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Fiddler crab chimneys keep intruders out

Australian researchers have found that fiddler crabs build chimneys around their burrows to keep intruders out.

A team at the Australian National University in Canberra reports its findings on Uca capricornis in the journal Biology Letters.

Fiddler crabs live on tidal mudflats and lock themselves away in air-filled sealed burrows when the tide comes in, says team member Rachel Slatyer, a behavioural ecology student, who carried out the research under the supervision of Dr Patricia Blackwell.

She says a small percentage of these burrows are surrounded by mud mounds, or chimneys, and biologists have long wondered why this is so.

Slatyer and colleagues studied 70 crabs at a site in Darwin and tried to detect differences between those that build chimneys and those that don't, hoping the difference might shed light on the role of the chimneys.

They studied how much time crabs spent in and out of their burrows, how far they travelled from their burrows, and how much time they spent feeding or having aggressive interactions.

The only difference the researchers noticed was that those with chimneys didn't feed as much as those who didn't have chimneys.

The team hypothesised crabs that built chimneys didn't need to eat as much because they already had enough food - and thus enough energy to build chimneys.

Other crabs appeared to not have enough food, and had to spend their time and energy looking for more food rather than building chimneys, says Slatyer.

Intruders

Slatyer and colleagues then conducted an experiment to see what impact chimneys had on keeping intruders out of burrows.

Crabs generally try to defend their burrows, says Slatyer, and if they find themselves taken away from their own burrows they will attempt to hide in someone elses.

So the researchers set up plastic arenas around burrows, some of which had chimneys and some of which didn't.

They put a foreign crab into the arena and then measured the time it took them to find the burrow.

"Most of the time they didn't find the burrows with a chimney at all," says Slatyer. "It was quite surprising."

The researchers conclude that it is possible that some crabs build chimneys to conceal the entrance of their burrows and reduce the risk of losing it to an intruder.

Slatyer says it is possibile that only some "classes" of fiddler crabs build and therefore understand the significance of chimneys, but further research would be required to investigate this.

Gender preference

Previous research on different species of fiddler crabs, have found that chimneys are built by reproductive males, who use chimneys to attract females and hide females they have mated with.

But Slatyer says, young U. capricornis crabs, as well as reproductive crabs built burrows, and female fiddler crabs were more likely to build chimneys than males.

Importantly, rather than being used to attract other crabs, the chimneys built by U. capricornis seem to hide the burrow, she says.

"It appears these structures can have a lot of functions even though they look similar to us," says Slatyer.