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.

However, such a solution would waste time, according to Ratnieks and his Nature study co-authors, Duncan Jackson and Mike Holcomb, both of the University of Sheffield Department of Computer Science. A better solution, they write, is to use polarized trails.

The phrase describes trails that incorporate information that acts like the signs on a freeway entrance, indicating the differing directions ants must travel to find food or their nest.

Researchers who have studied ant pheromone trails in the past have failed to uncover such directional signage, or polarity, the Sheffield team said.

Such inquiries assumed polarity would be indicated by the differences in pheromone concentration on the trail network—not simple geometry, as the English researchers suspected.

To test their theory, the scientists compared the ability of individual ants to orient themselves on a straight trail and a trail with branches of 55 degrees.

The ants on the straight trail were unable to reliably determine direction. But when placed on a branching trail, about 45 percent of the ants correctly oriented themselves at the first fork they encountered.

Network Flow

The experiment proved that trail geometry supplies the polarity Pharaoh's ants require.

The team then studied trails that branched at various angles between 30 and 120 degrees. The scientists found that the optimal angle of a trail fork for ants to correctly reorient themselves is about 60 degrees.

The angle is not only the same one used by Pharaoh's ants but also one that is found in the natural trail networks of other ant species.

Scientists know that natural branching networks, such as plant roots and animal cardiovascular systems, evolved to minimize the energy needed to distribute resources, be they plant nutrients or oxygen from lung tissue.

The University of Sheffield researchers conclude that ant trail networks may have evolved for a similar reason: the efficient flow of food to the nest. That ants use this specialized geometry to orient themselves may be a secondary benefit.

"They may have built it for one reason and, for free, get something else out of it," Ratnieks said.

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