Isotopic Incorporation of Carbon and Nitrogen in Invasive Burmese Pythons (Python molurus bivitattus)

Isotopic Incorporation of Carbon and Nitrogen in Invasive Burmese Pythons (Python molurus bivitattus) | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 7 July 2025 V1 Latest version Share on Isotopic Incorporation of Carbon and Nitrogen in Invasive Burmese Pythons (Python molurus bivitattus) Authors : Katherine Davis 0000-0002-8090-3749 [email protected] , Andrea Currylow 0000-0003-1631-8964 , Amy Yackel Adams , Christina Romagosa , and Hannah B. Vander Zanden 0000-0003-3366-5116 Authors Info & Affiliations https://doi.org/10.22541/au.175191395.50705083/v1 224 views 62 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract ¬¬The carbon and nitrogen stable isotope composition of animal tissues provide valuable insights into foraging ecology and trophic interactions. However, dietary isotopic composition varies widely among organisms and tissues. This study established carbon and nitrogen trophic discrimination and tissue-to-tissue conversion factors in Burmese pythons (Python molurus bivittatus) and explored tissue turnover dynamics. Trophic discrimination conversion factors (Δ 13 C diet-tissue and Δ 15 N diet-tissue ) were established by analyzing shed skin from captive pythons and comparing to known diet values. Tissue-to-tissue conversion factors between muscle, dermis, and scale and muscle and shed were determined from paired tissue samples from the remains of stored snake tail clips. Tissue turnover time was investigated with opportunistically collected samples: red blood cells from wild snakes held in captivity on a known diet. The Δ 13 C diet-tissue and Δ 15 N diet-tissue values between diet and shed were significantly different between adult and subadult snakes. For Δ 13 C diet-tissue , adult shed averaged 0.8‰ higher than diet while subadult shed averaged -1.4‰ lower than diet. Both age groups had higher mean δ 15 N values than their diet, resulting in positive Δ 15 N diet-tissue values (subadults = 2.3 ± 0.4 ‰, adults = 4.3 ± 0.2 ‰). Conversion equations were established between muscle, dermis, scale, and shed skin. Tissue turnover time could not be determined over the duration of available samples. Our results highlight the importance of taking ontogeny into consideration when interpreting Burmese python stable isotope composition. The isotopic metrics measured here can be used in future ecological research on Burmese pythons and expand the limited information on isotopic dynamics of herpetofauna. Isotopic Incorporation of Carbon and Nitrogen in Invasive Burmese Pythons ( Python molurus bivitattus ) Abstract.\(-\)­­The carbon and nitrogen stable isotope composition of animal tissues provide valuable insights into foraging ecology and trophic interactions. However, dietary isotopic composition varies widely among organisms and tissues. This study established carbon and nitrogen trophic discrimination and tissue-to-tissue conversion factors in Burmese pythons ( Python molurus bivittatus ) and explored tissue turnover dynamics. Trophic discrimination conversion factors ( ∆ 13 C diet-tissue and ∆ 15 N diet-tissue ) were established by analyzing shed skin from captive pythons and comparing to known diet values. Tissue-to-tissue conversion factors between muscle, dermis, and scale and muscle and shed were determined from paired tissue samples from the remains of stored snake tail clips. Tissue turnover time was investigated with opportunistically collected samples: red blood cells from wild snakes held in captivity on a known diet. The ∆ 13 C diet-tissue and ∆ 15 N diet-tissue values between diet and shed were significantly different between adult and subadult snakes. For ∆ 13 C diet-tissue , adult shed averaged 0.8‰ higher than diet while subadult shed averaged -1.4‰ lower than diet. Both age groups had higher mean δ 15 N values than their diet, resulting in positive ∆ 15 N diet-tissue values (subadults = 2.3 ± 0.4 ‰, adults = 4.3 ± 0.2 ‰). Conversion equations were established between muscle, dermis, scale, and shed skin. Tissue turnover time could not be determined over the duration of available samples. Our results highlight the importance of taking ontogeny into consideration when interpreting Burmese python stable isotope composition. The isotopic metrics measured here can be used in future ecological research on Burmese pythons and expand the limited information on isotopic dynamics of herpetofauna. Introduction.–Stable isotope analysis (SIA) is a useful tool for ecologists interested in understanding facets of resource partitioning, trophic position, and dietary influence that may otherwise be challenging to measure (Gannes et al., 1997; Newsome et al., 2007; M. J. Vander Zanden et al., 1999; Wainright et al., 2021). Identifying sources of variation in the isotopic values within and among species as well as differences between consumer and diet is fundamental to applying SIA in trophic ecology (Boecklen et al., 2011; DeNiro & Epstein, 1978; Martínez del Rio & Carleton, 2012). Trophic discrimination factors, tissue-tissue conversion factors, and tissue turnover times, which are components of interpreting animal diet and habitat use, have frequently been measured in mammals and birds (Kelly, 2000; Lesage et al., 2002; Roth & Hobson, 2000; Tieszen et al., 1983) but are largely unknown for herpetofauna, particularly in snakes (Dalerum & Angerbjörn, 2005; Durso et al., 2022; Fisk et al., 2009; Sandfoss et al., 2021; M. J. Vander Zanden et al., 2015). Our study addresses the limited availability of reptilian diet and tissue isotopic metrics by combining a series of Burmese python ( Python molurus bivittatus) datasets . Burmese pythons are large constrictor snakes native to southeast Asia (Pope, 1961). They were introduced into Florida through the pet trade, and unintentional escapes coupled with intentional releases have resulted in a large invasive population over much of south Florida (Dorcas & Willson, 2011; Enge et al., 2007). Pythons in this system are highly effective predators, which has motivated a number of studies to characterize their impact to native species (Dorcas et al., 2012; Dove et al., 2011; Romagosa et al., 2023). Pythons in south Florida do not experience the seasonal fasting events due to major seasonal shifts in prey availability that they would in their native southeast Asian ecosystems. In Florida, invasive pythons have constant access to prey and thus feed year-round, rarely experiencing a fasting event, which may result in faster metabolic rates (Card et al., 2018; Dorcas et al., 2012). Determining isotopic metrics in pythons will provide tools to better understand their effects on native animal communities in this ecosystem, which may reveal trends or information to effectively manage their spread or lessen their impact in areas of new spread. Trophic Discrimination Factor The relative ratios of carbon and nitrogen isotopes present in consumer tissues reflect diet. Isotopic composition is influenced by factors such as diet source, metabolic breakdown, and assimilation of prey (DeNiro & Epstein, 1978; Gannes et al., 1997). The shift in isotope ratios during these metabolic processes is the basis for measuring the trophic discrimination factor or TDF (Bastos et al., 2017). Accurate TDFs are needed in applications to quantify dietary resource use, trophic positioning, and primary producers of a system. Organismal carbon isotope compositions, or δ 13 C values, are largely driven by diet source at the primary production level in the system (C 3 -based, C 4 -based, marine-based or a mixture) (Fry, 1988; Post, 2002; Stephens et al., 2023a). The nitrogen isotope composition, or δ 15 N values, tend to increase from diet to consumer, and trophic position is the primary driver (Minagawa & Wada, 1984; Post, 2002). In the past, discrimination factors of 3-4‰ per trophic level for nitrogen, and 1‰ for carbon (Caut et al., 2009; Post, 2002) have been commonly applied in diet reconstruction studies using isotopic analysis. However, many researchers suggest moving away from applying these general estimates to instead using species-specific TDFs in order to avoid substantial errors in the quantifying the dietary proportions or trophic position of wild animals using stable isotope approaches (Bastos et al., 2017; Davis & Vander Zanden, n.d.; Healy et al., 2018; Newsome et al., 2010; Stephens et al., 2023b). A recent meta-analysis of TDFs reported large ranges in Δ 13 C diet-tissue (−5.1‰ to 9.1‰) and Δ 15 N diet-tissue values (−3.3‰ to 9.7‰) across taxa and among different tissues of the same species (ranging >9‰), further underscoring that values from the same, or similar, species may not be appropriate substitutions (Stephens et al., 2023b). Trophic discrimination can also vary with growth rate and body size, resulting in differences between age classes within species (Radloff et al., 2012; Sweeting et al., 2007; H. B. Vander Zanden et al., 2012; Villamarín et al., 2018). Thus, an accurate TDF is essential in mixing models to elucidate resource partitioning and usage. Calculating TDFs from animals with known dietary proportions and isotopic compositions is ideal, which typically only occurs in captive animals(Beltran et al., 2016). Directly observing foraging behavior in the wild can be challenging for many reasons, and a controlled captive setting eliminates several uncertainties surrounding the source and composition of wild diet. Tissue-Tissue Conversion Factors Utilizing δ 13 C and δ 15 N values as nutritional biomarkers can provide long-term information about diet (from prey sources consumed over months or years), which may be particularly useful when investigating the effects of invasive species (McCue et al., 2020; Pujol-Buxó et al., 2019; M. J. Vander Zanden et al., 1999). However, δ 13 C and δ 15 N values vary widely among organisms and among the tissues of an individual organism because of life stage, metabolic routing, nutritional status, and diet composition (Martínez del Rio et al., 2009). Muscle tissue is often selected for SIA due to its lower isotopic variability compared to most other tissues, large available tissue biomass, and because its metabolic fractionation behavior is comparatively well understood amongst tissues (Pinnegar & Polunin, 1999; Stephens et al., 2023b; Sweeting et al., 2007). Sampling muscle tissue in a living organism requires an invasive biopsy, though it is not problematic if sampling is conducted on carcasses. Invasive tissue sampling is not ideal if the study species is rare or endangered, cryptic, or if the study requires multiple samples from the same individual over time (Vašek et al., 2017). Less invasive tissue types, such as fin and scale clips in fish (Fincel et al., 2012; Vašek et al., 2017) and hair in mammals (Roth & Hobson, 2000; Tieszen et al., 1983), are correlated with muscle, allowing these tissues to be used as a substitute for muscle, occasionally with mathematical correction, depending on the correlation between the tissues. Information on the relationship between isotope values of muscle and other tissues that can be collected less invasively is still lacking for many species (Mancuso et al., 2022; Sinnatamby et al., 2008). Snake sheds provide an easily accessible and routinely discarded large piece of tissue, though the time between sheds varies between species and throughout growth stages (Rutland et al., 2019), and the foraging window during which the tissue was formed should be taken into consideration. Tissue-to-tissue conversion factors are usually determined via linear regressions that relate one tissue to another, enabling the integration of data collected from diverse tissues (Fincel et al., 2012; H. B. Vander Zanden et al., 2014; Winter et al., 2019). However, the metabolic nature of a tissue (inert after growth or continuously turning over) and equilibrium conditions might vary among tissues. Conversion factors not only facilitate the combination of information but also open new avenues for future research, particularly when sampling museum or other archived collections with varying tissue sources. Tissue Turnover Turnover time provides a temporal context to the isotopic values in consumer tissues. Understanding the time it takes for consumed diet to be assimilated into tissues is essential to make reliable estimates of consumer foraging, and this period often reflects a longer window of integration compared to traditional gut content analysis. The isotopic half-life is the time it takes for tissue to reach 50% equilibration with the diet, and turnover time is approximated by four half-lives (Hobson & Clark, 1992; Rosenblatt & Heithaus, 2013; Thomas & Crowther, 2015). A comprehensive literature synthesis on tissue turnover estimates revealed significant effects of body mass and taxon group for ectotherms, though reptiles only represented 7% of this sample, with values considered for only a single species of snake (M. J. Vander Zanden et al., 2015). Therefore, more controlled studies on turnover time in snakes would benefit the interpretation of isotopic data from wild populations. Study Objective Given the lack of published reptilian isotopic incorporation metrics that that could be applied to pythons, we expect this study will benefit future research of the invasion ecology of this large invasive constrictor in south Florida. This study had three objectives: 1) to determine the TDF between shed skin and diet in captive Burmese pythons, 2) to determine the tissue-to-tissue conversions for δ 13 C and δ 15 N stable isotope values between shed, scale, dermis, and muscle for wild Burmese pythons, and 3) to investigate the turnover time in red blood cells from opportunistically collected samples from captive Burmese pythons. Methods Sample Collection .\(-\) Trophic discrimination factor Shed skin (a combination of cornified ß- keratin epidermal scale and flexible α- keratin between scales (Landmann, 1986)), and dietary records from captive pythons were obtained from The Conservancy of Southwest Florida (n = 4), Cleveland Metro Parks Zoo (n = 1), and Little Rock Zoo (n = 2). From the seven individuals, two (and in one case, three) sheds were collected for a total of 15 sheds from both subadults (< 185cm SVL) and adults (≥ 185 cm SVL). Cleveland Metro Parks Zoo and Little Rock Zoo both purchase their feeder items from RodentPro, and we purchased and homogenized equivalent feeder items (large mice, extra-large mice, extra-large rats, Guinea pig pups, and medium rabbits from RodentPro in April 2023. RodentPro lists the nutritional composition of each organism and maintains organisms on consistent diets, as they are a primary provider of feeder animals to North American zoos (Dierenfeld et al., 2002). The Conservancy of Southwest Florida ordered their feeder items from Lindsey Reptiles, and hair samples from their frozen diet items (large mice, rat pups, small rats, medium rats, and large rats) were obtained from the Conservancy’s supply in July 2023. Tissue-tissue conversion factors Subsamples were taken from U.S. Geological Survey (USGS)-provided tail clips (the ~2-6” terminal part of the body) cut from 35 randomly selected Burmese python carcasses that were removed from the wild in south Florida between 2013-2021 and stored frozen until October 2022. Ten tail clips were from pythons that were shedding at time of death. All individuals had a snout vent length (SVL) >185cm and were considered adults (Currylow et al., 2022; Willson et al., 2014). A 0.5-2” piece of tail containing bone, muscle, dermis, scale, and/or shed, was subsampled using meat shears sanitized with 70% isopropyl alcohol swabs between samples. Individual subsamples were stored in Whirlpak bags and transported to the University of Florida on ice, where they were stored frozen until sample preparation. In this study, “scale” is defined as the largely unpigmented layer of cornified tissue on the skin surface comprised almost exclusively of ß- keratin, and “dermis” is the entire layer beneath the scale including the fibrous connective tissue that connects to the muscle (Landmann, 1986). Muscle, dermis, and scale were collected from the tail clips of non-shedding individuals using a scalpel and/or forceps. If shedding at time of death, scale and dermis were not collected due to uncertainly of where the python was in the shedding process, so only muscle and shed were collected for those nine individuals. Tissue turnover We collected 32 subsamples of red blood cells from four individual snakes that were live-captured in 2019 from the wild and subsequently held in captivity in Homestead, FL. Initial blood samples were prior to being fed in captivity. The initial blood sample from each python occurred at varying intervals before the captive diet was initiated (EPE.07: 20 days, EPE.10: 12 days, EPS.50: 21 days, EPS.54: 19 days,). In subsequent analyses, “Day 0” represents the first day of the captive diet, when all snakes ate the food offered. Blood samples were collected from each individual on eight occasions, with the last one ranging from 170-174 days from the first food eaten in captivity. Samples were centrifuged in lithium heparin to separate blood fractions, and red blood cells were stored at room temperature in tubes with 90-99% ethanol. Lithium heparin had no measured effect on the stable isotope values of red blood cells in elasmobranch studies (Kim & Koch, 2012). Ethanol had no significant effect on whole blood samples of quail or sheep (Hobson et al., 1997). In November 2022, sub-samples of red blood cells were taken with stainless steel spatula from LH tubes and placed in glass vials with 95% ethanol. The spatula was decontaminated with 95% ethanol and sterilized in a microbead sterilizer at 300°C between samples. The feeder items (rats) given during the captive period were sourced from RodentPro, and hair samples from rodents in this order were collected in April 2023. Samples were stored at the University of Florida until processing. Sample processing .\(-\) Tissue-tissue conversion factors All tissues were dried at 60°C for at least 24 hours and homogenized with a mortar and pestle (the whole body of diet items (Davis & Vander Zanden, n.d.) and muscle and red blood cells from pythons), or prior to drying, diced with a scalpel blade (hair from diet items and dermis, scale, and shed from pythons). Between 0.45 and 0.60 mg of each sample was placed in individual 5 × 9-mm tin capsules for analysis. The C:N ratios among all python tissues (Table 1) were less than the ratio suggested for lipid removal or mathematical correction for terrestrial animals (Post et al 2007); thus, no tissues were lipid extracted. Hair samples of diet items were not lipid extracted but were later corrected mathematically to represent the whole organism (see data analysis section). Stable isotope analysis All samples were analyzed at the Stable Isotope Mass Spec Lab in the Department of Geological Sciences at the University of Florida using a continuous-flow Delta V Advantage isotope ratio mass spectrometer (ThermoElectron, Bremen, Germany) coupled with a Conflo II interface (ThermoFinnigan) linked to a Carlo Erba NA 1500 CNS (Thermo Scientific, Milan, Italy) elemental analyzer. Sample stable isotope ratios relative to the isotope standard were expressed in the following conventional delta ( δ ) notation in units per mil: δ = ([R sample /R standard ] – 1) where R sample and R standard are the corresponding ratios of heavy to light isotopes ( 13 C/ 12 C and 15 N/ 14 N) in the sample and standard, respectively. Vienna Pee Dee Belemnite was used as the standard for 13 C and atmospheric N 2 for 15 N. The reference materials USGS40 (L-glutamic acid with isotopic composition of δ 13 C = –26.3 ‰ and δ 15 N = – 4.5 ‰) and USGS41 (L-glutamic acid enriched in 13 C and 15 N with isotopic composition of δ 13 C = 37.6 ‰ and δ 15 N = 47.6 ‰) were used in a linear normalization of results, and bovine liver and porcine keratin standards were used to examine precision in matrix-matched sample tissues (Table 2). Data Analyses .\(-\) Trophic discrimination factor Using the known diet information associated with sheds (Tables S1 and S2), proportional diet averages were calculated. For example, prior to the shed collection in December 2022, python “7932” from Little Rock Zoo had a diet of one small rat, one large mouse, and one Guinea pig. The average weights of these diet items from weighing five individuals of each were 49.9g, 20.0g, and 65.0g, respectively (Table S1). Thus, the total diet weight was 134.91g and the proportional contribution of each diet item and their respective isotope values were used to calculate the overall isotopic value of the diet. For diet δ 13 C values: (0.5 * \(-\)25.1) + (0.4 *\(-\)16.2) + (0.1 * \(-\)21.5) = 21.3‰ and for diet δ 15 N values: (0.497 * 4.7) + (0.37 * 3.7) + (0.13 * 4.7) = 4.3‰ (Table S2). Isotopic discrimination ( Δ X) was estimated as δ X tissue – δ X diet for each python and averaged across the 15 individual sheds. Pythons were grouped by length for subadults (< 185 cm SVL, n = 6) and adults (≥ 185 cm SVL, n = 9), and the TDFs for each group were compared with two-tailed t-tests. Hair δ 13 C values from diet items were mathematically adjusted to be representative of whole-body δ 13 C values by applying an offset of \(-\)1.93 ± 0.37 ‰ (Davis & Vander Zanden, n.d.). The δ 15 N values of hair were not adjusted because they do not differ from whole body values (Davis & Vander Zanden, n.d.). Tissue-tissue conversion factors The relationship between muscle and dermis, muscle and shed, muscle and scale, and dermis and scale were calculated using linear regressions in the R statistical software package lme4 (Bates et al., 2015) for both δ 13 C and δ 15 N values. Tissue turnover Tissue isotope values were plotted as days since diet switch, and individual trajectories were visualized using line plots. We were not able to apply a model to estimate isotopic turnover as a function of time, so we estimated turnover from a relationship that predicts half-life as a function of body mass using ectotherm whole blood (M. J. Vander Zanden et al., 2015) : ln (half-life) = 0.22 * ln (body mass) + 3.08 + / \(-\ \)0.15 This equation was applied to each individual python using their initial body mass at the start of captive conditions. Results Trophic Discrimination Factor .\(-\)­­Adult and subadult diet-shed TDFs were significantly different in both Δ 13 C ( P = 0.032) and Δ 15 N values ( P = 0.004), so we report the TDFs separately for the two size classes (Table 3, Table S2). Tissue-Tissue Conversion Factors .\(-\)­­The linear relationships between muscle and dermis, muscle and scale, and dermis and scale of Burmese pythons were significant for both δ 13 C and δ 15 N values ( P < 0.001), and the linear relationship between muscle and shed was significant for δ 13 C ( P < 0.001) but not δ 15 N values ( P = 0.110, Fig. 1). Thus, regression equations (Table 4) can be used to convert between all tissue values except δ 15 N values between muscle and shed. Tissue Turnover .\(-\)­­Pythons were offered food weekly or every other week through day 156 (2020-03-05), and each python ate on at least two offerings. Two pythons did not eat past day 22 despite being offered food throughout the duration of the study (Fig. 2). Python red blood cell δ 13 C and δ 15 N values varied over the sampling period, but they did not shift unidirectionally toward an isotopic equilibrium, which would be expected if the captive diet were isotopically different than the previous diet in the wild (Fig. 3). Therefore, we did not attempt to model red blood cell isotopic turnover as a function of time. Discussion This study used captive pythons to establish TDFs for adult and subadult pythons and used tissue from wild invasive pythons to determine the tissue-tissue conversion factors between four types of tissue. Though tissue turnover time was investigated, the opportunistically collected samples did not span a period long enough to estimate turnover. These metrics can be used for further investigation on invasive python diet and niche in south Florida ecosystems. Trophic Discrimination Factor .\(-\)Only two other studies have quantified TDFs in snakes (Fisk et al., 2009; Pilgrim, n.d.). In adult pygmy rattlesnake ( Sistrurus miliarius ) scale clips, Δ 13 C values ranged from –0.2 ‰ to 4.0 ‰ depending on the controlled diet of the prey consumed, and Δ 15 N values ranged from 1.2 ‰ to 3.2 ‰ regardless of prey consumed (Pilgrim, n.d.). In juvenile corn snakes ( Elaphe guttata guttata ), Δ 13 C values were 1.7 ‰ (liver), 2.3 ‰ (blood) and 2.3 ‰ (muscle), and Δ 15 N values could not be determined (Fisk et al., 2009). Both studies had a single age class of snakes. Our results from python shed show similar heterogeneity in Δ 13 C values (–1.4 ‰ to 0.8 ‰) and trophic enrichment in Δ 15 N values (2.3‰ to 4.3‰) similar to S. miliarius . The trophic discrimination factor between an organism and its diet is due to the metabolic breakdown, assimilation, and routing of nutrients (Martínez del Rio et al., 2009; Olive et al., 2003). It is unclear why Δ 13 C values between adults and subadults differ. One study found no ontogenetic shift in carbon values in one species but significant ontogenetic shifts in another and suggested it could be a result of a general versus specialist diet (Willson et al., 2010), though it seems routing of carbon isotopes is not fully understood. The difference in Δ 15 N values between subadults and adults was expected, as the growth of subadults results in less discrimination between their tissue and diet, largely driven by structural protein synthesis (Wolf et al., 2009). Similarly, in other herpetofauna, Δ 15 N values increase with body size and slower growth rate (Radloff et al., 2012; Villamarín et al., 2018; Wolf et al., 2009). Due to the sparse availability of TDFs for snakes, previous studies have substituted TDFs from literature averages or from sea turtles, which may not accurately reflect the discrimination in snakes (Durso & Mullin, 2017; Perkins et al., 2020; Post, 2002). Zoos provide opportunistic conditions to measure TDFs when controlled, consistent diets are offered and reliable feeding records are available. We were able to determine the TDF of pythons in captivity using non-invasively collected sheds due to the availability of diet records. These TDFs can now be used in subsequent ecological studies examining diet and will allow for more accurate estimates to dietary proportions in isotopic mixing models for this species. Tissue-Tissue Conversion Factors .\(-\)This is the first study quantifying tissue-tissue conversion factors in snakes, and we are unable to predict if these equations would be similar in other snake species. However, the regression equations (Table 4) can be utilized when converting between different tissue types in other python studies. Tissue to tissue conversion between δ 13 C values in shed skin and muscle should be used with caution due to the small sample size (n=10). We did not find a significant relationship between δ 15 N values in shed skin and muscle, and we suggest further research be conducted with a larger sample of paired shed skin and muscle. In other reptiles, some tissues, such as epidermis (Vanderklift et al., 2020) and dermis (H. B. Vander Zanden et al., 2012) have been reported to have higher variability than other tissues from the same species. The conversion equations provided in this study allow isotopic datasets using different tissue types to be combined in previously published and subsequent studies of Burmese python ecology. Tissue Turnover .\(-\)None of the four pythons had blood isotope values exhibiting and isotopic shift that would allow us to definitively calculate half-life. Assuming the two diets were isotopically distinct, we expected a consistent and gradual isotopic change in the red blood cell composition of the captive pythons. Because the isotopic values were relatively stable over the period of captivity, there are a few potential and non-mutually exclusive explanations for these observations. First, it is possible that our opportunistically collected samples spanned a period that was insufficient to measure isotopic turnover (170-174 days between the first captive feeding and the last blood sample). Second, it is possible that the wild and captive diets were not isotopically different. We operated on the assumption that the two diets would be sufficiently distinct, but we could not confirm or refute this assumption without direct knowledge of the wild diet. Third, two of the pythons did not eat after the first month in captivity despite repeated offerings of food, so it is possible that the captive diet was not administered for a sufficient duration to observe an isotopic shift to a new isotopic equilibrium. The only potential explanation for a lack of an isotopic shift that we could explore further was the insufficient observation time. A previous metanalysis proposed linear equations to estimate turnover time based on body mass. For whole blood tissue in ectotherms—which we expect to be similar to red blood cell turnover time—this equation(M. J. Vander Zanden et al., 2015) estimates a turnover time of 652 days (4 x half-life of 163 days) calculated using the average mass (9.6 kg) of 1333 adult pythons removed from south Florida (Guzy et al., 2023). In this study containing smaller individuals, the estimated turnover time ranged from 544 to 586 days (Table 5). Using this equation to estimate turnover time in a captive feeding trial designed to examine isotopic turnover would be helpful to plan the study duration. Though we were unable to calculate a definitive tissue turnover time in this opportunistically obtained sample set, our observations and subsequent estimation are consistent with the slow turnover rates measured in other reptiles (Fisk et al., 2009; Murray & Wolf, 2012; Rosenblatt & Heithaus, 2013). Despite maintaining a more active digestive system in south Florida compared to their native counterparts (Card et al., 2018), we do not have any indication that tissue turnover rate is higher in Florida pythons. Conclusion. \(-\)This study increases the availability of stable isotopic metrics in reptiles that are needed for more complete and accurate diet reconstruction. Additionally, the steps taken to acquire the necessary information to address these metrics highlight the benefits of collaborations with zoological institutions that serve as valuable animal populations that can be utilized for data collection in a controlled setting. Literature Cited Bastos, R. 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Functional Ecology , 23 (1), 17–26. https://doi.org/10.1111/j.1365-2435.2009.01529.x Tables TABLE 1: Sample size, mean percent (± SD) carbon and nitrogen, and carbon to nitrogen ratios from invasive Burmese python tissues collected in south Florida, USA Muscle 35 46.2 ± 2.1 14.5 ± 0.8 3.2 Dermis 25 45.6 ± 2.4 16.0 ± 1.3 2.9 Scale 25 47.9 ± 0.7 14.9 ± 0.5 3.2 Shed skin 10 49.0 ± 1.7 13.9 ± 0.4 3.5 TABLE 2: Standard deviation (SD) for δ 13 C values and δ 15 N values in two matrix-matched reference materials Bovine liver 12 0.1 ‰ 0.1 ‰ Porcine keratin 10 0.1 ‰ 0.4 ‰ TABLE 3: Mean (± SD) trophic discrimination factors between shed skins and diet from multiple sheds of seven subadult (< 185cm SVL) and adult (≥ 185 cm SVL) Burmese pythons Subadult 6 \(-\)1.4 ± 1.6 ‰ +2.3 ± 0.4 ‰ Adult 9 +0.8 ± 2.0 ‰ +4.3 ± 1.6 ‰ TABLE 4: Tissue-to-tissue conversions in the form of linear equations between Burmese python tissue types Muscle, dermis 25 δ 13 C: <0.001 δ 15 N: <0.001 δ 13 C muscle = 1.0* δ 13 C dermis \(-\)0.7 δ 15 N muscle = 0.9* δ 15 N dermis + 0.4 Muscle, scale 25 δ 13 C: <0.001 δ 15 N: <0.001 δ 13 C muscle = 0.6* δ 13 C scale \(-\)10.2 δ 15 N muscle = 0.8* δ 15 N scale + 0.2 Muscle, shed 9 δ 13 C: <0.001 δ 15 N: 0.115 δ 13 C muscle = 0.5* δ 13 C shed \(-\) 12.3 Not significant Dermis, scale 25 δ 13 C: <0.001 δ 15 N: <0.001 δ 13 C dermis = 1.3* δ 13 C scale + 6.6 δ 15 N dermis = 0.7* δ 15 N scale + 1.5 TABLE 5: Initial weights and turnover time estimates for Burmese python blood using equation: ln (half-life) = 0.22*ln (body mass) +3.08 +/\(-\ \)0.15(M. J. Vander Zanden et al., 2015) EPE.07 5600 580.7 EPE.10 4160 544.4 EPS.50 5525 584.8 EPS.54 5850 586.4 FIG. 1: Linear relationships and equations between Burmese python (a) muscle and dermis, (b) muscle and scale, (c) muscle and shed, and (d) dermis and scale δ 13 C (left panel) and δ 15 N (right panel) values. The shaded region around the solid line is the 95% confidence interval. Each point represents a different individual snake, and the dotted line depicts a 1:1 relationship. FIG. 2: Diet records of four individual Burmese pythons and days they ate (circle) or did not eat (triangles) during the captive feeding period. FIG. 3: Burmese python red blood cell δ 13 C (left panel) and δ 15 N values (right panel) during the captive period. Information & Authors Information Version history V1 Version 1 07 July 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords burmese pythons reptiles stable isotopes tissue offset tissue turnover trophic discrimination factor Authors Affiliations Katherine Davis 0000-0002-8090-3749 [email protected] University of Florida View all articles by this author Andrea Currylow 0000-0003-1631-8964 University of North Carolina Wilmington Department of Biology & Marine Biology View all articles by this author Amy Yackel Adams USGS View all articles by this author Christina Romagosa University of Florida View all articles by this author Hannah B. Vander Zanden 0000-0003-3366-5116 University of Florida View all articles by this author Metrics & Citations Metrics Article Usage 224 views 62 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Katherine Davis, Andrea Currylow, Amy Yackel Adams, et al. Isotopic Incorporation of Carbon and Nitrogen in Invasive Burmese Pythons (Python molurus bivitattus). Authorea . 07 July 2025. 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