SUMMARY
Traversing gaps with different orientations within arboreal environments has ecological relevance and mechanical consequences for animals. For example, the orientation of the animal while crossing gaps determines whether the torques acting on the body tend to cause it to pitch or roll from the supporting perch or fail as a result of localized bending. The elongate bodies of snakes seem well suited for crossing gaps, but a long unsupported portion of the body can create large torques that make gap bridging demanding. We tested whether the three-dimensional orientation of substrates across a gap affected the performance and behavior of an arboreal snake (Boiga irregularis). The snakes crossed gaps 65% larger for vertical than for horizontal trajectories and 13% greater for straight trajectories than for those with a 90 deg turn within the horizontal plane. Our results suggest that failure due to the inability to keep the body rigid at the edge of the gap may be the primary constraint on performance for gaps with a large horizontal component. In addition, the decreased performance when the destination perch was oriented at an angle to the long axis of the initial perch was probably a result of the inability of snakes to maintain balance due to the large rolling torque. For some very large gaps the snakes enhanced their performance by using rapid lunges to cross otherwise impassable gaps. Perhaps such dynamic movements preceded the aerial behavior observed in other species of arboreal snakes.
FOOTNOTES
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Supplementary material available online at http://jeb.biologists.org/cgi/content/full/215/15/2611/DC1
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FUNDING
This work was supported by a grant from the National Science Foundation [IOS 0843197 to B.C.J.].
LIST OF SYMBOLS AND ABBREVIATIONS
- COM
- center of mass
- COMgap
- center of mass of the unsupported region of the snake
- g
- acceleration due to gravity
- Gapmax
- maximum gap distance as a percentage of SVL
- I
- moment of inertia
- lexcess
- lgap divided by maximum gap distance, expressed as a percentage
- lgap
- length of the unsupported body of the snake in the gap at contact
- m
- mass of the unsupported region of the snake
- r
- length of the moment arm from the COM of the unsupported portion of the snake to the edge of the gap
- R
- distance between origin and snout at maximum height
- SVL
- snout–vent length
- TL
- total length
- vr,max
- maximum resultant velocity
- vx,max
- maximum velocity in x
- vy,max
- maximum velocity in y
- x1
- distance from snout at lunge initiation to top center of destination in x
- y1
- distance from snout at lunge initiation to top center of destination in y
- Δlgap
- change in length of the unsupported body during a lunge
- Δy2–1
- vertical displacement of snout from start to crest of a lunge
- Δy2–3
- vertical displacement of snout from crest of a lunge to position at contact
- Δθ3–1
- change in the angle of the vector between the origin and snout during a lunge
- θ
- angle with respect to vertical
- θ1
- angle with respect to straight line between the two perches at lunge initiation
- τbend
- bending torque
- τpitch
- pitching torque
- τroll
- rolling torque
- ω
- angular velocity
- ω̇
- angular acceleration
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