Dive into the mysterious world of the common goldfish! From incredible cell regeneration and being able to see in far-red, goldfish have an unusual - and highly entertaining - way of communicating affection. Subscribe: bit.ly/BBCEarthSub #GoldfishFlirting #PetsWildAtHeart #BBCEarth Watch more: Planet Earth bit.ly/PlanetEarthPlaylist Blue Planet bit.ly/BluePlanetPlaylist Planet Earth II bit.ly/PlanetEarthIIPlaylist Planet Dinosaur bit.ly/PlanetDinoPlaylist Pets: Wild At Heart We may think of our pets like another member of the family - but they exist in a secret world that we would hardly recognise. In this series new filming techniques reveal our pets' wild behaviour and natural abilities. Using the latest filming techniques, from spycams to thermal imaging and moving x-rays, this series reveals why just a small select group of animals have the perfect characteristics to become human companions. The first episode shows how our pets use play to...