Pantherophis obsoletus (Say, 1823)
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Pantherophis obsoletus, Elaphe obsoleta
Basis for Listing
Although widespread across the central and southern Great Plains, the Western Ratsnake (Pantherophis obsoletus) has been documented in only a few locations in southeastern Minnesota (Paleozoic Plateau Section). This limited Minnesota distribution makes the species vulnerable to the loss of woodland habitat to agricultural, commercial, industrial, and residential development. The species is susceptible to local extirpation by the destruction of den sites, where large numbers of snakes may congregate to overwinter. The Western Ratsnake is also vulnerable to collection for the pet trade and indiscriminate killing by humans. Until recently, it has remained largely unstudied in Minnesota. Due to the scarcity of records and limited information on its distribution, abundance, and habitat requirements, the Western Ratsnake was listed as a special concern species in 1984. Since that time, although extensive surveys have been conducted, only three additional records in Houston and Fillmore counties have been found. With the confirmation of its rarity in the state and the continued presence of the aforementioned threats, the Western Ratsnake was reclassified as a threatened species in Minnesota in 2013.
Adult Western Ratsnakes range in size from 107-183 cm (42 -72 in.) long with a maximum recorded length of 256.5 cm (101.0 in.) (Ernst and Barbour 1989; Conant and Collins 1998). They rival the Gophersnake (Pituophis catenifer) as the longest snake in Minnesota. Dorsal coloration is black or sometimes brown with white, yellow, orange, or red pigment that often produces an obscure blotched pattern on the body. The top of the head and the tail are typically solid black. The ventral surface is gray or brown with some white, yellowish, or reddish diffusions. The chin and throat are white or cream. The young are distinctly patterned with dark blotches on a lighter gray background color. The scales are weakly keeled, and the anal plate is divided. Young Western Ratsnakes are easily confused with the young of several other snake species in Minnesota particularly young Western Foxsnakes (Pantherophis ramspotti). The most reliable method for separating the two species is by counting the ventral scales. Western Ratsnakes have 221 or more ventral scales and Western Foxsnakes have 216 or less (Moriarty and Hall 2014). Young North American Racers (Coluber constrictor) and Eastern Milksnakes (Lampropeltis triangulum) are similarly patterned, but their scales are smooth (not keeled). Eastern Milksnakes also have single anal plates (Moriarty and Hall 2014).
The Western Ratsnake is a woodland species preferring low forested valleys (mesic hardwood forest and fire-dependent forest); forested rocky bluffs and rock outcrops; and woodland edges. All of the Minnesota records for this species have been located on wooded bluffs within the Rochester Plateau and The Blufflands Ecological Subsections of the Paleozoic Plateau.
Biology / Life History
The Western Ratsnake overwinters communally in bluff top rocky crevices below the frost line and emerges from winter dormancy in late April and early May (Oldfield and Moriarty 1994). Upon emergence, individuals may bask high up in trees in the vicinity of the den for a few days before moving away from the den site. Movement may take place terrestrially or arboreally. Summer range is unknown, but reports indicate that some Western Ratsnakes travel to the valley floor in mid-summer. Western Ratsnakes are extremely arboreal and often found high up in trees. They seek shelter in cavities of large dead or hollow trees, especially oaks (Quercus spp.), which are an important summer habitat component. Very little is known about Western Ratsnake reproduction in Minnesota, though breeding probably occurs in May and early June (Moriarty and Hall 2014). Females may lay 5-44 eggs (Fitch 1970) in late June, July, or early August (Johnson 1987; LeClere 2013). Nesting may be communal; large hollow oak trees may provide suitable nesting habitat for multiple females. Other nesting sites include hollow or rotting logs and stumps. Eggs hatch in late August or September (Ernst and Barbour 1989), and hatchlings are 29-36.8 cm (11.4-14.5 in.) long. The Western Ratsnake feeds on small mammals, nestling birds, birds’ eggs, treefrogs, and lizards (Ernst and Barbour 1989), and kills larger prey by constriction. Adults have few predators and are most often killed by humans and large raptors. Juveniles are eaten by other snakes, carnivorous mammals, and birds of prey (Oldfield and Moriarty 1994). The Western Ratsnake relies on its dark cryptic coloration to remain undetected, but if harassed it will vibrate its tail, raise its head, and strike at the aggressor (Oldfield and Moriarty 1994). While Western Ratsnakes are harmless to humans, a bite from a large individual can penetrate the skin.
Conservation / Management
Minnesota Western Ratsnake populations are highly susceptible to habitat destruction, alteration, and fragmentation. The development of forested bluffs for human housing increases the potential for snakes to be killed from interactions with humans and on roads. Forested, limestone bluffs provide denning sites, and quality mature oak-hickory woodland along river valleys is important summer habitat (LeClere 2013). Large old hollow oak trees should be left in place as critical refugia for this species (LeClere 2013). This snake is often misidentified as a rattlesnake since it will vibrate its tail when threatened. Because of this, people who encounter them often feel threatened and kill them. A public education effort would help in this regard.
As of 2015, Minnesota was one of nine states with confirmed cases of Snake Fungal Disease (SFD) in naturally occurring snake populations (Allender et al. 2015). Population-level impacts of SFD on the Western Ratsnake are difficult to assess due to the cryptic and solitary nature of the species.
Conservation Efforts in Minnesota
The Western Ratsnake is often difficult to detect because of its arboreal habits, and historically there have been few records of this species in Minnesota. After the state record was documented in 1942 (Breckenridge 1944a, 1944b), the Western Ratsnake received little attention in Minnesota with roughly 40 years passing until the next documented sighting (Weiss 1982). Between the late 1980s and mid-1990s, general amphibian and reptile inventories conducted within the species' range by the Department of Natural Resources’ Minnesota Biological Survey and the Minnesota Herpetological Society were unsuccessful in obtaining new records. Then, in the late 1990s and early 2000s, targeted surveys focusing on historic locations were successful in locating additional records and identifying important tracts for conservation (Brecke 1997; Hall and LeClere 2013).
Protection of large forested tracts near active dens will help ensure the survival of the Western Ratsnake in Minnesota. From 2010 to 2013, a study through a State Wildlife Grant resulted in the acquisition and protection of land utilized by one Western Ratsnake population (Hall and LeClere 2013). The Nongame Wildlife Program has also developed brochures and videos to educate the public about large snakes, including Western Ratsnakes, in southeastern Minnesota.
James E. Gerholdt, 2008; Jeffrey B. LeClere, MNDNR, 2017
Brecke, B. J. 1997. Documentation of ratsnake (Elaphe obsoleta) dens in southeastern Minnesota. Report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources, 4 pp.
Breckenridge, W. J. 1944. Reptiles and amphibians of Minnesota. University of Minnesota Press, Minneapolis, Minnesota. 202 pp.
Breckenridge, W. J. 1944. The pilot black snake in Minnesota. Copeia (1):64.
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Coffin, B., and L. Pfannmuller, editors. 1988. Minnesota's endangered flora and fauna. University of Minnesota Press, Minneapolis. 473 pp.
Conant, R., and J. T. Collins. 1998. A field guide to reptiles and amphibians of eastern and central North America. Third edition. Houghton Mifflin Company, New York, New York. 616 pp.
Ernst, C. H., and R. W. Barbour. 1989. Snakes of eastern North America. George Mason University Press, Fairfax, Virginia. 282 pp.
Fitch, H. S. 1963. Natural history of the black rat snake (Elaphe o. obsoleta) in Kansas. Copeia 1963(4):649-658.
Fitch, H. S. 1970. Reproductive cycles in lizards and snakes. University of Kansas Museum of Natural History, Miscellaneous Publications 51:1-247.
Hall, C., and J. LeClere. 2013. Surveys and development of monitoring techniques for reptile species in greatest conservation need in Minnesota. Minnesota State Wildlife Grant Final Report, 42 pp.
Johnson, T. R. 1987. The amphibians and reptiles of Missouri. Missouri Department of Conservation, Jefferson City, Missouri. 368 pp.
LeClere, J. B. 2013. A field guide to the amphibians and reptiles of Iowa. ECO Publishing, Rodeo, New Mexico. 350 pp.
Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 394 pp.
Moriarty, J. J., and C. D. Hall. 2014. Amphibians and reptiles in Minnesota. University of Minnesota Press, Minneapolis. 370 pp.
Oldfield, B., and J. J. Moriarty. 1994. Amphibians and reptiles native to Minnesota. University of Minnesota Press, Minneapolis, Minnesota. 237 pp.
Prior, K. A., and P. J. Weatherhead. 1996. Habitat features of black rat snake hibernacula in Ontario. Journal of Herpetology 30:211-218.