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Figures
- Fig. 1.
Study species and high-speed video recording. (A) Cataglyphis bombycina in the sand dune habitat of the Saharan desert near Douz, Tunisia. The ant is shown in typical cooling posture on an elevated structure – here, a small succulent plant. The violet frame marks data obtained in C. bombycina in subsequent figures. (B) Cataglyphis fortis in a salt pan near Menzel Chaker, Tunisia. The homing forager is carrying a food crumb in its mandibles. The grey frame marks data obtained in C. fortis in subsequent figures. (C) Tripod gait in C. bombycina. A frame from a high-speed video showing an ant during walking at a speed of v=561.4 mm s−1. Several consecutive step cycles were recorded for each leg in the corresponding video sequence (sampling rate 500 Hz). Tripods formed by the left front (L1), hind (L3) and right middle (R2) legs are in blue; tripods formed by the opposite set of legs (R1, L2, R3) are in orange. Stride length (SL) is indicated for the right middle leg, as determined by the distance between two successive footfalls. Stride amplitude can be estimated by subtracting body displacement (BD) from stride length (SL) for the respective swing phase (stride amplitude=SL−BD).
- Fig. 2.
Morphometric comparison of C. bombycina and C. fortis alitrunk and leg pair (Lp) lengths. Cataglyphis bombycina is significantly smaller than C. fortis with respect to mean alitrunk length (11% smaller; t-test, ***P<0.001) and mean leg length (first leg pair, 19% shorter, t-test, ***P<0.001; second leg pair, 18% shorter, U-test, ***P<0.001; third leg pair, 18% shorter, U-test, ***P<0.001). Morphometry data of C. fortis were kindly provided by S. Sommer and R. Wehner.
- Fig. 3.
Footfall geometry. (A) Average footfall positions of the six legs with respect to the petiole [origin of the plot (0/0)]; x- and y-axes were normalized to the body size of the animals (alitrunk length). For each footfall position, 10 videos with three step cycles each were evaluated, with walking speeds of 490–600 mm s−1 for C. bombycina and 490–580 mm s−1 for C. fortis. Anterior extreme positions (AEP) are shown for the left body side (L1, left front leg; L2, left middle leg; L3, left hind leg); posterior extreme positions (PEP) are shown for the right body side (R1, right front leg; R2, right middle leg, R3 right hind leg). (B) The spread of footprints was quantified by calculating standard deviations the of footprint clusters shown in A. Footprint positions show similar spreads in C. bombycina and C. fortis [Levene's-test, F–value AEP=1.3<1.5 (critical value) and F-value PEP=1.4<1.5 (critical value)].
- Fig. 4.
Walking parameters. The most significant walking parameters are plotted as functions of walking speed. (A–D) Each data point represents the mean value of all three leg pairs in one video sequence (n=134 for C. bombycina; n=305 for C. fortis). (A) Stride length (linear regressions: C. bombycina, y=0.02x+4.72, R2=0.91; C. fortis, y=0.02x+6.04, R2=0.92); (B) stride frequency (power functions: C. bombycina, y=1.43x0.52, R2=0.90, C. fortis, y=0.81x0.59, R2=0.95); (C) swing phase duration (power functions: C. bombycina, y=0.14x−0.32, R2=0.69; C. fortis, y=0.16x−0.33, R2=0.74); (D) stance phase duration (power functions: C. bombycina, y=1.22x−0.80, R2=0.89; C. fortis, y=1.58x−0.78, R2=0.94). (E–H) Individual values for the three leg pairs (Lp1, front legs; Lp2, middle legs; Lp3, hind legs). (E) Stride amplitude in C. bombycina (linear regressions: Lp1, y=0.0048x+3.02, R2=0.67; Lp2, y=0.0054x+2.91, R2=0.73; Lp3, y=0.0051x+2.94, R2=0.70); (F) stride amplitude in C. fortis (linear regressions: Lp1: y=0.0035x+4.42, R2=0.30; Lp2: y=0.0067x+4.75, R2=0.63; Lp3: y=0.0027x+4.30, R2=0.18); (G) swing speed in C. bombycina (linear regressions: Lp1, y=1.27x+75.47, R2=0.95; Lp2, y=1.44x+79.60, R2=0.94; Lp3, y=1.34x+69.19, R2=0.95) – table below provides angular velocities during slow (79–82 mm s−1), medium (335–367 mm s−1) and fast (701–855 mm s−1) walking; (H) swing speed in C. fortis (linear regressions: Lp1, y=1.48x+71.68, R2=0.95; Lp2, y=1.80x+86.43, R2=0.93; Lp3, y=1.40x+69.01, R2=0.94) – table below provides respective angular velocities during slow (75–81 mm s−1), medium (340–380 mm s−1) and fast (590–620 mm s−1) walking.
- Fig. 5.
Leg coordination parameters. (A) Mean tripod coordination strength (TCS, see Materials and Methods) plotted as a function of walking speed (logarithmic regression: C. bombycina, y=−0.015lnx+0.85, R2=0.02; C. fortis, y=0.097lnx+0.18, R2=0.46). (B) Phase relationships of the six legs analysed for C. bombycina (violet; circle represents 30.3 ms) and C. fortis (grey; circle represents 35.6 ms) with regard to swing phase onset (Bi) and stance phase onset (Bii). Cycle phase rotates counter-clockwise. The left hind leg (L3) served as reference (phase 0). For each species, 20 videos (60 step cycles) were analysed at speeds from 300 to 400 mm s−1. Each data point represents the onset of swing or stance with respect to L3. The length of the mean phase vector indicates the variance of data points. Statistically significant differences between the two species emerged during swing onset (Bi) for L2 (Kuiper two-sample test, P=0.002) and R2 (Kuiper two-sample test, P=0.001) with a time shift of around 3.3 ms, as well as during stance phase onset (Bii) for L1 and R1 (Kuiper two-sample test, both with P=0.05) with a time shift of about 2.6 ms. (C,D) Duty factor plotted as a function of walking speed for the three leg pairs in (C) C. bombycina (linear regressions: Lp1, y=−0.0004x+0.49, R2=0.61; Lp2, y=−0.0004x+0.54, R2=0.56; Lp3, y=−0.0003x+0.49, R2=0.54) and (D) C. fortis (linear regressions: Lp1: y=−0.0005x+0.59, R2=0.66; Lp2: y=−0.0004x+0.66, R2=0.61; Lp3: y=−0.0006x+0.57, R2=0.73).
- Fig. 6.
Podograms and quantification of gait patterns. (A,C) Sample podograms (top) recorded in C. bombycina (A) and C. fortis (C) at similar walking speeds (about 350–360 mm s−1), and the corresponding colour (middle) and number indices (bottom) plotted along the same time axis. Podograms illustrate the footfall patterns of the six legs; black bars indicate swing and white bars indicate stance phases (L, left; R, right body side; 1, 2 and 3, front, middle and hind legs). Each frame in the video was assigned a colour (middle) and a number index (bottom) according to leg coordination in the particular frame (see colour and number code keys at the bottom of the figure). (B,D) Quantitative analyses according to walking speed class (<50 to >750 mm s−1) for C. bombycina (B; n=71) and C. fortis (D; n=70). No data were available for the <50 mm s−1 speed class in C. bombycina, and for the 650 to <750 mm s−1 and ≥750 mm s−1 speed classes in C. fortis. Colour index analyses (Bi,Di) are shown as normalized frequency plots, and number index data (Bii,Dii) as averaged index numbers (ordinates) for the respective walking speed bins (abscissae). Cataglyphis bombycina reaches its maximum gait index beyond about 250 to <350 mm s−1; there were no statistically significant differences (of neighbouring speed bins) as indicated by the results of an ANOVA on ranks. Cataglyphis fortis shows a gradual increase in the number indices over the entire walking speed range; there were statistically significant differences between neighbouring speed bins (one-way ANOVA): <50 mm s−1 and 50 to <150 mm s−1 (***P≤0.001); 50 to <150 mm s−1 and 150 to <250 mm s−1 (*P≤0.017); 150 to <250 mm s−1 and 250 to <350 mm s−1 (**P=0.010); 350 to <450 mm s−1 and 450 to <550 mm s−1 (*P=0.025); 450 to <550 mm s−1 and 550 to <650 mm s−1 (*P=0.013).
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