This work derives and fits stochastic models to the trajectories of mammals moving about in a heterogeneous landscape. The basic data are locations of 53 Rocky Mountain elk ({\it Cervus elaphus}) estimated approximately every two-hours for nine months. The elk roam about the Starkey Experimental Forest and Range in eastern Oregon. Elk movements may be affected by explanatory variables such as the locations of fences, of roads, of cover, of water, of forage and other habitat characteristics. Wildlife biologists are interested in questions like how an elk's movement relates to such explanatories. In the work a model was developed in successive stages. First equations of motion were set down motivated by the idea of a potential function. Then the functional parameters appearing in the equations were estimated nonparametrically. Statistical questions arising involved how to include explanatory variables in the equations and how to decide which variables are significant? Residual plots proved useful. Time of day was found to play a fundamental role and distance to nearest road enters as well. Future work will include other explanatories.