‘Wavy Dave’ is a beefy-armed robot crab on a mating mission

A tiny robot fiddler crab is helping environmental scientists better understand the complexities of animal mating rituals and rivalries. And while their initial findings published August 5 in Proceedings of the Royal Society B are helping solve these ecological mysteries, the data was only obtained at considerable peril to ‘Wavy Dave.’

Male fiddler crabs are engaged in a constant, literal arms race. The males are known for asymmetrically sized pincers, with a dramatically larger major claw compared to its smaller one. The reason for this sexual dimorphism is mainly twofold—mating and fighting. Female fiddlers generally opt for the male with the largest major claw, which the latter advertises by waving it at potential partners more quickly than his competitors. Meanwhile, aggressive suitors will often use these oversized appendages to duke it out if they feel threatened.

Biologists have long wondered if male fiddler crabs adjust their claw choreography when the field gets crowded. But it’s not like they can simply ask crustaceans to vary their claw dances on command, then record the results.

“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 Joe Wilde, an environmental scientist at Biomathematics and Statistics Scotland (BioSS).

While at the University of Exeter, Wilde and colleagues at the institution’s Center for Research in Animal Behavior (CRAB, coincidentally) decided to further explore the subject. And boredom may have been the necessity of this crab-bot invention. During the COVID-19 lockdowns in spring 2020, Wilde began to teach himself how to use 3D printers. At one point, he came across online fiddler crab schematics, which he then adapted for a tiny machine that could wave its claw via Bluetooth controls. Thus, Wavy Dave was born.

Fast forward about five years. Wilde and collaborators took Wavy Dave on a trip to the beaches of southern Portugal, where thousands of fiddler crabs live in oceanside mudflat burrows. There, they placed the robot roughly a foot away from actual male crab residences. They waited for the crustaceans to emerge, and recorded the interactions as Wavy Dave greeted them at various claw speeds.

In general, the male crabs responded by waving their own major claws for extended periods of time, and were less likely to retreat into their holes if their appendage was larger than the robot’s. That said, Wavy Dave’s speed didn’t seem to influence the speed of the fiddler crabs. Wilde’s team theorized the male crabs likely reciprocated the robot’s claw movements because they took them as a cue that a female was nearby. In order to conserve energy, the real crabs only quickened the pace if they saw the females firsthand.

“If you own a shop and your rivals start selling things really cheaply, you might have to change how you run your business,” said Wilde. “The same might be true for males signaling to attract females–and our study suggests males do indeed respond to competition.”

According to the study, male fiddler crabs appear to adjust their behavior in subtle ways during these dynamic, competitive mating environments. They will only put in additional energy when they believe it’s most rewarding. But while Wavy Dave elicited valuable new data on crustacean behavior, the robot wasn’t quite as successful in the mating market—and in at least one instance, it suffered at the claws of a rival.

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

 

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