Communication is ubiquitous in the animal world and is often striking and elaborate. Ethologists studying animal behavior in the field have long been intrigued about the origin and function of the displays they observe. Classic signal theory was based upon Darwin’s principles (1872) and later elaborated by Tinbergen (1951, 1963), Marler (1984), Huxley (1966), and other ethologists who refined the Darwinian notion that signals evolved from incidental and involuntary expressions of internal states like emotions, a process known as ritualization. If an incidental result of emotional arousal and autonomic state activity involved an observable effect (i.e., a facial or vocal expression or erection of hair or feathers), through ritualization this movement could evolve into an effective signal that carries important information. Each signal is rigidly linked to a specific function, hence the term “fixed” signal. The information conveyed by the signal was thought to pertain to the animal’s internal state, rather than some external condition of the environment, hence Darwin’s title The Expression of the Emotions in Man and Animals.