Tonight's Natural World special is certainly a big one, as we take a sneaky peek at the life of one of the world's bigger mammals. Zoological and marine mammal reproductive physiologist Holley Muraco has been trying without success for the past six years to breed Pacific walruses, with only 15 pups having been born in captivity in the last 80 years. To discover more about their behaviour she heads from her base in California to their breeding grounds in northern Alaska to see if the answers can be found in the wild.