1. Cohn, M.: A. Rev. Immunol.12, 1 (1994)
2. Fitzgibbon, C. D., Fanshawe, J. H.: Behav. Ecol. Sociobiol.23, 69 (1988)
3. Mayr, E.: Am. Nat.121, 324 (1983)
4. Thornhill, R., in: Interpretation and Explanation in the Study of Animal Behavior, p.31 (M. Bekoff, D. Jamieson, eds.). Boulder: Westview 1990
5. Williams, G. C.: Adaptation and Natural Selection. Princeton, N. J.: Princeton Univ. Press 1966