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Science: Trail-blazing ants prefer the straight and narrow

点击量:   时间:2019-03-01 10:16:01

By STEPHEN DAY in INDIANA How do ants find the shortest route from their nest to a food source? By avoiding routes with sharp turns in them, say researchers in Belgium. R. Beckers, Jean-Louis Deneubourg and S. Goss at the Free University in Brussels observed how black garden ants (Lasius niger) select a route to food. First, a scout lays a pheromone trail from the food to the nest. Foraging ants then head outwards along the scout’s trail, strengthening it with their own pheromone. The trail is very weak initially, so foragers are easily lost. They get to the food along alternative routes, laying pheromone along these. But eventually, all the ants select just one path – the one that is marked most heavily with pheromone. As more pheromone is added to it, this path soon becomes irresistible to the ants. To discover how ants settle on the shortest path, Beckers and colleagues provided ants with a sugar solution which they could get to only by crossing a horizontal bridge (see Diagram). The bridge forked in the middle, so that there were two routes bending around a kite-shaped hole. The route along top of the kite was 14 centimetres long, while the route along the bottom was 28 centimetres long. The researchers found that the ants normally chose the shorter route, although they were unable to see the bridge from above and so compare the length of the two routes. However, the strategy the ants used was was not perfect: in 2 out of 14 experiments, the foragers selected the longer, bottom route (Journal of Theoretical Biology, vol 159, p 397) Beckers and his colleagues found that in the first few minutes of each experiment, ants that were forced to change direction made either a U-turn or continued, laying down less pheromone. With the aid of a computer model, Becker’s team was able to show that these two responses are sufficient to explain the success of the ants at choosing the short branch. When ants arrived at the bridge, they had to change direction by 30 degrees, whether they took the bottom fork or the top one. Few ants turned back at this fork. Ants choosing the longer branch of the bridge encountered a turn of 45 degrees after 2 centimetres. About half of these ants did a U-turn at this corner, while the rest laid only about half as much pheromone as before. In contrast, only about 15 per cent of ants turned back before completing the short branch of the bridge. Of the ants that made U-turns, about half tried the other branch of the bridge. The rest returned to the nest. Ants coming back after collecting food faced a symmetrical set of turns and behaved in a similar way. Clearly, the route around an obstruction which is likely to collect the most pheromone and attract the most ants is the one that bends away least from the direction of the main path,