Task partitioning in insect societies: bucket brigades

There are many ways in which social insect foragers may organise the collection of resources and their transportation back to the nest. One way is to partition the task into a number of sequential stages in which material is passed from one worker to another in a relay fashion. This relatively new concept is known as task partitioning. In this study, we focus on a particular form of task partitioning, bucket brigades, which we define as a multistage (i.e., three or more stages) partitioned transport scheme that uses only direct transfer between individual workers and without any predetermined transfer locations, other than the first or last stages. We first consider the potential costs and benefits of bucket brigades compared to other transportation schemes. We then use theory and computer simulation to analyse some of these aspects in detail. In one empirical study of a bucket brigade, foragers were generally found to be sequenced from smallest (near the food source) to fastest (nearest the nest). This exactly matches what dynamical systems theory would predict as an ergonomically efficient solution. However, we also demonstrate that a single and simple local rule - larger ants win fights over material - will generate this sequencing as an epiphenomenon that is not necessarily optimal. We use the behaviour of bucket brigades to reveal some general points about the optimality of task partitioning in more detail.