Division of labour in a crisis: task allocation during colony emigration in the ant Leptothorax unifasciatus (Latr.)

Division of labour during colony emigration is widespread in ants. An important problem is how tasks are allocated during colony movement from one nest site to another. The generally favoured view is that emigrations are organised by a minority group of individuals, which either work unusually hard at tasks (elites) or have the exclusive task of carrying out the emigration (moving specialists). Five consecutive emigrations of a Leptothorax unifasciatus (Latr.) colony showed that the number of transporters, i.e. the individuals that took an active part in the emigration by transporting brood and ants, was smaller than it would have been if allocation of this task was random during each emigration. However, single emigrations of another three colonies, for which the spatial distribution and behaviour of the workers had been observed for a week prior to the emigration, demonstrated that the transporters did not form a homogeneous group. They differed in their spatial positions and tasks before the emigration. There was also no evidence that transporters worked harder or less hard than their nestmates before the emigration. Therefore, the individuals which carry out emigrations in L. unifasciatus colonies appear to be neither moving specialists nor elites. We propose that task allocation during emigrations of L. unifasciatus colonies is based on a feedback mechanism that involves learning.     

Brood sorting by ants: distributing the workload over the work-surface

Summary Leptothorax unifasciatus ant colonies occupy flat crevices in rocks in which their brood is kept in a single cluster. In artificial nests made from two glass plates sandwiched together, designed to mimic the general proportions of their nest sites in the field, such colonies arrange their brood in a distinct pattern. These patterns may influence the priority with which different brood are tended, and may therefore influence both the division of labour and colony demography. Different brood stages are arranged in concentric rings in a single cluster centred around the eggs and micro-larvae. Successively larger larvae are arranged in progressive bands away from the centre of the brood cluster. However, the largest and oldest brood items, the prepupae and pupae, are placed in an intermediate position between the largest and most peripheral larvae and the larvae of medium size. Dirichlet tessellations are used to analyze these patterns and show that the tile areas, the area closer to each item than its neighbours, allocated to each type of item increase with distance from the centre of the brood cluster. There is a significant positive correlation between such tile areas and the estimated metabolic rates of each type of brood item. The ants may be creating a domain of care around each brood item proportional to that item's needs. If nurse workers tend to move to the brood item whose tile they happen to be within when they have care to donate, they may apportion such care according to the needs of each type of brood. When colonies emigrate to new nests they rapidly recreate these characteristic brood patterns.     

Testing the limits of social resilience in ant colonies

Social resilience is the ability of Leptothorax ant colonies to re-assemble after dissociation, as caused, for example, by an emigration to a new nest site. Through social resilience individual workers re-adopt their spatial positions relative to one another and resume their tasks without any time being wasted in worker respecialisation. Social resilience can explain how an efficient division of labour can be maintained throughout the trials and tribulations of colony ontogeny including the, often substantial, period after the queen dies when the ability to conserve worker social relationships may be essential for efficiency to be maintained. The mechanism underlying social resilience is, therefore, expected to be robust even in the absence of many of the colony’s components, such as the queen, the brood and even a large proportion of the workers. Such losses are likely, given the ecology of this genus. Using sociotomy experiments, we found that social resilience can occur in the absence of the queen. Furthermore, the spatial component of social resilience can occur even when the queen, the brood, as well as a large proportion of the workers, are all absent simultaneously and hence many of the tasks are missing. We conclude, therefore, that social resilience is indeed robust. This does not, however, preclude worker flexibility in response to changes in task supply and demand. We propose a possible sorting mechanism based on worker mobility levels which might explain the robustness underlying this phenomenon. Received: 25 October 1999 / Accepted: 1 April 2000     

Nest-seeking rock ants (Temnothorax albipennis) trade off sediment packing density and structural integrity for ease of cavity excavation

We investigated excavation and nest site choice across sediment-filled cavities in the ant Temnothorax albipennis. Colonies were presented with sediment-filled cavities, covering a spectrum from ones that should be quick to excavate but will form a weak enclosing wall to those that should be slow to excavate but form a strong wall. Overall, colonies only showed a significant preference for cavities that were fastest to excavate over those that were slowest. The speed of decision making and moving appears paramount over the suitability of the sediment for forming an enclosing wall. The mechanism behind the choice is the differential between the rates at which alternatives are excavated and accumulate ants. The rates for a particular type of cavity were unaffected by the type with which it was paired. This suggests that there is no significant competition between sites during the decision-making process. Certain colonies were able consistently to discriminate across more closely matched alternatives. These colonies required a greater number of ants to be present and took longer before beginning to move. A race is run between alternatives to become habitable but the process may be tuned across colonies such that it may run for longer and an incorrect or split decision is less likely.