The influence of thermal biology on road mortality risk in snakes

• We investigated the relationship between ambient temperature and road occurrence in seven snake species in southeastern Louisiana by comparing observations of live snakes on a black-topped road, across measurements of air temperature and road temperature on survey days.
• We compared how air and road temperatures affected occurrence on the road between sexes in the Nerodia fasciata, N. cyclopion, Thamnophis proximus, and Pantherophis obsoletus.
• Males and females of the viviparous species N. fasciata, N. cyclopion, and T. proximus diverged significantly in temperature preferences while males and females of the oviparous species P. obsoletus did not. Road temperature was also the strongest driver of differences between sexes.
• Significant influences of body size on detection temperatures were documented for N. fasciata, N. cyclopion, and T. proximus where larger individuals where associated with warmer temperatures.

Road mortality is a significant threat to terrestrial vertebrates in many areas, and the novel thermal environment of black-topped roads may represent ecological traps for some species and demographic groups. We investigated the relationship between ambient temperature and on-road detection in a snake assemblage in southeastern Louisiana by comparing observations of live snakes on a black-topped road, across measurements of air temperature and road temperature on survey days. Analyses indicated on-road detection of snakes was significantly influenced by ambient temperature conditions for five snake species. Additionally, road temperatures, and the difference between air and road temperatures, were strong drivers of on-road snake detections. Permutation analysis methods revealed that significant temperature related group (species or sex) structure exists in occurrences of snakes on the roadway, and that road temperature was the strongest driver of species differences. We also compared how air and road temperatures affected occurrence on the road between sexes in the colubrid snakes Nerodia fasciata, Nerodia cyclopion, Thamnophis proximus, and Pantherophis obsoletus. Males and females of the viviparous species N. fasciata, N. cyclopion, and T. proximus diverged significantly in temperature preferences, with females found under warmer conditions, while males and females of the oviparous species P. obsoletus did not. Road temperature was also the strongest driver of differences between sexes. Our results indicate that black-topped roads are an ecological trap that is heavily influenced by sex, reproductive condition, and species specific thermoregulatory requirements, particularly for viviparous species.