Notes and Comments Alternative Models for the Evolution of Offspring Size

Eleven years ago, I proposed a simple model for the evolution of offspring size, pointing out a general correlation between parental care and relatively large offspring size consistent with that model (Shine 1978). My basic premise came from Williams (1966): natural selection should adjust the life history such that organisms spend little time in life-history stages characterized by high mortality. Hence, optimal egg sizes should be determined by the relative survival rates of eggs and free-living juveniles. If the egg stage is a "safe harbor" (as in species with parental care), whereas juvenile life is hazardous, selection should favor an increase in egg size and thus a decrease in the duration of the high-risk juvenile phase. This safe-harbor hypothesis for the evolution of offspring size has recently been criticized on several grounds by Nussbaum (1985, 1987), Sargent et at. (1987), and Nussbaum and Schultz (1989). Important alternative ideas on the topic have also been developed by Ito (1980) and Ito and Iwasa (1981). Analysis ofthese contributions suggests at least four alternative explanations for the observed correlation between parental care and offspring size: (1) parental care favored the evolution of larger eggs (Shine 1978); (2) larger eggs favored the evolution of par~ntal care (Nussbaum 1985, 1987); (3) parental care and offspring size coevolved, with an increase in one favoring an increase in the other (Nussbaum and Schultz 1989); or (4) some third factor simultaneously selected for parental care and larger egg size. Although I maintain that several criticisms of the safe-harbor hypothesis are in error, I also believe that the general correlation between parental care and large offspring size may be due to a combination of processes rather than the single process I envisaged in my 1978 paper. This note outlines my objections to recent criticisms and proposes a method for testing these competing explanations.