Ants Follow Forks in Their Roads to Find Home

John Roach
for National Geographic News
December 15, 2004

Forget GPS, forget road signs. Foraging Pharaoh's ants employ a simpler means to find their way home: geometry.

Researchers in England report that a simple rule the ants employ when building their trail networks helps to guide the ants home: Outbound trails always fork at a 60-degree angle, or thereabouts.

The geometry (picture a capital Y) allows the ants to reorient themselves if they become lost, said Francis Ratnieks, a professor of plant and animal sciences at the University of Sheffield.

Here's how it works: At trail junctions ants traveling away from their nest will find two trails that branch at small angles, about 30 degrees, relative to their current heading. (Imagine an ant crawling up that Y, away from its nest.)

Conversely, the trail intersections homeward-bound ants encounter have branches that fork at a sharp angle and a gentle one. (Still thinking of that Y?) The gently veering fork always leads toward home.

"Ants have a road network like the Romans; all roads lead to Rome. For ants, all roads lead to the nest," said Ratnieks, who co-authored the study, published in tomorrow's issue of the science journal Nature.

Precisely how the ants reorient themselves remains a mystery, the researchers said.

Scientists already knew that ants used chemical, visual, and magnetic cues to determine direction. But the finding that ants also use trail geometry comes as a surprise, said Rudolf Jander, a professor in the department of ecology and evolutionary biology at the University of Kansas in Lawrence.

He said trail geometry would only be useful in a natural setting for an ant that was lost from the trail, lacked alternative orientation cues, and had no other ants to follow.

"There is no knowledge whatsoever how often … naturally foraging ants need to use this newly discovered skill in order to increase their efficiency in foraging [if at all]," Jander said.

Polarity

Once native to North Africa but now a common household pest, Pharaoh's ants mark their trails by releasing chemical substances known as pheromones. In theory, a lost ant could simply encounter a pheromone trail and, if necessary, follow the scent trail to its end—either a nest or food—to reorient.

Continued on Next Page >>


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