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Journal of Experimental Biology partnership with Dryad

The effects of three-dimensional gap orientation on bridging performance and behavior of brown tree snakes (Boiga irregularis)
Greg Byrnes, Bruce C. Jayne

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

  • 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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