Bipartite network analysis of ant-task associations reveals task groups and absence of colonial daily activity

Abstract

Division of labour is the key factor leading to higher-order systems such as cooperative animal groups. How division of labour is achieved without a central control is one of the most fascinating questions in behavioural ecology and complex systems science. Social insects are one of the best examples of complex self-organised systems through local interactions. However, it is difficult to comprehensively understand division of labour due to the chronological and individual variation in behaviours, and the differences between social environments. Thus, it is imperative to quantify individual behaviours and integrate them into colony levels. Here, we demonstrate that network analyses on individual-behaviour relationships can be valid for characterising the task allocation patterns. We recorded the behaviours of all individuals with verified age in colonies of monomorphic ant and analysed the individual-behaviour relationship at individual, sub-network, and network-levels. We detected the "task groups" including subsets of individuals and behaviour, and the patterns of task groups were consistent throughout the day. We also found a relationship between the age and the characteristics of individuals within the network. These findings suggest that bipartite network analyses can untangle the complex task allocation patterns and provide novel insights into understanding how division of labour is achieved.How the simple elements at a lower level, for example, a single cell and an individual, can evolve to higher-level systems, for instance, multicellular organisms and animal groups, through natural selection has been a central question in evolutionary biology [1]. Such systems often exhibit division of labour through efficient task allocation, in which different elements perform different tasks [2]. It has been considered that division of labour can be a primary advantage for social evolution [3,4]. Therefore, it is crucial to understand how they achieve division of labour.Social insects are one of the most sophisticated examples of division of labour because individuals exhibit a reproductive division of labour between queen(s) and workers; and among workers various tasks are allocated. For example, task allocation for nursing, foraging, nest defence, or cleaning is accomplished without central controls in a colony [5][6][7]. To elucidate how such colonies achieve task allocation through interactions among individuals, we need to reveal the behavioural rules underlying it. However, it is difficult not only to determine the behavioural rules but also to describe and quantify the individual behavioural patterns. The difficulty in measuring these parameters arises from the fact that a large number of individuals comprise a colony, and that the behavioural differences depend on internal states, experience, colony size, demands from the nestmates, or environment, that can change across time [8][9][10][11][12][13][14]. Moreover, colonies generally consist of individuals with different characteristics such as morphology, age, ...