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How did the pronghorn cross the road?

 

Fig 1 The fastest land mammal in North America has a transportation problem.  It doesn’t like to cross certain roads.  Specifically, they avoid heavily-traveled roads lined with fences.  Since pronghorn live in the grasslands that also serve as cattle grazing allotments, nearly all paved roads through pronghorn habitat have fences to prevent cattle from wandering into the road and becoming a safety hazard.  At first this may seem beneficial to pronghorn as well.  If the fences keep them out of the roads, they can’t get hit and killed by vehicles.  However, while cattle can be brought across roads by ranchers, pronghorn can be completely cut off from valuable resources and needed migration or escape routes.

Fig 2 This fragmentation of habitat can have a very serious impact on pronghorn herds.  They may not be able to obtain enough food or water reducing survival or their ability to produce offspring and keep the herds from declining in numbers.  In the past, large numbers of pronghorn have perished because fences and roads prevented them from moving to lower elevations away from blizzard conditions.  A less obvious consequence of the isolation is the inbreeding that can occur with fewer individuals contributing to each subpopulation.  The resulting lack of genetic variation greatly increases the population’s susceptibility to disease and climate change.

In order to mitigate these effects of our growing highway infrastructure, we must determine the best way to safely get pronghorn across our roads.  The Arizona Department of Transportation is funding research (implemented by Arizona Game and Fish Department’s Research Branch) so this question can be addressed as Fig 2improvements are made to highways within pronghorn habitat.  The first step in this process is figuring out where pronghorn would be most inclined to cross the road.  Using GPS-equipped radio collars, we are monitoring the movements of pronghorn on both sides of US Highway 89.   We will determine the permeability of the road, and probably more important in this case, where the pronghorn are coming closest to the road.  We are also collecting genetic samples to investigate the extent to which the highway has impacted gene flow. 

Using this data and past studies of pronghorn-road-crossing behaviors, we will make recommendations to ADOT for placement and design of crossing structures.  Since pronghorn rely heavily on their field of vision, underpasses such as those used for elk and deer on highway 260 may not be utilized for fear of predators.  Instead recommendations may include overpasses where the pronghorn cross the highway above the grade of traffic

For more information contact:
Scott Sprague, Arizona Game and Fish Department
5000 W. Carefree Highway, Phoenix, AZ 85023
Phone: (623) 236-7252           E-mail: ssprague@azgfd.gov

 



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