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First published online March 31, 2007
Journal of Experimental Biology 210, 1413-1423 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02747
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Take-off and landing forces and the evolution of controlled gliding in northern flying squirrels Glaucomys sabrinus

Keith E. Paskins1,*, Adrian Bowyer1, William M. Megill1 and John S. Scheibe2

1 Centre for Biomimetic and Natural Technologies, Department of Mechanical Engineering, University of Bath, Bath, BA2 7AY, UK
2 MS 6200, Department of Biology, Southeast Missouri State University, 1 University Plaza, Cape Girardeau, MO 63701, USA


Figure 1
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  		Fig. 1. A diagrammatic representation of the experimental set-up, showing the instrumentation used to measure take-off and landing forces including the carpet-covered landing pole, the rope-covered take-off branch and the location of the strain gauge pairs on each. Also shown are the measurements used to analyse the results, relative to an approximate squirrel trajectory. (A) The controlled horizontal distance between the end of the take-off branch and the vertical landing pole, (B) the total distance covered in the glide calculated from the exact horizontal distance (C) and the drop (D).

 

Figure 2
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  		Fig. 2. Video stills of the squirrels in the three postures observed. (A) The forelimbs being abducted prior to the hindlimbs leaving the substrate during take-off, and how the take-off angle, {theta}, is calculated as the angle between the branch and the major axis of the best-fitting ellipse to the squirrel (excluding its tail). (B) Normal gliding flight, (C) landing from the side and (D) a ventral view landing on the pole on the left. In C, the last few frames of a landing sequence have been superimposed onto one image to demonstrate the landing behaviour, although the penultimate frame had to be omitted for clarity. In this short, 1 m jump, the squirrel initially pitches upwards and flattens its body and tail against the direction of motion. Immediately prior to landing, the head is tilted backwards while the limbs are all pushed forwards with the tail simultaneously rotated back so that it is parallel with the ground.

 

Figure 3
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  		Fig. 3. A box plot directly comparing the landing forces with the corresponding take-off forces for three of the flying squirrels (young females YF1 and YF2, and young male YM) at each horizontal range (from the end of the take-off branch to the landing pole). Asterisks and circles show values that were outside the interquartile range, the former being statistically significantly far away.

 

Figure 4
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  		Fig. 4. A graph showing the output from the mixed model statistics of mean take-off and landing forces at each horizontal range. The error bars represent the 95% confidence intervals. By ignoring the few jumps at 2 m, when the squirrel consistently landed at the more rigid base of the pole, the square of the Pearson product moment correlation coefficient (the r2 value) improves from 0.61 to 0.99.

 

Figure 5
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  		Fig. 5. Average take-off angle for each individual flying squirrel as a function of the horizontal distance travelled in the jump (labelled C in Fig. 3). This implies that the squirrels are planning ahead, which in turn may imply that they are considering their landing. Asterisk and circles, see Fig. 3.

 

Figure 6
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  		Fig. 6. Percentage of body weight supported by lift during gliding as a function of horizontal range in flying squirrels (young females YF1 and YF2, and young male YM). Values are means for each squirrel at each range ± 1 s.d.

 

Figure 7
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  		Fig. 7. Scatter plot showing how the glide angle increases with horizontal range until it reaches approximately 45°, represented by a broken line, after which the glide ratio begins to improve slightly. High take-off angles and limited time spent in the air are the factors responsible for the low vales of glide angle across low ranges. Glide angle is strongly negatively correlated with range above 4 m (r=–0.816, P<0.001) where higher glide speeds enable northern flying squirrels to exhibit superior lift to drag ratios.

 




© The Company of Biologists Ltd 2007