Multiple resources and the optimal balance between size and number of offspring

We extend the classical Smith-Fretwell model for the optimal size of an offspring to the case of allocation of two or more fitness enhancing resources. Unlike the results of the single-resource model, the new model predicts that the optimal allocations will depend on the resource pool sizes. We apply this new model to the problem of carbon and nitrogen allocation to seeds and conclude (1) that the optimal seed size (carbon allocation) should be positively correlated with the ratio of the size of the carbon and nitrogen pools available for investment to offspring (C/N ratio) and (2) that there should be a negative correlation between seed size and absolute seed nitrogen content. These results may account for some of the within- and between-plant variation in resource allocation to seeds.