Size, But Not Experience, Affects
the Ontogeny of Constriction
Performance in Ball Pythons
DAVID A. PENNING*
AND SCHUYLER F. DARTEZ
Department of Biology, University of Louisiana at Lafayette, Lafayette, Louisiana
Smaller organisms typically face greater competitive and
predation pressures than their larger counterparts (Carrier,
'96). This differential selective pressure can lead to juvenile
organisms being capable of high levels of performance to offset
their size disadvantage (Herrel and Gibb, 2006). If juveniles are
unable to achieve high levels of performance, some will use
different behaviors than adults in order to accomplish similar
goals. For example, juveniles may use different modes of
locomotion or habitats (Irschick et al., 2000), perform closer to
their maximum capacity (Irschick, 2000), or feed on different
prey (Herrel and Gibb, 2006). This behavioral ﬂexibility can be
an important component of the organism's phenotype (Mehta
and Burghardt, 2008). However, some organisms lack the
behavioral plasticity to employ different behaviors (Greene and
ABSTRACT Constriction is a prey-immobilization technique used by many snakes and is hypothesized to have
been important to the evolution and diversiﬁcation of snakes. However, very few studies have
examined the factors that affect constriction performance. We investigated constriction
performance in ball pythons (Python regius) by evaluating how peak constriction pressure is
affected by snake size, sex, and experience. In one experiment, we tested the ontogenetic scaling of
constriction performance and found that snake diameter was the only signiﬁcant factor
determining peak constriction pressure. The number of loops applied in a coil and its interaction
with snake diameter did not signiﬁcantly affect constriction performance. Constriction
performance in ball pythons scaled differently than in other snakes that have been studied,
and medium to large ball pythons are capable of exerting signiﬁcantly higher pressures than those
shown to cause circulatory arrest in prey. In a second experiment, we tested the effects of
experience on constriction performance in hatchling ball pythons over 10 feeding events. By
allowing snakes in one test group to gain constriction experience, and manually feeding snakes
under sedation in another test group, we showed that experience did not affect constriction
performance. During their ﬁnal (10th) feedings, all pythons constricted similarly and with
sufﬁciently high pressures to kill prey rapidly. At the end of the 10 feeding trials, snakes that were
allowed to constrict were signiﬁcantly smaller than their non-constricting counterparts. J. Exp.
Zool. 9999A:XX–XX, 2016. ©2016 Wiley Periodicals, Inc.
How to cite this article: Penning DA, Dartez SF. 2016. Size, but not experience, affects the
ontogeny of constriction performance in ball pythons (Python regius). J. Exp. Zool. 9999A:XX–
J. Exp. Zool.
Conflicts of interest: The authors declare no conflicts of interest.
Current address: Schuyler F. Dartez's current address is Louisiana,
Department of Wildlife and Fisheries, White Lake Wetlands Conservation
Area, Gueydan, LA 70542.
Correspondence to: David A. Penning, Department of Biology,
University of Louisiana at Lafayette, Lafayette, LA 70504-43602.
Received 8 December 2015; Revised 23 January 2016; Accepted 25
Published online XX Month Year in Wiley Online Library
©2016 WILEY PERIODICALS, INC.
Burghardt, '78), relegating all individuals, regardless of their
size, to the same behavior. For these stereotyped behaviors,
body size likely has a strong inﬂuence on performance
Constriction likely played a key role in the evolution and
radiation of snakes (Greene and Burghardt, '78), making it an
evolutionarily and functionally important mechanism of prey
handling. Constriction movements vary among snakes, and in
general are produced by a highly complex muscle–tendon system
(Mosauer, '35; Moon, 2000; Moon and Mehta, 2007) and involve
muscle contractions that exert pressure on the prey (Greene and
Burghardt, '78; Greene, '97). Constriction pressures have been
hypothesized to prevent breathing (McLees, '28), cause circulatory
arrest (Hardy, '94; Moon, 2000; Moon and Mehta, 2007; Boback
et al., 2015), and spinal damage (Rivas, 2004). High constriction
pressures have also been hypothesized to force blood towards the
brain and disrupt its function(Penning et al., 2015), a phenomenon
known to occur when pilots experience brief blood pooling in the
head (i.e. the red-out effect; Balldin, 2002; Williams et al., 2008).
Constriction postures vary among snake lineages (Greene and
Burghardt, '78) and ontogenetically within some lineages (Mori
'94; Mehta 2003, 2009). Mori ('94) and Mehta (2003) showed that
hatchling colubrid snakes are less effective than adults at forming
constriction coils, and argued that experience may be necessary
for proﬁcient coiling and rapid subjugation of prey. However, this
may not be the case in booid snakes, because they are typically
more stereotyped in their constriction behaviors than colubrid
snakes (Greene and Burghardt, '78), although some booid snakes
may exhibit more plasticity than previously thought (Mehta and
The abilities of young snakes to improve their constriction
performance rapidly may have consequences for growth because
constriction requires energy, time, and risks of injury (Cruz-Neto
et al., 2001; Canjani et al., 2003; Moon and Mehta, 2007).
However, for snakes that display little behavioral variation,
constriction performance may be more dependent upon their size
than on past experience with prey. To our knowledge, only one
study has evaluated intraspeciﬁc scaling of constriction pressures
in snakes (Penning et al., 2015), and the results differed
signiﬁcantly from those on interspeciﬁc scaling (Moon and
Mehta, 2007). The scaling exponent for constriction performance
in two species of large pythons (Python molurus bivittatus and
Python reticulatus) was lower than in previous interspeciﬁc
comparisons (Moon and Mehta, 2007; Penning et al. 2015). Here,
we evaluate the effects of size, sex, and prior experience on
constriction performance in ball pythons (Python regius), and
compare our results to previous work. In the ﬁrst experiment, we
used a large sample size to test the ontogenetic scaling of
constriction performance in ball pythons. In a second experi-
ment, we used multiple clutches of snakes to test the effects of
experience on constriction performance in hatchling ball pythons
over 10 feeding events.
MATERIALS AND METHODS
This research was approved by the University of Louisiana at
Lafayette's Institutional Animal Care and Use Committee. For the
scaling experiment, we used ball pythons (Python regius;N¼40)
ranging from 0.062 kg to 2.8 kg in mass, 42.3cm to 153.6 cm in
snout–vent length (SVL), and 2.0 cm to 8.2 cm in maximum
diameter. Our sample size was determined by the number of
animals available to us in ﬁve private collections. We fed snakes
live and pre-killed rodents (Mus musculus and Rattus norvegicus)
ranging from 2% to 13% of the snake's mass (7.5 0.63%,
mean s.e.m.) based on the owner's feeding regimen. Using
masking tape and duct tape, we attached to each rodent a water-
ﬁlled 2 mL rubber pipette bulb that was connected via ﬂexible
tubing to a Harvard Apparatus Research Grade Blood Pressure
Transducer (Model 60-3002, Harvard Apparatus, Holliston, MA,
USA). In a pilot study, internal and external sensors produced
similar results, and both internal (Moon, 2000; Boback et al.,
2012, 2015) and external (Mehta, 2005; Moon and Mehta, 2007;
Penning et al., 2015) sensors have been used to measure
constriction performance. For pre-killed prey, we systematically
shook the prey using long forceps in order to elicit maximal
constriction effort. Both live and dead prey can elicit powerful
constriction, particularly if dead prey are manually moved to
simulate a struggling response (Moon, 2000; Boback et al., 2012).
During constriction, we counted the number of loops in the coil,
and after the snake released the coil, but before it swallowed the
prey, we measured maximum snake diameter. We removed the
tape and bulb as soon as a coil was released, and all snakes
completed the feeding event.
We used multiple ordinary least-squares (OLS) regression to
analyze the scaling of constriction performance (Smith, 2009).
Peak constriction pressure was the dependent variable, and
snake size (diameter) and the number of loops in the
constriction coil were independent variables. We removed
non-signiﬁcant factors (number of loops used in a coil) from
the analyses and arrived at the ﬁnal model that included only
peak pressure and snake diameter. To evaluate potential
differences between sexes while controlling for the effect of
size, we added sex as a categorical variable to the regression
and tested for differences in the slopes and intercepts between
males and females. To compare our scaling results to previously
published results, we used t-tests on OLS regression slopes from
log-transformed data (Zar, '84).
For the experiment on experience and growth, we used
hatchling ball pythons (N¼22) from eight separate clutches. At
the onset of trials, snakes ranged in mass from 45.5 g to 72.6 g and
in SVL from 41.4 cm to 48.3 cm. All of the snakes were na€
prey and had no constriction experience prior to the trials. Snakes
were randomly assigned to one of two feeding categories: snakes
that would gain experience with prey-handling through time
(“constricting group”) and a control group that would not use
prey-handling behaviors (“non-constricting group”). Snakes in
2PENNING AND DARTEZ
J. Exp. Zool.
the two groups were not signiﬁcantly different in mass at the
beginning of trials (non-constricting group ¼59.4 2.2 g; con-
stricting group ¼55.1 1.8 g; t
¼1.1, P>0.14). We fed all
snakes pre-killed mice for the duration of the experiment, and
controlled relative prey mass (the ratio of prey mass to snake
mass) at ca. 10% (non-constricting group ¼9.1 0.31%; con-
stricting group ¼10.8 0.51%). For the constricting group, we
used the methods described above for the scaling experiment. We
fed each snake one meal per week for 10 weeks, allowing them to
gain both constriction experience and mass under natural
conditions. In rare cases in which the snakes showed no interest
in prey, we lightly sedated and manually fed them using the
methods described below for the non-constricting group in order
to ensure that food intake was comparable to that of all other
individuals. For the non-constricting group, we anesthetized
each snake before each feeding using the open-drop method
(Blouin-Demers et al., 2000), which involved placing each snake
in a plastic enclosure containing a piece of tissue soaked with
1 mL Isoﬂurane anesthetic, which does not signiﬁcantly impact
metabolic rate (McCue, 2006). We left snakes in the chambers
until they ﬁrst lost the righting reﬂex. Under this light sedation,
we manually fed each snake a whole mouse of appropriate size.
During feeding, each snake maintained strong and steady
breathing and heartbeats. We performed this procedure for
9 weeks, during which snakes gained mass but not constriction
experience. During the 10th week of feedings, non-constricting
snakes were allowed to feed and constrict their prey under normal
conditions, during which we recorded peak constriction pressures
using the methods described above.
For the experiment on experience and growth, we had two
analyses. Snakes were not signiﬁcantly different in hatchling
mass at the onset of trials. We used a two-sample t-test to test for a
difference in ﬁnal mass between the snake groups. To evaluate
peak constriction pressure at the end of the feeding trials, and to
capture variation caused by any differences in ﬁnal snake mass,
we performed an OLS regression with snake mass as the
independent variable, pressure as the dependent variable, and
treatment group as a categorical predictor. We used Past 3.08
(Hammer et al., 2001) and RStudio (version 0.99.441; RStudio
Team, 2015) for analyses and considered models signiﬁcant
whenever P<0.05. When applicable, data are presented here as
mean standard error (s.e.m.).
All snakes in the scaling trials readily struck at and constricted
their prey using coils of 1–2 loops. In the anterior part of the coil,
the snake's ventral surface pressed against the prey, whereas in the
posterior part of most coils, the snake's body twisted such that a
lateral surface pressed against the prey (Fig. 1). Once the coil was
applied, no snake considerably readjusted its coil. Peak
constriction pressures ranged from 6.82 to 59.59 kilopascals
(kPa), which incapacitated the prey and in many cases caused
bleeding in the snout of the prey. The number of loops applied
(t¼6.4, P¼0.5; Fig. 2) and their interaction with snake diameter
(t¼1.2, P¼0.45) were not signiﬁcant factors in a full multiple-
regression model (pressure snake diameter loop number;
¼31.51, P<0.0001). For the ﬁnal model (pressure snake
diameter), elevation and slope did not markedly change with the
removal of outliers or high-leverage data points, so we retained
them in the analysis and ﬁgures. As snakes increased in size
(diameter), they exerted signiﬁcantly higher pressures (pressure
6.08 snake diameter 1.49; R
P<0.0001; Fig. 2). Ball pythons exhibit differences in size, diet,
and habitat use based on their sex (Luiselli and Angelici, '98).
Despite these differences, and accounting for differences in mass
by incorporating it into the model, sex did not impact peak
constriction pressure (t¼1.4, P¼0.17; Fig. 2).
Our log-transformed slope for ball pythons (b¼1.08) was
signiﬁcantly lower than the interspeciﬁc slope from Moon and
Mehta (2007) (b¼1.39, t
¼2.82, P<0.005), but signiﬁcantly
higher than that from Penning et al. (2015) for two closely related
species (P. molurus bivittatus b¼0.33 and P. reticulatus b¼0.25,
¼7.57, P<0.0001 and t
¼6.84, P<0.0001 respectively).
Pythons in the constricting group exerted pressures of at least
4 kPa on their ﬁrst constriction attempt. For the model on ﬁnal
peak constriction pressure, ﬁnal snake mass (t¼0.51, P>0.6)
and the interaction (ﬁnal snake mass treatment; t¼0.62,
P>0.54) were not signiﬁcant factors, and were removed from the
model. Despite the increase in statistical power due to removal of
these variables, prior constriction experience did not have a
Figure 1. Juvenile ball python (Python regius, 88 cm snout–vent
length) constricting an adult mouse (Mus musculus) and
generating a peak pressure of 27.9 kPa.
J. Exp. Zool.
CONSTRICTION PERFORMANCE IN BALL PYTHONS 3
signiﬁcant effect on peak constriction pressure (constricting
group ¼12.8 1.4 kPa; non-constricting group ¼14.1 1.5
¼0.37, P>0.54; Fig. 3). The result is the same when
using diameter as a covariate instead of body mass (F
P>0.46). Interestingly, the two groups of snakes were not
signiﬁcantly different in hatchling mass (t
¼1.53, P>0.14), but
snakes that constricted their prey were signiﬁcantly lighter
(75.2 2.25 g) than snakes that were not allowed to constrict
(85.2 3.3 g; t
¼2.45, P<0.03; Fig. 4) at the end of 10 feeding
events. These results are consistent with the high energetic cost of
constriction (Cruz-Neto et al., 2001; Canjani et al., 2003).
Ball pythons consistently constricted prey, displaying behaviors
similar to those reported for Boa constrictor (Mehta and
Burghardt, 2008), P. molurus bivittatus and P. reticulatus
(Penning et al., 2015) and other adult booids (Frazzetta, '66;
Greene and Burghardt, '78). Their stereotyped constriction differs
from the more-variable behaviors reported in Loxocemus, erycine
snakes (Mehta and Burghardt, 2008), and colubrid snakes (Mori,
'94; Mehta, 2003).
Peak constriction pressure in ball pythons depended only on
snake diameter, with larger diameter snakes exerting higher
pressures. There are several possible reasons for the differnt slope
of pressure against diameter in ball pythons than in other species
(Moon and Mehta, 2007; Penning et al., 2015). First, interspeciﬁc
scaling patterns may not reﬂect intraspeciﬁc patterns (Penning
et al., 2015). Second, snakes may use submaximal but fully
sufﬁcient effort in response to prey cues. For example, pressures
exerted by boas constricting sedated rats (20.79 1.99 kPa from
boas of 1.53 0.13 kg; Boback et al. 2015) are signiﬁcantly lower
than those from similarly sized ball pythons in this study
(31.42 2.52 kPa from ball pythons of 1.29 0.15 kg; t
P<0.001). Prey movement affects constriction performance (de
Queiroz, '84; Moon, 2000; Moon and Mehta, 2007; Boback et al.,
2012) and it is possible that the lack of a struggling from sedated
prey led the boas to use submaximal constriction performance.
Ball pythons did not exert higher pressures when they used
more loops of the body, a ﬁnding similar to that of other pythons
(Penning et al., 2015) but different from interspeciﬁc observa-
tions (Moon and Mehta, 2007). Why is peak constriction pressure
unaffected by the number of loops that pythons use? Snake axial
musculature is extraordinarily complex, involving serially
repeated muscles and tendons that span multiple joints, overlap,
interconnect, and appear to act in parallel (Mosauer, '35; Gasc,
'81; Jayne '82; Moon and Gans, '98; Moon, 2000). With such
anatomy, using more of the body in a coil may add more muscles
in parallel and therefore increase force exertion. However, the
increasing surface area over which the force is applied could keep
the pressure exerted on the prey constant. Another possibility is
that variation in individual performance obscures the true
Figure 2. Peak constriction pressures measured in ball pythons of
different sizes (N¼40). The black line indicates the ordinary least-
squares regression (see Materials And Methods). The horizontal
gray bar represents the normal systolic (top of bar) and diastolic
(bottom of bar) blood pressures for Mus musculus and Rattus
norvegicus (Flindt, 2003). Pressures above the shaded bar are likely
to disrupt circulatory and brain function. The number of loops used
in the constriction coil is indicated by marker size.
Figure 3. Mean peak constriction pressures (kPa; black bars) for
snakes with no prior constriction experience (non-constricting
group; diamonds) and snakes with 9 weeks of prior constriction
experience (constricting group; triangles). Each marker represents
an individual data point.
J. Exp. Zool.
4PENNING AND DARTEZ
relationship between loop number and constriction pressure.
However, this is the third python species to display this trend:
P. molurus bivittatus (N¼17; Penning et al., 2015), P. reticulatus
(N¼48; Penning et al., 2015), and P. regius (N¼40; this study). A
third possibility is that not all loops contribute equally to pressure
exertion. The ventral and lateral bending in different parts of a
ball python's coil (Fig. 1) probably use different muscles, or
perhaps some of the same muscles but with different angles of
force exertion that would result in different degrees of force
transmission. Furthermore, using selective muscle contractions
to focus exertion on targeted areas could conserve energy, which
may be beneﬁcial because constriction is energetically expensive
(Cruz-Neto et al., 2001; Canjani et al., 2003).
The high constriction pressures exerted by ball pythons are
likely to impede circulatory function, which would impair the
prey before suffocation (McLees, '28; Hardy, '94, Moon, 2000;
Boback et al., 2015). Stopping low-pressure venous ﬂow is
functionally equivalent to cardiac arrest because it stops all
blood ﬂow (Parmley, '91; Moon and Mehta, 2007). A
constriction pressure of ca. 22 kPa has been shown to reduce
heart rate, increase central venous pressure, and decrease
peripheral arterial blood pressure in rats (Boback et al., 2015). In
this study, 23 of the ball pythons produced pressures similar to
or higher than the pressures shown to disrupt circulation in
prey. In ball pythons of ca. 1 m SVL, constriction pressures
exceeded systolic blood pressures in the rodents and were 50%
higher than the values shown to impede heart function (Boback
et al., 2015). The occurrence of nasal bleeding in the rodents
during many constriction events indicated that blood vessels in
the head can be ruptured during constriction, which suggests
that additional ruptures can occur elsewhere in the head.
Furthermore, pressure can travel through tissues, and neural
tissues are susceptible to pressure (Toth et al., '97; Zhang et al.,
2004; Courtney and Courtney, 2009). Some larger ball pythons
exerted pressures (49–59 kPa) similar to transient pressures
(53–220 kPa for 10–20 ms) that can cause inju ry, neural
damage, and immediately incapacitate mammals (Toth et al.,
'97; Zhang et al., 2004; Courtney and Courtney, 2009), and
pythons can sustain their constriction pressures for several
minutes. It is likely that some constriction pressures are high
enough to not only interfere with circulatory function, but also
force blood towards the head and increase pressure in the brain
(Balldin, ‘02; Williams et al., 2008; Penning et al., 2015).
Unlike many young colubrid snakes (Mori, '94; Mehta, 2003),
ball pythons have sufﬁcient constriction strength from hatching.
Ball pythons constricted with pressures of at least 4 kPa on their
ﬁrst attempt, which are probably sufﬁcient to stop venous blood
ﬂow in the prey (Hardy, '94; Moon, 2000). Our experiments have
shown that constriction pressures exerted by ball pythons are
high enough to kill prey by circulatory arrest and possibly by
forcing blood to the brain. Constriction strength in ball pythons is
solely dependent upon their size, and independent of the number
of loops used in a coil, sex, and their prior experience with
We thank B. Clark, M. Miles, T. Lyon, and N. McCorkendale for
providing access to pythons and B. Moon for providing
equipment, reviewing and editing drafts of the manuscript, and
for fruitful discussions on constriction. We also thank P. Leberg
and J. Neigel for their contributions to the experimental design of
the experience trials and N. Haertle for his advice on snake
anesthesia. We would also like to thank C. Denesha, P. Hampton, I.
Moberly, M. Perkins, and A. Rabatsky for their helpful support,
guidance, and suggestions.
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