Optimal Egg Size and Clutch Size: Effects of Environment and Maternal Phenotype

Abstract

In this paper, we develop a series of models for predicting optimal egg size and clutch size in different environments and for different maternal phenotypes. The models investigate the interaction between three components of fitness: (1) the intrinsic effect of egg size, (2) the density effect, the effect of the density of competing offspring (including competition with sibs and with non-sibs), and (3) the hierarchy effect, the effect of egg size relative to the sizes of competing eggs laid by other females. The environmental effects that we consider are the intensities of sib and non-sib competition, the number of egg-laying females, and some aspects of seasonal development. The particular aspects of maternal phenotype examined are foraging efficiency and the gametic reserve available at the time of egg laying (broadly equivalent to female size). Regarding environmental effects, increasing intensities of sib competition select for the production of smaller clutches of larger eggs, and increasing intensities of non-sib competition select for even smaller clutches of even larger eggs. We show, however, that this occurs because viability depends on egg phenotype, not because competition depends on density. In addition, we show that in seasonal environments the production of smaller clutches of larger eggs is favored by slower rates of preadult development and shorter overall season lengths. Regarding maternal phenotype, we derive the following conclusions. The intrinsic effects of egg size promote the production of eggs of a constant size by all females, so that larger females lay larger clutches. Sib competition (considered a density effect with no hierarchy component) favors constancy of clutch size for all females, so that larger females lay larger eggs. If, however, sib competition is coupled with circumstances in which females have high gametic reserves and/or low search costs for future oviposition sites, egg size and clutch size should vary with respect to female size, but larger females should lay more clutches. Non-sib competition acting through total competitor density (the density effect) favors the production of a constant clutch size by all females, such that larger females lay larger eggs. The action of non-sib competition on egg size relative to the sizes of others' eggs (the hierarchy effect), although tending to increase egg size overall, may cause smaller females to produce eggs larger than those produced by larger females. This latter effect, however, is confined to cases in which there are few competing females; typically, the hierarchy effect will favor approximately constant egg sizes for all females, so that larger females lay larger clutches.