<p> Simple environmental features can shape complex behavior. Identifying key aspects of the environment (e.g., temperature, structure, toxins) that lead to widespread consequences is of central importance in a changing world. The primary objective of my dissertation is to investigate how relatively simple aspects of the environment can influence small groups of animals in profound and complex ways. In the first three chapters, I report on experiments showing how small changes in the environment can affect the expression of behavior at different points in development and can have important physiological consequences for litters of mouse pups. I then report on two sets of experiments showing how subtle changes in the environment can dramatically affect spacing patterns and social dynamics of small groups of adult zebrafish. Together, my results emphasize the ways that subtle changes in the environment can have profound impacts on individuals and small groups. In both lines of work, I have found that a more accurate characterization of the phenomena, infant rodent development and zebrafish social behavior, requires the use of individual and group measures and that temperature, density, and pollutants can have a powerful effect on group responses. These results are important because they show that the physical environment can have profound effects on the phenotype, and that with a changing physical environment or anthropogenic change, dramatic differences may be observed in the behavior of groups.</p>