Dolphins make whistles, clicks, and screeching sounds to communicate with each other, and for echolocation. In many species each individual dolphin makes a unique 'signature whistle'. Human-made underwater sounds, such as used with sonar images, can interfere with dolphins' communication, and even injure tissues used for hearing and air intake.

Dolphins also help each other in various ways. If a dolphin is sick and can't raise to the surface to get air, another one supports it at the surface so it can breathe. Or, a dolphin may stay near an injured or sick one as a companion. Dolphins also defend an injured one against a threat such as a boat.

Male and female dolphin take part in courtship, which involves playing, caressing, and 'songs'. Gestation lasts about a year, and then a single calf is born. Calving usually occurs once every two years. Dolphin mother produces very high-fat milk for her young. The calf is weaned aroud 6 months to two years.

Certain dolphin species live quite long, 50 years or more, while the bottlenose dolphin and common dolphin typically live around 20-25 years.

Dolphin Classification:
Class Mammalia
Order Cetacea
Suborder Odontoceti

Names:
male: bull
female: cow
young: calf
group: pod

Sources:

Microsoft ® Encarta ® Reference Library 2005
Encyclopedia Britannica