead predators, and a baking sun all complicate carefully scripted experiments.

Fiddler crabs live on the border between land and sea. Each male digs and defends a burrow that shelters him from extreme temperature swings, sudden rain, and the constant threat of herons or gulls.

The burrow is also his calling card for choosy females that cruise the flat at low tide. When females are likely watching, males emerge and begin rotating an enormous, energetically costly claw in the air.

Past research shows females prefer bigger claws, and faster wave speeds can boost a male’s attractiveness. The unanswered question was whether males react to how a nearby rival is advertising – not simply to the fact that a rival is present.

Creating the robotic crab

Because every male on a mudflat may be waving at once, a genuine test requires a focal subject to see a rival whose display parameters are under the researcher’s control. Wavy Dave provided exactly that.

Wilde found open-source 3D scans of fiddler crabs and printed a life-size shell. He then built a rotatable claw that could clip on in two sizes: a modest limb matching mid-ranking males and a giant showpiece that dwarfed most live competitors.

The team developed a phone app that could trigger the claw to rotate either slowly – one wave roughly every two seconds – or rapidly, doubling that pace.

Crabs face off with the robot

Fieldwork began by positioning the robot 12 inches from a resident male’s burrow. Thousands of real crabs share these flats, allowing the team to run dozens of independent 20-minute trials. Each trial was divided into four phases: no waving, slow wave, fast wave, and a final no-wave control.

Throughout, two high-definition cameras captured how long the subject stayed above ground, how often he waved and how fast, and whether he retreated when gulls shadowed overhead.

The behavioral shifts were striking. “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,” Wilde explained.

Competing in a dynamic environment

In the presence of a waving robot, males stayed outside longer and, crucially, they lengthened their signaling bouts. They did not, however, speed up individual waves. Wave rate matters – it’s costly, and adding strokes per minute uses more energy than waving longer at a normal pace.

Wilde interpreted the pattern through a shopkeeper analogy. “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 signaling 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 signaling when it is likely to be most profitable.”

Risk shifts with rival size

Risk assessment also shifted. Compared with baseline periods, males were less likely to dive into their tunnels – and they resurfaced sooner – when Wavy Dave brandished the smaller claw.

When the robot showed the gigantic claw, many focal crabs chose caution, cutting their display time or, in several cases, rushing forward to grapple the plastic intruder.

“The females realized he was a bit odd, and some of the males tried to fight him,” Wilde said. “One male broke Wavy Dave by pulling off his claw. We had to abandon that trial and reboot the robot.”

These interactive tests reveal two intertwined decision rules. First, males gauge relative weapon size: a bigger-clawed rival is probably hard to out-signal, so effort may be wasted or fighting danger high.

Second, the behavioral state of that rival matters. A rival that is already waving indicates that receptive females may be close, so extending one’s own bout becomes worthwhile – provided the competition looks beatable.

Robotics is reshaping animal research

The project showcases the power of DIY robotics in field ethology. The robotic crab began as what Wilde calls a “pipe dream” during lockdown: a printed shell, a micro-servo, a hobbyist Bluetooth module.

Yet the budget build yielded high-resolution insights that traditional staging or restrained-rival methods struggle to match. As off-the-shelf parts get cheaper and software more intuitive, researchers foresee fleets of programmable animal proxies probing complex social contests from coral reefs to bird leks.

For now, the Portuguese mudflat has returned to ordinary business, males rotating their claws in the sun as a new generation of larvae drifts out to sea.

But the experiment leaves a lasting message: even creatures with tiny brains monitor their marketplace and update tactics on the fly. They balance energy, danger, and opportunity in ways that are not so different from our own strategic choices.

The study is published in the journal Proceedings of the Royal Society B: Biological Sciences.

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