implement aspect in least cost path analysis to account for downhill and neutral ground movement

I am creating a least cost path model for large species that migrate over large distances across a rugged landscape (elk, deer, etc.)

To calculate movement cost I am using a slope raster but I realize that I should also take into consideration aspect to figure out if an animal is moving downhill, uphill or just along a contour. The idea is to have no penalty for moving downhill.

I know ArcGIS has a least cost path analysis but I am not sure how to implement direction of travel and aspect. This paper http://www.sciencedirect.com/science/article/pii/S0168159100001015explains it well but does not give any details on how it was implemented ArcGIS. Is there something already built-in ArcMap or R that I can use?