30 December 2008

How do fiddler crabs find their way home?

A seething mass of fiddler crabs takes over some of our sandy shores at low tide.
Orange fiddler crab
Countless crabs are busy feeding, fighting or courting. But as soon as you take a step closer, they disappear almost instantly into their respective burrows, leaving a barren shore. How do they do this?

A recent study suggests that the crabs literally take it in their stride!

This finding also suggests that we should not pick up fiddler crabs as they might get lost even if we put them back on the shore.

To Find Way Home, Some Crabs Find It’s All in the Stride
Henry Fountain, New York Times 29 Dec 08;
Most animals have the ability to return home from a foraging trip, even if home is nowhere in sight. They do this at least partly through path integration — using information about direction and distance to return to a starting point. (Sailors and others refer to this as dead reckoning.)

But how animals measure distance is largely a mystery. The honeybee has been shown to use the flow of the passing landscape across its field of vision. Some other animals may be able to gauge linear acceleration and use that to determine distance.

Now Michael L. Walls and John E. Layne of the University of Cincinnati provide direct evidence of yet another method. In a report in Current Biology, they show that the fiddler crab Uca pugilator uses its stride to gauge distance.

The researchers devised an experiment in which they put a slippery sheet of acetate in the path of a crab heading toward its burrow. Crabs took the same number of steps as if they were on a normal surface, but since they made no progress with some of the steps they ended up short of their burrow. Since the crabs were not moving, Dr. Layne said, this shows that they were using leg movements as a cue to gauge distance, rather than acceleration or the movement of the landscape across their vision.

The researchers found that this stride integration was quite flexible. With both slipping crabs and those that encountered no slippery patches, they found that the number of strides depended both on the distance to the burrow and the length of the stride, which could vary. “Our theory after doing this is that they don’t count steps at all,” Dr. Layne said. “They just add them up no matter how big or small they are.”

He added that the crab’s behavior must involve neuronal signals either to or from the legs.

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