Species Description: Drymarchon couperi, the eastern indigo snake, is a heavy-bodied, docile, and non-venomous snake. The longest native snake in North America, it grows to lengths of 1.88 - 2.65 m (6.2 - 8.7 feet) (Ashton and Ashton 1981; King and Krysko 2000). Body color is a uniform iridescent blue-black to black color except for a patch of red to reddish-cream around the chin, throat, and cheeks.
The extent and vibrancy of this patch varies with geographic location. Scales are large and smooth, with the central 3 - 5 scale rows keeled in adult males. There are 17 scale rows measured at mid-body, and the anal plate is undivided. Juveniles measure 43 - 61 cm (17 - 24 inches) at hatching, are typically black, have narrow whitish to bluish bands along the body, and are more red or cream colored around the head (USFWS 1999; Conanat and Collins 1991).
Potentially Misidentified Species: Juveniles may be easily confused with southern black racers (Coluber constrictor priapus) due to the pale patches around the chin and cheeks. Black pine snakes (Pituophis melanoleucas lodingi) are similar in color and size, but lack the iridescent sheen of the indigo snake. They also retain a faint crossbanding pattern on the tail that indigo snakes lack.
Regional Occurrence: The historic range of Drymarchon couperi included the coastal plains of Georgia, Alabama, Mississippi, and possibly Southern South Carolina. Currently indigo snakes primarily range throughout Florida, including the Florida Keys, and into southern Georgia (Lazell 1989; Lawler 1977).
IRL Distribution: Drymarchon couperi occurs throughout the IRL.
Age, Size, Lifespan: Eastern Indigo snakes are the longest native snakes in the U.S. and grow to lengths of 1.88 - 2.65 m (Ashton and Ashton 1981; King and Krysko 2000). Juveniles measure 43 - 61 cm (17 -24 inches) at hatching.
Little information is available regarding the lifespan of wild eastern indigo snakes; however, captive snakes have been documented to live more than 25 years (Shaw 1959).
Abundance: Though they occur throughout Florida, eastern indigo snake populations are declining to the point where they are considered rare in many areas.
Reproduction: Drymarchon couperi reaches sexual maturity at approximately 3 - 4 years of age (Hallam et al. 1998). In North Florida, breeding occurs November - April. Wild females lay clutches of 4 - 12 eggs in May or June (Moler 1992), while captive females have an average clutch size of 9 -20 (Speake et al. 1987). Hatching takes place after approximately 3 months, with peaks occurring August - September. In south-central Florida, breeding occurs June - January, with egg laying taking place April - July. Hatching generally occurs from mid-summer through early Fall (USFWS 1999).
Females can apparently store sperm and delay fertilization of eggs. Carson (1945) reported a on a female indigo snake that laid a clutch of 5 eggs after having been isolated for more than 4 years. There have also been anecdotal reports of parthenogenetic reproduction in virginal snakes (USFWS 1999).
Temperature: Drymarchon couperi is cold-sensitive and requires shelter when winter temperatures below approximately 10°C (50 °F). Most often refuge is taken in gopher tortoise burrows (Bogert and Cowles 1947; Speake et al. 1978).
In wetter habitats where gopher tortoises do not occur, indigo snakes take refuge in hollow logs and the burrows of rodents, land crabs, and armadillos (Lawler 1977; Moler 1985a; Layne and Steiner 1996).
Other Physical Tolerances: Desiccation: Under laboratory conditions, Drymarchon couperi showed a high susceptibility to desiccation, and thus seeks out cool, underground burrows or similar cover as shelter against extreme heat and dryness (Bogert and Cowles 1947; Speake et al. 1978; Moler 1985a).
Trophic Mode: As they lack venom and do not constrict, eastern indigo snakes attack any vertebrate small enough to be overpowered and killed with their strong jaws. They have been observed to flush prey from cover, then give chase (Layne and Steiner 1996). They also occasionally climb trees or shrubs in search of prey. The diet includes toads, frogs, lizards, other snakes (including venomous types), turtles, turtle eggs, gopher tortoises, small alligators, birds, and rodents. Juveniles consume primarily invertebrates (USFWS 1999; Layne and Steiner 1996; Steiner et al. 1983).
Activity Time: Diurnal.
Habitats: Drymarchon couperi utilizes a variety of habitat types and shows some preference for open, undeveloped uplands. Typical habitats include pine flatwoods, scrubby flatwoods, high pine, dry prairie, tropical hardwood hammocks, marshes, coastal dunes and scrub, and mangrove forests (Steiner et al. 1983; USFWS 1999).
In Georgia and Northern Florida, indigo snakes can be temperature-restricted to sandhill areas where gopher tortoises burrows are available to shelter in when winter temperatures drop below approximately 10°C. In peninsular Florida, where winter temperatures are not typically challenging, indigo snakes are found in all terrestrial habitats that are not densely developed. Along the coast, indigo snakes frequently use sandy ridges and hammock areas (Moler 1985b).
Drymarchon couperi move between habitats frequently, especially during the summer and fall (Moler 1985a), and thus have extensive home ranges. Speake et al. (1978) reported home range sizes for indigo snakes in Georgia averaging 4.8 ha in winter (December - April) and 42.9 ha in late spring and summer (May - July). Moler (1985a) reported that adult males utilize larger areas then either females or juveniles, perhaps encompassing 224 ha. Layne and Steiner (1996) reported male home range size as averaging approximately 74 ha (with a maximum of 199.2 ha), while females averaged approximately 19 ha (with a maximum of 48.6 ha).
Associated Species: Drymarchon couperi can often be found in association with gopher tortoises (Gopherus polyphemus) and other organisms that share tortoise burrows, including the eastern diamondback rattlesnake (Croatalus adamanteus).
Special Status: Drymarchon couperi has been listed as Threatened in the state of Florida since 1971. It has been Federally listed as Threatened since Jan 31, 1978. The decline of eastern indigo snakes is primarily the result of over-harvesting for the pet trade, but habitat loss and degradation, and the gassing of tortoise burrows to collect rattlesnakes have also heavily impacted the species (Speake and Mount 1973; Speake and McGlincy 1981).
Law enforcement of prohibitions against unauthorized take has reduced pressure on the indigo snake, but has not eliminated it (Moler 1992). Because this snake has a large home range, it may be especially susceptible to habitat loss and fragmentation (Lawler 1977; Moler 1985a). Some estimates suggest that habitat losses of approximately 5% per year continue to occur (Lawler 1977). Increased human population growth also increases the possibility of increased snake mortality due to deaths from property owners and domestic pets. It is expected that the increasing trend toward altering natural areas for agricultural, residential and commercial purposes will result in a large number of isolated groups which cannot support a sufficient number of individuals to ensure continued survival.
Recovery of Drymarchon couperi requires protection and preservation of large expanses of unaltered habitat (USFWS 1999). However, relatively little is currently known about the minimum population size required to maintain and recover the species, though research and population modeling efforts are underway to address these issues.
Management activities being currently undertaken include prescribed burning to maintain optimum habitat quality, maintenance of a captive breeding colony, public outreach and education, and landowner cooperation to conserve snake populations on privately held lands.
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Bogert, C. and R. Cowles. 1947. Results of the Archbold expeditions. No. 58. Moisture loss in relation to habitat selection in some Floridian reptiles. American Museum Noviciates 1358:1 - 55.
Carson, H. 1945. Delayed fertilization in a captive indigo snake with a note feeding and shedding. Copeia 1945(4):222-224.
Conant R. and J.T. Collins. 1991. A Field Guide to Reptiles and Amphibians. Houghton Mifflin Company, Boston, MA.
Hallam, C.O., K. Wheaton, and R. Fischer. 1998. Species profile: Eastern indigo snake (Drymarchon corais couperi) on military installations in the Southeastern United States. Technical report SERDP-98-2, March 1998. Strategic Environmental Research and Development Program, U.S. Army Corps. of Engineers, Waterways Experiment Station, Vicksburg, MS.
King, F.W. and K.L. Krysko. 2000. Online Guide to the Snakes of Florida. Florida Museum of Natural History, University of Florida, Gainesville, FL. 266 pp. Available at http://www.flmnh.ufl.edu/natsci/herpetology/ fl-guide/onlineguide.htm.
Lawler, H.E. 1977. The status of Drymarchon corais couperi (Holbrook), the eastern indigo snake, in the southeastern USA. Herpetological Review 8(3):76-79.
Lazell, J. 1989. Wildlife of the Florida Keys: a Natural History. Island Press. Washington, DC.
Moler, P.E. 1985a. Home range and seasonal activity in the eastern indigo snake, Drymarchon corais couperi, in northern Florida. Final performance report, Study E-1-06, III-A-5. Florida Game and Fresh Water Fish Commission; Tallahassee, FL.
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Speake, D.W. 1993. Indigo snake recovery plan revision. Final report to the U.S. Fish and Wildlife Service; Jacksonville, Florida.
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Speake, D.W., J.A. McGlincy, and T.R. Colvin. 1978. Ecology and management of the eastern indigo snake in Georgia: A progress report. Pages 64-73 in: R.R. Odum and L. Landers, (eds.), Proceedings of rare and endangered wildlife symposium, Georgia Department of Natural Resources, Game and Fish Division, Technical Bulletin WL 4.
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Steiner, T.M., O.L. Bass, Jr., and J.A. Kushlan. 1983. Status of the eastern indigo snake in southern Florida National Parks and vicinity. South Florida Research Center Report SFRC-83/01, Everglades National Park; Homestead, Florida.
U.S. Fish and Wildlife Service USFWS). 1982. Eastern indigo snake recovery plan. U.S. Fish and Wildlife Service, Atlanta, Georgia.
U.S. Fish and Wildlife Service (USFWS). 1999. South Florida multi-species recovery plan. Atlanta, GA. 2,172 pp.