Locating the source of an advected chemical signal is a common challenge facing many living organisms. When the advecting medium is characterized by either high Reynolds number or high Peclet number, the task becomes highly nontrivial due to the generation of heterogeneous, dynamically changing filamental concentrations that do not decrease monotonically with distance to the source. Defining search strategies that are effective in these environments has important implications for the understanding of animal behavior and for the design of biologically inspired technology. Here we present a strategy that is able to solve this task without the higher intelligence required to assess spatial gradient direction, measure the diffusive properties of the flow field, or perform complex calculations. Instead, our method is based on the collective behavior of autonomous individuals following simple social interaction rules which are modified according to the local conditions they are experiencing. Through these context-dependent interactions, the group is able to locate the source of a chemical signal and in doing so displays an awareness of the environment not present at the individual level. This behavior illustrates an alternative pathway to the evolution of higher cognitive capacity via the emergent, group-level intelligence that can result from local interactions.collective intelligence | olfactory search | cooperation T hroughout the natural world, organisms are constantly faced with the challenge of locating the resources required for their survival. Often this means navigating their environment based on spatiotemporally varying information such as advected chemical cues, thermal gradients, or magnetic fields. It has been noted that collective behavior can greatly assist animal navigation. One explanation for this, known as the "many wrongs" principle (1), is that inherent noise in the environment is dampened due to multiple sampling by individuals within a group. A quantitative study of an effect of this type was made by Grünbaum (2) and the benefits of sociality clearly illustrated. However, this effect does not fully capture the potential emergent properties of social aggregations, which often display complex behaviors entirely absent at the individual level (3-6). In this context, complex systems, such as fish schools or animal herds, can be viewed as information-processing entities with a collective awareness of their environment (7). Understanding their capacity for performing search tasks will have important consequences for the development of distributed technologies, such as olfactory robot swarms, with applications in the detection of explosives, landmines, or locating people in search-and-rescue operations (8-10).The use of chemical signals by organisms in order to gain information about their environment is a ubiquitous behavior commonly seen in aquatic animals or flying insects and observed in a diverse range of species over a range of scales. For low Reynolds number, viscous environments chemotaxis in o...