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.
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.
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.
Honest signaling has given rise to two forms of mimcry: Batesian mimicry, in which a dangerous signaler is imitated by a harmless mimic, and Mullerian mimicry, in which two dangerous species evolve mutual imitation to the benefit of both. A good example of Batesian mimicry is that of the Pseudotriton ruber salamander, which closely resembles another salamander, Notophthanlmuus viridescens. The Notophthalmus salamander is unpalatable to birds, and they soon learn to avoid these salamanders. The same birds also avoid the Pseudotriton salamander due to its close resemblance to that of its unpalatable cousin. Mullerian mimicry often occurs when two similar species, both of which are dangerous, have some overlapping habitat. By resembling each other, a predator that learns to avoid one will be more likely to avoid the other. In this case, the predator saves itself a hard lesson learned twice, and the mimics avoid a sacrificial encounter.
There are inherent problems in defining communication, which is generally concerned with intentional signals given to alter the behavior of the receivers. Some signals are not intended for this purpose and so we will exclude them from communication. But determining the intent of a signal can be difficult. For instance, the Cleaner wrasse fish performs a display to attract bigger fish, which line up to have their parasites picked out of their gills. This is a mutualistic symbiotic relationship; the bigger fish have their parasites removed and the Cleaner wrasse gets a nice meal. But is the display really communication? Certain bats hunt Tungara frogs, which have two main vocalizations--a high pitched whine and a low pitched chuck. Bats can only hear the chuck, and female Tungara frogs are more attracted by this sound. The signal is intended to attract females, but it also clues a hungry bat in to the location of a frog. The bat's behavior is modified as a result, but the signal was not intended for the bat. Consider a signal to an intended receiver where the response will be mutually beneficial. Flower colors, for example, have evolved to attract specific pollinators. The result is mutually beneficial--the pollinator enjoys a meal and the flower has its pollen spread, but would we really say the flower is communicating with its pollinator? These are some of the problems we face when defining communication, and as a result there can be no hard and fast definition.