Sweeping and striking: a kinematic study of the trunk during prey capture in three thamnophiine snakes
Michael E. Alfaro
Committee on Evolutionary Biology, 1025 E. 57th Street, University of
Chicago, Chicago, IL 60637, USA and Field Museum of Natural History,
1400 South Lake Shore Drive, Chicago, IL 60805, USA
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Fig. 1. Phylogenetic relationship of the tribe Thamnophiini (family Colubridae).
Shown is the majority rules consensus of 90 000 post-burnin states visited by
a million generation Bayesian Markov Monte Carlo reanalysis of previously
published data (Alfaro and Arnold,
2001 ) performed using MrBayes
(http://morphbank.ebc.uu.se/mrbayes/info.php).
Taxa sampled in this study, indicated by arrows, represent two of the three
major thamnophiine groups. Numbers above branches are the Bayesian posterior
probabilities for the clade.
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Fig. 2. Digitizing protocol for kinematic analysis. From dorsal footage of feeding
strikes (A), points along the trunk midline were digitized at 1-2 cm
intervals. For each frame, a quintic spline was fit to these trunk points.
From the spline fitting, 11 points spaced equally along the trunk were
calculated (B) and retained for analysis in
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Fig. 3. Thamnophis couchii strike in dorsal view. t = time in
seconds. At t=0, the typical prey-strike posture with a linear
arrangement of body loops is evident. The strike proceeds as the anterior-most
loops straighten, followed by straightening of the two larger posterior loops.
T. couchii strikes were rapid and usually involved a large proportion
of the trunk.
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Fig. 4. Head velocity (A), segment angle (B), path angle (C) and forward
displacement (D) profiles for Thamnophis couchii. Graphs have been
standardized to the time of peak velocity so that maximum velocity is reached
at t=0. Error bars represent 1 S.E.M. Anterior body points
are yellow, posterior points are blue. T. couchii strikes showed the
highest velocities of the three species measured. Segment straightening was
apparent for most positions along the trunk. Segment angles generally did not
exceed 90°, indicating that the anterior ends of all segments along the
trunk were pointed in the direction of the strike. Head acceleration was
accompanied by substantial angular rotation in segments 1-4. Path angles for
most anterior segments were under 90° and decreased with increasing
velocity, indicating that these segments traveled close to the calculated
strike vector. Path angles exceeded 90° for posterior segments shortly
before maximum velocity was achieved. This may have been the result of
backwards displacement of posterior body segments in reaction to
head-accelerating forces generated by the anterior trunk. Rearwards
displacement of the posterior segments was sometimes observed in video
sequences. Forward displacement was substantial and decreased in an anterior
to posterior direction.
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Fig. 5. Head sweeping by Thamnophis elegans. t = time in seconds. As the
snake travels forward, the head is swung from side to side primarily by
movements of the anterior trunk. Sweeping was the slowest of the behaviors
observed in this study.
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Fig. 6. Diagrammatic view of point position along the trunk over the course of a
sweep in Thamnophis elegans. t = time in seconds. The point of
maximum head velocity is set as t=0. Colors distinguish various
times. Head sweeping involves large lateral excursions of the anterior trunk
while the posterior trunk remains relatively static.
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Fig. 7. Thamnophis elegans strike. Shown is a sequence from a predatory
strike in dorsal view. t = time in seconds. At t=0, a strike
is elicited from a motionless individual in an ambush position. Note the
presence of small amplitude loops in the neck. Most of the long axis of the
body is directed away from the direction of the strike. In the first 100 ms,
head acceleration is accomplished by straightening of small loops in the
anterior trunk as well as by the initial uncoiling of a large loop in the
posterior trunk. As the strike proceeds, the large posterior coils continue to
straighten, driving the largely straight anterior trunk towards the prey.
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Fig. 8. Head velocity (A), segment angle (B), path angle (C) and forward
displacement (D) profiles for Thamnophis elegans. Graphs have been
standardized to time of peak velocity so that maximum velocity is reached at
t=0. Error bars represent 1 S.E.M. Anterior body points
are yellow, posterior points are blue. Head velocity during strikes was higher
than in sweeps but still lower than in the other species examined. Segment
straightening was apparent for positions 2 and 3 as the head approached peak
velocity. Head segment angle was variable during the initial stages of head
acceleration, decreasing shortly before the head reached peak velocity.
Segment 1 segment angle decreased rapidly after peak velocity. More-posterior
segment angles decreased slightly after peak velocity. Path angles for the
three anterior-most positions dropped sharply as the head accelerated, while
positions 4-8 showed little change from an initial path of 90°. Forward
displacement was greatest at the snout and positions 1 and 2.
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Fig. 9. Dorsal view of a predatory strike by Nerodia rhombifer. t = time
in seconds. Prior to strike initiation, the snake is at rest on the bottom of
the tank with the head out of the water in a typical ambush position
(t=0). At t=0.017, the mouth is opened and the head is swung
laterally towards the prey. As the strike proceeds, more-posterior portions of
the trunk become involved in sweeping the head and prey laterally, although
over two-thirds of the total length remains kinematically inactive.
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Fig. 10. Head velocity (A), segment angle (B), path angle (C) and forward
displacement (D) profiles for Nerodia rhombifer. Graphs have been
standardized to time of peak velocity so that maximum velocity is reached at
t=0. Error bars represent 1 S.E.M. Anterior body points
are yellow, posterior points are blue. The snake achieves peak velocity in
approximately 50-60 ms. Starting head segment angle is approximately 90°,
indicating that the head is not closely aligned to the direction of the strike
at the onset of this behavior. The head and segment 1 show a sharp decrease in
segment angle as the head is accelerated. Segment angle in these segments
continues to decrease after peak velocity. More-posterior segments undergo
relatively little angular change. Head and segment 1 path angle also markedly
decrease with head velocity, while more-posterior segment angles are largely
unchanged. Forward displacement is greatest at the snout, followed by body
position 1. More-posterior positions experience minor displacement, with
positions 5-10 undergoing periods of rearwards movement.
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Fig. 11. Summary of mean species differences in three kinematic variables. (A)
Starting segment was high in N. rhombifer and T. elegans,
suggesting that these species struck at prey from a greater range of positions
than did T. couchii. (B) Minimum segment angle over the course of a
strike was lowest for all species at the head (H). T. couchii had
significantly lower segment angles than did the other two species. (C) Minimum
path angle was also lowest at the head for all three species and was
significantly lower in T. couchii than in either of the other two
species.
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© The Company of Biologists Ltd 2003
