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ORIGINAL PAPER
Cold-induced mortality of invasive Burmese pythons
in south Florida
Frank J. Mazzotti
•
Michael S. Cherkiss
•
Kristen M. Hart
•
Ray W. Snow
•
Michael R. Rochford
•
Michael E. Dorcas
•
Robert N. Reed
Received: 17 March 2010 / Accepted: 1 June 2010 / Published online: 15 June 2010
Ó The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract A recent record cold spell in southern
Florida (2–11 January 2010) provided an opportunity
to evaluate responses of an established population of
Burmese pythons (Python molurus bivittatus)toa
prolonged period of unusually cold weather. We
observed behavior, characterized thermal biology,
determined fate of radio-telemetered (n = 10) and
non-telemetered (n = 104) Burmese pythons, and
analyzed habitat and environmental conditions expe-
rienced by pythons during and after a historic cold
spell. Telemetered pythons had been implanted with
radio-transmitters and temperature-recording data
loggers prior to the cold snap. Only one of 10
telemetered pythons survived the cold snap, whereas
59 of 99 (60%) non-telemetered pythons for which we
determined fate survived. Body temperatures of eight
dead telemetered pythons fluctuated regularly prior to
9 January 2010, then declined substantially during the
cold period (9–11 January) and exhibited no further
evidence of active thermoregulation indicating they
were likely dead. Unusually cold temperatures in
January 2010 were clearly associated with mortality of
Burmese pythons in the Everglades. Some radio-
telemetered pythons appeared to exhibit maladaptive
behavior during the cold spell, including attempting to
bask instead of retreating to sheltered refugia. We
discuss implications of our findings for persistence and
spread of introduced Burmese pythons in the United
States and for maximizing their rate of removal.
Keywords Python molurus Florida Everglades
Cold temperatures Invasive species
Mortality Thermoregulation
Introduction
Invasive alien reptiles present an increasing challenge
to conservation of biological diversity (Wilcove et al.
1998; Kraus 2009). The United States dominates the
world trade in live reptiles (Hoover 1998; Franke
and Telecky 2001), and the state of Florida alone
currently hosts more established alien reptiles than
any other state or nation (Meshaka et al. 2004; Kraus
F. J. Mazzotti (&) M. S. Cherkiss M. R. Rochford
Ft. Lauderdale Research and Education Center, University
of Florida, 3205 College Ave., Davie, FL 33314, USA
e-mail: fjma@ufl.edu
K. M. Hart
US Geological Survey, Southeast Ecological Science
Center, 3205 College Ave., Davie, FL 33314, USA
R. W. Snow
National Park Service, Everglades National Park,
40001 State Road 9336, Homestead, FL 33034, USA
M. E. Dorcas
Department of Biology, Davidson College, P.O. Box
7118, Davidson, NC 28035, USA
R. N. Reed
US Geological Survey, Fort Collins Science Center,
2150 Centre Ave., Fort Collins, CO 80526, USA
123
Biol Invasions (2011) 13:143–151
DOI 10.1007/s10530-010-9797-5
2009). Many of the more than 40 exotic reptile
species established in Florida are confined to urban or
otherwise manmade habitats such as backyards and
canals; however, several species (e.g., Burmese
python, Python molurus bivittatus) have successfully
invaded natural habitats. The Burmese python is
native to Southeast Asia and is established in natural
areas in southern Florida such as Everglades National
Park (ENP) (Snow et al. 2007a).
Burmese pythons are habitat and dietary general-
ists (Reed and Rodda 2009). Considerable concern
has been expressed over their impacts on south
Florida ecosystems, particularly in ENP, where
Burmese pythons consume primarily birds and
mammals (Snow et al. 2007b). Ecological and
economic impacts of invasive animals such as
pythons depend on types and magnitude of impacts
(e.g., predation on or competition with native species)
and geographic extent of invasion (Pimentel et al.
2005; Kraus 2009).
Although climate is often proposed as a primary
factor limiting potential geographic extent of invad-
ing species, predicting potential range of an invasive
species is difficult because of a poor understanding of
predictors of invasive ranges (Hayes and Barry 2008),
observations that native range climate may under-
predict invasive range distribution (Fitzpatrick et al.
2007; Duncan et al. 2009), and methodological or
statistical uncertainties (Randin et al. 2006; Beau-
mont et al. 2009; Phillips et al. 2009; Reed and Rodda
2009). As an example of the latter, several attempts
have been made to use native-range climatic vari-
ables to predict potential distribution of Burmese
pythons in the United States, but results have been
inconsistent and even contradictory (Pyron et al.
2008; Rodda et al. 2009; van Wilgen et al. 2009).
The native range of Burmese pythons extends
from tropical zones in Southeast Asia (including
Vietnam, Cambodia, Laos, and Thailand) to warm
temperate zones in China and Nepal (Groombridge
and Luxmoore 1991; Zhao and Adler 1993; Whitaker
and Captain 2004). Winter temperatures may exert
some influence over northern range limits of Burmese
pythons. However, little is known about thermal
physiology and thermoregulatory behavior of pythons
in native habitats (Alexander 2007; Reed and Rodda
2009). A recent record cold spell in southern Florida
(NOAA 2010) provided an opportunity to evaluate
responses of an existing Florida population of
Burmese pythons to a prolonged period of unusually
cold weather.
During 2–11 January 2010, south Florida experi-
enced record cold temperatures (NOAA 2010). During
9–11 January, air temperatures remained at or below
10°C for at least 48 h. A low of 1.6° C on the morning
of 10 January tied an all-time record low for Miami.
Record low maximum air temperatures (9°C) were
recorded in Miami and Naples on 10 January 2010
(NOAA 2010). On the morning of 11 January,
monitoring stations across south Florida recorded air
temperatures ranging from
-4to0°C. West Palm
Beach and Miami both set record lows (0.5 and 2.2°C,
respectively) for 11 January. Between 2 and 11
January, West Palm Beach and Naples set records
for number of days (10) with air temperature lows at or
below 7.2°C (NOAA 2010). A combination of dura-
tion and extremes of this historic cold period resulted
in extensive press coverage of mortality of native and
alien wildlife including manatees, sea turtles, croco-
diles, numerous species of fish, iguanas, and pythons
(Fantz 2010; Quinlan 2010; Waters 2010).
Our objectives were to report on behavior, thermal
biology, and fate of telemetered pythons (n = 10),
fate of non-telemetered pythons (n = 104), and
habitat and environmental conditions experienced
by pythons during and immediately after the historic
cold spell in southern Florida.
Methods
Adult Burmese pythons used for telemetry (n = 10)
were obtained from ENP. For each python we
measured snout-vent length (SVL), total length (TL),
tail girth, and mass at the time of transmitter implan-
tation. Each python was implanted intraperitoneally
(Reinert and Cundall 1982; Hardy and Greene 1999,
2000) with two VHF radio transmitters obtained from
Holohil Systems Ltd. Small transmitters (11 g,
40 9 11 mm) were used for snakes less than 16 kg
and larger transmitters (25 g, 45 9 11 mm) were used
in larger snakes. Transmitter weights were less than
0.5% of each snake’s body mass. We also inserted one
temperature-recording data logger into each snake.
Pythons less than 16 kg received a smaller (3 g,
16 9 6 mm) data logger (Maxim Integrated Products)
and larger snakes received a larger (30 g, 30 9 40 9
10 mm) data logger (Onset Computer Corporation).
144 F. J. Mazzotti et al.
123
Each data logger was programmed to record body
temperature (Tb) every 30 min for one year. Simul-
taneously, we deployed a temperature data logger in
each of two biophysical snake models constructed of
105 9 5 cm copper pipe painted black. Biophysical
snake models have similar thermal properties to live
snakes and allow detailed interpretation of thermal
data (see Peterson et al. 1993 for explanation). Most
pythons were acclimated to the wild well before the
onset of these record cold temperatures and had the
transmitters surgically implanted 2 weeks to 9 months
before this cold event. The two most recent surgeries
were conducted 2 weeks and four and a half weeks
prior to the cold event.
As part of an ongoing python radio-tracking
project, we located each telemetered snake weekly
using fixed-wing aircraft flying at an altitude of
150 m and a speed of 175 kph. During the cold spell,
we supplemented our weekly fixed-wing flights with
helicopter flights and/or on the ground tracking of
each snake 1–2 times per week until confirmation of
death or survival. We ‘walked in’ on all snakes that
had been located from the air for visual identification
and to obtain a GPS location. We accessed snakes on
the ground either via helicopter or by foot from the
nearest vehicle access point. When we sighted a
snake we recorded details on surrounding habitat
(e.g., tree island, marsh, hammock, road, levee),
position, and health. We took in situ pictures of each
python and recovered carcasses of dead snakes for
necropsy and to remove temperature data loggers.
Python Tb’s were compared to model temperatures
using linear regression.
To search for other Burmese pythons throughout
the Everglades landscape, we surveyed hammocks,
ponds, tree islands, canals, levees, roads, and trails by
air, vehicle, boat, and foot between 2 January 2010
and 4 February 2010 (Fig. 1); we also solicited
Fig. 1 Locations in south Florida of python search efforts, python captures (live and dead), and environmental data stations
Cold-induced mortality of invasive Burmese pythons in south Florida 145
123
observations from colleagues and ENP staff. Addi-
tional pythons were encountered while radio-tracking
snakes on the ground.
To determine correlates with python mortalities,
environmental data (air temperature, surface water
temperature, and rainfall) were obtained from ENP
weather station records. No single weather station in
ENP recorded all three environmental variables;
hence, we obtained data from the closest station that
recorded each variable. We used air temperature
recorded at Royal Palm, surface water temperature
from station P35, and rainfall from NP44.
Results
Nine of 10 telemetered pythons (90%; all 8 females
and 1 of 2 males) died during the cold period of 2–11
January 2010. All 10 telemetered pythons were found
on the surface of the ground or in vegetation. The
lone survivor was found in a hardwood hammock
(forest). Six of the nine dead pythons were found on
tree islands, one in a marl prairie, one along an
ecotone between mangrove forest and sawgrass
marsh, and one in a hardwood hammock. Four of
the nine dead pythons were partially covered by
vegetation and one had small bite marks on its body,
presumably from a rodent. Two of the pythons
appeared dead when found but then partially revived
after being transferred to a laboratory maintained at
23°C; one of these died within 24 h, and the second
never fully recovered and was subsequently
euthanized.
One hundred and four non-telemetered pythons
were found between 2 January and 4 February 2010.
Among those 99 had a date and fate (alive or dead)
associated with them. Fifty-nine (60%) were found
alive and 40 (40%) were found dead. Among the dead
were 13 individuals whose death could not be
connected to the cold snap; these snakes were found
after being run over by a mower (n = 2), while being
carried around by an alligator (n = 3), dead on
roadways (n = 3), or killed by humans (n = 5)
(Fig. 2). One hundred and one non-telemetered
pythons were documented as being associated with
a specific habitat type. Among these, 84 (83%) were
found in artificial habitats such as levees, canals, and
roads; of these, 32 (38%) were found dead. Seventeen
(17%) non-telemetered pythons were observed in
natural habitats, and 9 (53%) of these were dead.
Overall, 52 (87%) of 60 surviving non-telemetered
pythons were found associated with artificial habitats.
Sex was determined for 50 of the 104 non-tele-
metered snakes (48%); of the 12 females identified 2
were dead, and of the 38 males identified 4 were
dead. Recovered dead snakes averaged 260.4 cm
total length (TL) (53.7 SD) with a range of 167–
433 cm TL, representing juveniles and adults of both
sexes.
Air and surface water temperatures in ENP for the
record cold spell in January 2010 are presented in
Fig. 3. During a 14-day period starting on 2 January
2010, daily minimum air temperatures fell below
10°C for 12 days and below 15°C for all 14 days.
Maximum daily water temperatures remained below
15°C for most of the period. Both air and water
temperatures fell to their lowest on 11 January
(Fig. 3), and ice was observed on the surface of
shallow water south of Florida City adjacent to
southeastern ENP.
Fig. 2 Summary of
Burmese pythons found
alive and dead between 2
January and 4 February
2010. Cause of death for
pythons summarized here
were as a result of the cold
snap and other
circumstances (snakes
found after being run over
by a mower, while being
carried around by an
alligator or found dead on
roadways)
146 F. J. Mazzotti et al.
123
We recovered temperature data loggers from eight
dead telemetered pythons. Among these individuals,
Tb fluctuated daily with minimum temperatures
below 10°C and maximum temperatures above
30°C prior to 9 January (Fig. 3), similar to data
collected from telemetered pythons in similar habitats
over previous winters and indicative of active ther-
moregulation. The maximal daily python tempera-
tures reflect increased Tb likely from basking
behavior (i.e., Tb matched snake model tempera-
tures). Body temperatures declined substantially
during the coldest period (9–11 January) so that
maximum snake temperatures were \10°C and in
some snakes \5°C on the morning of 11 January.
After 11 January, Tb of the dead individuals tracked
more closely with air temperature, indicating a lack
of active thermoregulation (Fig. 3). Comparison of
Tb of dead pythons and model temperatures between
1,000 and 1,700 h when snakes typically actively
thermoregulate for 4 days before and 4 days after the
cold snap indicated active thermoregulation before
the cold snap (R
2
= 0.277) and a lack of thermoreg-
ulation afterwards (R
2
= 0.025). Although the exact
time snakes died is unknown, a lack of thermoreg-
ulatory behavior after 11 January indicates that the
snakes were either dead or incapacitated at that time.
Fig. 3 Summary of air temperature (Royal Palm), along with
hourly python body temperature (Tb) (a and b), surface water
temperature (P35), rainfall (NP44), and snake model and mean
python Tb from within Everglades National Park from 2
January until 16 January 2010 (c). No one station recorded all
variables, so the three closest stations were used for this
summary. Note that although air temperature rarely exceeded
20°C before 9 Jan, snakes frequently maximized body
temperature during the daytime (i.e., snake temperature
matches snake model temperature). From 9 to 11 Jan, snake
temperatures dropped precipitously as environmental temper-
atures decreased to below 0°C on the morning of 11 January.
Snake thermal patterns began changing on 10 January (arrows)
and after 11 January snake thermal patterns changed so that
maximal snake temperatures more closely matched air
temperature indicating a lack of behavioral thermoregulation
by the snakes. Presumably, most of the snakes died sometime
between 10 and 13 January
Cold-induced mortality of invasive Burmese pythons in south Florida 147
123
Discussion
Unusually cold temperatures in January 2010 were
associated with mortality of Burmese pythons in the
Florida Everglades, and it is possible that the mortality
observed among telemetered pythons was typical of
pythons in natural habitats within ENP. However,
mortality may have been lower in artificial habitats, as
discussed below. It is unclear whether python mortal-
ity was exacerbated by the duration of the cold event,
the extremely cold temperatures at the end of the
period, the sequence of events including a long
persistent rain prior to plunging temperatures, or some
combination of these factors. However, both the lack
of active thermoregulation in telemetered pythons
during and immediately after 11 January (Fig. 3) and
the increase in sightings of dead pythons after 11
January (Fig. 2) suggest that mortality or incapacita-
tion of pythons occurred during this time period.
While it is indisputable that large numbers of pythons
died, attempts to interpret these deaths with respect to
python ecology are complicated by a number of
factors, including behavior, availability of refuges,
and unknown variation in detection probabilities.
Telemetered pythons appeared to exhibit maladap-
tive behaviors during the cold spell, as evidenced by
observations that all 10 individuals were found on the
surface rather than in sheltered refugia. Evidence
from other snakes (and crocodilians) suggests that a
major difference between tropical and temperate
species is their thermoregulatory behavior (Lang
1987; Shine and Madsen 1996). This difference in
behavior between temperate and tropical species has
been well described for crocodilians. Temperate
species such as American alligators (Alligator mis-
sissippiensis) are more likely to deliberately seek heat
and avoid cold than are tropical species such as
American crocodiles (Crocodylus acutus) or specta-
cled caimans (Caiman crocodilus) (Lang 1987;
Brandt and Mazzotti 1990). Lang (1987) described
both alligators and crocodiles basking to maintain Tb
on cool days; however, when temperatures cooled
further tropical crocodiles continued to bask despite
cold air temperatures, whereas warm-temperate alli-
gators avoided basking and instead retreated to
warmer refugia in the water. Brandt and Mazzotti
(1990) confirmed this observation for alligators and
caimans placed in an outdoor enclosure at the
Savannah River Ecology Laboratory in Aiken, South
Carolina. In that study, both species basked during
cool weather but alligators retreated to the water
during freezing temperatures whereas caimans did
not; the latter died as a result. Maladaptive behavior
of basking during freezing temperatures appeared to
be responsible for the deaths of at least some of the
Burmese pythons documented here. Avery et al.
(2010) observed similar results for Burmese pythons
maintained in an outdoor enclosure with thermal
refugia provided in Gainesville, Florida during the
same time period. Avery et al. (2010) ascribed
mortality of 7 out of 9 pythons to cold temperatures.
They also suggested that this cold related mortality
was related to maladaptive behavior of the captive
pythons that neither avoided cold temperatures nor
sought available warm temperatures.
If inappropriate behaviors or intolerance to cold
contributed to cold-induced python mortality, an
obvious question is whether either thermoregulatory
behavior during cold weather or physiological toler-
ance to cold is genetically based. If so, and if there is
heritable variation in behavior or physiology within
the Florida population of Burmese pythons, then the
cold event might have exerted selective pressure on
the population in favor of individuals with greater
physiological cold tolerances or appropriate thermo-
regulatory behavior (i.e., refuge-seeking). The native
range of Burmese pythons includes regions with
winter lows that regularly drop below freezing (Zhao
and Adler 1993; Schleich and Ka
¨
stle 2002), indicat-
ing that populations of this species can persist in
areas cooler than south Florida. However, most
Burmese pythons in the international live animal
trade are sourced from Southeast Asia (Groombridge
and Luxmoore 1991). If pythons from tropical areas
exhibit reduced tolerance to cold as opposed to more
temperate populations, and if Florida pythons are
sourced only from tropical areas, then the Florida
population may have a limited ability to spread
northward (Rodda et al. 2009).
In some snakes, thermoregulatory and other
behavioral tactics appear to be set early in life, and
exposure to novel thermal or habitat conditions later
in ontogeny can provoke maladaptive behaviors
(Kingsbury and Attum 2009; Aubret and Shine
2009). The python population in south Florida has
probably not previously experienced such cold tem-
peratures as those in early 2010; thus the population
may be thermally naı
¨
ve, providing an alternative
148 F. J. Mazzotti et al.
123
hypothesis for observations of cold-induced mortal-
ity. Because no young-of-the-year pythons (live or
dead) have been found since the cold event, it
remains to be seen whether recent experience with
cold weather will affect future behavior of pythons.
All populations of large-bodied pythons and boa
constrictors inhabiting areas with cool winters,
including northern populations of Burmese pythons
in their native range, appear to rely on use of refugia
to escape winter temperatures (Bhupathy and Vijayan
1989; Chiaraviglio et al. 2003; Alexander 2007;
Waller et al. 2007). These refugia are usually burrows
or other subterranean retreats, but deeper water may
also be used. All of the 10 radio-telemetered pythons
were resident in natural habitats of the Everglades
ecosystem, an area characterized by perennially high
water tables and seasonal flooding. Although holes in
the karst limestone underlying much of this area are
abundant, most such holes remain flooded (Loftus
et al. 2001). Dry refugia in ENP tend to be limited to
uprooted trees and dense clumps of grass, two types
of thermal refugia that could easily have their
capacity to insulate reduced by precipitation during
a cold persistent rainfall. The apparently inappropri-
ate behaviors exhibited by some pythons, including
remaining on the surface during inclement weather,
may be the result of a lack of suitable thermal refugia
in Everglades habitats: the sole surviving telemetered
python was found in a large hardwood hammock with
thick vegetation and leaf litter, a habitat that may
have been drier than the small tree islands and marl
prairie used by most pythons that died. In contrast,
artificial habitats, especially raised levees associated
with canals and roads, have abundant refugia (bur-
rows, erosional holes, etc.) that would be more likely
to remain dry and thermally secure. If availability of
refugia in areas with high water tables limits ability to
escape from cold, then we can anticipate occasional
python population reductions in future severe cold
snaps in the Everglades and similar habitats. How-
ever, we would expect higher survival in drier natural
areas with burrows and large tree hollows and in
artificial habitats as described above; paradoxically,
this could allow relatively higher python survival in
areas outside the Everglades which are mostly
located farther to the north. Our observations of
non-telemetered pythons were especially helpful in
illustrating this point: most pythons known to have
survived the cold event were found in elevated areas,
and all 6 pythons found in higher areas to the
northwest of ENP (Collier-Seminole State Park, Big
Cypress National Preserve) were alive. Using radio-
telemetric data from Everglades National Park alone
would likely have provided a skewed view of overall
mortality rates. Data from the non-telemetered
pythons are also likely to be skewed, as a result of
differences in detection probabilities among live and
dead snakes; however, these data provide valuable
insight on habitat-based variability in mortality rates.
If mortality of telemetered pythons is due to
maladaptive behaviors or genetically fixed cold intol-
erance rather than unavailability of suitable refugia,
then range expansion hypotheses for Burmese pythons
in the United States may warrant re-evaluation. The
inappropriate behavior by Burmese pythons during the
cold spell was more like that of a tropical reptile whose
geographic range extends into warm temperate or
subtropical areas than that of a warm temperate reptile
whose geographic range extends into subtropical or
tropical areas (e.g., American alligators). Therefore
we hypothesize that Burmese pythons are not likely to
reach the distributional limits of alligators in the US,
with the caveat that Burmese pythons from a different
genetic background may respond differently. Consis-
tent with this hypothesis, Rodda et al. (2009) stated
that Burmese pythons found in temperate areas of the
native range appear to hibernate in the winter. Our
results suggest that at least some Burmese pythons in
southern Florida did not seek refuge during the cold
spell, but that others appeared to use refugia. We could
not distinguish between python use of refugia for
thermal or other purposes.
Adult female pythons died during the cold event,
likely reducing overall recruitment in the population
in 2010. Removal of reproductively mature individ-
uals from the population via direct mortality or
reduced capacity to reproduce will suppress popula-
tion growth rates (Caswell 1982; Heppell 1998), and
population growth may be further reduced if there is
significant juvenile mortality. Future population
modeling efforts to predict impacts of extreme cold
events should consider the effect of such unantici-
pated removal of individuals on survival and fecun-
dity. Such modeling efforts should also include
stochastic freeze conditions at a rate equivalent to
the historic rate of freezes in south Florida (Storey
and Gudger 1936; Storey 1937), with the caveat that
climate change scenarios should also be incorporated.
Cold-induced mortality of invasive Burmese pythons in south Florida 149
123
Knowledge of detectability of an invasive species is
important for planning control and eradication pro-
grams (Christy et al. 2010). Pythons are less detectable
in natural areas than in artificial habitats, but are more
detectable in both areas after a cold spell. That pythons
are apparently more detectable in artificial habitats is
likely a result of a combination of accessibility and
visibility. Pythons may also have been more visible to
human researchers during this study because the
snakes appeared to increase the amount of time spent
basking after the cold event. Because maximizing
removal rates is an important component of invasive
species control (Christy et al. 2010), timing rapid
responses (Stanford and Rodda 2007) in suitable
habitats during and after unusual climatic events might
increase removal rates of pythons.
Acknowledgments This research was supported by the US
Geological Survey Priority Ecosystems Science program, the
US National Park Service Critical Ecosystems Studies
Initiative, and the South Florida Water Management District.
We thank T. Kiechkefer and T. Hill for tracking and collecting
pythons, J. Vinci for making figures, S. Williams for formatting
the manuscript, and R. Harvey for editing the manuscript.
Everglades National Park agents, park staff, park partners, and
visitors assisted by reporting observations and helping to
recover pythons. We are especially indebted to B. Hill of the
South Florida Water Management District for his reports. This
manuscript was greatly improved by comments from H.
Waddle, P. Schofield and both anonymous reviewers. Permits
and approvals required for this research were obtained from the
US National Park Service and the Animal Research Committee
at the University of Florida. References to non-USGS products
and services are provided for information only and do not
constitute endorsement or warranty, expressed or implied, by
the US Government, as to their suitability, content, usefulness,
functioning, completeness, or accuracy.
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which
permits any noncommercial use, distribution, and reproduction
in any medium, provided the original author(s) and source are
credited.
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