Penguins are a group of aquatic, flightless birds living almost exclusively in the southern hemisphere. The number of penguin species is debated. Depending on which authority is followed, penguin biodiversity varies between 17 and 20 living species, all in the subfamily Spheniscinae.The largest living species is the Emperor Penguin : adults average about 1.1 m tall and weigh 35 kg or more. The smallest penguin species is the Little Blue Penguin (also known as the Fairy Penguin), which stands around 40 cm tall and weighs 1 kg.