Blue whales typically breed during the winter or early spring after they have undergone a journey of several thousand miles, leaving the cold waters of the northern and southern hemispheres in favor of the warmer waters near the equator. The male and female engage in a complicated courtship ritual, rolling around each other and diving to the depths before engaging in sexual intercourse as they return to the surface.
Blue whales reach sexual maturity between the ages of 5 and 10. As with other mammals, blue whales reproduce through sexual meiosis. The male and the female each produce haploid gametes, or reproduction cells. The female gamete is the egg, and the male gamete is sperm. The male deposits his sperm inside the female with his penis, which is the largest of its kind found on Earth. If fertilization is successful, a fetus begins to develop inside the mother's uterus, and after an 11- to 12-month pregnancy, the baby blue whale calf is born near the surface of the water. It emerges tail-first and weighs around 6,000 pounds at birth. The calf promptly swims to the surface to take its first breath, aided by its mother if necessary. The calf typically stays with its mother for a year or longer before striking out on its own.