The internal computer
In the featureless, sun-scorched salt pans of the Sahara, the desert ant Cataglyphis fortis forages for heat-stricken insects. Its nest is a tiny, inconspicuous hole in the ground, nearly impossible to find by sight. After wandering on a long, tortuous path, sometimes for hundreds of meters, the ant must find its way back before it succumbs to surface temperatures that can exceed 60°C (140°F). To do this, it computes a direct, straight-line path back to the nest entrance. This computation is achieved by path integration, a form of dead reckoning. For decades, researchers at the University of Zurich, led by neurobiologist Rüdiger Wehner, have conducted elegant experiments to understand how the ant's tiny brain—with fewer than one million neurons—accomplishes this. The ant's brain continuously combines two separate streams of information: direction and distance. The result is an updated "home vector" that tells the ant the precise direction and distance to its nest from any point in its journey.
A celestial compass and a step counter
The ant's directional information comes from a sophisticated celestial compass. Specialized photoreceptors in the dorsal rim of its compound eyes can detect the polarization pattern of sunlight in the sky. This pattern, invisible to humans, provides a reliable directional cue even when the sun itself is not visible. Research from the University of Zurich has shown that this polarization compass is the dominant directional tool for the ants, even more so than the sun's position.
The distance component of the ant's navigation system is an internal odometer. The definitive proof for this came from a now-famous experiment conducted by Matthias Wittlinger, Rüdiger Wehner, and Harald Wolf. Researchers trained ants to walk 10 meters down a channel to a feeder. Once an ant collected food, the scientists captured it and performed a delicate procedure. Some ants had pig bristles glued to their legs, creating stilts. Others had a small segment of their lower legs severed, creating stumps. A control group was left unaltered.
When released to return home, the ants with stilts consistently overshot the nest location, walking about 10.55 meters before starting to search. The ants on stumps undershot the nest, walking about 10.25 meters. The control ants stopped almost exactly at the 10-meter mark. This demonstrated that the ants were not using elapsed time or energy expenditure to measure distance. Instead, they were using an internal mechanism that functions like a pedometer, integrating the number of strides taken to calculate distance traveled. The brain uses this step count with the directional data from the sky compass to calculate its dead-reckoning path and find its way home.