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EASTERN INDIGO SNAKE (Drymarchon corais couperi)

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Eastern indigo snake picture
Family: Colubridae

Order: Squamata

Suborder: Serpentes

STATUS: Threatened

The Eastern indigo is the longest North American snake, with a maximum recorded length of 2.6 m. It is glossy blue-black with smooth iridescent scales. The chin and throat are commonly tinged with a reddish or orange-brown coloring. Some individuals have white blotched throats. The young are usually similar to the adults, but some may be lighter and show a blotched dorsal pattern. The scales of the indigo snake are smooth, although adult males typically show a partial anterior keel on the scales of the middorsal 3 to 5 scale rows. The anal scale is undivided in this species.

The only snake commonly confused with the Eastern indigo snake is the black racer (Coluber constrictor). The black racer is a smaller snake, seldom exceeding 1.2 m; it is dull black in coloration, with white (brown in some areas) on the throat and lower labial scales. Unlike the indigo snake, the black racer has a divided anal scale.

At one time, the indigo's range included all of the Florida peninsula, north to Georgia, South Carolina, Alabama, and Mississippi. In contrast to past years, the indigo's range and abundance are considerably more restricted to Georgia and Florida. In southern and central Florida, indigo snakes may be found in habitats ranging from mangrove swamps and wet prairies to pinelands and xeric scrub; whereas, in Georgia and the panhandle of Florida, the species is almost always associated with high sandhill habitats. Here, it overwinters in the burrows of gopher tortoises, where winter temperatures seldom drop below 50 degrees F. But because of overexploitation and habitat degradation, gopher tortoise populations have also declined in many areas, and with them have gone the burrows on which the indigo snake depends. Although the indigo snake still occurs throughout most of Florida, it is now rare in most of the panhandle.

During the warmer months, they range widely, individuals utilizing activity areas of 50 to 110 ha or more. Males are territorial, at least during the breeding season, and confrontations sometimes lead to combat or cannibalism. During the winter months, indigo snakes usually stay fairly close to some sort of deep shelter (e.g., gopher tortoise burrow, stump hole, land crab burrow). Activity area during this period is usually less than 10 hectares. Shedding occurs with surprising frequency in this species, about every 30 or 45 days. Indigo snakes typically become inactive for a period of 10 to 14 days immediately prior to shedding.

Breeding occurs from November through April. Up to 11 eggs may be laid under logs, leaf litter, or in the burrow of a gopher tortoise during May or June. Eggs are incubated in about 90 days. Hatchlings average between 45 and 60 cm in length. Maturity is reached in about 2 to 3 years after hatching. Indigo's can produce fertile eggs up to 4 1/2 years after a single mating. The eggs and young often fall prey to skunks, raccoons, and other snakes. Indigo snakes usually feed during the cooler hours of the day, early morning and early evening. Diet includes a variety of amphibians, reptiles, birds, and small mammals. Other prey species include fish, toads, lizards, small turtles, birds, and mice.

The Eastern indigo snake was formerly collected heavily for the pet trade. In order to curtail the commercial collecting of this species, the federal government classified the Eastern indigo snake as a "threatened" species in 1978. A technique of rattlesnake hunting, which involved pouring gasoline into tortoise burrows, previously took a heavy toll on indigo snakes; this practice, however, became illegal in 1978. The most insidious and far reaching threat to the survival of this species is habitat loss or degradation. The indigo snake is critically dependent upon the availability of appropriate winter and summer refugia. Historically, these refugia were most often provided by gopher tortoise burrows and stump holes. However, gopher tortoises are now becoming rarer and hundreds of "lighted" stumps have been removed by the resinous wood industry.

Elsewhere, the indigo snake has suffered from the increasing fragmentation of its habitat. Indigo snakes are large, highly visible, and wide-ranging. Whereas, some species of snakes are secretive and able to survive in small wooded lots, indigo snakes require relatively large tracts of suitable habitat. Even very low density development can seriously impact populations of indigo snakes, since the activity area of an individual snake may be from 400 hectares to over a 1000 hectares. In such situations, indigo snakes are especially vulnerable to vehicles, domestic dogs, and insensitive landowners.

The gopher tortoise is crucial to the survival of a number of other listed species in Collier County because its burrows provide important refuges for a much larger wildlife community (Speake 1981, Franz 1986). As gopher tortoise habitat decreases, so do many of the species that utilize their burrows. The eastern indigo snake and gopher frog are all listed species that occur within Collier County and prefer xeric habitat with gopher tortoise burrows.

The preservation strategy for the above listed species is to follow the management guidelines outlined for gopher tortoises with the following modifications to address the additional habitat needs; a viable population is 25-30 which translates to 75-90 active and inactive burrows, with 25 or more acres depending on the suitability of habitat size, shape, type and adjacent environs and maintaining habitat area with a canopy cover <25%. Management for gopher tortoise habitat should consider and/or maintain: (1) the presence of well-drained, sandy soils which allow easy burrowing, (2) an abundance of herbaceous ground cover, and (3) an open canopy (<25%) with sparse shrub cover to allow sunlight to reach the ground layer.

Habitat protection intended to benefit the indigo snake should be focused on large tracts of land, generally of at least 1000 ha. Small, isolated habitat tracts have little potential for sustaining populations of indigo snakes long-term. When indigo snakes habitat is unavoidably to be eliminated as a result of development activities, mitigation funds should be pooled in "mitigation land banks" in order to accumulate adequate funds to allow for the acquisition of preserves of adequate size. The indigo snake will benefit from efforts to rebuild gopher tortoise populations. This may require a combination of tortoise restocking and active habitat management (primarily through prescribed fire). Also, existing prohibitions against the "gassing" of tortoise burrows should be strongly enforced.

Layne, J.N. and T.M. Steiner. 1984. Sexual dimorphism in occurrence of keeled dorsal scales in the Eastern Indigo (Drymarchon corais couperi). Copeia 1984(3):776-778.

Moler, P.E. 1992. Eastern indigo snake. Pp. 181-186. In P.E. Moler (ed.), Rare and Endangered Biota of Florida. Amphibians and Reptiles. University of Florida Presses, Gainesville, FL.

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. Report SFRC-8 3/01, South Florida Research Center, Everglades National Park, Homestead, FL.

U.S. Fish and Wildlife Service. 1982. Eastern indigo snake recovery plan. U.S. Fish and Wildlife Service, Atlanta, GA. 23pp.

Content updated Date March 4, 2005