A study using a robotic crab has revealed how male fiddler crabs compete for mates.
These crustaceans vie for the attention of females. The male fiddler crabs use their one oversized claw to attract females, waving it with a frantic energy outside their burrows.
It’s a very important signal. But what happens when another male shows up?
Researchers at the University of Exeter’s Centre for Research in Animal Behaviour (CRAB) wanted to find out. To do this, they built their own robotic crab named Wavy Dave.
The robotic crab was made with a 3D-printed body and a waving claw.
This little robot was deployed on a mudflat along with fiddler crabs in southern Portugal.
The robotic impersonator was programmed to wave its claw, allowing the researchers to observe the reactions of the real crabs.
“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.
“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,” Wilde said.
Crab competition
For each experiment, the robotic crab was positioned 30cm away from a real male’s burrow, and two cameras were used to record the results.
The researchers wanted to see how the real crabs would react to this mechanical rival. Would they wave back? Would they get scared?
The results revealed intriguing behavior. When Wavy Dave waved, the male fiddler crabs in the vicinity upped their game.
They waved their claws for longer durations and were less inclined to retreat into their burrows. Interestingly, this response was particularly noticeable when the robot was sporting a smaller claw.
Wilde explained the findings through an analogy relatable to our local markets and businesses.
“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,” Wilde explained.
Attack on Wavy Dave
Fiddler crabs might interpret another male’s waving as a sign that a female is close, but they don’t fully commit to attracting her until they see her.
In addition to rival avoidance, males retreat into their burrows to rest, stay safe from predators, and wet their gills.
In case a female does enter a male’s burrow, he fertilizes her eggs, and the resulting larvae later float out to sea.
Male fiddler crabs were less likely to compete with rivals with larger claws, possibly out of fear of losing or being attacked.
Not all the interactions were so scientific, though. Some crabs still confronted the robotic crab.
One particularly feisty male attacked Wavy Dave, pulling off his claw and ending the experiment.
“The females realised he was a bit odd, and some of the males tried to fight him. One male broke Wavy Dave by pulling off his claw. We had to abandon that trial and reboot the robot,” said Wilde.
The findings were published in the journal Proceedings of the Royal Society B Biological Sciences.