How Crabs Find Their Way Home
so they cannot get home drunk, ah 🙂 ?
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
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
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.
I take the two things: receptors and path integration – let’s see if humans do the same.