The evolution of cooperation is a fundamental problem in biology, especially for non-relatives, where indirect fitness benefits cannot counter within-group inequalities. Multilevel selection models show how cooperation can evolve if it generates a group-level advantage, even when cooperators are disadvantaged within their group. This allows the possibility of group selection, but few examples have been described in nature. Here we show that group selection can explain the evolution of cooperative nest founding in the harvester ant Pogonomyrmex californicus. Through most of this species’ range, colonies are founded by single queens, but in some populations nests are instead founded by cooperative groups of unrelated queens. In mixed groups of cooperative and single-founding queens, we found that aggressive individuals had a survival advantage within their nest, but foundress groups with such non-cooperators died out more often than those with only cooperative members. An agent-based model shows that the between-group advantage of the cooperative phenotype drives it to fixation, despite its within-group disadvantage, but only when population density is high enough to make between-group competition intense. Field data show higher nest density in a population where cooperative founding is common, consistent with greater density driving the evolution of cooperative foundation through group selection.
The objective of this experiment is to study a colony’s seed preference following previous exposure to a seed type in the seed harvester ant Pogonomyrmex californicus. It was hypothesized that foragers would demonstrate a measurable preference for the seed type they had previously experienced over the novel seed type. The cuticular hydrocarbon profile is suspected to be an influence in the foragers’ seed selection. Following an incubation period with the designated seed type, a series of preference trials were conducted over the course of two days for two experiments in which each colony fragment was given a seed pile with a 1:1 ratio of niger and sesame, after which any seeds moved off the seed pile were determined to be chosen, as well as if the workers were observed moving the seeds off the pile from the video recordings. Using video recordings, the seed selections of individual foragers were also tracked. The results partially support the hypothesis, however, in some cases, the ants did not collect enough seeds for the preference to be significant, and not all colony fragments had preferences that lined up with what they had previously experienced according to their treatment. Familiarity with the hydrocarbon profile of the seed type the colony had experienced is a possible proximal explanation for why colonies had seed preferences that aligned with their treatment, the seed they were designated to experience. Due to the low quantity of seeds collected during preference trials, seed preference amongst individual foragers remains unclear due to many different foragers selecting a seed during only one trial, with very few foragers returning to forage for seeds over the course of the experiment.