The nervous system of the desert ant Cataglyphis fortis, with around 100,000 neurons, is about 1 millionth the size of a human brain. Yet in the featureless deserts of Tunisia, this ant can venture over 100 meters from its nest to find food without becoming lost. Imagine randomly wandering 20 kilometers in the open desert, your tracks obliterated by the wind, then turning around and making a beeline to your starting point—and no GPS allowed! That’s the equivalent of what the desert ant accomplishes with its scant neural resources. How does it do it?
“Jason,” a graduate student studying the development of human and animal cognition, discusses a remarkable series of experiments on the desert ant on his blog The Thoughtful Animal. In work spanning more than 30 years, researchers from Rüdiger Wehner’s laboratory at University of Zurich Institute of Zoology carefully tracked the movements of ants in the desert as the insects foraged for food. One of the researchers’ key questions was how the ants calculated the direction to their nest. To correct for the possibility that the ants used landmarks as visual cues, despite the relatively featureless desert landscape, the researchers engaged in a bit of trickery. They placed a food source at a distance from a nest, then tracked the nest’s ants until the ants found the food. Once the food was found, the ants were relocated from that point so that the way back to their nest was a different direction than it would have been otherwise. The relocated ants walked away from the nest, in the same direction they should have walked if they had never been moved. This suggested that the ants are not following features, but orienting themselves relative to an internal navigation system or (as turned out to be the case) the position of the Sun in the sky.
No matter how convoluted a route the ants take to find the food, they always return in a straight-line path, heading directly home. The researchers discovered that the ants’ navigation system isn’t perfect; small errors arise depending on how circuitous their initial route was. But the ants account for these errors as well, by walking in a corrective zigzag pattern as they approach the nest.
So how do the ants know how far to travel? It could still be that they are visually tracking the distance they walk. The researchers tested this by painting over the ants’ eyes for their return trip, but the ants still walked the correct distance, indicating that the ants are not using sight to measure their journeys.
Another possibility is that the ants simply count their steps. In a remarkable experiment published in Science in 2006, scientists painstakingly attached “stilts” made of pig hairs to some the ants’ legs, while other ants had their legs clipped, once they had reached their food target. If the ants counted their steps on the journey out, then the newly short-legged ants should stop short of the nest, while stilted ants should walk past it. Indeed, this is what occurred! Ants count their steps to track their location. (If only you had remembered to do this before you started on your 20-kilometer desert trek…)
But other creatures have different navigation puzzles to solve. In a separate post, Jason explains a study showing how maternal gerbils find their nests. When a baby is removed from the nest, the gerbil mother naturally tries to find and retrieve it. Researchers placed one of the babies in a cup at the center of a platform, shrouded in darkness. When the mother found the baby, the platform was rotated. Did she head for the new position of her nest, with its scents and sounds of crying babies? No, she went straight for the spot where the nest had been, ignoring all these other cues. For gerbils, clearly, relying on their internal representation of their environment normally suffices, so the other information goes unheeded.
Migratory birds, on the other hand, must navigate over much larger distances, some of them returning to the identical geographic spot year after year. How do they manage that trick? One component, University of Auckland researcher and teacher Fabiana Kubke reports, is the ability to detect the Earth’s magnetic field. Though we’ve known about this avian six sense for some time, the location of a bird’s magnetic detector is still somewhat of a mystery. Last November, however, a team led by Manuela Zapka published a letter in Nature that narrowed the possibilities. Migratory European robins have magnetic material in their beaks, but also molecules called “cryptochromes” in the back of their eyes that might be used as a sort of compass. The team systematically cut the connections between these two areas and the robins’ brains, finding that the ability to orient to compass points was only disturbed when the connection to cryptochromes was disrupted.
Much remains to be learned about how birds can successfully migrate over long distances. Unlike ants and gerbils, they can easily correct for large displacements in location and still return to the correct spot. As researchers learn more about how animals—including humans—navigate, look for more discussion of their results on ResearchBlogging.org.
Front page image courtesy of Jim Champion.
Originally published February 10, 2010