Fiddler crabs keep cool with giant claws

Fiddler crab (c) M Z Darnell

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Male fiddler crabs use their giant claws to keep cool as well as attract females, say scientists.

The crabs' oversized limbs are waved to signal strength to potential mates.

US researchers discovered that crabs with an intact major claw also cool faster than those without, showing that the claws help regulate temperature.

Scientists suggest this previously unknown benefit could help offset the energy cost of growing such a large sexual ornament.

There are about 100 species of fiddler crab found in tropical regions around the world.

Males are recognisable by their assymetrical claws, whereas females' claws are equally sized.

Watch a male fiddler crab waving to attract females.

To attract the attention of potential mates, males perform elaborate claw-waving displays.

Dr Zachary Darnell and Assistant Professor Pablo Munguia from the University of Texas, US investigated whether the giant claws had a function beyond attracting mates.

Their study focussing on the Gulf coast fiddler crab (Uca Panacea) was recently published in the American Naturalist journal.

In the experiment, researchers shone lamps on crabs with an intact major claw and on crabs that were missing theirs.

They measured the crabs' body temperature every 10 minutes and found that the crabs lacking the large claw took much longer to cool down.

"The major claw likely functions like a heat sink, with heat being transferred from the body to the claw and dissipated into the surrounding air through convective heat transfer," said Dr Darnell.

Although the giant limbs effectively attract the attention of females, they had previously been considered a physical burden.

KEEPING COOL

  • Elephants' ear-flapping helps them to keep cool. Blood passed to the ears is cooled by the breeze and then circulated around the rest of their body
  • Toucans are able to radiate body heat from their bills which can measure up to a third of their body length
  • Goats likewise lose heat from their horns as do certain species of horned beetle

"The large claw is metabolically costly, it hinders feeding because it is cumbersome for this task, and it reduces endurance capacity when crawling on the sand," Dr Darnell explained.

"Male crabs are both heat-stressed and hungry while on the surface, foraging and performing the claw-waving display."

But Dr Darnell says the team's results are the first evidence to show how the enlarged structure benefits both reproduction and survival.

"With the large claw acting as a heat sink male fiddler crabs can remain on the surface longer, foraging and performing the waving display," he told BBC Nature.

Dr Darnell suggests that the dual function of fiddler crab claws could provide insight for other species.

"A thermoregulatory function of the major claw should change the way scientists think about this structure, and reconsider the costs and benefits associated with the claw and other [sexual] traits."

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