Crotalus horridus

Pisani et al. (1972) conducted a multivariate analysis of variation in C. horridus and concluded that characters tended to be clinal and recommended against recognition of the two subspecies. Brown and Ernst (1986) countered that morphology in the eastern part of the range supported recognition of coastal plain and montane subspecies. Clark et al. (2003) identified three mitochondrial DNA lineages separated by the Appalachian and Allegheny Mountain ranges that did not correspond with the classic arrangement of subspecies within C. horridus (Crother 2017).

The traditional view of rattlesnake taxonomy that recognizes the two monophyletic sister genera Crotalus and Sistrurus (e.g. Brattstrom 1964) has recently been challenged. Stille (1987) and McCranie (1988) presented data that suggested Sistrurus is not monophyletic and rendered Crotalus paraphyletic. Parkinson (1999) found Sistrurus monophyletic but its position rendered Crotalus paraphyletic. Knight et al. (1993) used mtDNA to defend the traditional generic taxonomy, but in order to do so ignored the most parsimonious tree. The genus Crotalus is monophyletic when including the Mexican C. ravus (Murphy et al. 2002), and is supported as such in most recent phylogenies, as well as being the sister taxon to a monophyletic Sistrurus (e.g., Pyron et al. 2013) (Crother 2017).

Large range in the eastern United States, but occurrence is spotty in most regions; most populations have been depleted or exterminated by humans, and most are declining.

The range extends from central New England to northern Florida, and west to eastern Texas, central Oklahoma, eastern Kansas, southeastern Nebraska, southern and eastern Iowa, and southeastern Minnesota (Martin, in Tyning 1992; Ernst and Ernst 2003; Campbell and Lamaer 2004). The distribution is spotty along the western and northern edges of the range. Sizeable populations still occur in the Appalachian Mountains from Pennsylvania though the Virginias, across eastern Kentucky and Tennessee to northeastern Alabama, in the Ouachita and Boston mountains of Arkansas and extreme eastern Oklahoma, in heavily wooded sections of the southeastern Coastal Plain from North Carolina to northeastern Florida and west to Louisiana and southern Arkansas, and in the Piedmont in the Uwharrie National Forest of central North Carolina and Pine Mountain of west-central Georgia (Martin, in Tyning 1992). See Martin (in Tyning 1992) for a detailed range map and further details on current known distribution. Dens occur at elevations of up to about 5,000 feet in the southern Appalachians, 2,200 feet in southern New England, and about 1,300 ft in northeastern New York, Wisconsin, and Minnesota; individuals may range to higher elevations in summer (Martin, in Tyning 1992).

This species is represented by a large number of occurrences (see Martin, in Tyning 1992), though most may not represent populations that are viable in the long term.

Summary of primary threats: loss of habitat; habitat fragmentation and isolation of populations, which may become small and nonviable; and direct mortality caused by humans (including illegal snake hunters) and vehicles as habitat is encroached upon by urban/residential development (Brown, in Tyning 1992; Brown 1993).

1. HABITAT DESTRUCTION: In states where the snake is legally protected, housing developments near rattlesnake dens are causing the most serious problem. Current laws generally do not mandate habitat protection for endangered or threatened species. In some areas, persons encountering rattlesnakes in new developments have cooperated with protection efforts by calling persons authorized to catch and transport live rattlesnakes. This is an important factor in successfully protecting rattlesnake populations near developments.

2. MARKET HUNTING: Bounty systems have caused a high level of deleterious exploitation and significant reduction or extirpation of populations by a mere handful of people (W. S. Brown, unpubl. data). In some areas (e.g., Pennsylvania) bounty hunting led to rattlesnake hunting among the general population and became a major outdoor activity promoted by sports and civic groups. Commercial collecting for the pet trade is an ever-present current threat, despite some decline in recent years. Single individuals have been responsible for removal of several thousand snakes (Stechert 1980). Today, a growing number of persons maintain reptiles in private collections. Timber rattlesnakes, beautiful and easily kept in captivity, are much sought-after. Private collectors are supplied by an often illicit network of collectors, dealers, and buyers.

3. SNAKE HUNTING FOR "SPORT," ORGANIZED SNAKE HUNTS OR "ROUND-UPS." In Pennsylvania, organized snake hunts caused injury and cruelty to captured snakes, displacement from familiar range, removal of gravid females from already-depleted populations, and habitat destruction by snake hunters (Galligan and Dunson 1979, Reinert 1988). Despite regulations, timber rattlesnake populations in Pennsylvania were legally "harvested" at a nonsustainable level, leading to the collapse of most den populations (Martin et al. 1990). In response, Pennsylvania regulations were amended such that the open season extends from the second Saturday in June through July 31, with daily bag limit of one snake. These regulations should protect snakes at den sites, prevent stockpiling of snakes, and discourage rattlesnake hunting altogether.

4. SHADING-OVER: In some regions, several investigators believe that "shading over" by the growth of large trees on and near a den may be causing conditions that are incompatible with long-term viability for timber rattlesnakes. According to this view, there is a need for an open, lightly wooded or brushy early successional plant association to provide an optimal denning environment. In contrast, Martin (1990, pers. comm.) says: "Shading over of the den site does not present a problem for snakes at emergence time when the trees are bare or just starting to leaf. The problem is shading over of the rocks that are used as gestating and birthing rookeries." Similarly, in Connecticut, Hammerson (pers. obs.) found that heavy shading (in summer) did not discourage den use, but he observed reduced use of a gestation/birthing site after it was shaded by growing vegetation. The possible threat of shading-over deserves further study, but it seems likely that shading has temporary, localized effects that are insignificant over the long term on a landscape scale.

5. LOGGING: Commercial tree removal may not necessarily cause long-term harm to a timber rattlesnake population's habitat, but can pose a direct threat to the snakes if conducted during the active season (mainly April-October).

6. ROAD MORTALITY: New Jersey Pine Barrens populations suffer from excessive mortality of gravid females due to vehicular traffic on roads and trails (Zappalorti and Reinert, in Tyning 1992). Ill-placed developments and associated new roads and increased traffic can lead to increased mortality of rattlesnakes even in areas several miles from the development.