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Biomechanics
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Debunking the viper’s strike: harmless
snakes kill a common assumption
David A. Penning, Baxter Sawvel and Brad R. Moon
Research
Cite this article: Penning DA, Sawvel B,
Moon BR. 2016 Debunking the viper’s strike:
harmless snakes kill a common assumption.
Biol. Lett. 12: 20160011.
http://dx.doi.org/10.1098/rsbl.2016.0011
Received: 6 January 2016
Accepted: 24 February 2016
Subject Areas:
behaviour, biomechanics, ecology
Keywords:
acceleration, strike, velocity
Author for correspondence:
David A. Penning
e-mail: [email protected]
Department of Biology, University of Louisiana at Lafayette, Lafayette, LA 70504, USA
DAP, 0000-0002-5368-9900
To survive, organisms must avoid predation and acquire nutrients and
energy. Sensory systems must correctly differentiate between potential predators and prey, and elicit behaviours that adjust distances accordingly.
For snakes, strikes can serve both purposes. Vipers are thought to have
the fastest strikes among snakes. However, strike performance has been
measured in very few species, especially non-vipers. We measured defensive
strike performance in harmless Texas ratsnakes and two species of vipers,
western cottonmouths and western diamond-backed rattlesnakes, using
high-speed video recordings. We show that ratsnake strike performance
matches or exceeds that of vipers. In contrast with the literature over the
past century, vipers do not represent the pinnacle of strike performance in
snakes. Both harmless and venomous snakes can strike with very high accelerations that have two key consequences: the accelerations exceed values that
can cause loss of consciousness in other animals, such as the accelerations
experienced by jet pilots during extreme manoeuvres, and they make the
strikes faster than the sensory and motor responses of mammalian prey
and predators. Both harmless and venomous snakes can strike faster than
the blink of an eye and often reach a target before it can move.
1. Introduction
For many organisms, defence and feeding involve different behaviours or
different levels of performance during the same behaviour [1–4]. For many
snakes, striking can be used both to catch prey and defend against predators
[1,2]. Scientific descriptions of viper strikes date at least as far back as the
early nineteenth century [5], and one of the first animal behaviours viewed
with high-speed imagery was a rattlesnake strike [6,7]. For much of the twentieth century, the assumption that a viper strike represents ‘the fastest thing
in nature’ has dominated our understanding of strike performance in snakes
[5– 8]. This assumption was tested with high-speed photography in 1954,
which showed markedly slower strike velocities in rattlesnakes than generally
expected [7]. However, the belief persists that vipers have the fastest strikes
among snakes [9,10]. In order for a strike to be successful—regardless of the
species involved—a snake must contact prey before it escapes or deter a
threat before it causes harm. We used high-speed video recordings to test
whether or not harmless ratsnakes can strike as fast as two species of vipers
that often feed on similar prey and encounter similar kinds of predators.
We compared defensive strike distances, durations, and peak accelerations
and velocities among species using data from 14 Texas ratsnakes (Pantherophis
obsoletus; mean mass + s.e. ¼ 348 + 71 g, snout – vent length ¼ 91 + 5.6 cm),
6 western cottonmouth vipers (Agkistrodon piscivorus; 273 + 15.8 g, 68 +
2.4 cm), 12 western diamond-backed rattlesnakes (Crotalus atrox; 634 + 38 g,
95 + 2.0 cm) and previously published studies [2,4,10,11]. We discuss the accelerations of snake strikes in relation to the known physiological effects
& 2016 The Author(s) Published by the Royal Society. All rights reserved.
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experienced during high accelerations, and we compare strike
durations with mammalian startle-response times.
88 ms
116 ms
0 ms
88 ms
108 ms
2. Material and methods
3. Results and discussion
All snakes struck with very high accelerations (range ¼ 98–
279 ms – 2) and velocities (2.10–3.53 ms – 1), over short distances
(8.6–27.0 cm), and with short durations (48–84 ms). Strike performance was not significantly different among species for
three of the four variables (figure 1; table 1). Snakes displayed
similar strike accelerations (F2,28 ¼ 1.5, p . 0.23), velocities
(F2,28 ¼ 1.8, p . 0.17) and durations (F2,28 ¼ 2.6, p . 0.09).
However, peak strike distance differed significantly (F2,26 ¼
7.2, p , 0.01), with rattlesnakes striking shorter maximum distances than ratsnakes. The lack of corresponding differences in
duration or velocity was owing to peak values coming from
different and variable strikes. There was no difference among
species in maximum accelerations (F2,27 ¼ 0.91, p . 0.38) or
velocities (F2,27 ¼ 1.9, p . 0.16) in models when both mass
and strike distance were included as covariates. Ratsnake
strikes matched or exceeded the performance of viper strikes
in other studies (table 1).
Strike accelerations were similar and impressively high in
all three species that we studied (table 1) and are similar to
those of feeding strikes [1]. Strike accelerations are probably
more important than the peak velocities [2] because strikes
4 cm
Figure 1. Video images of defensive strikes by Pantherophis obsoletus (top)
and Crotalus atrox (bottom) recorded at 250 frames s – 1 with a Keyence
camera (Itasca, IL, USA). (Online version in colour.)
do not involve a chase. These accelerations have two sets of
important consequences. First, the high accelerations keep
the strike durations shorter than the response times of mammalian predators and prey. Mammalian startle responses can
activate muscles in 14–151 ms, and produce observable
movement in as little as 60 –395 ms [12,13]; non-mammalian
response times are not well known. Our results demonstrate
that both harmless and venomous snake strikes can reach
their targets in ca 50 –90 ms, which is often faster than mammals can respond. These strikes are literally faster than the
blink of an eye, which takes 202 ms in humans [14]. However,
strike performance and prey capture in nature may not
always be this high [15]. If strike durations are longer than
the response times of the targets, then strike accelerations
and reaches must be high enough to overcome the predator
or prey once it has initiated a response. Our two highest
strike accelerations (274 ms – 2 from a ratsnake and 279 ms – 2
from a rattlesnake) were approximately an order of magnitude
greater than the jumping accelerations of black-tailed jackrabbits [16], and 30% faster than those of kangaroo rats [17,18],
whose escape behaviour may have evolved in response to
snake predators [18]. The impressive strike performance
across species indicates that selection for rapid strike performance acts on many snakes. Snakes that defend against similar
kinds of predators or feed on similar kinds of prey, such as
small mammals, probably all need to have comparably high
accelerations and short strike durations.
A second important consequence of strike performance
involves physiological tolerances to high accelerations.
Blood flow to the brain may be reduced during rapid headfirst accelerations [19], such as those in snake strikes.
Humans rarely experience accelerations as high as those of
snake strikes. Fighter-jet pilots launching from an aircraft
carrier experience take-off accelerations of only 27 –49 ms – 2
[20]. Without the aid of anti-gravity suits, pilots can lose consciousness at accelerations that are 21 –23% of the values
achieved by our snakes [19]. Even with anti-G suits, pilots
lose the ability to stand up from sitting at accelerations of ca
30 ms – 2 and lose the ability to move their limbs when
Biol. Lett. 12: 20160011
We presented each snake with a target (stuffed glove) and
recorded 4 – 8 defensive strikes per snake. We performed all
trials at 278C. To record strikes, we used a Redlake (San Diego,
CA, USA) MotionScope high-speed camera set at 250 Hz and a
shutter speed of less than or equal to 0.004 s. Depending on
their size, we recorded snakes in an arena measuring 30 30 60 cm or 65 95 57 cm with a scale grid visible in the same
plane as each snake’s strike.
We analysed only strikes that were perpendicular to the
camera. From the high-speed videos, we derived strike distance
as the linear distance between the snake’s snout and the target
at the onset of the strike, and strike duration as the total time
from the initiation of strike movement to first contact with the
target. Maximum velocities and accelerations are the highest
single (frame-to-frame) values obtained from analyses of filtered
coordinates ([2]; 50 Hz cut-off Butterworth filter). We analysed
peak values for each variable (often from different strikes [2])
from each snake and obtained the variables by digitizing
videos using Tracker 4.87 software (Open Source Physics,
http://www.opensourcephysics.org/index.cfm).
We log-transformed the data and treated strike duration (s),
distance (m), maximum acceleration (ms – 2) and maximum velocity (ms – 1) as dependent variables. We used an ANCOVA for
each dependent variable with species as the independent variable
and body mass as the covariate. We also compared maximum
accelerations and velocities with more complex models (one for
acceleration and one for velocity) incorporating mass, species,
strike distance (from the same strike that produced the maximum
values) and their interactions, and subsequently excluded nonsignificant interactions. All model assumptions were tested and
met. For maximum strike distance, two outliers were removed
(standardized residual . 2) to meet assumptions. We used JMP
Pro 11.0.0 and RStudio (0.99.441) for analyses, and determined significance whenever p , 0.05. We lack sufficient data for analysing
muscle cross-sectional areas in these species.
2
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0 ms
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acceleration (ms – 2)
velocity (ms – 1)
distance (cm)
duration (ms)
Pantherophis obsoletus
190 + 11.5 (191)
2.67 + 0.1 (2.7)
16.7 + 1.1 (17)
74 + 2 (75)
Agkistrodon piscivorus
Crotalus atrox
173 + 14.1 (175)
169 + 13.4 (157)
2.98 + 0.1 (3.1)
2.95 + 0.1 (2.7)
13.6 + 1.4 (14)
12.7 + 1.1 (11)a
66 + 5 (66)
71 + 2 (69)
Bitis arietans [10]
72
2.6
21
87
Bothrops sp. [11]
Gloydius shedaoensis [4]
—
—
1.23
1.32
12.6
13
81
—
Trimeresurus albolabris [2]
62.1
1.52
12
85
Indicate pairwise significant differences between P. obsoletus and each viper.
accelerations reach 78 ms – 2 [19]. Acceleration duration and
heart-to-head distance are important in physiological
responses to acceleration [19], as is size [21], which complicates
our ability to understand how acceleration affects other animals. The long distances between the heart and head in
many snakes could make them susceptible to impaired cranial
blood flow during strikes, similar to the impairment that can
occur during climbing [22], but the very short strike durations
may preclude such physiological disruptions. The events at the
end of a strike may have additional effects [19], but the effects
of rapid deceleration and impact on a soft-bodied target are
well tolerated by snakes.
Despite statements in the literature [9,10], vipers do not
strike faster than all other snakes. Ratsnakes and vipers alike
have similarly impressive strikes. Such high accelerations disrupt the physiology of other animals, but are well tolerated
by the snakes and allow them to make contact before their targets can respond. Selection for high strike performance may be
heavily influenced by the target’s sensory and motor response
capacities, which are an understudied aspect of predator–prey
interactions. With so few snakes having been studied thus far, it
seems likely that future research will reveal a greater range of
performance in these diverse and successful predators.
Ethics. This work was approved by the University of Louisiana at
Lafayette’s Institutional Animal Care and Use Committee.
Data accessibility. Complete dataset (https://figshare.com/s/
156f2bf10d5767051cc9).
Authors’ contributions. D.A.P. conceptualized the project, collected data
and drafted the manuscript. B.S. helped with data collection and
writing. B.R.M. helped with project design, data collection and writing. All authors agree to be held accountable for the content herein
and gave approval for publication.
Competing interests. We have no competing interests.
Funding. Supported by NSF IOS-0817647 to B.R.M., Louisiana Board of
Regents (RD-A-34 and ENH-TR-77 to B.R.M. and Doctoral Fellowship
to D.A.P.) and a UL Lafayette STEP grant (to T. Pesacreta).
Acknowledgement. We thank T. Cole, M. Fulbright, P. Hampton, N. Kley,
T. LaDuc, M. McCue, I. Moberly, N. Haertle, M. Perkins, T. Pesacreta,
K. Wadsworth and the UL Lafayette Microscopy Center.
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a
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