Dolphin Communication

Echolocation

The term echolocation refers to an ability that odontocetes (and some other marine mammals and most bats) possess that enables them essentially to "see" with their ears by listening for echoes. Odontocetes echolocate by producing clicking sounds and then receiving and interpreting the resulting echo. Sound waves travel through water faster than through air. These sound waves bounce off objects in the water and return to the dolphin in the form of an echo. High frequency sounds don't travel far in water. Because of their longer wavelength and greater energy, low frequency sounds travel farther.

Echolocation is most effective at close to intermediate range, about 16 to 656 feet. Dolphins produce a sound in the air passages in their heads, an oil-filled organ or melon in the forehead enables dolphins to focus and beam clicks through the water in front of them. Upon striking an object, the sounds will reflect, or "echo" back. By interpreting these echoes,dolphins can distinguish the direction, distance, speed, size,shape, and even the density of an object. The closer the dolphin is to a target, the more information he can acquire.
(MacGillivary Freeman's Dolphins)

Vocal

Bottle-nose dolphins communicate mainly by means of sounds and are almost always constantly vocal. These sounds includes high-pitched whistles or squeals and short, pulse-type or squawks eminating from their blowholes, with an average of 300 sounds per pulse. But they also use breaching (jumping and falling back into the water with a loud splash) and pectoral fin (or flipper) and tail (or fluke) slaps (hitting the flipper or fluke on the water surface). Body posturing and jaw popping also have a role in communication.

Bottle-nose dolphins identify themselves with a signature whistle. A mother dolphin may whistle to her calf almost continuously for several days after giving birth. This acoustic imprinting helps the calf learn to identify its mother. Dolphins regulate their sounds by shunting air throughout the air sacs beneath the blowhole. Tissue structures in this area slap together (much like a trumpet player's lips) to produce the clicks. These sounds often extend into the ultrasound region.

Dolphins can get specific information about a target by altering the rate and frequency of the clicks they produce. Although the clicks are definitely used in echolocation, the other sounds seem to be involved in certain forms of limited communication. Both whistles and burst-pulse sounds often fall below 20 kHz. Burst - pulse sounds seem to be related to a dolphin's emotional state. For example, those sounds often associated with aggression have a "squawking" or "barking" quality. Dolphins might produce "squeaking" tones when engaging in playful activities. "Chirping" sounds can sometimes be heard during sexual interaction.
(The Marine Mammal Center)

Body Posturing

Body posturing or "body language" is an obvious way in which dolphins exchange a wealth of information. Bodily contact itself is one important way dolphins communicate fellowship. Dolphins are tactile animals and are often observed touching and caressing each other. Much of this physical contact is likely to reaffirm friendly relations among individuals and groups. Dolphins can be seen stroking or petting each other with their pectoral fins, rostrum, and flukes. Genital contact is prevalent among dolphins regardless of age, gender, or hormonal influences.

Synchronous movements are another way in which dolphins reveal solidarity. Dolphins surfacing together, turning simultaneously, swimming in sync, and even jumping together display their close association. This type of behavior is especially common during courtship displays.

Aggression is often communicated by an open mouth, exaggerated movements, jaw clapping, and inverted swimming. Agressive posturing is frequently a prelude to biting, buting, or tail-slapping (fluking). The number of scratches from other dolphins' teeth (rake marks), are a tale-tell sign of an individual's fighting skill.

Dolphins will sometimes produce sounds by slapping the surface of the water with the pectoral fins or flukes. They will aslo jump into the air and land on one side for a louder effect. Such slapping and breaching may communicate an animal's location, frame of mind, or even used as an alarm to warn against danger.
(Dolphin Doctor)