Unway sign: ant pheromone stops traffic.

Researchers say that they've discovered a new kind of traffic sign on ant highways--a chemical "Do not enter" that lets the insects avoid wasting time on paths that don't lead to food.

Ant science has for decades focused on chemical attractants that define trails, says Elva J.H. Robinson of the University of Sheffield in England. However, the new tests give evidence of a repellent pheromone, which hasn't yet been identified, she and her colleagues report in the Nov. 24 Nature.

"Nobody believed that such a thing existed," says Robinson.

There has certainly been resistance to the idea over the years, says Nigel Franks of the University of Bristol in England. In the 1990s, he and his colleagues mathematically modeled ant trails. Complementing attractants with a hypothetical repellent to block useless trails in a model system "vastly increased its efficiency," he says, but other scientists' reviews of that model were "scathing."

Robinson says that she wasn't thinking about repellents when she started her laboratory experiments on foraging trails in pharaoh's ants (Monomorium pharaonis). "We got some quite unexpected results," she says. Some of the ants started zigzagging or doing U-turns when approaching a trail that only Robinson knew didn't lead to food. "It looked as if ants had suddenly developed psychic abilities," she says.

She and her colleagues set up two-pronged, paper-covered platforms where ants could forage. One setup had a feeder on one prong but no food on the other. After ants had used it for a while, the researchers moved the paper from the no-food prong to one prong of a different platform that had previously had a working ant trail and feeder on each prong. The researchers put a neutral piece of paper--one from an area of the ants' lab home that had no trail--on the second prong, which had also carried a feeder.

Of the ants in the new setup that came to the fork and made a choice, some 70 percent avoided the branch with the paper from the no-food prong. Something on the paper must have turned away traffic, the researchers concluded.

The prong's paper was most repellent near the fork. Also, the ants often changed course some 15 body lengths before the fork.

Chemical ecologist David Morgan of Keele University in England says that biologists 'lust haven't really looked" for negative pheromones on ant trails, but the new paper "might now start a great flood of interest."

As for do-not-enter signs in other ant species, "I would be very shocked indeed if they didn't find them," says Franks.