Flirting 101, for Fiddler Crabs

Flirting 101, for Fiddler Crabs

Yoo-Hoo!
A male fiddler crab issues a grand, "yoo-hoo" type wave by stretching his large claw to the side, slowly lifting it high above his body and then dropping it back down to the resting position. Observations and experiments conducted on a population of mud flat-dwelling fiddler crabs at Bowling Green Bay in Queensland, Australia, revealed the males didn't stretch out their claws as much when the females were nearby.

When Elvis shook his hips, girls went wild, and so it is that the sight of a male

fiddler crab waving its single giant claw can send female crabs into a sexy swoon, according to a new study.

The study is one of the first to show that receiver distance affects visual signals in non-human animals: the males wave differently, depending on how close or how far away the female onlooker is.

People do this all of the time, such as when someone is trying to catch the attention of a loved one at an airport and jumps up and down while waving to do so. If the intended recipient of the signal is nearby, the person jumping up and down and waving would waste energy and look rather stupid.

Crabs too change their tactics, which also alters the meaning of the signals.

"I think that the long-range claw-waving signal is essentially saying, 'I'm a male Uca perplexa and I'm over here!,'" said Martin How, lead author of the study. "It acts as a beacon towards which receptive females can move."

How, a researcher in the Center for Visual Sciences at The Australian National University in Canberra, thinks the short-range claw waving display is different.

"In this case, the information of the signal changes, and probably says something along the lines of, 'I'm a big healthy crab with a nice burrow to raise a brood and would make an ideal mate!'" How told Discovery News.

He explained that the yoo-hoo type wave happens when the male stretches his large claw to the side, slowly lifts it high above his body and then drops it back to the resting position. Observations and experiments conducted on a population of mud flat-dwelling fiddler crabs at Bowling Green Bay in Queensland, Australia, revealed the males didn't stretch out their claws as much when the females were nearby.

In fact, when females were right in front of the males, the males really switched strategies by performing a less sweeping wave while lifting their bodies off the ground with their unflexed legs. How suggests that's a macho move for male crabs trying to show off their size to its best advantage.

The findings have been accepted for publication in the journal Animal Behavior

.

The male crabs have their work cut out for them because, if the big wave catches a female unawares, she may run for her life.

"Fiddler crabs carry their eyes on long vertical stalks in a fixed position above the body and the eye-stalks always stand upright," How explained.

They "can therefore simplify their visual world into things they see above the horizon and things they see below the horizon."

Since the crabs equate above-the-horizon sights with predators, like birds, the females can sometimes mistake the male's waving for the movements of a hungry threat.

"This could be a good thing, though, in that it taps into the anti-predator response of females, causing them to freeze and direct their attention towards the displaying male," How said. "But it could also backfire and cause the female to run away."

John Christy, a researcher in the Smithsonian Tropical Research Institute located both in Panama and Washington, D.C., told Discovery News that the new study "provides a detailed quantitative description" of crab communication.

Christy thinks the broader wave might allow females to choose among a variety of males, while the less dramatic wave, when she's up a bit closer, might serve to lure her into a particular burrow.

In the future, How hopes a big robotic claw might be designed to better test what specific information females glean from the sign language of their potential mates.