Pronghorn and Mule Deer Use of Underpasses and Overpasses along US Highway 191

The seasonal migrations of ungulates are increasingly threatened by various forms of anthropogenic disturbance, including roads, fences, and other infrastructure. Although roadway impacts (e.g., wildlife-vehicle collisions and landscape permeability) to species such as mule deer (Odocoileus hemionus) can largely be mitigated with underpasses and continuous fencing, similar mitigation may not be effective for pronghorn (Antilocapra americana) or other ungulate species that are reluctant to move through confined areas. The Wyoming Department of Transportation recently installed 6 underpasses and 2 overpasses along 20 km of U.S. Highway 191 in western Wyoming, USA, where we evaluated species-specific preferences by documenting the number of migratory mule deer and pronghorn that used adjacent overpasses and underpasses for 3 years (autumn 2012 to spring 2015) following construction. We also measured the amount of back-and-forth movement across the highway for each species through time. We documented 40,251 crossings of the highway by mule deer and 19,290 crossings by pronghorn. Of those highway crossings, 79% of mule deer moved under, whereas 93% of pronghorn moved over the highway. These strong species-specific differences were evident at both sites and support the notion that overpasses are more amenable to pronghorn than underpasses. Concurrently, we documented a 64-306% increase in the amount of back-and-forth movement of mule deer and pronghorn across the highway during migration periods. Such movement flexibility is presumed to improve their ability to respond to changing environmental conditions by easily accessing habitats on either side of the highway. Our results highlight that species-specific preferences are an important consideration when mitigating roadway impacts with wildlife crossing structures. (C) 2016 The Wildlife Society.