Physical constraints in biological collective behaviour

Many biological systems require the coordinated operation of a large number of agents linked together by complex interactions in order to achieve their function reliably. Because of the complex relationship between individual laws and system-level behaviour, theory is needed to assess which emergent phenomena result from fine-tuning or adaptation, and which follow from logical or physical constraints set by the system's design. Here we illustrate this crucial role of theory through recent examples from the collective motion of bird flocks. In some cases abstract theoretical laws explain the emergence of some apparently surprising traits, without the need to invoke new assumptions. Conversely, quantitative theoretical predictions sometimes show that general mathematical and physical laws are incompatible with otherwise mundane observations, forcing us to reconsider our assumptions and leading us to discover new principles.