The backtrackers

“Viewed as a geometric figure, the ant’s path is irregular, complex, and hard to describe,” wrote Herbert Simon, an American psychologist. But, he added, this is really down to a “complexity in the surface of the beach, not the complexity in the ant”. Or is it?

Ants have the animal kingdom’s biggest brains, relative to their bodies. Brains account for up to 15% of an ant’s total mass in some species (humans weigh in at a meagre 2%). This goes some way to explaining their uncanny knack for finding their way back home from foraging forays. But entomologists have, like Simon, long believed that this apparently complex behaviour is the result of sticking to a handful of simple rules. Ants keep track of distance (for example with an internal pedometer) and direction (based on the position of the sun or scent, of pheromones, for example). If they lose it, they switch to a second startegy and move in a spiral around a centre they think is the nest. If the nest is not found in the first one, then the ant increases the radius and tries another.

Now Antoine Wystrach, of the University of Sussex, proposes that there is more to ants than mindless adherence to simple instincts. As he and his colleagues report in the Proceedings of the Royal Society, at least one species of ant appears to display some hallmarks of intelligence: the ability to integrate different strategies based on experience.

Dr Wystrach captured Melophorus bagoti ants just before they reached their nests after a foraging trip and shunted them into straight tubes which led to random spots about 50 metres away. On exiting the tube, the ants invariably turned around and headed straight back in the direction of the nest. They could not be following the pheromone trail, as that was enclosed in the tube. Nor were they using visual cues: they appeared to backtrack just as well with with their eyes closed. (Cruel as it may sound, the researchers used an opaque paint to cover ants’ eyes.) In other words, the ants appeared to have some sort of internal compass.

Oddly, ants do not always use this device. When Dr Wystrach put ants into the tube when they were farther than two metres away from their nest, however, they used the two basic methods to find home. But when he repeated the procedure, but dropped the ants close to the nest for a few seconds before setting them down the tube, they backtracked just as they had in the first experiment. Backtracking, in other words, appeared to be triggered only when ants possessed a recent memory of their nest. Complexity in the ant, it seems, is a tad greater than Simon would have allowed.

First published on

Image credit: Reverend Barry


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