Worker size-related task partitioning in the foraging strategy of a seed-harvesting ant species

Messor bouvieri is a seed-harvesting ant species in which workers forage in trails from the nest to a search area. A previous observation of seed transfer events between workers returning to the nest suggested potential task partitioning. In this study, we describe seed transportation and analyze the role of task partitioning in the foraging strategy of this species in terms of seed intake efficiency in relation to costs and benefits based on transport speed and task reliability. We assess the harvesting efficiency of task partitioning by comparing cooperative seed transport (CST) and individual seed transport (IST) events. Our results show task partitioning in the form of a sequence of transfer events among workers going from the search area to the nest. Importantly, and despite the weak worker polymorphism of this species, this sequence involved workers of different sizes, with seeds usually being passed along from smaller to larger workers. In addition, we show that small workers are better at finding seeds (spend less time finding a seed), and large workers are better at transporting them (were faster when walking back to the nest and lost fewer seeds). However, we failed to demonstrate that workers of different sizes are specialized in performing the task in which they excel. Overall, sequential CST in M. bouvieri results in a greater seed intake because seed search time decreases and task reliability increases, compared to IST. The determinants and adaptive benefits of CST are discussed.