Male wink crabs lure females into their construction for mating by waving with their oversized scissors. They wave significantly longer when waving competitors are nearby, as a study shows. In the experiments, the male animals reacted to the behavior of a robot crab and waved more persistently to impress potential mating partners – especially if the artificial rival had a smaller claw and quickly swung it. The study thus proves for the first time that invertebrate animals such as angular crabs, also adapt their courtship behavior flexibly and dynamically to the competitive situation and their prospect of success.
Female European wink crabs (Afruca Tangeri) are picky when searching for a partner: They prefer males with a larger main scissors and those that quickly swivel this oversized scissors. At low tide, the males wave rhythmically with their scissor arm to lure females into their underground building in the mudflats. The closer the crab females come, the faster the arm movement is. If a female accepts this invitation and enters the cave of a male, it fertilizes its eggs. The female then puts the eggs off in their own building. As soon as larvae have slipped out of them, they drive into the sea with the flood and grow up there. Between these exhausting courtship rituals, males withdraw into their cave – among other things, to rest from the swivel of the heavy arm and to avoid predators and fight with rivals.
But how do the crabs react to competitors? “We know that many animals adapt their sexual performances when rivals are nearby, but it is less known about whether wink crabs do this and how they react to the performances,” says Joe Wilde from the University of Exeter. Flexible courtship behavior, depending on the competitive situation, has so far been known from primates, birds and fish, but not of invertebrates like crabs.
Wink competition with a robot crab
In order to follow this, researchers about Wilde carried out an experiment with a robot crab. The robot with the nickname “Wavy Dave” was made by 3D printing and equipped with a five or seven centimeter scissors that can be operated and moved via an app via Bluetooth. The biologists placed these robot crabs during the breeding season in a watt in the south of Portugal, in which thousands of wink crabs live. For each test, they set up the robot 30 centimeters from the construction of a male, either equipped with the large or small claw. Two cameras then filmed the animals’ reaction to the wave of the supposed rival for 20 minutes – without a female being present.
The evaluation showed that when the robot waved, the males waved longer than usual and rarely retired to their caves – especially if their supposed rival had a smaller scissors and quickly waved. The wink crabs then waved more persistently, but not faster than usual. According to the biologists, this indicates that male crabs notice the behavior of rival and react to the competition. The team suspects that the crabs interpret a waving rival as a sign of a female nearby and wave to him as a precaution to keep up. However, they apparently wait until they see the female themselves and it is nearby before they make full effort and then wave faster, as previous tests suggested.
First proof of flexible courtship behavior in invertebrates
Some crabs also attacked the robot. “Some of the males tried to fight him,” reports Wilde. “A male even broke Wavy Dave off his scissors. We had to stop this attempt and restart the robot.” However, the animals attacked the artificial rivals less frequently and retired when he had a larger claw – probably because they then calculated with a defeat in the fight and balm competition, the team suspects. “Our results show the subtle way of how these crabs adapt their behavior to be competitive in a dynamic environment. They invest more in the courtship signals if this is probably the most profitable,” concludes Wilde. “For our knowledge, this is the first proof that a invertebrate animal adapts its sexual signal behavior as a direct reaction to changes in the signal behavior of supposed rivals,” explains the team.
Source: Joe Wilde (University of Exeter) et al.; Proceedings of the Royal Society B Biological Sciences, DOI: 10.1098/RSPB.2025.1570
