Hooded seals breed on drifting pack ice, which can break up at any time, and the nursing period is compressed to about four days on average, the shortest lactation known in mammals. Milk fat content is exceptionally high, commonly reported in the range of 60 per cent or more toward the end of the period.
The pup, called a blueback for its slate-blue dorsal coat, is born at around 20 kilograms and roughly doubles that by weaning. It is then abandoned. The mother's investment is delivered almost entirely as stored energy transferred at extreme rate, and she loses a substantial fraction of her own mass in the process.
Why this species compresses lactation so much further than related seals is not settled. Ice instability is the standard explanation, but harp seals breed on comparable ice and nurse for around twelve days. The hooded seal is currently listed as Vulnerable, with the Northeast Atlantic stock much reduced from historical levels.