Blue whales mate in the warm waters of the tropics, and this is where the calves are born. At birth the baby may be up to 8.6m in length and weigh about 7,000 kg. After that, it increases its weight by about 90.7 kg a each day. In fact, the blue whale, from the time of conception until twelve months old, can claim the most rapid growth rate of any plant or animal species in the world.