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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 Southeastern Naturalist A.J. Wolf, T.M. Walters, M.R. Rochford, R.W. Snow, and F.J. Mazzotti 2016 36 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 Southeastern Naturalist 37 A.J. Wolf, T.M. Walters, M.R. Rochford, R.W. Snow, and F.J. Mazzotti 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. Southeastern Naturalist A.J. Wolf, T.M. Walters, M.R. Rochford, R.W. Snow, and F.J. Mazzotti 2016 38 Vol. 15, Special Issue 8 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. Literature Cited Brashears, J.A., and D.F. DeNardo. 2013. Revisiting python thermogenesis: Brooding Burmese Pythons (Python bivittatus) cue on body, not clutch, temperature. Journal of Herpetology 47:440–444. Cunnington, G.M., and J.E. Cebek. 2005. Mating and nesting behavior of the Eastern Hognose Snake (Heterdon platirhinos) in the northern portion of its range. American Midland Naturalist 154:474–478. Hutchison, V.H., H.G. Dowling, and A. Vinegar. 1966. Thermoregulation in a brooding female Indian Python, Python molurus bivittatus. Science, New Series 151:694–696. King, R.B. 1986. Population ecology of the Lake Erie Water Snake, Nerodia sipedon insularum. Copeia 757–772. Powell, C., D.J. Stevenson, M. Smith, and J.B. Jensen. 2010. A new clutch-size record for the Mud Snake (Farancia abacura). Southeastern Naturalist 9:177–178. Pyron, R.A., F.T. Burbrink, and T.J. Guiher. 2008. Claims of potential expansion throughout the US by invasive python species are contradicted by ecological-niche models. PLoS ONE 3:e2931. doi: 10.1371/journal.pone.0002931. Ramesh, C., and S. Bhupathy. 2010. Breeding biology of Python molurus molurus in Keoladeo National Park, Bharatpur, India. Herpetological Journal 20:157–163. Sakai, A.K., F.W. Allendorf, J.S. Holt, D.M. Lodge, J. Molofsky, K.A. With, S. Baughman, R.J. Cabin, J.E. Cohen, N.C. Ellstrand, D.E. McCauley, P. O’Neil, I.M. Parker, J.N. Thompson, and S.G. Weller. 2001. The population biology of invasive species. Annual Review of Ecology and Systematics 32:305–332. Southeastern Naturalist 39 A.J. Wolf, T.M. Walters, M.R. Rochford, R.W. Snow, and F.J. Mazzotti 2016 Vol. 15, Special Issue 8 Shine, R., T.R.L. Madsen, M.J. Elphick, and P.S. Harlow. 1997. The influence of nest temperatures and maternal brooding on hatchling phenotypes in Water Pythons. Ecology 78:1713–1721. Snow, R.W., K.L. Krysko, K.M. Enge, L. Oberhofer, A. Warren-Bradley, and L. Wilkins. 2007a. Introduced populations of Boa constrictor (Boidae) and Python molurus bivittatus (Pythonidae) in southern Florida. Pp. 416–438, In R.W. Henderson and R. Powell (Eds.). The Biology of Boas and Pythons. Eagle Mountain Publishing, Eagle Mountain, UT. 438 pp. Snow, R.W., V.M. Johnson, M.L. Brien, M.S. Cherkiss, and F.J. Mazzotti. 2007b. Python molurus bivittatus: Nesting. Herpetological Review 38:93. Snow, R.W., A.J. Wolf, B.W. Greeves, M.S. Cherkiss, R. Hill, and F.J. Mazzotti. 2010. Thermoregulation by a brooding Burmese Python (Python molurus bivittatus) in Florida. Southeastern Naturalist 9:403–405. Walsh, T., and J.B. Murphy. 2003. Observations on the husbandry, breeding, and behavior of the Indian Python. International Zoo Yearbook 38:145–152.