Offspring size and reproductive allocation in harvester ants

A fundamental decision that an organism must make is how to allocate resources to offspring, both with respect to size and to number. The two major theoretical approaches to this problem, optimal offspring size and optimistic brood size models, make different predictions that may be reconciled by including how offspring fitness is related to size. We extended the reasoning of Trivers and Willard (1973) to derive a general model of how parents should allocate additional resources with respect to the number of males and females produced, and among individuals of each sex, based on the fitness payoffs of each. We then predicted how harvester ant colonies should invest additional resources, and tested three hypotheses derived from our model, using data from three years of food supplementation bracketed by six years without food addition. All major results were predicted by our model: Food supplementation increased the number of reproductives produced. Male, but not female,
size incre...