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Walking on Inclines: How Do Desert Ants Monitor Slope and Step Length

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Walking on Inclines: How Do Desert Ants Monitor Slope and Step Length

Tobias Seidl et al. Front Zool.

Abstract

Background: During long-distance foraging in almost featureless habitats desert ants of the genus Cataglyphis employ path-integrating mechanisms (vector navigation). This navigational strategy requires an egocentric monitoring of the foraging path by incrementally integrating direction, distance, and inclination of the path. Monitoring the latter two parameters involves idiothetic cues and hence is tightly coupled to the ant's locomotor behavior.

Results: In a kinematic study of desert ant locomotion performed on differently inclined surfaces we aimed at pinpointing the relevant mechanisms of estimating step length and inclination. In a behavioral experiment with ants foraging on slippery surfaces we broke the otherwise tightly coupled relationship between stepping frequency and step length and examined the animals' ability to monitor distances covered even under those adverse conditions. We show that the ants' locomotor system is not influenced by inclined paths. After removing the effect of speed, slope had only marginal influence on kinematic parameters.

Conclusion: From the obtained data we infer that the previously proposed monitoring of angles of the thorax-coxa joint is not involved in inclinometry. Due to the tiny variations in cycle period, we also argue that an efference copy of the central pattern generator coding the step length in its output frequency will most likely not suffice for estimating step length and complementing the pedometer. Finally we propose that sensing forces acting on the ant's legs could provide the desired neuronal correlate employed in monitoring inclination and step length.

Figures

Figure 1
Figure 1
Speed of locomotion of C. fortis (filled boxes) and F. pratensis (empty boxes) at different inclinations. Boxplots with 25%, 50% and 75% percentiles; whiskers extend to max. 1.5 times the inter-quartile range. Notches indicate significant difference in pairwise comparisons.
Figure 2
Figure 2
Relationship between speed, step length, and stepping frequency at different inclinations in C. fortis (a) and F. pratensis (b). Dots: values plotted in 3D space; lines: projected linear fits of data on two parameters; green: -60°, blue: -30° ; red: 0° ; magenta: 30°, cyan: 60°.
Figure 3
Figure 3
Footfall geometry of C. fortis (solid lines) and F. pratensis (dashed lines) during touch down (AEP) and lift off (PEP) at different inclinations. The intersection of two lines denotes the mean value, while the ends of the lines denote the standard deviation. Star: center of mass (COM); green: -60°, blue: -30° ; red: 0° ; magenta: 30°, cyan: 60°. Spatial resolution: 0.1 mm.
Figure 4
Figure 4
Influence of the input parameters on footfall geometry during the touch down (AEP) and the lift off (PEP) phase at different inclinations. The influences of speed (blue, per 0.1 m/s acceleration), species (green, from C. fortis to F. pratensis), head width (black, per 1 mm increase in head width), and inclination (red, per an increase of 10° in inclination) are plotted on the mean value of all analyzed runs. Arrows are attached to each other, i.e., if pointing in the same direction, one arrow has its base at the tip of the previous one.
Figure 5
Figure 5
Density distributions of the ants' nest search behavior after the ants had performed their outbound runs on slippery ground (except for the zero control ants) under different wind conditions (tail wind: n = 13, no wind: n = 15, head wind: n = 18, zero control: n = 20).
Figure 6
Figure 6
Schematic drawing of the setup used in experiment 1 (side view). While foraging from the nest to the feeder the ants passed a narrowed section of the channel, within which they were recorded from above (x-y plane, see arrow). The inclination of this section could be altered from level ground (φ = 0°) to almost vertical (φ = 90°).
Figure 7
Figure 7
Dorsal view of C. fortis foraging in the channel arrangement. For kinematic analysis the head-thorax joint and the petiolus were tracked as well as the contact point of the tarsi of all six legs with the ground (L1 through R3 denoting left and right legs, and front to hind legs, respectively).

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