State-Dependent Optimization of Litter Size

Four inter-related theories demonstrate that the quality or state of the parent influences its optimum litter size. A review of each model highlights their similar predictions, sets the stage for an expanded theory and demonstrates that three are really special cases of state-dependent optimization. The state-dependent theory helps to explain three paradoxes in life history. l. Why the most commonly observed litter size is often smaller than the one that yields the maximum number of recruits. 2. Why litter size may increase or decrease in high-quality habitat. 3. Why there may be no reproductive cost associated with increased litter size. Tests of state-dependent optimization are likely to be complicated because the essential state variables are often left undefined, and because many state variables may interact with one another to determine a parent's quality. Composite state-variables, such as female body size, may overcome some of the limitations that otherwise arise from interacting variables. In white-footed mice, significant body-size effects interact with the environment and with a variety of life-history characters. The resulting networks of interactions, complicated as they are, may be caused by a relatively small number of underlying processes. Density-dependent habitat selection appears, at least for white-footed mice, to represent a fundamental process accounting for much of the environmental state-dependent variation in litter size.