They are mostly black, often with a white belly and white markings underneath flippers and tail (flukes) that are unique to each whale. Humpbacks have fleshy knobs (tubercles) on their heads and lower jaws and sport paired blowholes on top of their heads. Females usually breed every two or three years starting at the age of five. The gestation period (pregnancy) is 11½ months. Calves nurse from their mothers, drinking 40 to 60 gallons of very rich fatty milk a day for about 10 months, after which they feed themselves. Male Humpbacks sing the longest and most complex songs in the animal kingdom which is thought to have a role in the mating ritual.