Reproductive Effort and Optimal Reproductive Rates in Small Rodents

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Abstract
The reproductive effort as a function of the reproducing individual''s weight and number of fetuses was estimated. This estimate was based on models for energy requirements of reproducing females of the common vole Microtus arvalis (Pall.). This function forms the basis for construction of age (or weight) specific demographic fitness sets. Based on these, optimal reproductive rates are predicted for stable and cyclic populations. The predictions thus deduced find support in available field information. The model explains the larger litters and earlier age of maturation in cyclic microtine populations as compared to stable populations, and it explains the increased litter size with increasing parity. A new pattern is revealed; the maximal realized litter size is reached earlier in fluctuating populations than in more stable populations. The ratio between litter size of old and young females is predicted to be largest in the most cyclic populations. This compares favorably with field data on several species.