<p>Colonies of the harvester ant, <i>Pogonomyrmex occidentalis</i>, vary in both the onset and duration of foraging as a function of the colony’s genetic diversity, but the underlying cause of this variation is unknown. The phenotypic diversity hypothesis proposes that colonies with a greater number of worker phenotypes will have a broader range of collective behavior. We tested two mechanisms for this hypothesis, variation in thermal sensitivity and variation in the tempo of activity, by comparing the activity of the earliest and last foragers collected in the field from colonies with different levels of genetic diversity. Using video imaging, we compared the activity of these foragers under standardized conditions over a broad temperature range: we demonstrated that the workers that are the first to forage have a higher tempo of activity than late foragers. Early and late foragers belonged to different subsets of the colony’s worker genotypes, and the range of activity phenotypes was directly related to the number of times that the queen had mated. High levels of multiple mating may be favored because they increase the probability of acquiring both high and low tempo phenotypes which leads to an expanded foraging range. We suggest that high tempo phenotypes may suffer performance trade-offs in endurance, accuracy or persistence that favor phenotypic diversity of the worker force.</p>