If you've ever watched a lone ant scurrying across your floor, you'll know that once its located a juicy snack, the tiny forager somehow recalls where home is and finds the most direct return route, regardless of the circuitous path it took. Intrigued by the navigational strategies ants use to negotiate their way through the world, Rüdiger Wehner has spent much of his career investigating foraging ant's navigational strategies. One species he focused on i Africa, Cataglyphis, relies on polarised light cues from the sky to orient themselves. But Wehner was curious whether a close relative, the wood ant, would also use the same strategy. Wehner knew that the tiny explorers are capable of detecting polarised light, but would they use these cues to find their way home? Teaming up with Tsukasa Fukushi in Miyagi University of Education, Japan, the began testing the ants' navigational responses, to see which strategy the Japanese ants used to find their way home(p. 3431). Fukushi explains that by intercepting ants as they return from a foraging trip, and watching their chosen path when released a short distance away, it's possible to discover which cues the insects use to get their bearings. Ants that rely on polarised light from the sky aren't able to adjust to their new orientation, so they continue following their original course, missing the nest entirely. But ants that rely on local cues are wise to the shift, and adjust their homeward journey to compensate for the inconvenience. Luring the ants with tempting honey water to an abandoned terrace on the campus, Fukushi intercepted the insects and released them a short distance from the feeder,tracking their return journeys to see how they responded. The displaced ants instantly set off in the direction where the nest lay, regardless of where they were released. They weren't relying on a celestial compass. But which landmarks were the insects fixing on to direct them home?
At this point, Fukushi explains, winter set in and he was forced to retreat to the comfort of his laboratory. Plotting the ants' return journeys relative to landmarks visible from the deserted terrace, Fukushi realised that the ants weren't aiming directly at the nest site, but another landmark; a point visible on the ants' skyline that lay between two large chestnut trees. Instead of turning to the sky for guidance, the ants were navigating relative to fixed local landmarks to find their way home.
Having discovered that the wood ants and their African cousins use completely different strategies, Wehner and Fukushi were curious to know which landmarks the wood ants use to orient themselves at different stages during the round trip. Intercepting the ants as they embarked from either the nest or feeder, Fukushi found that the ants set off resolutely towards their goal, no matter where they were released. But when he intercepted the ants that had arrived at the nest or feeder and displaced them, they became disoriented.`They staggered with frequent turns' says Fukushi, but then they slowly began moving in the direction of their goal. Surprisingly, the insects seemed to use different navigational strategies depending on which leg of the journey they were on.