Pantherophis obsoletus (Say, 1823)
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Pantherophis obsoletus, Elaphe obsoleta
Basis for Listing
Although widespread across the central and northern Great Plains, the Western Ratsnake (Pantherophis obsoletus) has been documented in only a few locations in the southeast corner of the state (Paleozoic Plateau Section). Its limited Minnesota distribution makes it vulnerable to the loss of woodland habitat to agricultural, commercial, and industrial development. The species is susceptible to local extirpation by the destruction of den sites, where large numbers of snakes aggregate for overwintering, The Western Ratsnake is also vulnerable to collection for the pet trade and arbitrary 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, extensive surveys have been conducted, however, only a few additional records in Houston and Fillmore counties were obtained (Brecke 1997; Hall and LeClere 2013). With the confirmation of its rarity within the state, and the continued presence of the threats listed above, 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 ground color. The scales are weakly keeled, and the anal plate is divided. Young Western Ratsnakes are easily confused with the young of several 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 forested, rocky bluffs and rock outcrops, woodland edge, and low forested valleys (mesic hardwood forest and fire dependent forest). 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). Individuals may bask 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. Extremely arboreal, Western Ratsnakes are often found high up in trees, where they seek shelter in 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, however, 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. The young are eaten by other snakes, carnivorous mammals, and birds of prey (Oldfield and Moriarty 1994). The Western Ratsnakes’ first line of defense is to rely on their dark, cryptic coloration to remain undetected, but if harassed they will vibrate their tails, raise their heads, and strike at their 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 and alteration. The development of forested bluffs for human housing causes fragmentation of habitat and increases the potential for snakes to be killed 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 refuge 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. Several specimens from Minnesota have shown signs of Snake Fungal Disease (SFD), a disease similar to ‘White-nose Syndrome in bats.
Conservation Efforts in Minnesota
Since its discovery in Minnesota in 1942 (Breckenridge 1944a and b), the Western Ratsnake has remained an understudied, rare, snake species in the state. It is often difficult to detect because of its arboreal habits. Surveys conducted within the range of this species by the Department of Natural Resources’ Minnesota Biological Survey as well as (MNDNR) Nongame Wildlife Program funded surveys by the Minnesota Herpetological Society were originally unsuccessful in documenting new populations. Then, in the late 1990s and early 2000s, targeted studies funded by the (MNDNR) Nongame Wildlife Program to survey historic locations of the Western Ratsnake were successful (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 State Wildlife Grant study 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.
Author: James E. Gerholdt, March 2001
Revised: Jeffrey B. LeClere, MNDNR, 2016
Breckenridge, W. J. 1944. Reptiles and amphibians of Minnesota. University of Minnesota Press, Minneapolis, Minnesota. 202 pp.
Burbrink, F. T. 2001. Systematics of the Eastern Ratsnake Complex (Elaphe obsoleta). Herpetological Monographs, Volume 15, pp. 1-53. Published by Herpetologists' League.
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. 1970. Reproductive cycles in lizards and snakes. University of Kansas Museum of Natural History, Miscellaneous Publications 51:1-247.
Johnson, T. R. 1987. The amphibians and reptiles of Missouri. Missouri Department of Conservation, Jefferson City, Missouri. 368 pp.
Oldfield, B., and J. J. Moriarty. 1994. Amphibians and reptiles native to Minnesota. University of Minnesota Press, Minneapolis, Minnesota. 237 pp.