1. Hamilton, W. D. J. theor. Biol. 7, 1–52 (1964).
2. Sherman, P. W. in Sociobiology: Beyond Nature/Nurture (eds Barlow, G. W. & Silverberg, J.) 505–544 (Westview, Boulder, 1980).
3. Bekoff, M. in Parental Care in Mammals (eds Gubernick, D. & Klopfer, P. H.) 307–346 (Plenum, New York, 1981).
4. Wilson, E. O. Sociobiology: The New Synthesis (Harvard University Press, 1975).
5. O'Hara, R. K. & Blaustein, A. R. Anim. Behav. (submitted).