1Department of Biology, Duke University, Box 90338, Durham, NC 27708, USA
2Institute of Primate Research, National Museums of Kenya, P. O. Box 24481, Karen 00502, Nairobi, Kenya
3Department of Evolutionary Anthropology, Duke University, Box 90383, Durham, NC 27708, USA
4Duke University Population Research Institute, Duke University, Box 90420, Durham, NC 27708, USA