Nutrient Composition of Prey Items Consumed by
Free-Ranging Drymarchon couperi (Eastern Indigo Snakes)
Ellen S. Dierenfeld, Terry M. Norton, Natalie L. Hyslop, and Dirk J. Stevenson
Southeastern Naturalist, Volume 14, Issue 3 (2015): 551–560
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Southeastern Naturalist
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22001155 SOUTHEASTERN NATURALIST 1V4o(3l.) :1545,1 N–5o6. 03
Nutrient Composition of Prey Items Consumed by
Free-Ranging Drymarchon couperi (Eastern Indigo Snakes)
Ellen S. Dierenfeld1,*, Terry M. Norton2,3, Natalie L. Hyslop4, and
Dirk J. Stevenson5
Abstract. Drymarchon couperi (Eastern Indigo Snake) has a threatened conservation status
and restricted range in the southeastern US. Evidence suggests it mainly consumes other
reptile species. Dietary nutrient analysis is a component of habitat/resource quality and
species health assessments, and the results provide guidelines for optimal captive-feeding
protocols. Native prey items (7 snakes, 1 tortoise, 1 rodent) had higher protein and lower
fat content, considerably higher concentrations of vitamins A and E, and variable mineral
content (high Ca, P, Na; low Cu, Mn) compared to the diets of commercially reared and
captive-fed rodents. Data suggest that diets for captive snakes may require modification to
better duplicate natural food sources. Further investigation of captive diets is warranted to
understand possible health implications for wild Indigo Snake populations.
Introduction
Drymarchon couperi (Holbrook) (Eastern Indigo Snake) is a large (up to 2.6
m total length [TL]; Conant 1998) colubrid currently federally listed as a threatened
species throughout its restricted range of Florida and the Coastal Plain of
Georgia (USFWS 2008). In southern Georgia, Eastern Indigo Snakes use the burrows
of Gopherus polyphemus (Gopher Tortoise) for protection from temperature
extremes and fire, for foraging, and potentially as nesting sites (Hyslop 2007,
2009; Hyslop et al. 2014; Newberry 2009; Speake and McGlincy 1981; Speake et
al. 1978). Adult Eastern Indigo Snakes have been documented feeding on a wide
variety (48 spp.) of vertebrates in Georgia and Florida including: amphibians (anurans,
14% of prey records), mammals (rodents, 15%), other snakes (45%), and
reptiles—notably juvenile Gopher Tortoises (15%) (Stevenson et al. 2010).
As a component of health assessments conducted simultaneously with population
monitoring and spatial ecology studies of Eastern Indigo Snakes on Fort
Stewart, GA, and adjacent private lands between 2001–2005 (Hyslop 2007; Stevenson
et al. 2003, 2009), we opportunistically collected Eastern Indigo Snake prey
samples for nutrient analysis. Chemical information on prey items offers details
that can be interpreted in terms of habitat/resource quality for wildlife-management
purposes, provide useful guidelines for development of captive-feeding protocols,
and contribute to evaluation of biochemical health parameters.
1Ellen S. Dierenfeld, LLC, St. Louis, MO 63128. 2The Georgia Sea Turtle Center, Jekyll
Island Authority, Jekyll Island, GA 31527. 3St. Catherines Island Foundation, Midway, GA
31320. 4The University of North Georgia, Gainesville, GA 30566. 5The Orianne Society, The
Indigo Snake Initiative, Athens, GA 30605. *Corresponding author - edierenfeld@aol.com.
Manuscript Editor: Max Nickerson
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Materials and Methods
We obtained prey specimens either as fresh road-killed snakes of species known
or suspected to be eaten by Eastern Indigo Snakes (Stevenson et al. 2010), or from
the gastrointestinal tracts of Eastern Indigo Snakes during postmortem examination.
We placed all samples on ice in the field and stored them frozen at -20 °C (for
less than 1 year) until transferred to the St. Louis Zoo Animal Nutrition Laboratory,
St. Louis, MO. We then partially thawed, sectioned into smaller segments using
either a band saw or meat cleaver, and ground whole prey items in a commercial
meat grinder. In the case of the 3 viperids Crotalus horridus (Eastern Timber Rattlesnake),
Agkistrodon contortrix (Southern Copperhead), and A. piscivorus (Eastern
Cottonmouth; n = 4 individuals), we removed and discarded heads and venom
glands prior to grinding. We used only the head and tail from 1 Farancia abacura
(Eastern Mudsnake) and only the front half from a second Mudsnake due to extensive
tissue damage. We also analyzed 2 intact Gopher Tortoise neonates obtained
from the stomach of 1 adult Eastern Indigo Snake and a partially digested rodent
and snake sample from the stomach contents of another Eastern Indigo Snake.
Fat-soluble components
Following homogenous grinding and mixing, we saponified duplicate 1-g subsamples
and extracted fat-soluble components for analysis using the methodology
of Taylor (1976) as detailed in Douglas et al. (1994). Instead of conducting analyses
immediately, we drew 1-mL extracts drawn from the hexane layer (top), evaporated
the samples under nitrogen, then reconstituted with 0.25 mL ETOH containing
0.2% BHT and stored them at -20 °C until shipment to Arizona State University,
Tempe, AZ, where they were analyzed using high-performance liquid chromatography
(HPLC, McGraw et al. 2006) to quantify vitamin E (as α-tocopherol) and total
carotenoids. We measured vitamin A (as all-trans retinol) with absorbance spectrophotometry
(in hexane, using quartz cuvettes and the extinction coefficient of 1600)
at λ max = 277 nm. We calculated vitamin E activity as 1 mg α-tocopherol = 1.49
IU vitamin E and vitamin A activity as 0.3 μg all-trans retinol = 1 IU vitamin A.
Proximate nutrients and mineral content
We placed remaining homogenates in aluminum pans, and weighed and freezedried
them to determine water content. We ground freeze-dried samples in a
coffee grinder, placed the products in individual plastic bags, and sent them for
proximate composition (crude protein, crude fat, ash) and mineral analysis using
standardized methodology (AOAC 2004) for animal products (Dairy One Forage
Lab, Ithaca, NY). Minerals were not analyzed in the partially digested rodent prey
as the sample was too small.
Results
We analyzed 7 known and 1 unidentified snake species, along with 2 Gopher Tortoises
and an unidentified rodent species eaten by Eastern Indigo Snakes. Proximate
nutrient composition, calculated vitamins A and E, and total carotenoids in prey
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items are listed in Table 1. Pituophis melanoleucus mugitus (Florida Pinesnake) appeared
to be unique in fat content within the snake species examined, with fat levels
measuring 1.7 to 8-fold higher than other species (Table 1). The partially digested
snake prey (Table 1) displayed the lowest protein content, likely due to digestive
processes. Further substantiation of digested tissue (primarily protein, possibly
fat) was also evident from the relatively higher ash (bone) content in the partially
digested snake sample: 42% ash compared with 16–24% in all other potential prey
items. Mineral contents of whole prey are listed in Table 2, where higher levels of
Ca, Mg, P, and Mo are apparent in the partially digested snake-prey sample; this is
again indicative of tissues other than bone being digested.
Discussion
Proximate nutrient composition of prey eaten by Eastern Indigo Snakes is
similar in many aspects to values reported in a wide variety of other vertebrate
prey items. Water content ranges from ~60 to 85% in most whole vertebrate prey,
with median values averaging 70–80% (Dierenfeld et al. 2002a), similar to most
whole-prey snakes analyzed (Table 1). Gopher Tortoise neonates and Florida
Pinesnakes—the latter species a previously undocumented but suspected prey of
Eastern Indigo Snakes (Stevenson et al. 2010)—could provide as much as 20% less
water in a meal, which may be physiologically important to Eastern Indigo Snakes
in dry areas or during hot seasons. Additionally, obligate carnivores can meet their
water needs through metabolic breakdown of dietary fats; however, the primary
food items of Eastern Indigo Snakes appear limited to those low in fat content.
Hence, the free water itself could be critical in maintaining hydration and electrolyte
balance.
Across the prey-snake items analyzed, crude fat content ranged from about
2% to 17% of dry matter (DM); by comparison, fat content in laboratory rodents
(primarily mice and rats) fed to captive snakes typically ranges from 20 to 30%
(or higher, depending on husbandry and species; Dierenfeld et al. 2002a). Snakeprey
items contained some of the lowest fat concentrations measured across a wide
variety of vertebrate prey analyzed; only anurans display whole-body fat content
consistently lower than 15% of DM (Dierenfeld et al. 2002a). This finding may
be important for overall energetics and nutritional balance in the Eastern Indigo
Snake because poikilotherms have a lower metabolic rate than homeotherms, and
hence require fewer calories per unit body mass. High-fat diets can be linked with
obesity and associated health problems of captive-managed snakes (Frye 1984).
Our data suggest that lower-fat, higher-protein diets may be more characteristic of
foods consumed by the free-ranging Eastern Indigo Snake. Lower nutrient-density
dietary components may be entirely appropriate for this species, but actual nutritional
requirements remain unknown.
Importantly, low dietary fat may impact the uptake of fat-soluble nutrients such
as vitamins A and E in this species. To our knowledge, these are the first data on
the vitamin A and E content of whole snakes utilized as prey. Vitamin A levels tend
to increase with age/maturity in vertebrate-prey species through accumulation in
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Table 1. Water, crude protein, crude fat, ash, calculated vitamins A and E, and total carotenoids in prey (or potential prey) items consumed by Eastern
Indigo Snakes (Drymarchon couperi). Crude protein, crude fat, and ash are presented as % DM (mean ± SD), vitamin A as IU/g DM, vitamin E as IU/kg
DM, and carotenoids as μg/g DM. *denotes heads not included in analyses; na = not analyzed.
Total
Sample n Water (%) Crude protein Crude fat Ash Vitamin A Vitamin E carotenoids
Snakes
Heterodon platirhinos (Latrielle) 2 77.07 ± 2.40 69.85 ± 5.02 9.60 ± 0.57 18.11 ± 3.15 na na na
(Eastern Hognose)
Pituophis melanoleucus Barbour 2 63.43 ± 0.60 63.75 ± 1.48 16.65 ± 0.78 20.70 ± 1.01 888.74 ± 116.82 179.15 ± 49.83 0.26 ± 0.05
(Florida Pinesnake)
Farancia abacura (Holbrook) 3 77.87 ± 4.27 83.03 ± 6.82 3.60 ± 3.21 15.83 ± 3.93 1592.88 ± 356.38 163.81 ± 147.55 0.06 ± 0.08
(Eastern Mudsnake)
Agkistrodon contortrix (L.)* 2 71.51 ± 0.49 71.05 ± 1.77 4.90 ± 1.41 22.88 ± 0.66 1371.66 ± 261.91 203.61 ± 94.37 0.01 ± 0.02
(Southern Copperhead)
Coluber constrictor L. 1 72.05 75.30 3.00 24.16 1343.20 118.74 0.73
(Southern Black Racer)
Crotalus horridus L.* 1 76.20 73.20 2.10 22.88 1908.33 593.85 0.40
(Timber Rattlesnake)
Agkistrodon piscivorus Lacepede* 1 73.73 67.80 9.20 19.89 2150.20 1041.81 0.07
(Eastern Cottonmouth)
Snake Average ± SD 73.30 ± 5.60 72.89 ± 7.73 7.28 ± 5.31 19.81 ± 3.62 1475.97 ± 430.61 306.02 ± 301.03 0.39 ± 0.69
Tortoises
Gopherus polyphemus (Daudin) 2 67.85 ± 2.51 67.80 ± 0.85 15.15 ± 0.78 20.44 ± 3.97 1062.04 ± 31.33 148.99 ± 39.41 3.68 ± 0.19
(Gopher Tortoise) neonates
Partially digested prey
Rodent (species unknown) 1 70.00 na 11.90 na 1059.85 182.95 1.66
Snake (species unknown) 1 71.42 57.80 2.90 41.57 947.30 227.97 2.24
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body stores, particularly the liver (Dierenfeld et al. 2002a). Calculated vitamin
A values measured in this study, regardless of species, should be considered exceptionally
high in foods eaten by the Eastern Indigo Snake compared with other
assayed whole-prey reported in the literature. Five rodent species fed to captive
carnivores had whole-body vitamin A concentrations ranging from ~16,000 IU/kg
to ~600,000 IU/kg DM, 2 bird species contained ~36,000 IU/kg DM to 70,000 IU/
kg DM, and 2 lizard species contained ~5000 IU/kg DM to ~39,000 IU/kg DM,
whereas 2 anurans displayed vitamin A levels ranging from ~16,000 IU/kg DM to
38,000 IU/kg DM (Dierenfeld et al. 2002a). By comparison, the 7 snake species
we analyzed in this study averaged about 1,475,000 IU/kg DM vitamin A; Gopher
Tortoise neonates also contained vitamin A levels >1,000,000 IU/kg DM. It may be
that these seemingly excessive levels of vitamin A in whole prey are necessary due
to the low dietary-fat content, but this speculation remains to be investigated.
All whole prey analyzed would appear not only to exceed the vitamin A dietary
requirements established for obligate carnivores or domestic carnivores (cats,
~4000 IU/kg DM, dogs: 11,000 IU/kg DM; NRC 2006), but even exceed presumed
upper safe limits for this nutrient (33,000 IU/kg for canids, up to 100,000 IU/kg for
felids; NRC 1987). These data suggest possible unique, perhaps seasonal cycling
of this nutrient in Eastern Indigo Snakes and further studies to better understand
species requirements are warranted. Furthermore, to avoid potential toxicities, it is
possible that snakes, like other carnivores, store vitamin A in specialized hepatic
cells (Leighton et al. 1988) and/or mobilize it throughout body tissues safely packaged
in retinyl esters, a phenomenon particularly associated with sporadic feeding
(Schweigert et al. 1991).
We found no references in the literature on the ability of snakes to convert dietary
carotenoids to active forms of vitamin A; given the quite low levels of total
carotenoids detected in whole prey consumed by Eastern Indigo Snakes along with
the high preformed vitamin A detected, we suspect that this metabolic route is of
little consequence to overall nutritional health of this species. However, total carotenoid
concentrations, in the Gopher Tortoises, even though they were neonates,
were about 10-fold higher than in the snake-prey items, reflecting the more herbivorous
habits of the tortoises, and subsequent deposition of carotenoids into eggs and
developing embryos (Dierenfeld et al. 2002b).
Vitamin E concentrations for all prey (except viperid snakes), whether intact or
partially digested, averaged ~200 IU/kg DM, and were generally higher than most
captive-reared rodent or avian prey (25–174 IU/kg DM, excluding neonatal rats) but
similar to values reported from amphibian prey sampled from a comparable locale
(82–370 IU/kg DM) (Dierenfeld 2002a). While there was some variability across
species, the 3 viperids (in which heads were removed prior to grinding/analysis)
contained the highest vitamin E concentrations (3- to 5-fold higher for the Cottonmouth
and Timber Rattlesnake, respectively). There may be species-specific differences
in vitamin E need, utilization, or metabolism in snakes. Reduced hemolytic
action of viper venoms has been correlated with human vitamin E supplementation
(Mukherjee et al. 1998). Speculatively, perhaps an affiliated endogenous protective
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mechanism exists in some snakes, such as the Eastern Indigo Snake, that consume
venomous prey. Average vitamin E concentrations in the prey items of the Eastern
Indigo Snakes consistently exceeded dietary vitamin E recommendations established
for domestic carnivores (30–50 IU/kg DM for canids, up to ~80 IU/kg DM
for felids; NRC 2006). Recommendations vary depending upon amounts and types
of dietary fats, as well as physiologic stage; high dietary polyunsaturated fats, for
example, may increase the dietary vitamin E requirement up to 5-fold due to high
oxidative potential of these fats. Vitamin E deficiency, manifested as steatitis, has
been only occasionally reported in captive snakes (Dierenfeld 1989).
Regarding mineral nutrition, the generally elevated levels of Ca and P in the
whole prey eaten by Eastern Indigo Snakes likely reflects a high proportion of
skeleton to body mass in the prey-snake species. Although the Ca:P ratio remains
approximately 1.7–1.8:1 in both snake and tortoise prey-items, the concentrations
of Ca and P (ranging from an average of 4.1% to 7.3% DM, and 2.4% to 4.0%
DM, respectively, in snakes) are considerably higher than Ca (0.9–5.9% DM) or
P (1.1–3.4%) reported in other vertebrate prey (Dierenfeld et al. 2002a). These
high levels of Ca and P may be needed to properly support the skeletal tissues
or oogenesis of Eastern Indigo Snakes, or may pose an excessive load requiring
mineral excretion bound to urates (Shoemaker and Nagy 1977); quantitative
Ca requirements have not been determined. Evaluation of dietary seasonality,
particularly relative to reproduction, may elucidate some of the physiology involved
with Ca metabolism in this species. Presumably, whole prey contains
adequate stores of preformed vitamin D to support uptake and utilization of both
Ca and P, but we did not quantify vitamin D levels in this study. Pantherophis
guttatus (Corn Snake) exposed to artificial UV lighting for 28 days demonstrated
increased circulating 25-OH vitamin D levels (from 57 to 196 nmol/L, Acierno
et al. 2008). Environmental as well as dietary sources of vitamin D may well be
required for optimal nutritional status. Macromineral requirements established for
growing mammal and bird species (Ca, 0.4–1.1%; Mg, 0.04–0.1%; P, 0.4–0.8%;
K, 0.3–1.4%; and Na, 0.1–0.2%; AAFCO 2012, NRC 1994) appear to be met by
any of the whole prey items described in this study, but the actual requirements of
Eastern Indigo Snakes remain unknown.
Wide variability in trace-mineral composition within samples was evident and
likely due to multiple factors including limited sample size and possible habitat
and/or primary-prey item (dietary) differences that impacted the whole body
(Table 2). Dietary requirements for Cu range from about 3 to 5 mg/kg DM for domestic
carnivores (AAFCO 2012, NRC 2006). Snake species, in particular, appear
to contain low body levels of Cu (all 7 spp. had ≤10 mg/kg), as opposed to the
Gopher Tortoise neonates at 22 mg/kg DM. By contrast, mammalian whole prey
(9 spp.) Cu concentrations ranged from 2 to 62 mg/kg, 4 avian and 4 amphibian/
reptile species exhibited levels of ≤11 mg/kg DM, and 1 anuran species contained
>100 mg/kg Cu (Dierenfeld et al. 2002a). It appears that a mixed prey diet may
optimize Cu intake for Eastern Indigo Snakes. In contrast, iron concentrations
ranged more than 5-fold across the prey species analyzed (87–535 mg/kg DM).
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Table 2. Mineral concentrations in prey (or potential prey) items consumed by Eastern Indigo Snakes (Drymarchon couperi). All nutrients presented on a
dry-matter basis (mean ± SD). *denotes heads not included in analyses. na = not analyzed.
Cu Fe Mn Mo Zn
Sample n Ca (%) K (%) Mg (%) Na (%) P (%) Ca:P (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg)
Snakes
Eastern Hognose 2 5.12 0.96 0.12 0.72 2.86 1.79 5.50 163.00 2.00 0.40 135.00
± 1.49 ± 0.08 ± 0.01 ± 0.20 ± 0.64 ± 0.71 ± 33.94 ± 0.00 (n = 1) ± 16.97
Florida Pinesnake 2 4.15 0.99 0.11 0.48 2.58 1.61 2.00 535.00 11.00 0.30 108.50
± 0.56 ± 0.07 ± 0.01 ± 0.02 ± 0.28 ± 0.00 ± 258.80 ± 1.41 ± 0.28 ± 3.54
Eastern Mudsnake 3 4.26 1.03 0.10 0.57 2.38 1.79 3.00 482.67 3.33 0.27 144.00
± 0.97 ± 0.08 ± 0.01 ± 0.10 ± 0.40 ± 1.73 ± 434.37 ± 2.31 ± 0.06 ± 21.79
Southern Copperhead* 2 7.01 0.70 0.15 0.63 3.75 1.87 8.00 156.00 7.00 0.55 113.50
± 0.01 ± 0.01 ± 0.01 ± 0.03 ± 0.03 ± 1.41 ± 32.53 ± 1.41 ± 0.21 ± 6.36
Southern Black Racer 1 7.27 0.99 0.16 0.53 3.98 1.83 3.00 87.00 7.00 0.40 158.00
Timber Rattlesnake* 1 5.73 0.75 0.14 0.84 3.38 1.70 10.00 254.00 11.00 0.70 117.00
Eastern Cottonmouth* 1 5.74 0.75 0.13 0.62 3.15 1.82 9.00 410.00 4.00 0.60 104.00
Snake Average ± SD 5.34 0.91 0.12 0.61 3.00 1.77 5.17 325.58 6.00 0.42 127.08
± 1.37 ± 0.15 ± 0.02 ± 0.13 ± 0.65 ± 0.09 ± 3.04 ± 267.62 ± 3.69 ± 0.19 ± 21.16
Tortoises
Gopher tortoise neonates 2 3.29 0.62 0.08 0.48 1.87 1.76 22.00 365.50 10.00 1.00 109.50
± 1.16 ± 0.05 ± 0.02 ± 0.06 ± 0.60 ± 5.66 ± 123.74 ± 0.00 ± 0.00 ± 16.26
Partially digested prey
Rodent (species unknown) 1 na na na na na na na na na na na
Snake (species unknown) 1 12.83 0.68 0.18 0.53 6.48 1.98 less than 1 82.00 4.00 2.90 146.00
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Iron requirements for carnivorous species range from ~30 to ~100 mg/kg DM
(AAFCO 2012, NRC 2006) depending on physiologic stage of development,
and excessive levels may interfere with absorption and utilization of other trace
minerals, including Cu (Puls 1988). Again, we noted some species-specific differences
in Fe content that might reflect habitat/substrate or diet differences among
prey-snake species. Recommended levels of dietary Mn and Zn (5–7.5 mg/kg, and
50–75 mg/kg, for dogs and cats, respectively; AAFCO 2012, NRC 2006) could be
achieved through consumption of various individual food items or combinations.
However, 71% (5 of 7) of the prey-snake species contained marginal or deficient
levels of Mn, if consumed as sole diet items.
Until further studies are conducted to clarify the nutrient requirements of Eastern
Indigo Snakes, the ranges measured in local prey items from native habitats can
provide guidelines for nutritional assessment. Based on body composition analyses,
snakes and Gopher Tortoise neonates eaten by Eastern Indigo Snakes represent less
calorically dense (low fat, high ash) prey items compared with the chicks, rodents,
or rabbits commonly fed to snakes in captivity. In a review of Eastern Indigo Snake
prey items observed in the field from 1940–2008, 46% of 185 recorded prey items
were snakes (Stevenson et al. 2010); these primary prey items may better represent
optimal diets and nutritional profiles for Eastern Indigo Snakes. In contrast to energy,
fat-soluble vitamin A and E concentrations, as well as Ca, P, and possibly Na
contents in native prey were higher than expected based on published values for
commercially reared whole-prey species. Of the trace minerals, Cu and Mn levels
in some native whole prey appeared deficient. These data suggest that the diets of
captive Eastern Indigo Snakes may need to be modified to better duplicate the diets
of this species in the wild. Additionally, these observations may warrant further
investigation to understand any underlying health implications for Indigo Snake
populations. Future studies could examine seasonal variation in prey composition
consumed by Eastern Indigo Snakes, not only due to a choice of species consumed
but also variability within prey species, and correlate findings with natural history,
behaviors, and physiologic status.
Acknolwedgements
We thank Larry Carlile for this opportunity, as well as Beth Willis-Stevenson and
Frankie Snow for field sampling help, and April Braddy and Kevin McGraw for laboratory
preparation and analytical assistance.
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