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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