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Scientific name: Elaphe obsoleta obsolete
Common name: 
Black Rat Snake

(Information in this species page was compiled by Phaedra Wray in Biology 220W, Spring 2003, at Penn State New Kensington).

The black rat snake (Elaphe obsoleta obsolete) is one the most impressive animals found in the biotic community of our Nature Trail. Individuals of this species may reach lengths of 7 to 8 feet and is, thus, the longest snake naturally occurring over its broad, geographic range of the eastern United States (west to Wisconsin and parts of Texas) and southern Ontario. Its long, relatively thin, but very muscular body and straight sides (which gives it a "loaf-like" appearance) make it a very recognizable species despite its wide range of possible color variations (black (of course), but also gray and even yellow). Most typically the dorsum (back) of this species is solid black and the venter (belly) is gray along most of its body length. The gray belly coloration changes into a solid white at the throat. There may also be a series of white spots and speckles running along its sides.

Sexual Dimorphism
The male black rat snake, unlike many of the other snake species described at this website, is typically larger than the female. Males also reach sexual maturity earlier than females (7 years vs. 9 years). Temperature and the length of the active, feeding season influence the growth rate, maturation rate, and ultimate size of these snakes. Females are thought to be shorter than males because of the energy cost involved in the production of eggs.

Habitats favored by black rat snakes are varied. A deciduous forest with many field and grassland edges (“ecotones”) is probably ideal. This complex patchwork of sub-habitats provides the snake with a rich food supply (mostly small rodents, birds, bird eggs, other snakes, and amphibians), protective cover, thermoregulation sites (both sunny basking sites and shady, cool refuges), winter hibernaculae, and ovipositional (“egg laying”) sites. The black rat snake is an excellent climber and may use trees to flee from potential predators and to seek out birds and bird nests for food. This snake is also a frequent resident of farm buildings and may function as an important control agent for rodents in barns, corn cribs, and other outbuildings.

Mating and Reproduction
Black rat snakes are oviparous (egg laying) and iteroparous (with repeating reproductive cycles). The typical breeding season runs from May to late June. Within a given locale, most males will begin to display active, mate-searching behaviors at about the same time each year. During this active season, males venture far from their normal ranges in a search for females. These males fight each other for dominance and the right to mate with receptive females. Females lay between 6 and 24 eggs sometime in mid to late July. The females put a significant amount of their total body weight (up to one third of their mass!) into the production of these eggs. The number of eggs produced is directly related to the size of the female. A female is capable of reproducing only once every two or three years in cold climate zones but possibly every year in more moderate regions. The eggs are deposited under logs, in compost, manure or sawdust piles, and in hollow trees. Females about to lay eggs are found in the ecotone components of their habitats more frequently than are non-gravid females. These ecotone habitats are very complexly structured and provide not only abundant ovipositional sites, but also a range of warm basking sites and cool shady sites for the gravid female to use while maintaining her delicate and important thermoregulation balance. The eggs take anywhere from 37 to 51 days to develop and hatch.

The burying beetle (Nirtophorous pustulatus) parasitizes black rat snake eggs. The adult beetles lay their eggs in the snake eggs and the beetle larvae feed on the developing snake embryos. This parasitism is thought to be a significant cause of mortality in black snake eggs.

Development of Young
Black rat snake hatchlings are quite long (the males are over 13 inches long and the females are just under 13 inches in length). The hatchlings remain near their hatching sites for one or two years and during this time will use their birth nest as winter hibernaculae. There is a high rate of winter mortality (from freezing and dehydration) in these young snakes due to the poor, over-all quality of these ovipositional sites as hibernation dens. Also, many predators (including other snakes and hawks) readily take these young and adolescent snakes for food. Hawks and other raptors readily prey on adult snakes, too. This predator pressure explains the importance of dense cover in the snakes preferred habitats. Humans are also a major cause of young and adult snake mortality.

Black rat snakes are powerful constrictors that are capable of using their excellent chemosensory systems to locate prey organisms. After killing its prey, it is not uncommon for the black snake, now covered in the prey’s scent, to continue to follow the rich chemical trails of the prey species. This behavior may even involve the snake abandoning its fresh kill as it goes off after other individuals. The snake soon will return to its kill via chemosensory location and consume its meal. These search behaviors allow the snake to consume significant numbers of prey items in a single feeding event.

Black rat snakes may use underground sites, caves, decaying trees and tree holes, and even sheltered sites in buildings for their winter hibernaculae. Anywhere from 10 to 60 individuals may form a hibernating group. Members of this group are mostly adults and may even include snakes of other species (copperheads and rattlesnakes). Upon emergence from the hibernaculum in the spring the black rat snake spends several days basking in its favored, protected sub-habitats. These basking sites are also used following a large meal and when they are undergoing ecdysis (skin shedding).

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