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Homing in wood ants, Formica japonica: use of the skyline panorama

Tsukasa Fukushi^*

Department of Biology, Miyagi University of Education, Aramaki-aza-Aoba, Aoba-ku, Sendai 980-0845, Japan

View larger version (25K): [in a new window]   Fig. 1. (A) Inbound (return) and (B) outbound (foraging) paths of individual ants shuttling back and forth on the terrace platform between the feeding site (F9.5; open circle with cross) and the nest (N; filled circle). Only the eastern half of the terrace is shown. The northeast corner of the terrace is taken as the origin of the x,y coordinate system. The positions of the ants were recorded every 10s. N=15 (A) and N=11 (B).

View larger version (35K): [in a new window]   Fig. 2. Homeward (return) paths of ants displaced from F9.5 to various release sites (open circles). (A) Paths from R1.9, R5.7 and R13.3. (B) Paths from R17.1. (C) Paths from 20.9. N=10 in each experiment. N, nest.

View larger version (20K): [in a new window]   Fig. 3. Topographical layout of the terrace platform and its surroundings. The mean homeward courses of the ants starting at F9.5 and at the various release (R) sites (see Fig.1, Fig.2) and their extensions beyond the nest (N) area are included. The positions of trees, bushes and other conspicuous objects are marked. The lines labelled A, B and C indicate the positions of the screen used in the deprivation experiments. For further details, see text.

View larger version (70K): [in a new window]   Fig. 4. (A) Photograph depicting the experimental area taken from behind F9.5, where the feeder can be seen in the foreground. The hemispherical bush in the centre is a rhododendron plant (white arrow; R.sp.1 in Fig.3), and the surrounding trees are horse chestnuts (black arrows labelled with individual numbers corresponding to those in Fig.3). The ants returning from F9.5 walk in a direction coinciding approximately with the right edge of R.sp.1. The yellow dots on the ground running back from the feeder are vial caps placed on the path taken by the ant. (B) The same landscape but with screen A mounted in the experimental area. (C) The same landscape but with screen C mounted in the experimental area.

View larger version (53K): [in a new window]   Fig. 5. Horizontograms as seen from the nesting site (Nest), the feeding site (F9.5) and the various release sites (R1.9–R20.9). The abscissa shows the azimuthal direction; south defines 0° in a counterclockwise (positive) sense of rotation. Elevation angles of the skyline (ordinate) were measured every 10° of azimuth using a transit (Tracon S-25, Ushikata Co., Ltd, Japan). As the measurable angle of elevation using the transit was up to 50°, all elevation angles higher than 50° are represented as 50°. The numbers at the top of each column corresponds to the numbers of chestnut trees in Fig.3 (A.t.1–A.t.13). The asterisk shows the mean homeward direction of ants starting from their respective feeding or release site (see Table1). The high elevation angles for the north side were due to the building positioned next to the terrace, and those for the south to west side in the plots for R17.1 and R20.9 were due to a cherry tree (10.0m in height) located near the southwest corner of the terrace (x=24.5 and y=5.6; not shown in Fig.3), the branches of which overhung the west side of the terrace. For reference, vertical dotted lines are drawn at 0°, 50° and 100°.

View larger version (17K): [in a new window]   Fig. 6. Deprivation experiments. The frontal fields of view of the ants were obscured by screens (for exact positions, see Fig.3). The heavy dotted line (in B and C) indicates the normal homing direction. (A) Homeward paths of ants deprived of the lower parts of their frontal fields of view (screen A positioned 1m behind the nest; see Fig.4B). N=20. (B) Homeward paths of ants deprived of their entire frontal fields of view (screen B, positioned 3.1m from F9.5, as indicated by the heavy black line; height 1.8m). N=13. (C) The same screen as in B, but raised by 10cm, so that the ants were able to pass underneath it. N=13. N, nest.

View larger version (26K): [in a new window]   Fig. 7. Deprivation experiments. The upper parts of the frontal fields of view of the ants were obscured by a screen. Screen C (oblique heavy black line) was positioned 6.8m from F9.5 in the direction of the nest (N) (see Fig.4C). Assuming that the ant walks along its normal homeward course (y=-0.548x+7.206) from F9.5, the positions where the top of a chestnut tree drops out of sight behind the screen and reappears under the screen were calculated by advancing the position of the ant by a 5.0cm step along the x-axis. Horizontal bars indicate the top parts of individual chestnut trees (A.t.3–A.t.7; see Fig.3) that could not be seen by the ants on their homeward courses. (A) Paths of all runs tested. N=36. (B) Paths exhibiting a characteristic sigmoid bend, selected from A. N=22. (C) The remaining paths shown in A but not in B. N=14.

View larger version (49K): [in a new window]   Fig. 8. Two examples each of search trajectories of ants displaced from F9.5 to either a distant football field (A,B) or the roof of the building attached to the terrace (C,D). The ants were always released at the centre of the test area (the crossing point of the two heavy black lines). Walking time recorded: (A) 8min 22s, (B) 8min, (C) 7min 40s and (D) 6min 47s.

View larger version (19K): [in a new window]   Fig. 9. Mean heading directions (computed from individual path segments) of ants displaced from F9.5 to either the football field (A) or the roof (B). Quadrant A is centred about the fictive home direction. Numbers in parentheses refer to the direction of the initial path segment of each run. N=35 (A) and 22 (B) runs. For details see Materials and methods.