Dynamics of multiple signalling systems: animal communication in a world in flux.

The ubiquity of multiple signalling is a long-standing puzzle in the study of animal communication: given the costs of producing and receiving signals, why use more than a single cue? Focusing on sexually selected signals, I argue that dynamic variation in selection pressures can often explain why multiple signals coexist. In contrast to earlier research, which has taken a largely static view of the world, new insights highlight how fluctuations in ecological and social environments, as well as non-equilibrium dynamics intrinsic to coevolutionary systems, can maintain both multiple redundant and non-redundant signals. Future challenges will include identifying the circumstances under which environmental fluctuations lead to multiple signalling, and the consequences of such fluctuations for speciation in multiple-signalling species. 2009 Elsevier Ltd. All rights reserved.