THE EVOLUTION OF OFFSPRING SIZE AND NUMBER - A TEST OF THE SMITH-FRETWELL MODEL IN 3 SPECIES OF CRICKETS

Most models of parental investment in offspring assume a trade-off bet ween propagule size and number, and an increasing ''concave down'' fun ction relating offspring fitness to propagule size. In this study, we test these two fundamental assumptions, using three closely related sp ecies of crickets, Gryllus firmus, G. veletis, and G. pennsylvanicus. Egg weight, 35-day fecundity and 35-day egg biomass were estimated in a population of each species, and the relationships between these repr oductive traits and date of egg laying and body size were estimated. T he relationships between egg weight and offspring survival were also s ought for eggs buried at different depths, soil moistures, and soil ty pes (G. firmus and G. veletis), as well as in the field (G. pennsylvan icus). A trade-off between egg weight and 35-day fecundity was reveale d in a multivariate analysis taking into account among-species variati on in egg weight and body size. Independent of the environmental condi tions affecting the eggs, a positive correlation existed between the n umber of larvae that emerged from the soil and propagule weight in eac h species. Therefore, these findings provide partial support for the a ssumptions considered in the models mentioned above. A single optimal egg size was favored in two out of the three sets of conditions in whi ch the functions relating egg weight to larval survival could be deriv ed. The conditions encountered by the eggs, however, influenced the av erage survival of the larvae, as well as the shape of the relationship between egg weight and offspring survival. This suggests that cricket eggs frequently face heterogeneous environments with respect to egg a nd hatchling survival; the implication of habitat heterogeneity on the evolution of an optimal egg size is considered. The relationships bet ween the reproductive components and female age and size, as well as b etween egg size and variation in cricket life-history, are discussed i n an ecological and evolutionary context.