1. Center for Comparative Medicine, Department of Medicine, University of Virginia Health System, and Department of Biology, University of Virginia, Charlottesville, Virginia 22904-0326, USA
2. US Geological Survey, Fort Collins Science Center, 2150 Centre Ave., Bldg C, Fort Collins, Colorado 80526-8118, USA
3. Animal Reproduction and Biotechnology Laboratory, Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80523, USA