This research is part of my Ph.D. dissertation. I gratefully thank my dissertation director, E.H. Lindsay for his guidance throughout this project. I would also like to give my special thanks to the late C.A. Repenning for his consultation on arvicolines. I thank D. Bently at the Image Lab of the University of Arizona for his help with the SEM examination. I have received great help in the field from the late W.R. Downs, K.F. Downing, L.E. Park, L.J. Roe, O. Fejfar, J.L. Pederson, C. Pederson, K. Iris, J.H. Harrison, E.E. Erickson, and K. Lindsay.
This research was supported by the NSF grant EAR 9526386.