The document ranks and describes 15 of the smartest animal species, including sheep, pigeons, rats, octopuses, squirrels, pigs, cats, dogs, parrots, crows, elephants, monkeys, whales, dolphins, and great apes. It notes several traits that demonstrate intelligence for each species, such as memory, tool use, social behaviors, communication abilities, and self-awareness. The top three smartest animals are identified as great apes, dolphins, and whales.