Dolphins are relatively long-lived, with some individuals living more than 20 years. Most species do not begin to breed until ages of 5 to 8 or more. The female bears a single offspring, called a calf, after a gestation period of 8 to 11 months, depending on the species. As soon as the calf is born, the mother nudges it to the surface for its first breath of air. The young stay with the mothers for at least one and sometimes as long as two years, while continuing to nurse.
Most dolphins have their young in the warm months of the year.
The river dolphins are native to South America and southern Asia. River dolphins can be distinguished from ocean-dwelling dolphins by their smaller brains and more limited interactions with each other. Their eyes can detect light but do not form an image. As with dolphins, social interaction among porpoises is common. However, porpoises do not form enormous schools like some of the dolphins, generally remaining in coastal areas in small schools of two to four.