Alternative Models for the Evolution of Offspring Size

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Abstract
The work of Ito (1980), Ito and Iwasa (1981), Nussbaum (1985, 1987), Sargent et al. (1987), and Nussbaum and Schultz (1989) has done much to clarify possible reasons for the general correlation between parental care and large offspring size. On the basis of this literature, I suggest that at least four explanations for the above correlation are plausible: parental care favors an increase in offspring size; increased offspring size favors the evolution of parental care; the two variables coevolve; or some third factor (perhaps size-dependent mortality of eggs and embryos) simultaneously selects for both parental care and larger offspring size. Tests between these alternative models should be feasible and would be of great interest. These discussions also illuminate more-general models for offspring size, because any comprehensive model for evolutionary shifts in offspring size must explain the consistent correlation of this variable with parental care. In my opinion, the net effect of the work reviewed above has been to support the safe-harbor hypothesis and to extend and clarify its nature, rather than to challenge its basic validity. It is perhaps unfortunate that most discussion of the safe-harbor model has been focused so strongly on the role of parental care in the evolution of offspring size. It seems not to have been widely appreciated that the safe-harbor hypothesis-the idea that optimal offspring size is determined by relative viabilities of eggs versus free-living juveniles-is applicable to the evolution of offspring size in any situation.

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