Crotalus horridus Linnaeus, 1758
Click to enlarge
Basis for Listing
The timber rattlesnake occurs in the eastern and central United States. Minnesota is on the northwestern periphery of its range, which extends north along the Mississippi River from Illinois to Minnesota. Timber rattlesnake specimens have been collected from 8 counties in southeastern Minnesota, but survey efforts in the late 1990s and early 2000s found that populations were substantially reduced or extirpated from several areas where they occurred historically (Keyler and Fuller 1999; Keyler and Oldfield 2003). The major cause of the timber rattlesnake's decline is its vulnerability to systematic and willful destruction by humans. Although the species is secretive and not aggressive, people's fear of it led to the creation of a bounty in Minnesota in 1909. Records for Houston County indicate a marked decline in that county's timber rattlesnake population, with nearly 5,000 bounties paid in 1970, but only 191 in 1987 (Keyler and Oldfield 1992). Habitat destruction, road mortality, and collection for the pet trade are other factors in the species decline. In 1984, the timber rattlesnake was designated a special concern species in Minnesota, and in 1989 the bounty was repealed. Because of declining populations, the timber rattlesnake was reclassified as threatened in 1996.
The timber rattlesnake is a large snake, averaging 80-122 cm (31.5-48 in.) in length (Oldfield and Moriarty 1994). It has a broad, triangular-shaped head and narrow neck, a distinctive barred body pattern, significantly keeled scales, a gray to tan rattle with 1 to over 13 segments, and a single anal plate. Timber rattlesnakes also have a sensory pit, visible on each side of their head, between their eye and nostril. Their eyes have vertically elliptical pupils. The background color of their bodies varies from gray to yellow to dark brown, but their tail is always solid black. The timber rattlesnake is 1 of 2 venomous snakes in Minnesota, the other being the eastern massasauga (Sistrurus catenatus). While the eastern massasauga also has a gray to tan rattle, it is a smaller snake (47-76 cm; 18.5-30 in. long) and lacks the distinctive banded pattern of the timber rattlesnake. Several species of harmless snakes, including the milksnake (Lampropeltis triangulum), western foxsnake (Pantherophis vulpinus), and gophersnake (Pituophis catenifer), may be confused with the timber rattlesnake. However, none of these species have facial sensory pits or a rattle, their body markings are blotched rather than barred, and their pupils are round rather than elliptical. The buzzing sound of a timber rattlesnake's rattle is generally a diagnostic characteristic; however, other snakes will rapidly vibrate their tails when threatened, often resembling the sound of a rattlesnake.
In Minnesota, the ideal habitat for timber rattlesnakes includes forested bluffs, south-facing rock outcrops, and bluff prairies, particularly in the Mississippi River valley. Bluff prairies located on steep, south or west-facing hillsides, with rock outcroppings and ledges, are essential habitat components because over-wintering dens are often located in these areas. Surrounding forests, prairies, and agricultural lands are used as summer feeding grounds. Two necessary habitat components for this species are open areas for thermoregulation and dens for over-wintering.
Biology / Life History
Timber rattlesnakes live within small home ranges that include three seasonal components: summer range, transient habitat, and over-wintering sites. Summer habitat is used for foraging and loafing, and encompasses an area from 300 m (0.19 mi.) up to 6.4 km (4 mi.) from the den site, depending on the sex and reproductive status of the snake (Minnesota DNR 2009). Gravid females stay closer to the den site, using open, rocky areas that are thought to provide higher ground temperatures needed for gestation. Males and nongravid females travel farther from the den site, using wooded areas and woodland edges. Transient habitat, also referred to as a staging area, is the area used by snakes when moving between summer and winter habitat and includes open woodlands with exposed clearings and shelter rocks (Minnesota DNR 2009). Winter habitat includes a den, also known as a hibernaculum, usually located in rocky areas of cliff ledges, or talus slopes that provide crevices for protection from cold weather and predators (Brown 1993). In Minnesota, dens are typically located on south and west facing slopes, which allow for warmer surface temperatures (Minnesota DNR 2009). Den sites are communal and can be shared with other snake species including gophersnakes, North American racers (Coluber constrictor), and milksnakes (Oldfield and Moriarty 1994). Timber rattlesnakes typically use the same den year after year (Brown 1989), although some snakes have been found to use multiple dens within the same area (Adams 2005). Timber rattlesnakes emerge from their dens in late April or early May, and return to them in late September or early October.
Conservation / Management
The timber rattlesnake's relatively long lifespan, low reproductive rate, infrequent reproductive events, and high juvenile mortality leads to a population dominated by older age classes with variable annual recruitment (Minnesota DNR 2009). As a result, the impact of removing a single adult, especially a female, from a population can be significant (Brown 1993). Therefore, conservation of timber rattlesnakes must concentrate on protection of both individual snakes and habitat.
Conservation Efforts in Minnesota
Surveys for timber rattlesnakes in Minnesota have been funded and coordinated by a collaborative effort of the Minnesota DNR Nongame Wildlife Program (NWP), Division of Parks and Trails, and Minnesota Biological Survey for almost 20 years. Formal surveys began in 1990-1991 at 6 southeastern Minnesota State Parks. Since then, a number of surveys and research projects have been conducted, including surveys at sites on the periphery of the species' range (Keyler and Fuller 1999); habitat assessments (Fuller 2000); DNA haplotype and lineage research (Keyler 2000); reproduction research (Keyler and Wilzbacher 2002); a transient movement study (Hanson and Keyler 2003); and population surveys and habitat management (Keyler and Oldfield 2003; Fritcher and Quinn 2005; Keyler and LeClere 2005; Fritcher and Quinn 2008). The NWP has conducted annual timber rattlesnake surveys and bluff prairie restoration on private lands in southeast Minnesota since 2003, with funding from the federal Landowner Incentive Program. In 2008, a telemetry study to evaluate timber rattlesnake use of sites that have undergone habitat management was initiated. Combining all survey efforts conducted in Minnesota up to 2008, approximately 450 bluffs have been surveyed in southeastern Minnesota and at least 122 dens have been identified, although their viability is unknown (Minnesota DNR 2009).