Flirting crabs hit the dance-claw

How the Fiddler Crab gets the girl... or not. Robotic crabs help researchers understand the importance of tempo during mating
23 January 2018

Interview with 

Sophie Mowles, Anglia Ruskin University


The males of many species go to extraordinary lengths to attract a mate. Peacocks invest heavily in impressive plumage, lions grow a large mane, while humans pay for dinner and pretend they like cats. Fiddler crabs, on the other hand - or should that be claw - grow one very large pincer that they wave and drum against the ground to attract female attention. Talking to Katie Haylor, Sophie Mowles from Anglia Ruskin University, has discovered that, for these fiddler crabs, timing is everything...

Sophie - The display of the male fiddler crab served to advertise his physical fitness, so they not only wave this claw but they also drum it.

Katie - Romantic eh? The percussive crabs make sweet music by drumming these enormous claws vigorously against their leg. The vibrations carry through the ground, getting the attention of the females. Take a listen...

Sophie - What we found was that the males that waved and drummed more vigorously had better performance, so we actually put them in sprint trials -  we made them run. It’s a nice way to show whether stamina has been depleted.

Katie - Is the logic to this then: if  I can wave my big claw more quickly, I’m fitter therefore I’m going to be a better match for you?

Sophie - Exactly that. A better male. Firstly that he’s got good genes; maybe he is just genetically physically fit and if he’s fit enough to perform this waving display, then he might be fit enough to run quickly from a predator, or to defend his territory, or to forage effectively. These are all things that the female should want to provision her offspring with - genes for these characteristics.

Katie - So vigorous wavers were fitter wavers. Good genetic news for potential offspring. But it’s not just fast waving that gets female fiddler crabs’ hearts racing, it’s fast waving that is getter faster still. Slowing down your waving might indicate fatigue, whereas waving that’s gaining tempo indicates energy to spare, and more interest. But can the female crabs pick up on this increase in waving pace and how do you test this?

Sadly, male crabs aren’t that easy to train Sophie says, so she and her colleagues made a set of robotic crab arms fitted with a claw the same colour and size as a male crab’s claw and tested whether females preferred waving that maintained a speed, or sped up, or slowed down. These tests were done in Australia so I couldn’t see them for myself, but Sophie did have a video to hand...

Sophie - What we have here is the female in a shot glass. She can see the display at the point that we release her. We’ve got a male robot that’s speeding up, and a male robot that is displaying at a constant rate. If I press play on this video… there she is. She’s been released and she’s gone to the side, and she’s now looking at the two males. She makes a dash to the back because she gets distracted by a fly but she’s still interested and she’s edging closer to them and, finally, she runs and she touches the plate of the robot that she’s selected.

Katie - What can you conclude from this?

Sophie - These were conducted in Darwin in the Northern Territory which is great because there are hundreds of these crabs there - you can get great sample sizes. But what we can conclude from this is that the females are indeed paying attention to changes in rate. They dislike males that slow down. They don’t completely punish them because they’ve been fast before in our experiments, but when they’re faced with a choice of ones that’s speeding up, one that’s slowing down, and one that’s constant they will go for the one that’s speeding up at an escalating rate.

Given that we know that these males are more physically fit from our previous experiments, we can see that females are adapted to note these changes in rate. They can avoid the males that are becoming fatigued and they can select the physically fit specimens that have good stamina and good genes that they should give to their offspring.

It’s not just about the crabs. They’re a great system; they’re very tractable; you can study them very easily. But so many other animals have courtship displays like this that might be simple, like the waving of a claw, the chirping of a cricket, the movements of the wings, but they might be very, very complicated too. Even the display of a peacock, we would assume that it’s all about colour and prettiness, but he’s got to lift this huge train and wave it about in order to impress the female. So a lot of these motion displays that might advertise his physical fitness could be, throughout the animal kingdom we’ve possibly been a bit preoccupied by prettiness in the past when, in fact, it’s stamina that might be important to our females.


With thanks to BenSound for royalty free music:


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