Data from: 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 increased with food addition; the greatest increases in male size occurred in colonies that made small females. We discuss how use of a fitness landscape improves quantitative predictions about allocation decisions. When parents can invest differentially in offspring of different types, the best strategy will depend on parental state as well as the effect of investment on offspring fitness

The structure of foraging activity in colonies of the harvester ant, Pogonomyrmex occidentalis

The timing of activity by desert dwelling poikilotherms can be critical to survival. In the western harvester ant, colonies that have higher levels of genetic diversity forage for longer time periods in the morning than colonies with less diversity. We determined whether the advantage of early foraging colonies was consistent by examining foraging behavior at other times of day and year. We used a combination of activity monitoring and temperature measurement at the nest entrance to quantify foraging activity during the morning and evening summer foraging periods in both June and August. The duration of morning and evening foraging was significantly positively correlated both within and across seasons--some colonies have a consistent advantage in foraging. The temperature range over which colonies foraged was also consistent across time, suggesting that intercolony differences are a consequence of variation in the thermal ranges/preferences of the colony's workers. The duration of foraging during this study was correlated with the duration of foraging measured 6 years earlier, suggesting that it is an aspect of colony phenotype. Copyright 2010, Oxford University Press.

Inclusive fitness theory and eusociality

Arising from M. A. Nowak, C. E. Tarnita & E. O. Wilson 466, 1057-1062 (2010); Nowak et al. reply. Nowak et al. argue that inclusive fitness theory has been of little value in explaining the natural world, and that it has led to negligible progress in explaining the evolution of eusociality. However, we believe that their arguments are based upon a misunderstanding of evolutionary theory and a misrepresentation of the empirical literature. We will focus our comments on three general issues