Proceedings of the National Academy of Sciences of the United States of America. 7/15/2008, Vol. 105 Issue 28, p9739-9744. 6p. 1 Color Photograph, 5 Graphs.
Proceedings of the National Academy of Sciences of the United States of America. 3/4/2003, Vol. 100 Issue 5, p2456. 6p. 12 Color Photographs, 11 Graphs.