Communication occurs when an animal produces a signal and uses it to influence the behavior of another animal. A signal can be any behavioral, structural or physiological trait that has evolved specifically to carry information about the sender and/or the external environment and to stimulate the sensory system of the receiver to change their behavior. A signal is different from a cue in that cues are informational traits that have not been selected for communication purposes. For example, if an alerted bird gives a warning call to a predator and causes the predator to give up the hunt, the bird is using the sound as a signal to communicate its awareness to the predator. On the other hand, if a rat forages in the leaves and makes a sound that attracts a predator, the sound itself is a cue and the interaction is not considered a communication attempt.
Air and water have different physical properties which lead to different velocity and clarity of the signal transmission process during communication. This means that common understanding of communication mechanisms and structures of terrestrial animals cannot be applied to aquatic animals. For example, a horse can sniff the air to detect pheromones but a fish which is surrounded by water will need a different method to detect chemicals.