by Whit Gibbons

August 15, 2004

Science magazine, one of the top scientific journals in the world, does not often publish articles that most people, including many scientists, can comprehend through casual reading. But occasionally they do, and last month the journal had a couple of papers in it that presented information in easily understood language.

One article was about an ugly little marine fish called the plainfin midshipman. Ichthyologists already knew that both males and females of the species migrate from the deep ocean areas of the Pacific to shallow areas during the breeding season. The intriguing part to me was that midshipmen males attract the females by entering rocky areas and actually making a humming noise so loud that it sounds like a small motorboat. But the scientific revelation was that hearing capabilities in female midshipmen change in association with hormonal changes during the mating period that improve their hearing. Thus they can hear the males best during the breeding season.

Meanwhile, Markus Knaden and Rüdiger Wehner of the University of Zurich conducted a study on aggression in Saharan desert ants. These little creatures get along fine with members of their own colony. But they can get combative when encountering Saharan desert ants from nearby colonies, not unlike group behavior observed among some humans toward intruders from other neighborhoods.

The level of aggression changes dramatically based on how far an ant is from its nest. Desert ants often travel long distances to gather resources, sometimes more than the length of a football field. Once they get more than a few yards from their own nest, they become relatively passive and are less likely to pick a fight with another ant. Presumably the explanation for greater belligerence in the vicinity of the nest is a protective one: guarding their home and kin. But how do they know they are close enough to the nest that they should display their aggressive nature toward an interloping ant that's not from the 'hood? By landmarks in the vicinity? By sight or smell of the nest?

Based on intriguing experiments with desert ants, the investigators found that the answer lies in what they refer to as an ant's "navigational toolkit," which informs the ant how far away it is from home. Ants from four different colonies were used in the experiment, each being marked with a different colored dot to identify where it came from.

The study involved training ants to come to a feeding area about 60 feet from home, far enough away that they are no longer aggressive toward other ants. Ants were captured at the feeding area and transported to a site far away from their home nest. Upon being released at the remote site, an ant immediately began traveling in the compass direction of its home nest.

As the experiment continued, some of the released ants were removed after they had traveled 60 feet in the direction that, had they not been hijacked, would have taken them to their nest. Others were removed after traveling only 15 feet toward what they presumed was the direction of their nest. For the true test of aggression each ant was taken to the laboratory after removal, where pairs of ants from different colonies were placed together in a box and their behavior videotaped. Thus began the gladiator part of the study. In fact, the investigators referred to the test area in the box as the "battlefield."

The upshot of the tests was that displaced ants that had traveled 60 feet back to what they thought was home were more likely to attack the other ant. The ones that had traveled only 15 feet, which meant they "thought" they were farther away from home, were less likely to do so. In other words, an ant's aggressiveness is not based on its proximity to its home but on its perception of how close it is to home. The ant does not perceive its distance from home based on sights, smells, or landmarks. Instead, some internal, yet-to-be-understood feature of the Saharan desert ant's traveling toolkit reveals its location. When Swiss scientists aren't moving the ants around, their toolkits work fine and serve as another example of the complexity and mystery of bioecological systems.

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