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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. 
Habitat
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.
Parasitism
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.
Prey
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.
Hibernation
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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