Last mating season on the southern mudflats of Portugal, a robot crab disguised as a rival male stirred up fiddler crabs. Called “Wavy Dave”, the 3D-printed creature is designed to mimic the species Afruca tangeri, which takes part in claw-waving displays – rituals where males attract females by waving an ‘enormous crab claw’. The robot’s antics were good for a few laughs but didn’t fool the genuine crabs for long. One male attacked Wavy Dave and ripped off his claw, forcing researchers to pause the trial and reset the robot before continuing the study.

Robot Rival Reveals How Male Crabs Strategically Adjust Courtship Displays

According to a study published August 6 in Proceedings of the Royal Society B Biological Sciences, Wavy Dave helped scientists explore how male crabs assess and react to rivals. Designed by statistician Joe Wilde of Biomathematics and Statistics Scotland, the robot came with interchangeable claws—average-sized and large—and was controlled via Bluetooth. Waves were similarly affected by the social context: when released in Ria Formosa Natural Park, males close to Wavy Dave extended their hand-waving duration without affecting its speed, possibly a result of expectations built on the presence of a robot.

In some cases, crabs avoided conflict if their claw was smaller than Wavy Dave’s, perhaps perceiving him as dominating or fearing injury. Some stuck around a little longer—when his claw was smaller, he seemed a less important rival—and others slipped off into the darkness. Male crabs, however, were strikingly energy conservative unless it seemed like a bout was worth the effort, showing quite flexible behaviour in courtship rituals.

Waves were similarly affected by the social context: when released in Ria Formosa Natural Park, males close to Wavy Dave extended their hand-waving duration without affecting its speed, possibly a result of expectations built on the presence of a robot.

A study shows that male fiddler crabs modify their reproductive tactics in response to challenging contest competition, using a robotic opponent to investigate the evolution of fighting behaviour in a complex and changeable environment.