Gripping during climbing of arboreal snakes may be safe but not economical

Abstract

On the steep surfaces that are common in arboreal environments, many types of animals without claws or adhesive structures must use muscular force to generate sufficient normal force to prevent slipping and climb successfully. Unlike many limbed arboreal animals that have discrete gripping regions on the feet, the elongate bodies of snakes allow for considerable modulation of both the size and orientation of the gripping region. We quantified the gripping forces of snakes climbing a vertical cylinder to determine the extent to which their force production favoured economy or safety. Our sample included four boid species and one colubrid. Nearly all of the gripping forces that we observed for each snake exceeded our estimate of the minimum required, and snakes commonly produced more than three times the normal force required to support their body weight. This suggests that a large safety factor to avoid slipping and falling is more important than locomotor economy.

Keywords: gripping pressure; locomotion; normal force; prehensile; safety factor.

Figure 1.

Longitudinal distribution of normal forces for one locomotor cycle for (a) M. spilota and (b) B. irregularis. Horizontal bars indicate the total normal forces from four sensors at each longitudinal location. The time intervals between successive images in (a) and (b) are 1.4 and 1.8 s, respectively.

Figure 2.

Total observed normal force, F_obs. (a) With increased body mass, F_obs increased significantly. (b) Values of F_obs versus time for one trial with four locomotor cycles (denoted alternately by grey and white) of M. spilota (191 g). One cycle is the time between successive initiations of static contact for a single point on the snake. (c) Frequency distribution of F_obs for 10 locomotor cycles of M. spilota. The dashed lines indicate F_req.