Path integration may not be as widely known as the American Express card, but you'd better not leave the house without it. After all, path integration is the ability of animals, including humans, to return home from somewhere else.

How animals keep their bearings on hunting trips is somewhat of a mystery. Celestial navigation and electromagnetic fields help ants, honeybees, birds and sea turtles keep track of directions. More puzzling: how animals measure distances. A new study has found the first direct evidence that fiddler crabs monitor their travels by tracking their strides.

The obvious explanation for how fiddler crabs chart their path home is that they simply count their steps heading away from their burrow. But, well, that doesn't measure up. "If they took steps that were all the same size, and had that ingrained in their brain, they would convert distance into a number of steps," says senior study author John Layne, a biologist at the University of Cincinnati. "In fact, that’s not what they do."

Using a high-speed, high-definition video camera, Layne and his student, Michael Walls, observed fiddler crabs walking across both mud and a sheet of slippery plastic. "We were able to measure every step by every leg of every animal in this experiment, and since these are eight-legged animals, that's a lot," Layne says. Their findings: that crabs took short steps while moseying and longer strides when running.
Even small missteps confused the crabs dramatically. When allowed to run home, crabs that didn't cross the plastic sheet travelled far enough to reach their burrows. Those that slipped on the material came up well short of that distance. The more they slipped, the farther from their burrow they stopped. 

To Layne, the results, set to be published next month in Current Biology, indicate that fiddler crabs perform sophisticated math whenever they leave home. "We think they're summing steps," he says. They know how many strides they've taken and the length of each, and that magic number gives them their distance from home. 

The next step, says Layne, is to determine what, exactly, crabs are doing when they sum their strides. One possibility is that sensors called proprioceptors in the animals' legs somehow tally both the number and length of their strides. Other options, he says, are  that the brain itself is counting its outgoing commands to move the legs or even that the crabs can measure their own energy expenditure over distance.

"The fiddler crab paper is a beautiful example of what the great ethologist [and Nobelist] Nikolaas Tinbergen called 'physiology without breaking the skin,'" says Fred Dyer, chair of zoology at Michigan State University in East Lansing.  “It illustrates how behavioral evidence—careful measurements of what the whole animal does—can provide clues about the computations being carried out in the animal’s brain. Such evidence is  indispensable because of the difficulty of observing neuronal computations directly."