How do Highways Influence Snake Movement? Behavioral Responses to Roads and Vehicles

Roads affect animal survivorship and behavior and thereby can act as a barrier to movement, which exacerbates habitat fragmentation and disrupts landscape permeability. Field experiments demonstrated that interspecific differences in ecology and behavior of snakes affected responses of species when they encountered and crossed roads. The probability of crossing a road varied significantly among southeastern U.S. snakes, with smaller species exhibiting higher levels of road avoidance. Species also differed significantly in crossing speeds, with venomous snakes crossing more slowly than nonvenomous ones. All species crossed at a perpendicular angle, minimizing crossing time. A model incorporating interspecific crossing speeds and angles revealed that some species cannot successfully cross highways with high traffic densities. Individuals of three species immobilized in response to a passing vehicle, a behavior that would further prolong crossing time and magnify susceptibility to road mortality. Identifying direct and indirect effects of roads on snakes is essential for mitigating road impacts and for designing effective transportation systems in the future.

Figure 1.

Overhead view of single release site showing placement of the blind and the fence in relation to the release point at the road edge

Figure 2.

Road avoidance rates for nine southeastern snake species.Asterisks above bars represent species that significantly deviated from expected (P < 0.05). Black = individuals that retreated to the woods without entering the road (i.e., no attempt). Gray = individuals that attempted but did not cross the entire road (i.e., deter). Species had a highly significant effect on crossing probability (P < 0.0001). Sample sizes, in order by species, are C.c. 54, A.p. 25, E.g. 13, N.f. 20, E.o. 26, C.h. 16, H.p. 14, T.c. 8, D.p. 9

Figure 3.

Crossing speeds for all target species with n > 10 crossing occurrences.The effect of species on crossing speed was highly significant (P < 0.0001). Sample sizes, in order by species, are C.h. 20, A.p. 29, E.g. 13, H.p. 14, E.o. 17, N.f. 19, C.c. 73

Figure 4.

Probability of mortality for three different snake species at varying traffic densities based on observed crossing speeds.Traffic densities characteristic of low, medium, and high flows are presented for hypothetical roads. Note that densities achieved in cities, or in some areas during peak patterns (>15,000 vehicles/day) are not represented. Model is adapted from Hels and Buchwald (2001) in which p(death) = 1−e⁻(Na/v(cosα)) where N = traffic density (traffic/lane/day), a = kill zone width, v = average velocity of snake species (m/sec), and α = deviation from a perpendicular cross

Contributor Notes

Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, South Carolina 29802. (KMA) andrews@srel.edu; and (JWG) gibbons@srel.edu. Send reprint requests to KMA.

Accepted: 15 Jun 2005