river dolphins have a torpedo shaped body with a flexible neck, limbs modified into flippers, non-existent external ear flaps, a tail fin, and a small bulbous head. river dolphin skulls have small eye orbits, a long snout and eyes placed on the sides of the head. river dolphins are rather small, ranging in size from the long south asian river dolphin to the and amazon river dolphin. they all have female-biased sexual dimorphism, with the females being larger than the males. river dolphins are polygynous, meaning male river dolphins typically mate with multiple females every year, but females only mate every two to three years. calves are typically born in the spring and summer months and females bear all the responsibility for raising them.