Sexually mature female whales are referred to as cows. Mature males are often called bulls, and the offspring of a cow and a bull is called a calf.
Every year, some species of whales, such as blue whales, travel thousands of miles and cross oceans to reach their mating grounds. Female whales reproduce once every 1 to 6 years. Depending on the species, the gestation period lasts between 9 and 18 months. Because whales are mammals, the female whales carry the young in their womb before giving a live birth. When calves are born, they rely on their mother's milk for up to two years.