Sperm whales breed slowly, with females having their first calf when they reach nine years old. Pregnancy lasts from 14 to 16 months, and the newborn calf is 13 feet long and weighs 1000 kilograms. These babies will stay with their mothers for over a decade, and suckle for over three years. There have been some records of sperm whale calves nursing from their mothers (and other lactating females in the group) for up to 13 years. The milk produced by the mother whales is incredibly rich; sperm whale milk is 36% fat, compared to 4% fat in raw cow’s milk. This means the milk has a consistency closer to that of cottage cheese than to liquid milk, which has the added bonus of keeping the milk from dissolving and being lost in the water as the calf feeds.