Proceedings of the National Academy of Sciences of the United States of America. 4/29/2008, Vol. 105 Issue 17, p6302-6307. 6p. 1 Color Photograph, 5 Black and White Photographs, 4 Graphs.
Proceedings of the National Academy of Sciences of the United States of America. 6/24/2003, Vol. 100 Issue 13, p7853. 6p. 4 Color Photographs, 1 Diagram, 1 Chart, 10 Graphs.