Linear recruitment leads to allocation and flexibility in collective foraging by ants

Positive feedback in communication is central to the emergence of collective behaviour in animal groups. The nonlinearity hypothesis predicts that group behaviour will be defined in large part by the degree of cooperative interaction in this feedback. Strong interactions mean that the effectiveness of communication grows nonlinearly with signal quantity, leading to robust spatial patterning and consensus decision making. These predictions have been supported in many experimental systems, but the corresponding behaviour of linear systems, where signal effectiveness is proportional to signal quantity, has not been well explored. We examined the consequences of tandem running, a linear method of recruitment, for collective decision making by foraging colonies of the ant Temnothorax rugatulus. We found that colonies collectively chose the better of two unequal feeders, but they allocated foragers equally when feeders were identical. This result accords with theoretical predictions and contrasts with symmetry breaking seen in species with highly nonlinear trail pheromone recruitment. Colonies were also able to reallocate foragers when the quality of two unequal feeders was switched, again in accord with theoretical expectations. We built a model based upon the behaviour of individual ants and found that colony-level qualities of choice, allocation and flexibility can be predicted by two behaviours: quality-dependent linear recruitment and quality-dependent attrition. These experiments are the first thorough investigation of the consequences of linear recruitment for collective animal behaviour.