Nov. 28, 2005 — Ants scouting for food place a tiny scent marker on branches that do not lead to a reward, according to a study published on Thursday in Nature, the weekly British science weekly.

The pheromone acts like a "no entry signal" to other ants, telling them not to waste their time going down that route, it says.

The discovery was made by animal scientists at Britain's University of Sheffield.

In a lab, they got a scouting colony of Pharaoh ants (Monomorium pharaonis) to explore branches that led to no reward. Paper covers were placed at the branch's forks.

The paper covers were then taken off and transferred to another set of branches. This set had already been explored by another ant colony, which found a tasty meal of sucrose at the end of the trail.

Even though the food reward was still there, ants performed a U-turn or chose another direction whenever they came across the covers.

The team concluded that the ants use a repellent pheromone to mark unrewarding branches at "decision points," where branches fork.

"It provides advance warning, like human road signs situated before junctions," the authors suggest.

In a study published last December, the same team found that Pharaoh ants deposited pheromones wherever the trail forked at 60 degrees. The markers are like invisible signposts, helping ants to find their way home.

The latest finding amplifies this, showing that the ants also use scent as a negative signal to help them in their quest for food.