Blue whale (Balaenoptera musculus) showing the underside of its fluke. The blue whale is the worlds largest animal, and is probably the largest animal that has ever lived. It can reach lengths of 27 metres and weigh 120 tonnes, with females being larger than males. It is found in all the worlds seas, spending the winter in tropical waters and migrating to cooler polar oceans for the rest of the year. One calf is born after a gestation period of 10 to 11 months. A newborn calf measures roughly 7 metres in length. It nurses for 7 to 8 months, consuming over 350 litres of its mothers milk and gaining 90 kilograms in weight a day. Photographed in the Santa Barbara Channel, California, USA.