Incubation Temperature and Sex Ratio of a Python bivittatus (Burmese Python) Clutch Hatched in Everglades National
Park, Florida
Alexander J. Wolf, Theresa M. Walters, Michael R. Rochford, Ray W. Snow, and Frank J. Mazzotti
Southeastern Naturalist, Volume 15, Special Issue 8 (2016): 35–39
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35
Incubation Temperature and Sex Ratio of a Python bivittatus
(Burmese Python) Clutch Hatched in Everglades National
Park, Florida
Alexander J. Wolf
1, Theresa M. Walters1, Michael R. Rochford1, Ray W. Snow2,
and Frank J. Mazzotti1,*
Abstract - We describe characteristics of a Python bivittatus (Burmese Python) nest from
observations made from December 2008 through August 2009 in Everglades National Park,
Homestead, FL. The nest hatched on 28 July with a 77% hatching success. The female lost
54% of her body weight while breeding, laying, and incubating eggs, and abandoned the
nest 3 days before eggs began hatching. Egg-mass temperature was 26.29–31.41 °C (mean
= 28.07 °C), and was more stable than the temperature in direct sun above the nest, which
ranged from 20.81 °C to 45.70 °C (mean = 28.45 °C). Egg-mass temperature was likely
buffered from extreme heat by adjacent vegetation, where the temperature ranged from
23.19 °C to 30.48 °C (mean = 27.05 °C) and from extreme cold by shivering thermogenesis.
Of successful hatchlings, 9 were male and 8 were female.
Report and Discussion
Python bivittatus Kuhl (Burmese Python) is native to southeastern Asia. The
species is considered invasive in Florida and has established an expanding breeding
population in the southern part of the state (Snow et al. 2007a, b). Here we report
natural history data we collected from a breeding adult female Burmese Python that
we collected as part of a larger study to test trap prototypes (Fig. 1).
We radio-telemetered the female Burmese Python (snout–vent length [SVL] =
2.64 m; total length [TL] = 2.98 m; weight = 15.014 kg as of 02 December 2008),
and placed her in an outdoor enclosure (~ 100 m x 100 m) in Everglades National
Park (ENP), FL. We implanted 2 radio tags (Holohil AI-2, Holohil Systems, Tarp,
ON, Canada) and a temperature-data logger (TidbiT TBI32-20+50, Onset Computers,
Bourne, MA) in this snake before releasing her in the enclosure on 08 December
2008. We also radio-tagged 1 additional adult female and 2 adult male Burmese Pythons.
Additional wild Burmese Pythons may have also existed within the enclosure.
We checked the trap prototypes and used a Yagi antenna to approximate the location
of all Burmese Pythons in the enclosure each day. Researchers circled the enclosure
and checked the radio-transmitter signals from all sides to verify the presence of
all tagged Burmese Pythons in the enclosure. We were using this enclosure to test
the efficacy of trap prototype designs; thus, we tried to obtain visual confirmation
of the snakes’ locations when possible. However, in order to minimize contact with
the Burmese Pythons, this verification wasn’t always possible. On 23 July 2009, we
1University of Florida, Fort Lauderdale Research and Education Center, 3205 College Avenue,
Fort Lauderdale, FL 33314-7719. 2Everglades National Park, 40001 State Road 9336,
Homestead, FL 33034. *Corresponding author - fjma@ufl.edu.
Manuscript Editor: John Placyk
Everglades Invasive Species
2016 Southeastern Naturalist 15(Special Issue 8):35–39
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A.J. Wolf, T.M. Walters, M.R. Rochford, R.W. Snow, and F.J. Mazzotti
2016
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Vol. 15, Special Issue 8
observed the aforementioned female incubating a clutch of eggs, and we began daily
visual observations of the nest and female Burmese Python to collect much-needed
data on life-history characteristics of Burmese Pythons in Florida.
On 24 July 2009, the female was still coiled around eggs, but on 25 July 2009 we
discovered that she was no longer with the eggs. We constructed a small plywood
enclosure (~2.4 m x 2.4 m x 1.2 m high) around the nest and covered it with a screen
lid. We created a hole (~30 cm x 30 cm) in the middle of the screen lid and installed
a wooden ramp from the ground outside the plywood enclosure to the top of the enclosure.
With this design, we sought to retain any Burmese Python hatchlings while
still allowing the female to return to the nest. We inserted 3 temperature-data loggers:
1 between 2 eggs in the egg mass, 1 in adjacent vegetation, and 1 suspended
~50 cm above ground at nest. Loggers recorded temperatures every 30 min. On 28
July 2009, we checked the nest and observed at least 2 eggs pipped. The last egg
hatched on 30 July. We collected all hatchlings and unhatched eggs, and counted
the number of hatched shells to ensure that we recovered all hatchlings.
Egg-mass temperatures were similar to those recorded in vegetation next to the
egg mass (Table 1). The nest contained 22 eggs, of which 18 appeared developed
Figure 1. Timeline of events related to the female Python bivittatus (Burmese Python) and
nest described in this study.
Table 1. Mean, minimum (min), maximum (max), standard deviation (SD), and sample size (n) for
temperatures in the egg mass, in vegetation next to the egg mass, and temperature in direct sun above
a Python bivittatus (Burmese Python) nest. Temperatures were recorded every 30 min between 1828
on 25 July 2009 and 0758 on 30 July 2009.
Mean (°C) Min (°C) Max (°C) SD (°C) n
Egg mass 28.07 26.29 31.41 1.17 220
Vegetation 27.05 23.19 30.48 1.38 220
Above nest 28.45 20.81 45.70 6.71 220
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2016 Vol. 15, Special Issue 8
and 17 hatched successfully, resulting in a 0.77 success rate. For the 17 pythons that
successfully hatched, mean SVL was 51.9 cm (± 1.9 cm SD), mean TL was 59.7 cm
(± 2.2 cm SD), and mean mass was 120.1 g (± 5.3 g SD). Nine hatchlings were male
and 8 were female. We determined that 4 eggs either terminated early in development
or were unfertilized, because they were darker than viable eggs, shriveled, and
lacked a hatching slit. One egg failed to hatch; the hatchling inside the egg was 43.2
cm SVL, 49.7 cm TL, and appeared fully developed. Over the course of breeding
and incubation, the female lost 54% of her body weight—from a weight of 15.01 kg
on 02 December 2008 to 6.91 kg on 12 August 2009. We radio-tracked this female
to a location 18.8 m away from the nest on 25 July 2009, and she was 35.0 m from
the nest on 28 July 2009.
Allowing the nest to hatch in situ provided us with new data on egg survival,
hatching rate, clutch size, and sex ratio and size of hatchlings. These data represent
a small step in gaining information that moves us closer to constructing an accurate
population model that will help managers target age classes for effective Burmese
Python control (Sakai et al. 2001).
Nest temperatures and ambient air temperatures were similar to those recorded
at another wild Burmese Python nest in Florida in 2008 (Snow et al. 2010). Both
nests exhibited mean temperatures of ~28–29 °C, which is lower than the range
of 32–34 °C reported for captive nests (Hutchison et al. 1966). Pyron et al. (2008)
suggested that it is too cold for Burmese Pythons to breed in much of the US.
However, both wild Burmese Python nests in Florida for which temperatures were
recorded (our current study and Snow et al. 2010) successfully hatched offspring
despite lower mean temperatures than those recommended for captive incubation,
and experienced ambient air temperatures as low as 20.8 °C and nest temperatures
as low as 26.3 °C. Females likely insulate eggs from ambient temperatures
through thermogenesis (Snow et al. 2010), but there must be limits to their ability
to insulate. Our data and that of Snow et al. (2010) from wild nesting Burmese Pythons
in southern Florida indicate that the lower thermal limit for reproduction is
not being exceeded.
Lower mean temperatures of wild nests in Florida may account for the observed
hatching-success rate. Hatching success for this clutch (77%) was slightly lower
than observations of 95% in 2 nests of a related species, P. molurus (L.) (Indian
Python), in India (Ramesh and Bhupathy 2010). Although the rate we observed is
the same or higher than rates observed for other python species in laboratory studies
(Shine et al. 1997) and captivity (Walsh and Murphy 2003), and for other species of
snakes (Cunnington and Cebek 2005, Powell et al. 2010), it is possible that cooler
temperatures may have reduced hatching success in this nest relative to hatch rates
reported by Ramesh and Bhupathy (2010). However, it is unclear whether the relatively
low hatch-rate we observed is due to temperature, inbreeding, breeding with
an unfit male, use of stored sperm, or other unknown factors.
In India, female pythons left their nests earlier than we observed (11–13 d and
3 d prior to hatching, respectively). Ramesh and Bhupathy (2010) hypothesized females
left the nest to facilitate an increased oxygen demand by developing embryos.
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The female in this study lost 54% of her body mass post-parturition, which is greater
than what has been observed in other species such as Nerodia sipedon insularis
(L.) (Lake Erie Watersnake), which loses 28.2–45.5% (mean = 38.2%; King 1986).
The loss of mass we observed is also greater than the 30.0–38.1% (mean = 35.0%)
mass lost by Burmese Pythons in another study (Brashears and DeNardo 2013). Our
finding may be due to increased an metabolic rate during shivering thermogenesis
necessary for incubating eggs under cooler ambient temperatures in Florida (Snow
et al. 2010).
Prior to this study, numerous hatchlings had been recovered from ENP and surrounding
areas, but neither their age nor size upon hatching were known. Burmese
Python hatchling size, especially in relation to that of native snakes, presumably has
an effect on which native predators prey upon them, and thus, will impact Burmese
Python survival rates. Additionally, hatchling size may affect competition with
native snakes for prey; interspecific competition is often strong enough to cause
divergent body size among snake species (King 1986). Information associated with
this observation will serve to better inform risk assessments and management actions
for this invasive species.
Acknowledgments
We thank the South Florida Water Management District, the US National Park Service
(Critical Ecosystems Studies Initiative), the US Geological Survey (Priority Ecosystems
Sciences), and the University of Florida for their continued support of and funding for Burmese
Python projects in southern Florida.
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