For animals that forage or travel in groups, making movement decisions often depends on social interactions among group members 1,2 . However, in many cases, few individuals have pertinent information, such as knowledge about the location of a food source 3,4 , or of a migration route [5][6][7][8][9] . 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.Primary questions concerning the mechanism of information transfer in groups include how uninformed individuals recognize those that are informed, whether such recognition is actually necessary, and how groups can come to a collective decision when informed individuals differ in preference 10 . It is known that several animal species have evolved specific recruitment signals that help guide conspecifics. Most famous in this context is the waggle-dance of the honeybee that recruits hive members to visit food sources 5,7,8,11 . Furthermore, valuable experience may be correlated with age or dominance 1,2 , which can presumably be estimated by conspecifics of some species 12 . However, it remains questionable whether such explanations hold when migrating groups of fish, ungulates, insects and birds are considered, where crowding limits the range over which individuals can detect one another 1,2 . In pelagic fish schools, for example, individuals are usually less than one body-length apart 13 . Although it is likely that some species have a genetically determined propensity to migrate in a general direction 14,15 , or respond to abiotic cues such as thermal gradients that may aid migration 16,17 , it is likely for many species that experienced group members play an important role in guiding those that are less experienced or inexperienced. Relatively few informed individuals within fish schools are known to be able to influence the foraging behaviour of the group 3 and the ability of a school to navigate towards a target 4 . Similarly, very few individuals (approximately 5%) within honeybee swarms can guide the group to a new nest site 7 .Furthermore, for some animal groups such as large insect swarms or fish schools, it may be unreasonable to assume that group members have the capacity for individual recognition. Here we address two fundamental issues, both occurring in the absence of complex signalling mechanisms and when it is not possible f...

We present a self-organizing model of group formation in three-dimensional space, and use it to investigate the spatial dynamics of animal groups such as fish schools and bird flocks. We reveal the existence of major group-level behavioural transitions related to minor changes in individual-level interactions. Further, we present the first evidence for collective memory in such animal groups (where the previous history of group structure influences the collective behaviour exhibited as individual interactions change) during the transition of a group from one type of collective behaviour to another. The model is then used to show how differences among individuals influence group structure, and how individuals employing simple, local rules of thumb, can accurately change their spatial position within a group (e.g. to move to the centre, the front, or the periphery) in the absence of information on their current position within the group as a whole. These results are considered in the context of the evolution and ecological importance of animal groups. r

Recent models from theoretical physics have predicted that mass-migrating animal groups may share group-level properties, irrespective of the type of animals in the group. One key prediction is that as the density of animals in the group increases, a rapid transition occurs from disordered movement of individuals within the group to highly aligned collective motion. Understanding such a transition is crucial to the control of mobile swarming insect pests such as the desert locust. We confirmed the prediction of a rapid transition from disordered to ordered movement and identified a critical density for the onset of coordinated marching in locust nymphs. We also demonstrated a dynamic instability in motion at densities typical of locusts in the field, in which groups can switch direction without external perturbation, potentially facilitating the rapid transfer of directional information.