Some animal activities have become ritualized over the course of evolution so that they now serve a communicative function. Protective reflexes, for example, such as narrowing the eyes and flattening the ears prepare an animal in danger to protect sense organs. These movements also may indicate fear or anger to other animals. Intention movements such as these are incomplete behavior patterns that provide information about the activity a particular animal is about to perform. A bird will generally crouch, raise its tail, and pull back its head before it takes flight. If a bird takes flight without first performing these movements, it acts as an alarm signal, and the whole flock will suddenly take flight.

Ritualized behaviors allow for the evolution of a signal by increasing conspicuousness, stereotypy, and separation from its original function. An example of such increasing exaggeration can be found in bower birds. Males decorate their nest with blue objects. They will steal any blue object, including pieces of paper, plastic, and glass. This behavior began as nest building and has evolved to attract females.

The process of ritualization first involves the receiver noticing the correlation between the signal and the actions of the sender. The sender then ritualizes his signal to receive the optimal ideal response from the receiver and the receiver modifies his response to optimally benefit himself. As an example, a dog who is preparing to bite retracts his lips into the familiar growl snarl. This particular behavior began so that the dog does not bite his own lips as he bites. However at some point in evolutionary history, the receiver noticed that the snarling dog presented a danger to him. The signaling dog now notices the receiver often backs down before the fight even begins, and continues retracting his lips as a way to ward off the receiver. However, this ritualization can have a "dog who cried wolf" result, where the receiver will become so accustomed to the snarling without attack that he will no longer retreat.

The Effects of Natural Selection on Signaling

Conflict and Cooperation

Signals of conflict, such as that of predator to prey, involve a signaler who intends to manipulate the receiver. The receiver then interprets the signal as a warning sign and evolves resistance. The result is a coevolutionary arms race, which can lead to ever- exaggerated signals.

Cooperation, on the other hand, involves a mutual interest of the signaler and receiver. In terms of cooperation, the signaler and receiver both want to be able to communicate while remaining as little noticed as possible by potential predators. Such inconspicuous signaling offers a distinct selective advantage. Evolution therefore results in "conspiratorial whispers," where both signaler and receiver evolve to make the signal as inconspicuous as possible while still reaching its receiver without alerting unintended receivers.

Honest Signaling

Zahavi's handicap principle states that in order to be honest, a signal must be costly to the signaler. Thus, only the most fit individuals can afford to brandish an honest signal. For females looking for a mate, such a declaration of fitness will identify a particular male as a quality choice. For this reason, some signals, such as peacock's tails, become extremely exaggerated: males are trying to declare their fitness. Only the toughest males can survive with such a costly, conspicuous tail. Another example is the black bib of dominant male Harris sparrows. Only dominant males have this black bib. An experiment in which males were given a black bib by means of a magic marker showed that male was attacked by other sparrows. The male with an artificial black bib could not survive the attack; only the fittest males could have the black bib of dominance and not lose fights by challengers. There is currently much debate over whether the handicap principle is valid, and there is some evidence that it does not always hold true. However, in general, a costly signal such as a peacock tail that can serve no other purpose are honest indicators of fitness.