Should Egg Size and Clutch Size Decrease with Age?

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Abstract
We analyse a model in which females gather resources for eggs in a pre-reproductive phase, and are then, in the reproductive phase, subject to mortality during the production of successive clutches of offspring. We find that if clutch size is constrained, egg size should decline with maternal age. Where egg fitness shows diminishing returns with egg size, and maternal mortality is age-independent, the decline in egg size will be linear, and that rate of decline will be proportional to maternal mortality. All females with the same survival prospects follow the same trajectory of decline, but larger females enter the trajectory at a larger egg size. Similarly, if egg size is constrained, clutch size declines with maternal age, the rate of decline increases with maternal mortality, and larger females lay larger clutches initially. Support for the models is found in a wide variety of species, and in particular in a number of recent studies of butterflies.