An early draft was read by Edwin M. Banks, Peter Marler, and Michael J. A. Simpson. I am grateful to them for their comments. Thanks are also extended to C. Richard Dawkins, Richard F. Green, and David L. G. Noakes for reading the final version. The research was supported by grants GB 5314 and GB 6604 from the National Science Foundation.
Dept. of Zoology & Museum of Vertebrate Zoology, University of California, Berkeley, California 94720, U.S.A.