Nigel R. Franks

Effective leadership and decision-making in animal groups on the move

For animals that forage or travel in groups, making movement decisions often depends on social interactions among group members. However, in many cases, few individuals have pertinent information, such as knowledge about the location of a food source, or of a migration route. Using a simple model we show how information can be transferred within groups both without signalling and when group members do not know which individuals, if any, have information. We reveal that the larger the group the smaller the proportion of informed individuals needed to guide the group, and that only a very small proportion of informed individuals is required to achieve great accuracy. We also demonstrate how groups can make consensus decisions, even though informed individuals do not know whether they are in a majority or minority, how the quality of their information compares with that of others, or even whether there are any other informed individuals. Our model provides new insights into the mechanisms of effective leadership and decision-making in biological systems.

Self-organized lane formation and optimized traffic flow in army ants

We show how the movement rules of individual ants on trails can lead to a collective choice of direction and the formation of distinct traffic lanes that minimize congestion. We develop and evaluate the results of a new model with a quantitative study of the behaviour of the army ant Eciton burchelli. Colonies of this species have up to 200 000 foragers and transport more than 3000 prey items per hour over raiding columns that exceed 100 m. It is an ideal species in which to test the predictions of our model because it forms pheromone trails that are densely populated with very swift ants. The model explores the influences of turning rates and local perception on traffic flow. The behaviour of real army ants is such that they occupy the specific region of parameter space in which lanes form and traffic flow is maximized.

Speed versus accuracy in collective decision making

We demonstrate a speed versus accuracy trade–off in collective decision making. House–hunting ant colonies choose a new nest more quickly in harsh conditions than in benign ones and are less discriminating. The errors that occur in a harsh environment are errors of judgement not errors of omission because the colonies have discovered all of the alternative nests before they initiate an emigration. Leptothorax albipennis ants use quorum sensing in their house hunting. They only accept a nest, and begin rapidly recruiting members of their colony, when they find within it a sufficient number of their nest–mates. Here we show that these ants can lower their quorum thresholds between benign and harsh conditions to adjust their speed–accuracy trade–off. Indeed, in harsh conditions these ants rely much more on individual decision making than collective decision making. Our findings show that these ants actively choose to take their time over judgements and employ collective decision making in benign conditions when accuracy is more important than speed.

Individual and collective decision-making during nest site selection by the ant Leptothorax albipennis

Abstract. Social insect colonies possess remarkable abilities to select the best among several courses of action. In populous societies with highly efficient recruitment behaviour, decision-making is distributed across many individuals, each acting on limited local information with appropriate decision rules. To investigate the degree to which small societies with less efficient recruitment can also employ distributed decision-making, we studied nest site selection in Leptothorax albipennis. Colonies were found to make sophisticated choices, taking into account not only the intrinsic qualities of each site, but also its value relative to the available options. In choices between two sites, individual ants were able to visit both sites, compare them and choose the better one. However, most ants encountered only one site in the course of an emigration. These poorly informed ants also contributed to the colonys decision, because their probability of initiating recruitment to a site depended on its quality. This led to shorter latencies between discovery and recruitment to a superior site, and so created greater amplification via positive feedback of the population at the better site. In short, these small colonies make use of a distributed mechanism of information processing, but also take advantage of direct decision-making by well-informed individuals. The latter feature may in part stem from the limitations of their social structure, but may also reflect the stringent demand for unanimous decisions by house-hunting colonies of any size.

Strategies for choosing between alternatives with different attributes: exemplified by house-hunting ants

We tested the decision-making abilities of emigrating ant colonies. The colonies had to choose a new nest site when presented with two or more potential nest sites, each with different attributes or different combinations of attributes. For Leptothorax albipennis colonies in the laboratory, darkness of the nest cavity, internal height of the cavity and width of the entrance were all important attributes. The colonies ranked these attributes: darkness of the nest site was more important than internal cavity height, which in turn was more important than entrance width. These choices conform to the logic of transitivity. In addition, the colonies used a sophisticated decision-making strategy in which they took all alternatives and all attributes into consideration. Furthermore, the ants, in effect, weighed the different values of different attributes. They also chose the best nest when presented with only one excellent nest among four mediocre ones or one excellent nest in an array of one excellent, one good and one mediocre. Altogether, our results suggest that these ant colonies, in deciding upon a new home, used a weighted additive strategy, one of the most computationally expensive and thorough decision-making strategies. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

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.

Self-organizing nest construction in ants: sophisticated building by blind bulldozing

Abstract The pattern and process of building in an ant, Leptothorax (Myrafant) tubero-interruptus (F.), is described. This species naturally uses flat crevices in rock, so they do not have to construct the roof and floor of their nests. The ants simply build a perimeter wall around their colony. Such building is essentially two-dimensional and facilitates laboratory experimentation and statistical analysis. The ants apparently use very simple rules both to make their nest an appropriate size for their colony and to co-ordinate their building activities. A computer-simulation model of such building shows how the ants could use a simple self-organizing procedure based largely on bulldozing. Each builder, by pushing its building block into others, adds its work to existing structures. Building workers do not communicate directly but can communicate efficiently via the products of their successful labours.

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.

Self-organizing nest construction in ants: individual worker behaviour and the nest's dynamics

We examine nest construction in the ant Leptothorax tuberointerruptus at two levels: (1) the building behaviour of individual workers and (2) the collective properties (temporal and spatial) of the structures they create. We also explore, for the first time explicitly, the linkage between these two levels. Leptothorax tuberointerruptus nests occur in flat cavities which provide the roof and the floor of their dwelling places. Hence, they construct only a peripheral encircling wall, breached by one or more entrance passageways. The wall is constructed brick by brick. This facilitates experimental estimation of the probabilities of individual workers picking up and depositing building material in response to different stimuli. We incorporate both the qualitative and quantitative behavioural rules that works employ during building into a mathematical model. This model confirms that a surprisingly small and simple set of behavioural rules are not only sufficient for wall construction but also for the formation of one or more nest entrances. In addition, this model predicts that the nests of these ants are likely to exhibit interesting dynamics, in which, for example, the tendency to build a new larger nest may lag behind growth of the population that the nest has to house. We present experimental evidence that suggests that this prediction is valid.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour

On optimal decision-making in brains and social insect colonies

The problem of how to compromise between speed and accuracy in decision-making faces organisms at many levels of biological complexity. Striking parallels are evident between decision-making in primate brains and collective decision-making in social insect colonies: in both systems, separate populations accumulate evidence for alternative choices; when one population reaches a threshold, a decision is made for the corresponding alternative, and this threshold may be varied to compromise between the speed and the accuracy of decision-making. In primate decision-making, simple models of these processes have been shown, under certain parametrizations, to implement the statistically optimal procedure that minimizes decision time for any given error rate. In this paper, we adapt these same analysis techniques and apply them to new models of collective decision-making in social insect colonies. We show that social insect colonies may also be able to achieve statistically optimal collective decision-making in a very similar way to primate brains, via direct competition between evidence-accumulating populations. This optimality result makes testable predictions for how collective decision-making in social insects should be organized. Our approach also represents the first attempt to identify a common theoretical framework for the study of decision-making in diverse biological systems.