Oceanic dolphin another indication of animal intelligence. Killer whales have the second largest brain mass of any animal on earth, next to the sperm whale. The brain to body mass ratio in some is second only to humans. Self-awareness is seen, by some, to be a sign of highly developed, abstract thinking. Self-awareness, though not well-defined scientifically, is believed to be the precursor to more advanced processes like meta-cognitive reasoning (thinking about thinking) that are typical of humans. Research in this field has suggested that cetaceans, among others, possess self-awareness. The most widely used test for self-awareness in animals is the mirror
Blue whale of its life, a blue whale calf drinks approximately of milk every day. Blue whale calves gain weight quickly, as much as every 24 hours. Even at birth, they weigh up to —the same as a fully grown hippopotamus. Blue whales have proportionally small brains, only about , about 0.007% of its body weight, although with a highly convoluted cerebral cortex. The blue whale penis is the largest penis of any living organism and also set the Guinness World Record as the longest of any animal's. The reported average length varies but is usually mentioned to have an average length