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First published online February 4, 2005
Journal of Experimental Biology 208, 707-719 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01434

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Walking on inclines: energetics of locomotion in the ant Camponotus

Alexandra Lipp, Harald Wolf and Fritz-Olaf Lehmann^*

Department of Neurobiology, University of Ulm, 89069 Ulm, Germany

View larger version (30K): [in a new window]   Fig. 1. Different types of breathing behaviour occurred in resting Camponotus. (A-D) Various patterns of the discontinuous gas exchange cycle (DGC) in single decapitated minor worker ants. CO2 is released supposedly via a single spiracle (A), via two spiracles as indicated by the arrows (B), or via three spiracles (C). Alternatively, a single spiracle might open repeatedly within one breathing cycle. (D) Time-expanded DGC. Large CO2 spike represents tracheal opening phase. Small spikes of gradually increasing amplitude signify spiracle flutter phase. (E) DGC in an intact ant during rest. This breathing pattern is characterised by prolonged spiracle opening during flutter phase and no distinct opening phase. (F) Continuous breathing (or higher flutter frequency) behaviour in a decapitated minor. Continuous breathing mode was further observed in all ants during locomotion. C, closed phase, F, flutter phase and O, spiracle opening phase. Inset sketches illustrate decapitated and intact situations.

View larger version (44K): [in a new window]   Fig. 2. CO2 release during locomotor activity in a single ant. (A) The measured amounts of CO2 (black line) are confounded by the Doppler effect, due to the ant's movement in the air current of the respirometric chamber. Doppler time shifts were corrected according to the momentary position of the ant in the chamber (red line; details in Materials and methods). The red line is also shifted by a fixed offset, compensating for a small constant delay produced by the tubing which connected respirometric chamber and gas analyser. (B) Running speed of the ant, as calculated from the position (video) recording above. Shaded areas indicate that the ant was running against the direction of air flow during flow-through respirometry. Asterisks mark turns of the animal at the ends of the respirometric chamber. The broken line indicates the maximum running speed considered for linear regression in Fig. 4. (C) To correct time delays produced by chamber washout, and possible behavioural delays in CO2 release, we estimated the time shift between the CO2 release and the speed traces using cross-correlation. The cross-correlation shift was calculated in a sliding data window (75 data points, or 15 s), and tested for temporal shifts of up to 10 s between the two traces. Mean value for cross correlation is 2.4 s (broken line); see Fig. 3A.

View larger version (20K): [in a new window]   Fig. 3. Cross-correlation time shift between walking speed and CO2 release, due to both wash-out of the respirometric chamber and possible behavioural delays in gas exchange during breathing. (A) Histogram of cross-correlation time shifts calculated from 46 animals. Mean temporal shift between speed and metabolic rate traces was 2.4 s according to the Gaussian fit curve (black line). Values below 1.2 s shift were disregarded because they were mostly produced by small chance fluctuations. (B) Sample trace illustrating the shift of the CO2 release trace (red, Doppler-corrected trace) by the time delay (t, shaded area) obtained with the cross-correlation method (blue). The time correction results in approximately simultaneous fluctuations of CO2 release and running speed.

View larger version (30K): [in a new window]   Fig. 4. Relationship between CO2 release and running speed. Data are shown for the five different slopes of the respirometric chamber examined: (A) -60° (steep descent), (B) -30°, (C) horizontal, (D) 30°, and (E) 60° (steep ascent). See also inset sketches. Red data points in C give mean values obtained from all five inclines. Black lines represent linear fit regressions to the data up to 90 mm s-1 speed (A) y=1.64+9.7x10-3x, r2=0.71; (B) y=1.73+10.5x10-3x, r2=0.88; (C) y=1.50+11.4x10-3x, r2=0.91; (D) y=1.55+10.1x10-3x, r2=0.94; (E) y=1.37+13.0x10-3x, r2=0.82. Numbers of ants in the speed bins were 26 in each of the 6 lower bins (0-5 mm s-1 to 45-55 mm s-1), 25 (55-65 mm s-1), 25 (65-75 mm s-1), 24 (75-85 mm s-1), 22 (85-95 mm s-1), 18 (95-105 mm s-1), 12 (105-115 mm s-1) and 7 (115-125 mm s-1). Estimates for resting metabolic rates of intact (a) and decapitated (b) ants are indicated as blue dotted lines. The speed range up to 90 mms-1 is shaded (see Discussion for explanation). Values are means ± S.D.

View larger version (20K): [in a new window]   Fig. 5. Metabolic costs calculated from pooled data (five inclines in Fig. 4), as dependent on relative running speed (A) and incline of the walking substrate (B). (A) Differences (a-b) in metabolic rate between locomotion against (a) and with (b) the direction of air flow (CO2 release) are plotted against relative running speed (that is twice the running speed over ground). Airflow speed in the running tube was approximately 235 mm s-1. At low walking speeds the ants experience similar head and tail wind speeds, whereas at high running speeds head wind becomes progressively faster than tail wind (see text for details). A polynomial fit line approximates the data points following a `speed-squared' relationship. N=26, 26, 26, 26, 26, 25, 24, 22, 18, 13 ants from left to right. (B) Metabolic rate of Camponotus at the five different inclines of the running chamber, averaged for all running speeds. Data are weighted by the duration of the mean running period in each ant. N=30 (-60°), 30 (-30°), 45 (0°), 12 (30°), 12 ants (60° inclination). None of the values are significantly different from each other, except metabolic rates at 0° and -30° (P<0.05). Broken lines indicate metabolic rates of resting intact (a) and decapitated (b) ants, and during locomotion at walking speeds below 5 mm s-1 (c).

View larger version (30K): [in a new window]   Fig. 6. Differences (a-b) in metabolic rate (CO2 release) between running on (a) inclines of -60° (A), -30° (B), 30° (C), 60° (D) and (b) level walking. Differences are plotted as a function of running speed of the animal over ground and derived from the mean values for each speed bin (see Fig. 4). Dotted lines indicate both equal metabolic costs between inclined and level walking (horizontal line), and 90 mm s-1 walking speed (vertical line), which was the maximum speed used for linear regression fits. Values are means ± S.E.M.

View larger version (18K): [in a new window]   Fig. 7. Frequency distributions of running speed (A) and continuous running distance (B) in the respirometric chamber. (A) Histograms of running speeds measured in all 46 ants at the five different inclines: -60° (purple data points), -30° (green), 0° (black), 30° (red), 60° (blue). Speeds from 0 to ±5 mm s-1 were disregarded to avoid disruption of curves by resting periods. Positive and negative running speeds indicate locomotion against and with the direction of airflow, respectively. Speed distributions were approximated by Gaussian fits (curves with respective colours). Peak locations and standard deviations (0.85 times half width) of the Gaussian curves are (in mm s-1) 0.7, 78 (-60°); 1.1, 101 (-30°); 2.0, 87 (0°); -0.6, 94 (30°); -0.1, 78 (60°). None of the values are significantly different from each other (P>0.05). (B) Histograms of walking distances covered in continuous bouts by ants in the respirometric chamber (time segments >0.4 s during which the ant maintained running speeds >5 mm s-1; N=42, n=4038 running segments). Level walking, black; ±30° incline, red; ±60° incline, green. Length of respirometric chamber was 500 mm (vertical dotted line).