Egg Size Variation in Satyrid Butterflies: Adaptive vs Historical, "Bauplan", and Mechanistic Explanations

Egg size decreases over the oviposition period in the wall brown butterfly, Lasiommata megera. Experiments have failed to indicate any positive correlation between egg size and 1) egg mortality, 2) larval mortality, 3) larval development time, 4) pupal weight, 5) ability of newly hatched larvae to survive without food for up to 96 h, and 6) ability of newly hatched larvae to establish themselves on the coarse grass Festuca rubra. Since we find no evidence that the decrease in egg size over time is adaptive, a mechanistic explanation is advanced to account for the phenomenon. The decrease in egg size is strongly correlated with the age of females, and with the remaining egg mass of females, but not with the weight of females. Therefore we suggest a model for decreasing egg size based on the assumption that daily egg size constitutes a given fraction of the remaining reproductive mass of females. Part of the remaining reproductive mass is assumed to be gradually depleted with time, viz. protein and lipids which are ultimately derived from larval feeding, whereas part is replenished by the feeding of the adult female, viz. water and carbohydrates. The shape of the curve for decreasing egg size generated by this partial resource depletion hypothesis is in general agreement with the empirical curve for L. megera. Since the problem of adaptiveness of egg size variation largely depends on understanding what determines optimal egg size per se, we studied egg size in 10 species of satyrid butterflies. An adaptive explanation, based on the assumption that egg size should be related to the coarseness of the host plant, was refuted by data. Instead a strong Bauplan relationship was found, large butterfly species laying large eggs and small species laying small eggs. Also, egg size among the 10 species studied was in some agreement with systematic affinity suggesting that a historical explanation for egg size in satyrid butterflies may be applicable.