Male Fiddler Crabs Don't Know What Female Fiddler Crabs Want


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

Dueling crabs

Male fiddler crabs use their claws to fight, but also to attract mates. They're just not good at working out what females want. Australian National University

Male fiddler crabs often confuse a female's desire for shelter with a search for sex. Zoologists have been making the same mistake, but a paper in Royal Society Open Science has revealed the true motivations, along with the considerations male and female crabs engage in when they interact.

Professor Patricia Backwell of the Australian National University has made an extensive study of fiddler crab mating dynamics, seeing them as good test subjects for things other species do in more subtle ways. She's even built robot crabs that, like real male fiddler crabs, have giant claws they wave around to attract females. They do this so convincingly that the females show no ability to distinguish real crabs from Backwell's imitation versions.


Backwell used her robot crab army to explore the behavior when female crabs visit the burrows of multiple males.

This common behavior had previously been thought to represent females checking out what was on offer before deciding which male she would allow to fertilize her eggs. Male crabs apparently think this too, putting a lot of effort into waving their claws to convince the females they're the best bachelor on the block. 

Sometimes the hopeful males are right, but Backwell found the females are often not interested in sex at all. Instead, they are checking out burrows to locate where the best hiding places are in case of predator attack.

Male mating success depends on having a big claw and a nice burrow. ANU


Backwell placed female fiddler crabs well away from their burrows, forcing them to seek out new safety sites. She told IFLScience the mudflats where the crabs live are so crowded it is usually not possible for a crab to just dig itself a new burrow without invading the territory held by another crab and attracting a hostile response. Consequently, the females make use of the fact that males are very happy to share their burrows with a female who might be open to some canoodling.

The female crabs investigate multiple burrows as they roam around looking for a place to call their own. "If a bird attacks, female fiddler crabs can move quickly and directly back to the last burrow it visited," Backwell said in a statement. "Having this map of burrow positions is essential if they are to survive a bird attack, and this is true for females who are looking for a mate and those who are looking for a burrow."  

The challenge for Backwell was to distinguish the females who actually were looking to mate from those seeking safety. The only way she could do this was to observe a large group of females, and track them for weeks afterwards until some produced babies, revealing which ones had been carrying eggs that needed fertilizing. 

Males crabs are similarly unable to tell the females looking for sex from the ones seeking shelter, so they treat all visiting females the same, welcoming them into their burrows on the basis that they have little to lose, and much to gain, by being hospitable.


We’ll leave it to you to assess similarities in other species.

Can you work out what is on her mind? Andrea Westmoreland

  • tag
  • mating behavior,

  • fiddler crabs,

  • sex and shelter,

  • robot crabs