The emergence of reproductive division of labor in forced queen groups of the ant P ogonomyrmex barbatus

Abstract

One of the defining features of advanced eusocial groups is reproductive division of labor, where one or a few individuals specialize on reproduction while others perform strictly nonreproductive tasks such as brood care, defense and foraging. Recent theoretical work suggests that the rudiments of division of labor may originate spontaneously during initial group formation as an emergent property, rather than requiring a secondary adaptation. Empirical studies on nonreproductive tasks support the emergence hypothesis, but it is unclear whether this mechanism also extends to reproduction. To test whether reproductive division of labor can be produced as an emergent property, we assessed the extent and mechanisms of both nonreproductive and reproductive division of labor in forced associations of normally solitary queens of the harvester ant Pogonomyrmex barbatus. We find that division of labor in both types of tasks can be induced in groups of individuals with no evolutionary history of social cooperation. Specialization in excavation behavior was more pronounced than reproduction, which tended to be incomplete although significantly skewed. In addition to reproductive division of labor, enhanced productivity in forced pairs relative to solitary queens suggests that both queens contributed cooperatively to brood care despite unequal maternity. Thus, two of the three defining features of eusociality may have originated through self-organizing mechanisms concurrently with the evolution of grouping, exposing these social strategies to selection early on in the evolution of social life.