Why do not all workers work? Colony size and workload during emigrations in the ant Temnothorax albipennis

Abstract

Here, we study distribution of workload and its relationship to colony size among worker ants of Temnothorax albipennis, in the context of colony emigrations. We find that one major aspect of workload, number of items transported by each worker, was more evenly distributed in larger colonies. By contrast, in small colonies, a small number of individuals perform most of the work in this task (in one colony, a single ant transported 57% of all items moved in the emigration). Transporters in small colonies carried more items to the new nest per individual and achieved a higher overall efficiency in transport (more items moved per transporter and unit time). Our results suggest that small colonies may be extremely dependent on a few key individuals. In studying colony organisation and division of labour, the amount of work performed by each individual, not just task repertoire (which tasks are performed at all), should be taken into account.

A social mechanism facilitates ant colony emigrations over different distances

“…Differences at the colony level were underpinned by variation in individual behaviour between emigration distances. The finding that individual tandem running workload (leading and following tandem runs) decreased with the number of ants actively engaged (Figs 5 and 2D) is consistent with observations in analogous systems, and constitutes a commonly observed phenomenon in insect societies (Karsai and Wenzel, 1998;Dornhaus et al, 2008). Principally, the decrease in individual contributions when more ants are engaged may be explained by the reduced need for workers to be involved in a task, and the concomitant reduction in task stimuli (PinterWollman et al, 2012).…”

“…The relative likelihood and significance of such options is uncertain, as previous studies have shown complex and differing structures of work distribution within colonies. Many of the studies conducted to date support the concept that T. albipennis colonies have some 'specialist' workers in relation to certain task groups (Dornhaus, 2008;Dornhaus et al, 2008). It has been noted that task propensities are positively correlated within groups of related tasks, e.g.…”

Task Allocation in Ant Colonies

“…This is likely consistent with what has been observed in real ant colonies [4], where even during periods where tasks have a very high demand (such as nest migration) an important fraction of the ants remain idle where the rest perform the tasks necessary for the survival of the colony.…”

“…We will say an ant is idle at time t if it belongs to the set I(t) 4 . For succinctness we define the energy supply and the energy need at each task (both are defined in terms of a task assignment).…”

Computational Collective Intelligence

“…For cavity-nesting ants, scouts sense quorum by spending 1-2 minutes in a nest assessing the number of workers present via encounter rate, rather using than indirect cues such as pheromone concentration [34] [31]. Quorum threshold as a proportion of colony size is remarkably constant across a range of colony sizes (c3.5%) [13], and this is intriguing, because the relationship between colony size and cavity size is not simple positive correlation: although larger colonies do inhabit larger cavities in the wild, in laboratory tests both small and large colonies prefer larger cavities, presumably to allow for growth [33], [11] & [22].…”