Closed-loop system: experimental setup and animal tethering. (A) Cataglyphis fortis on the closed-loop treadmill with a styrofoam sphere of 5 cm diameter in the desert of Tunisia (complete overview). (B–F) Closed-loop configuration. (B) Top view, (C) front view, (D) side view. The air cup is tilted by 10 deg. Two wheels (W) are mounted on horizontal tip-supported axes and touch the sphere in the plane of its equator (EQ), thus preventing any yaw of the sphere. Motion of the sphere is monitored by two optical mouse sensors (S) that look at 90 deg of azimuth with respect to each other through lenses (L) to the equator of the sphere. Two somewhat tangentially shining LEDs enhance the contrast of the sphere surface sufficiently for safe sensor recordings. The two wheels can be moved horizontally (indicated by arrows in B) by micromanipulators, thus permitting centring of the sphere in the cup. (E) Prepared animal on the holder. (F) Schematic drawing of the holder of the closed-loop configuration.
Open-loop system: experimental setup and animal tethering. (A) Top view, (B) side view. The sphere is allowed to rotate about all three axes. The motion of the sphere is monitored by two optical mouse sensors (S) that look at 90 deg of azimuth with respect to each other through lenses (L) to the equator (EQ) of the sphere. Two somewhat tangentially shining LEDs enhance the contrast of the sphere surface sufficiently for safe sensor recordings. (C) Schematic drawing of tethering an animal by a small rod to a horizontal strip of paper. This way the animal is prevented from yaw and translation but allowed to adjust its height, roll and pitch to some degree on top of the sphere.
Homing behaviour of a foraging ant: angular orientation and the walking speed separated into approach and search phases. Example of a tracked homing walk of the C. fortis ant after a training distance of 10 m. (A) Frequency histogram of the walking angle for (B) the approach phase before the turning point and (C) the search phase after the turning point (TP). (D) Walking speed plotted against the running time, approach phase (start to TP) with an average speed of 13.9 cm s−1 and the search phase (TP to end) with an average speed of 10.4 cm s−1. The walking speed was smoothed with a moving average of a factor of 0.49. (E) Angular speed plotted against the running time, smoothed with a moving average of a factor of 2. The dotted line marks the time when reaching the 10 m point [fictive nest (FN)] whereas the dashed line indicates the turning point (TP).
Statistical analysis of the walking speed of tracked homing ants. (A) Walking speed of the homing runs of C. fortis under open sky separated into two phases according to the criterion of the first turning point (TP) (left panel, N=15) and separated by the fictive nest site position (FN) (right panel, N=12). (B) The homing runs are separated into three parts, depending on whether the ants first pass the fictive nest site before performing the TP (FN=10 m, left panel, N=6) or perform the TP before reaching the FN (right panel, N=5). Box-and-whisker plots give the 10th, 25th, 50th, 75th and 90th percentile distribution. Asterisks indicate statistical significance by the following analyses: (1) Wilcoxon paired, P<0.001; (2) Wilcoxon paired, P<0.001; (3) Friedmann test, P=0.004; (4) Friedmann test, P=0.040.
Approach and search phases of tracked homing walks of two different Cataglyphis species trained and tested under open-field or laboratory conditions. (A–D) First and second column show training and test setup for Cataglyphis fortis (A–C) and Cataglyphis bicolor (D). F, feeder position; N, nest position. Third column shows approach phases, fourth column shows search phases. For the third and fourth column, the trajectories of all ants have been superimposed; the first 20 m of the search phases are represented. The experiments in A and B were conducted under open sky in the desert of Tunisia. The animals were trained to visit a feeding site at a distance of 10 m. Homing runs were tested in an open test field (Control, N=20; A), and on the spherical treadmill (OUT-1, N=18; B). The experiments in C and D were conducted in the laboratory of Ulm. We trained the animal to walk to a feeding site through an aluminium channel of 4 m distance covered with a polarisation filter (represented as a black/white pattern) and tested their homing run on the spherical treadmill. (C) Cataglyphis fortis (LAB-1, N=9); (D) C. bicolor (LAB-2, N=8). A black cross marks the fictive nest position. The red dots show the search centres of every search phase. Corresponding analyses are shown for the approach phase: (E) length of the approach trajectories, (F) distance of first turning point to the nest, (G) index of straightness. For the search phase: (H) accuracy of nest search, (I) width of search, (J) index of straightness. For comparison, the laboratory data were normalised to 10 m distance. Box-and-whisker plots show the median as the box centre, plus 10th, 25th, 75th and 90th percentiles. Results of the statistical evaluation are indicated above the plots by letters; data sets with the same letters indicate an absence of significant difference, different letters indicate significant differences (for details, see text).
Course correction response to imposed changes of the compass directions by turning the entire apparatus back and forth under the open sky. (A) Example of a homing run tracked on the spherical treadmill while alternately rotating the apparatus 90 deg to the right (northbound) and 90 deg to the left (westbound). The ant's starting position is at the zero point (0,0) and the ant initially moved westward towards its fictive nest position. The black asterisk marks the time when the sky was covered. The grey dots indicate the times when the apparatus was rotated. (B,C) The angular orientation with respect to the ant's current walking direction of N=2 homing runs with (B) open (N=16 turns) and (C) covered sky (no celestial information, N=8 turns). The dashed line indicates the centre of the turning behaviour of the ants. Turns to the left were mirrored and the angular orientations were smoothed with a moving average of a factor of 5. The bold black line indicates the mean value; the grey area marks the span of the standard deviation.