by Whit Gibbons

September 17, 2006

Nobody likes fire ants, but no one is likely to suggest that ants are not intriguing little creatures that constantly amaze us. The more ants are studied, the more they reveal capabilities that exceed their small size.

For example, army ants have been said to comprise “the most cohesive societies on earth" because of their incredible cooperative behavior, such as the remarkable feat that occurs when they face a stream. To cross the water, the workers lock legs to form an ant-bridge of bodies for the others to walk on. Army ant colonies operate in many other ways like single organisms with a plan, like a problem-solving unit responding with a collective intelligence to special situations. Colonial bees, ants, and wasps function as individuals in the best interest of the colony. That’s why they sting us. Many mysteries remain about army ants, such as how they decide which direction the raiding colony will go each day.

Determining how animals navigate has been of long-standing interest to behavioral scientists. This past year, Matthias Wittlinger and Harald Wolf of the University of Ulm and Rüdiger Wehner of the University of Zurich conducted investigations that to me were as intriguing as the ants themselves. They put stilts on the legs of ants to find out how they found they way home after a long trip.

When Saharan desert ants forage, they return home in a straight line, though their route in search of food may have been a winding, circuitous one across flat desert with no visible landmarks. Based on earlier studies, the ants are known to use a directional compass involving the sun for orientation. Proper navigation also relies on a heretofore unknown mechanism for measuring the distance traveled in various directions. Even in the dark the ants are able to assess how far they have walked.

One hypothesis advanced many years ago is that ants somehow measure distance traveled by registering their leg movements. That is, by counting how many steps they have taken. To test the hypothesis the scientists conducted experiments in which ants were trained to follow a channel over sand to a food source. The top of the channel was open so that the ants were able to obtain compass information from the sky. The channel was about ten yards long, and upon reaching the food, experimental ants were captured and prepared for the test.

The ants were released to return home in another test channel that was parallel to the one they traveled from home. This assured they that did not use chemical cues from their previous route to find their way home, but they could still see the sky and keep their sun-compass orientation. But, before the ants were set on their way home, the researchers attached stilts (made from pig bristles) to an ant's legs to lengthen its gait. Or they shortened the ant's legs by literally cutting off the lower part of each leg. This apparently did not much bother any of the ants, because after its orthopedic surgery, each ant took some food and headed home.

As expected, on the trip home, ants with stilts took longer strides, and those with shortened legs took shorter ones than they had taken on the trip out. And sure enough, the ants with stilts walked beyond the point where they thought their home site would be whereas the ants with shortened legs did not go far enough. In other words, they were not using sight or smell to find their home but were counting how many steps they would have to take to return to it. In a separate experiment ants were outfitted with stilts or short legs before they began their ten-yard walk to the feeder, so that they had the same stride length in the outbound and homebound trip. This time the ants accurately assessed the homing distance on their return.

The investigators concluded that the ants measured the distance traveled by some mechanism whereby they count the number of steps taken, allowing them to navigate by integrating how far they have traveled in different directions. Ants are indeed intriguing, and so are scientists in figuring out unusual ways to study them.

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