Ana B. Sendova-Franks

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

SummaryLeptothorax 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 items 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.

Spatial relationships within nests of the antLeptothorax unifasciatus(Latr.) and their implications for the division of labour

This paper investigates how the movement zones of all the ants in a colony are organized inside the nest. The workers in nine colonies of the antLeptothorax unifasciatus(Latr.) were marked individually and their positions in the nest were recorded over 33 periods of observation spread throughout the year. Results from randomization tests demonstrated that the individual workers inL. unifasciatuscolonies had movement zones of limited area. These are termed spatial fidelity zones (SFZs). SFZs were specific to individuals. They occurred with partial overlap, in a sequence from the colony centre to the colony periphery. The size of SFZs increased from the centre of the colony towards the periphery. The median size of SFZ in a colony varied with the time of year; they expanded gradually after hibernation with a peak in May and then contracted gradually until the following hibernation. The frequency of a workers brood care behaviour was related to the amount of overlap between her SFZ and the spatial distribution of the brood. Individuals on the periphery of the colony were most likely to leave the nest. No clear segregation on the basis of age was observed. The division of labour inL. unifasciatuswas flexibly organized along the continuum of SFZs where each worker performed the tasks within her spatial fidelity zone.

Radio tagging reveals the roles of corpulence, experience and social information in ant decision making

Ant colonies are factories within fortresses (Oster and Wilson 1978). They run on resources foraged from an outside world fraught with danger. On what basis do individual ants decide to leave the safety of the nest? We investigated the relative roles of social information (returning nestmates), individual experience and physiology (lipid stores/corpulence) in predicting which ants leave the nest and when. We monitored Temnothorax albipennis workers individually using passive radio-frequency identification technology, a novel procedure as applied to ants. This method allowed the matching of individual corpulence measurements to activity patterns of large numbers of individuals over several days. Social information and physiology are both good predictors of when an ant leaves the nest. Positive feedback from social information causes bouts of activity at the colony level. When certain social information is removed from the system by preventing ants returning, physiology best predicts which ants leave the nest and when. Individual experience is strongly related to physiology. A small number of lean individuals are responsible for most external trips. An individual’s nutrient status could be a useful cue in division of labour, especially when public information from other ants is unavailable.

Social resilience in individual worker ants and its role in division of labour

This paper extends the notion of spatial efficiency in the organization of social insect colonies. We demonstrate for the first time that ants (individually marked workers in three colonies of Leptothorax unifasciatus (Latr.)) are not only faithful to particular positions within the nest, but they also quickly readopt these positions, relative to one another, when the colony emigrates to an entirely new nest site. This phenomenon, which we term social resilience, has implications for the role of learning in the maintenance of an efficient division of labour to which, in part, the great ecological success of social insects has been attributed. As we demonstrate with observations of another three colonies over a period of six months, workers change their positions asynchronously and different age cohorts are intermingled. Thus the reconstruction of colony spatial order cannot be accounted for by age-based task allocation (i. e. age polyethism), as at any one time the colony meshwork represents a heterogeneous mixture of different generations. These findings also show that ant colonies have a much more precise spatial structure and greater cohesion than previously assumed, and demonstrates the importance of detailed quantitative examination of the sociogenesis or developmental biology of these societies.

Improvement in collective performance with experience in ants

We show that entire ant colonies can improve their collective performance progressively when they repeat the same process. Colonies of Leptothorax albipennis can reduce their total emigration times over successive emigrations. We show that this improvement is based on experience and some memory-like process, rather than a coincidental developmental change or an increased general level of arousal. We demonstrate that the benefits of experience can be lost (i.e. forgotten) if the interval between successive emigrations is too long. We also show that the benefits of experience are more likely to be retained over a longer period if the collective performance has been repeated several times. This is a new demonstration of a process akin to learning in ants and we briefly discuss how it may involve not only improvements in individual performance but also improvements in the ways in which the ants interact with one another.

How experienced individuals contribute to an improvement in collective performance in ants

Certain groups of organisms are capable of improving their collective performance with experience. In a recent study, we demonstrated that, over successive emigrations, colonies of the ant Temnothorax albipennis are able to improve their collective performance by reducing the time taken to complete an emigration (Langridge et al., Behav Ecol Sociobiol 56:523–529, 2004). In this paper, by recording the performance of individually marked workers during repeated emigrations, we were able to analyse some of the ways in which time gains are achieved. We found that: (1) those transporters that also transported in the preceding emigration began to transport earlier in the current emigration and, in the majority of emigrations, transported more items than those transporters that had not transported in the preceding emigration; (2) the time that elapsed before the first item was transported into the new nest reduced over successive emigrations, and this first item was, in the majority of emigrations, carried by a transporter that had also transported in the preceding emigration; (3) the number of adults that were transported reduced over successive emigrations. Our results strongly suggest that the behaviour of transporters that also transported in a preceding emigration may be modified as a result of their experience and that, consequently, their efforts in the next emigration make a major contribution to the improved performance of the colony as a whole.

Algorithms for Building Annular Structures with Minimalist Robots Inspired by Brood Sorting in Ant Colonies

This study shows that a task as complicated as multi-object ‘ant-like annular sorting’ can be accomplished with ‘minimalist’ solutions employing simple mechanisms and minimal hardware. It provides an alternative to ‘patch sorting’ for multi-object sorting. Three different mechanisms, based on hypotheses about the behaviour of Leptothorax ants are investigated and comparisons are made. Mechanism I employs a simple clustering algorithm, with objects of different sizes. The mechanism explores the idea that it is the size difference of the object that promotes segregation. Mechanism II is an extension to our earlier two-object segregation mechanism. We test the ability of this mechanism to segregate an increased number of object types. Mechanism III uses a combined leaky integrator, which allows a greater segregation of object types while retaining the compactness of the structure. Its performance is improved by optimizing the mechanisms parameters using a genetic algorithm. We compare the three mechanisms in terms of sorting performance. Comparisons between the results of these sorting mechanisms and the behaviour of ants should facilitate further insights into both biological and robotic research and make a contribution to the further development of swarm robotics.

Brood-sorting by ants: two phases and differential diffusion

Leptothorax ant colonies sort their brood in concentric annuli with the smallest items in the middle and the largest on the periphery. Such brood sorting is a prime example of collective structure formation by social insects. Its underlying mechanism, however, is still not understood. We tested the hypothesis that brood sorting has two phases: the phase of clustering, proposed earlier, is followed by a phase of spacing, when ants move brood items away in a random direction but in a type-specific way so that items of different brood types spread out to a different extent. We hypothesized that in phase 2, spacing, items of the smallest brood type spread out the least and end up in the centre, whereas items of the largest brood type spread out the most and end up on the periphery. We found two distinct phases in the direction of brood movement during brood sorting associated with nest emigration. In phase 1, ants moved brood items in the direction away from the nest entrance. This was the clustering phase. In phase 2, ants moved brood items in a random direction. This was the spacing phase. Ants moved smaller items for longer than larger items. This is consistent with the hypothesis that ants put down brood items as a function of their weight. The diffusion coefficient and the frequency of movement were different for different brood types. The measure of the average spread (root-mean-square displacement) for each brood type was consistent with their order from the centre to the periphery of the sorted brood structure. The process underlying this spread, however, could not be simple diffusion since the movements of different brood types are interdependent. We discuss the mechanisms that could underlie the switch between the two phases.

Convergent evolution, superefficient teams and tempo in Old and New World army ants

Swarm raiding army ants, with hundreds of thousands or millions of workers per colony, have evolved convergently in the Old World and New World tropics. Here we demonstrate for the first time, to our knowledge, superefficient foraging teams in Old World army ants and we compare them quantitatively with such teams in New World army ants. Colonies of Dorylus wilverthi in the Old World and Eciton burchelli in the New World retrieve almost identical sizes of prey item and the overall size range of their workers is very similar. However, 98% of D. wilverthi workers are within the size range of the smallest 25% of E. burchelli workers. In E. burchelli larger workers specialize in prey retrieval, whereas in D. wilverthi workers form many more teams than in E. burchelli. Such teams compensate for the relative rarity of larger workers in Dorylus. The proportions of prey items retrieved by teams in Dorylus and Eciton are 39% and 5%, respectively. The percentages of all prey biomass retrieved by teams in Dorylus and Eciton are 64% and 13%, respectively. Working either as single porters or teams, Dorylus carry more per unit ant weight than do Eciton, but Eciton are swifter. However, these different ergonomic factors counterbalance one another, so that performance at the colony level is remarkably, although by no means completely, similar between the Old and New World species. The remaining differences are attributable to adaptations in worker and colony tempo associated with the recovery dynamics of their prey populations. Our comparative analysis provides a unique perspective on worker–level and colony–level adaptations and is a special test of the theory of worker caste distributions.

Can ant colonies choose a far-and-away better nest over an in-the-way poor one?

Nest choice in the ant Temnothorax albipennis is a model system for investigating collective decision making. Previous research has demonstrated the sophistication of this decentralized system, yet such studies have focused on binary choices in which alternative nest sites are equidistant from the colonys original nest. In nature, for example, a poor nest might be closer than a better one. Hence, to investigate the collective decision-making system of these ants further, we challenged colonies with a choice between a distant high-quality nest and a much closer and collinear poorer one. Colonies successfully emigrated to the better nest when it was two, three or even nine times further away than the collinear poorer one. Most often, colonies started emigrating simultaneously to both nests, and then they redirected all traffic exclusively to the better, more distant one. We show that this is a good strategy for minimizing exposure and risk. In principle these ants might compensate for distance effects by increasing recruitment latencies and quorum thresholds at nearby poor nests so that they are better able to find and use distant better ones. However, the simplest explanation is that scouts are more likely to begin to look elsewhere, at all stages of the decision-making and emigration process, whenever and wherever they have initially found a low-quality nest.