Wavy Dave and a fiddler crab waving their claws. Credit: Joe Wilde
Scientists from the Centre for Research in Animal Behaviour (CRAB) at the UK’s University of Exeter used an unconventional approach to study the bizarre sexual behaviours of male fiddler crabs.
Their robot crab, nicknamed “Wavy Dave”, mimics how male fiddler crabs compete over females – waving a singular oversized claw around in the air to entice them into their burrows.
The team dropped Wavy Dave into a mudflat in southern Portugal pockmarked with thousands of male fiddler crab burrows to see whether they changed their sexual displays in response to new competition.
“The females realised he was a bit odd, and some of the males tried to fight him,” says lead author Dr Joe Wilde, now a statistician and modeller in ecology and environmental science at Biomathematics and Statistics Scotland.
“One male broke Wavy Dave by pulling off his claw. We had to abandon that trial and reboot the robot.”
The research is published in Proceedings of the Royal Society B Biological Sciences.
Female fiddler crabs have been shown to prefer males with larger claws who wave them around at a faster rate. The major claw can make up as much as 30% of a male’s total body mass, so this requires lots of energy.
Signalling in vertebrates – such as primates, birds and fish – is known to change depending on the presence, characteristics and behaviours of other individuals. But is this also the case for invertebrates (animals without a spine) like 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,” says Wilde.
“If you own a shop and your rivals start selling things 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.”
In the Ria Formosa Natural Park on the southeastern coast of Portugal, the researchers propped Wavy Dave 30cm away from male crabs’ burrows and captured the resulting response on 2 cameras.
Wavy Dave’s claw size and waving speed was altered in each test.
They found that real male crabs waved for longer durations in response to Wavy Dave’s gesturing, but they did not wave faster.
Wilde says the crabs might interpret a waving rival as a signal that a female is around – but wait to see the female themselves before committing their full effort.
Male fiddler crabs. Credit: Joe Wilde
The male crabs were also more likely to stay above ground when the robot was waving, compared to when it was present but not moving. This makes sense, as the researchers suggest “it is in the best interests of a male not to enter his burrow when there is an immediate opportunity to attract a female”.
“The response to waving was strongest if that rival had a small claw [5cm long] rather than a large claw [7cm],” the authors write.
“When males did enter their burrow, they spent less time in there when the robotic rival was waving rather than not waving, and when the ‘fast’ wave speed treatment was presented first, and if the rival had a small claw rather than a large claw.”
The researchers think the male fiddler crabs were less likely to compete with a larger-clawed Wavy Dave because they expected to lose or were fearful of being attacked.
“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,” says Wilde.
“To our knowledge, this is the first evidence that an invertebrate animal adjusts its sexual signalling behaviour as a direct response to changes in the signalling behaviour of ostensible rivals,” the authors write.
“Our findings suggest that such flexibility may be more widespread in animal communication than previously thought.”