Pantherophis obsoletus (Say, 1823)
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Pantherophis obsoletus, Elaphe obsoleta
Basis for Listing
Although widespread across the central and eastern United States and into southern Ontario, the western ratsnake has only been documented in a few locations in southeast Minnesota. Its limited distribution makes it vulnerable to habitat destruction, and the loss of bluff prairie and woodland habitat to agricultural, commercial, and industrial development has been a major factor in the species' decline. The western ratsnake is also vulnerable to collection for the pet trade and indiscriminate killing. For these reasons, it was listed as a special concern species in Minnesota in 1984.
Adult western ratsnakes range in size from 107-183 cm (42 -72 in.) long, with the record size being 256.5 cm (101 in.) long (Ernst and Barbour 1989; Conant and Collins 1998). They are the longest snake in Minnesota. The dorsal, or upper, color is black with faint markings and blotches of yellow or white. The ventral, or belly, surface is gray or brown with some white or yellowish diffusions. The chin and throat are white or cream. The young have a pattern of dark blotches on a lighter gray ground color. The scales are weakly keeled and the anal plate is single. Young western ratsnakes are easily confused with the young of several snake species in Minnesota. North American racers (Coluber constrictor) and milksnakes (Lampropeltis triangulum) are similarly colored, but their scales are not keeled (Oldfield and Moriarty 1994). Young western ratsnakes so closely resemble young western foxsnakes (Pantherophis vulpinus) that the best way to tell them apart is to count their ventral scales. Western ratsnakes have 221 ventral scales or more, and western foxsnakes have 216 or less (Oldfield and Moriarty 1994).
The western ratsnake is a woodland species, preferring moist forests and forest edges in the summer and rock outcrops and bluffs in the fall and spring. All of the Minnesota records for this species have come from wooded bluffs.
Biology / Life History
The western ratsnake hibernates in rocky crevices below the frost line and emerges from hibernation in late April and May (Oldfield and Moriarty 1994). Very little is known about western ratsnake reproduction in Minnesota, but breeding probably occurs in May and early June (Oldfield and Moriarty 1994). Nesting may be communal, with 5-44 eggs laid per female (Fitch 1970) in late June or early July, often in hollow logs or stumps (Johnson 1987). Hatchlings emerge 60-75 days later (Ernst and Barbour 1989), and are 29-36.8 cm (11.4-14.5 in.) long. The western ratsnake feeds on small mammals, nestling birds, eggs, tree frogs, and lizards (summarized in Ernst and Barbour 1989), and kills its prey by constriction. Adults of this species have few predators, and are most often killed by humans and hawks. The young are eaten by other snakes, carnivorous mammals, and birds of prey (Oldfield and Moriarty 1994). The western ratsnake may hibernate in groups, and as noted above, may nest communally. This is an arboreal species, and is often found high up in trees where it may seek shelter in tree cavities. They rely on their coloration to remain undetected, but if harassed they will vibrate their tails, raise their heads, and strike at their aggressor (Oldfield and Moriarty 1994). Western ratsnakes are not venomous, but a bite from an adult snake can hurt.
Conservation / Management
Western ratsnake populations are highly susceptible to habitat destruction and alteration. The development of forested bluffs for home sites causes fragmentation of habitat and increases the potential for snakes to be killed on roads. This snake is often misidentified as a rattlesnake, since it will vibrate its tail when threatened. Because of this, people who encounter them feel threatened and often kill them. A public education effort would help in this regard. Other common names for this species include black ratsnake, pilot blacksnake and Texas ratsnake.
Conservation Efforts in Minnesota
The western ratsnake is often difficult to detect because of its arboreal habits. The DNR Nongame Wildlife Program has funded efforts to survey historic locations of the western ratsnake and document new den sites. Protection of large forested tracts near active dens will help ensure the survival of this species in Minnesota. The Nongame Wildlife Program has also developed brochures and videos to educate the public about large snakes, including ratsnakes, in southeastern Minnesota.
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.