Species Description
Eastern Indigo Snakes are one of the largest snakes in North America, reaching a maximum total length of 2.6 meters (8 feet, 7 inches). Most records of wild specimens measuring 2.4 m. (7 ft., 10 in.) or longer have been from southern Florida. The species is fairly stout-bodied and sexually dimorphic in size, with males attaining greater lengths than females. Large males are typically 2.0 to 2.3 m. (6 ft. 8 in. to 7 ft., 6 in.) and weigh 3.2 to 4.5 kilograms (7 to 10 pounds), while large females seldom exceed 2.0 m. (6.5 ft.) and weigh 1.8 to 2.7 kg. (4 to 6 lbs.).
Eastern Indigo Snakes are a uniform bluish or gunmetal black, both dorsally and ventrally. A wash of orange-red is often present on the chin, sides of the head and throat. There is variation in this trait, with some populations in northern Florida having black lip scales, a white patch in the center of the throat and lacking orange-red. The orange-red pigment is more prominent in male Eastern Indigos and may extend onto the belly of some south Florida snakes.
The common name for Eastern Indigo Snakes relates to the large, smooth scales of this species which appear iridescent purple in sunlight. Adult males have partial keels on the scales along the middle of the back. Hatchling Eastern Indigos may be may possess a faint pattern of cream speckling, forming indistinct lateral bands.
Taxonomy
The taxon was described by James Edward Holbrook in 1842 and named in honor of J. H. Couper who brought him the first specimen from south of the Altamaha River in Wayne County, Georgia. Historically, Drymarchon corais was considered a monotypic species with 8 to 12 subspecies including D. c. couperi. In the early 1990s, the Eastern Indigo Snake, Drymarchon corais couperi, was elevated to a full species (i.e., Drymarchon couperi), and most herpetologists have adopted this suggestion including the Society for the Study of Amphibians and Reptiles. Populations of Eastern Indigo Snakes are isolated from their nearest relative, the Texas Indigo Snake, Drymarchon melanurus erebennus, by approximately 1,000 km. (620 miles).
The Latin name for the genus Drymarchon roughly translates to “forest ruler,” from the Greek words drymos, meaning forest, and archon, meaning ruler. Eastern Indigo Snakes have a number of common names: Indigo, Blue Indigo Snake, Black Snake, Gopher Snake, Blue Gopher Snake and Blue Bull Snake.
Distribution
The Eastern Indigo Snake is a member of a species complex which is primarily tropical in distribution. Its ranging into temperate North America depends on the presence of deep animal burrows (e.g., Gopher Tortoise burrows) to survive cold winter temperatures. Eastern Indigo Snakes occur throughout most of Florida and much of the Coastal Plain of southern Georgia. The historic range included southeastern Mississippi, southernmost Alabama and possibly southeastern South Carolina. However, Eastern Indigo populations no longer inhabit these states.
The current strongholds for the species are peninsular Florida and southeastern Georgia. Eastern Indigo Snakes are rare and of very local occurrence in the Florida panhandle (west of Tallahassee) and in southwestern Georgia.
Habitat
Throughout its range, the Eastern Indigo Snake can be found in a wide variety of habitats depending on the region in which it is found. In peninsular Florida, the species occurs in diverse habitat types, including sandhills, oak scrub, sand pine scrub, mangrove swamps, wet prairies, cabbage palm-live oak hammocks and pine flatwoods. Some Eastern Indigo populations in south Florida inhabit vegetated, rock-strewn canal banks surrounded by sugarcane fields or citrus groves.
In the northern parts of the range (i.e., southern Georgia and northern Florida, including the Panhandle region) Eastern Indigo Snakes rely on Gopher Tortoise (Gopherus polyphemus) burrows for cool-season dens, and are restricted to sandy, Longleaf Pine habitats. In this region, Gopher Tortoise burrows play a central role in the ecology of these snakes, particularly during ecdysis (shedding of their skins). The presence of Gopher Tortoise populations and large numbers of tortoise burrows will be a necessity for repatriation sites at northern latitudes. Additional types of refugia used throughout the year include stump holes, armadillo burrows, small mammal burrows, hollow logs, root channels, limestone solution holes, and in south Florida land crab burrows.
In the Coastal Plain of Georgia, Eastern Indigo Snakes favor habitats underlain by wind-blown deposits of sand 3 to 9 m. ( 10 to 30 ft.) deep which are located along the northeastern sides of major blackwater streams (e.g., the Alapaha, Altamaha, Canoochee, Ohoopee and Satilla Rivers). These extensive, xeric sand ridges may be 8 km. (5 mi.) long and support barren environments vegetated with stunted turkey oaks and a patchy ground cover of saw palmetto, rosemary, mints and reindeer lichens. These sandhill areas are used by Eastern Indigos during the cooler months, while lowland habitats are important as foraging sites during the rest of the year. Wildlife corridors are important in linking these seasonally-used habitats.
Site fidelity, that is a “homing instinct” or tendency to return to the same tortoise colonies and burrows year-after-year, has been documented for adult Eastern Indigo Snakes at numerous sites in Georgia. This behavior emphasizes the importance of long-term burrow viability. In the cooler months, Eastern Indigos bask in the vicinity of those Gopher Tortoise burrows that they use as retreats. Basking often occurs at temperatures of 15-22°C (60-72°F) and at temperatures as low as 7°C (45°F).
Movement and Home Range
Eastern Indigo Snakes have one of the largest home ranges of any North American snake species. Home range size appears to fluctuate between south Georgia and north Florida and more southerly sites in central and south Florida, with snakes in southern Florida having smaller home ranges than those of their northern counterparts. In Georgia, females have been found to have a home range of around 100 ha., and males around 500 ha. (1,200 acres). In north Florida, female home ranges vary between 20-280 ha. (55-700 acres) while males have home ranges of approximately 185 ha. (460 acres).
Home range size and movement may be influenced by Eastern Indigo Snake population densities, mating opportunities, prey abundance and the distance traveled between winter refugia and seasonal foraging areas. Individual linear movements can also be significant. One Georgia specimen made a long-distance, interpopulation movement of 22 km. (14 mi.).
Eastern Indigo Snakes are most vagile during the warmer months of the year (April to October). During this time they move often and visit numerous habitats, especially low, shaded forests and wetlands where snake, frog and rodent prey are abundant. Eastern Indigos are inactive for roughly two weeks when undergoing ecdysis. During this time they are often found in Gopher Tortoise burrows or similar retreats.
Diet
Eastern Indigo Snakes are indiscriminate carnivores known to feed on virtually any vertebrate they can overpower. They are robust predators that overpower their prey by using strong jaws while pinning their prey items to the ground with a body coil. When feeding on snakes, they chew until the snake is immobilized and then swallow the snake head first. Eastern Indigos often consume rattlesnakes and cottonmouths and can do so because they have a high degree of immunity to the venom of these species and suffer no lasting injury if they are bitten by them.
Eastern Indigo Snakes are known to feed on fish, frogs, toads, small alligators, hatchling aquatic turtles, hatchling and juvenile Gopher Tortoises, lizards, other snakes, birds and their eggs, and small mammals. An adult Eastern Indigo found in southern Georgia regurgitated a Pigmy Rattlesnake (Sistrurus miliarius), a hatchling Gopher Tortoise, a Southern Hognose Snake (Heterodon simus) and a Southern Toad (Bufo terrestris). Eastern Indigos are sometimes cannibalistic, but observations documenting this for wild snakes are rare.
Reproduction
Eastern Indigo Snakes have been characterized as a “late-maturing colubrid snake.” These traits include high adult survivorship, high longevity, low to medium fecundity, small annual clutches, low juvenile survivorship, male-biased sexual size dimorphism, high ratio of mature to immature individuals in the population, and a significant proportion of the population that is older than four years old.
Sexual maturity in Eastern Indigos is reached in three to five years of age at a total length of1.5 to 1.8 m. (5 to 6 ft.). Male Eastern Indigos are estimated to reach sexual maturity in two to three years, and females in three to four years. In southeast Georgia, it is likely that males and females first breed when three to four years old. A male-biased sex ratio of 2:1 male to female appears to be the norm in Georgia and Florida. This may be the result of higher energetic costs associated with reproduction, higher predation rates and/or higher overwinter mortality in females.
Eastern Indigo Snakes are active in the winter, emerging from refugia during suitable temperatures to bask and breed. With the advent of cooling temperatures and shortening day lengths in autumn, adult Eastern Indigos concentrate on sand ridges and other upland habitats to breed. Competition among males for mates may be intense, resulting in male-male combat rituals. This involves the males intertwining as they wrestle and fight, sometimes biting each other on the neck and inflicting deep gashes. The height of the mating season is November through January, but breeding can occur from October through March.
Females lay a single clutch of 4 to 14 large eggs between April and June, usually in an open-canopied sandy microhabitat. Eggs measure approximately 7.6 cm. (3 in.) in total length. The eggs are soft-shelled, oval and granular-surfaced. Females have the ability to retain live sperm for prolonged periods, possibly over four years, for later release and egg fertilization. Reports of nest sites are rare, but those located in the field have been associated with Gopher Tortoise burrows including abandoned burrows. Eggs hatch in August to September, and neonates measure 40.6 to 61 cm. (1 ft. 3.98 in. to 2 ft.) upon hatching.
Conservation Concerns
Habitat loss and fragmentation are the most significant causes of Eastern Indigo Snake decline. The species requires large expanses of natural habitat for both overwintering and foraging. Due to the prevalence of private land and small parcel sizes, the southeastern Coastal Plain is a patchwork of different land uses with little continuity across the landscape. Specifically, urban and rural development, agriculture, roads, altered fire regimes, forestry and conversion to pine plantations have fragmented habitats, resulting in negative impacts to Eastern Indigo Snakes.
The effects of habitat fragmentation on Indigo Snakes are most severe in peninsular Florida, where human populations and development continue to increase. However, Eastern Indigo Snakes in Florida are often observed in fragmented or disturbed habitats, such as suburban developments or along canal banks. As a result, the relationship between population viability and habitat fragmentation for Eastern Indigo Snakes in Florida may be more complex than we realize. It is essential that we understand these peculiar issues so that we can determine the impacts of habitat fragmentation and where threats are most acute.
To accomplish this, the Orianne Society is conducting research in central and southern Florida on Eastern Indigo Snakes to understand how they respond to habitat fragmentation and non-natural landscapes. We are using radio telemetry to determine how individual Eastern Indigo Snakes respond to habitat fragmentation by measuring their movement patterns and habitat selection across different landscape types. These individual level responses will be used to develop population viability models that will allow us to determine if Eastern Indigo Snake populations in non-natural landscapes can persist into the future. We will then use genetic techniques to determine how habitat fragmentation and non-natural landscapes influence gene flow and connectivity among these populations.
Declines in Gopher Tortoise populations are likely having a negative effect on Eastern Indigo Snakes due to the snake’s reliance on tortoise burrows for overwintering sites, particularly in southern Georgia and northern Florida. The Orianne Society is conducting a thermal ecology study to determine how the importance of Gopher Tortoise burrows as refugia for Eastern Indigo overwintering habitat and how this varies across the range of the snake. Specifically, we are examining the thermal (i.e., temperature) environment that Gopher Tortoise burrows provide for Eastern Indigo Snakes, as well as corresponding surface temperatures. We are doing so year-round at multiple focal sites distributed across the snakes’ range. We have recently finished collecting over two years of environmental temperature data, as well as body temperature data from Eastern Indigo Snakes in southern Georgia and southern Florida. Over the next several months, we will analyze these data to determine what thermal constraints Eastern Indigo Snakes face across their range and how they use the available thermal environment to mitigate those constraints.