I thank K. Carpenter, J. Phillips, T. Pantzer, L. Witmer and two anonymous reviewers for helpful comments on earlier drafts; J. Farlow for encouragement to pursue these questions; C. Brochu, T. Holtz, K. Stevens, T. Rowe, M. Everhart and K. Sabath for discussion of various details; Indiana University's Center for the Integrative Study of Animal Behavior (NSF BIR 9413220) ; and D. Brainard (NIH EY 10016) for supporting me during the preparation of this manuscript.