Cooperative transport by the ant Novomessor cockerelli

While most ant species carry burdens solitarily, a few have evolved impressive skills at cooperative transport that are important to their ecological success. We examined one of these species, Novomessor cockerelli, with the goals of better understanding how transport groups are coordinated and how cooperation influences the efficiency of load transport. Ants were induced to carry standard artificial loads over a smooth horizontal surface, both in groups and as solitary individuals. Porters in groups were non-randomly distributed around the load, with most pulling from the front while walking backward. Porters were more persistent at the leading edge than at the rear, where high rates of leaving and joining were observed. Solitary porters also pulled the load, but they usually first attempted to walk forward and push, until their interactions with the unwieldy load rotated them into a pulling position. Cooperative transport entailed a loss of efficiency, in that solitary porters moved faster and on straighter paths than groups, even though carrying the same per capita weight. This led to a higher prey delivery rate for individuals than for groups. These findings contradict earlier claims of ‘superefficient’ transport for this species, but are consistent with the overall pattern across the ants, in which only a few swarm-raiding species show higher transport efficiency by groups than individuals. We interpret cooperative transport by N. cockerelli and many other species as the product of selection for competitor avoidance, in contrast to swarm raiders, where the special need for efficient prey retrieval has shaped distinctive transport tactics.