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The influence of beacon-aiming on the routes of wood ants

Paul Graham^*, Karine Fauria and Thomas S. Collett

School of Biological Sciences, University of Sussex, Brighton, UK, BN1 9QG

View larger version (27K): [in a new window]   Fig. 1. Route shapes are influenced by landmark position. (A) Ants were trained to go from a start position at one end of a large indoor arena to a feeder 3 m away at the other end. A black cylinder was placed either to the left or to the right of the direct path. (B) Ants headed to the left of the direct path (grey band) when the cylinder (filled circle) was on the left. Each dash marks the point of intersection of one trajectory with a semi-circle centred on the start with a radius of 20 cm, 40 cm, 60 cm or 80 cm [n (number of trajectories)=101, N (number of ants) =10]. Open triangles show the mean heading at each distance. (C) Ants tended to head to the right when the cylinder was on the right (n=136, N=14). Two ants were atypical and headed away from the cylinder; their trajectories were excluded from subsequent analysis. The grey band indicates the direct path from start to cylinder. (D) Trajectories of one ant trained with the cylinder on the left and (E) trajectories of one ant trained with the cylinder on the right.

View larger version (30K): [in a new window]   Fig. 2. Route shapes survive the removal or displacement of the cylinder. (A) Headings of trained ants when no landmark was present and (B) when the landmark was shifted to the opposite side from that in training. Data from ants trained with the landmark to the right of their route are mirrored to generate a single data set (no landmark: n=66, N=14; landmark opposite: n=23, N=8). Each dash marks the point of intersection of one trajectory with a semi-circle centred on the start with a radius of 20 cm, 40 cm, 60 cm or 80 cm. Open triangles show the mean heading at each distance. (C,D) Sample trajectories from one ant in each test condition. Open circles represent the position of the landmark during training, and the filled circle in D represents the position of the landmark in test runs.

View larger version (14K): [in a new window]   Fig. 3. Trajectories with displaced start position. Ants with no cylinder present were started from the normal position or 30 cm to its left (near) or to its right (far). Arrows show mean trajectory directions measured at 20 cm, 40 cm, 60 cm and 80 cm from the start. Trajectories within a start condition are sufficiently similar that r, the mean vector length, is greater than 0.9 at 20 cm, 40 cm, 60 cm and 80 cm from the start. The open circle indicates the normal position of the absent cylinder. The three triangles represent the areas enclosed by extending the three headings at 40 cm, 60 cm and 80 cm.

View larger version (24K): [in a new window]   Fig. 4. Retinal position of real and imaginary cylinder during a trajectory. (A) The horizontal path of the centre and of the left and right edges of the bottom of the cylinder across the retina is shown as the ant moves from the start to a point 80 cm away. Estimated horizontal, retinal position for a stated distance comes from the mean retinal position of all data (194 trajectories from 20 ants) within the interval 4 cm either side of the stated distance. (B) Imaginary retinal positions for tests with no cylinder present (71 trajectories from eight ants). (C) The distribution of retinal positions of the centre of the cylinder accumulated over the first 45 cm of the route. Left panel: trajectories with cylinder present (194 trajectories from 20 ants). Right panel: imaginary retinal positions in tests without a cylinder (71 trajectories from eight ants). (D) Distribution of retinal positions of the right edge of the landmark over the second 45 cm of the route. Left panel: trajectories with cylinder present. Right panel: imaginary retinal positions in tests without a cylinder. Data from ants trained with the cylinder on the left or right are combined and represented as though the cylinder is on the left.

View larger version (25K): [in a new window]   Fig. 5. Routes of ants trained with the cylinder at 2 m from the start. (A-C) Paths of three ants with the cylinder (filled circle) in place. Each ant followed a different route that was consistent from trial to trial. (D-F) Paths from the same three ants in tests with the cylinder removed (the open circle indicates the normal position of the absent cylinder). (G-J) Mean trajectories of individual ants trained with the cylinder 2 m from the start (G,H) are compared with a similar plot for ants trained with the cylinder 1 m from the start (I,J). Eight or more trajectories contribute to each mean. Trajectories are significantly more dispersed in G and H than they are in I and J. The grey bar shows the direct path from start to landmark.