The true seals (family Phocidae) produce large amounts of high-fat (ca. 30 60%) milk, and wean their young at a young age (4 days to 8 weeks).[1'2] During the shortest lactation of any mammal (4 days), the hooded seal provides 8 kg/d of 60%-fat milk, causing pups to increase in weight from 22 kg at birth to 45 kg at weaning. Such an enormous transfer of energy to the young, whether expressed relative to the metabolic size of the mother (4.0 MJ/kg075/d) or the metabolic size of the pup (14 MJ/ kg083/d), is unparalleled among other marine mammals (Table 1) or terrestrial mammals. This intensive lactation may have evolved to allow completion of lactation in a very short time because the pack ice on which pups are born is liable to disintegrate during storms. However, most phocids have both a short lactation and a high rate of milk energy transfer relative to other mammals (Table 1). This is an evolutionary compromise to resolve the conflict between needing to feed at sea and having to nurse out of the water, whether on land or on ice. Phocids are able to ingest and deposit large amounts of fat and protein in body reserves before giving birth, and are thus able to fast for much or all of lactation. A short lactation with rapid energy transfer to the young minimizes the time that mothers must remain ashore, and thus reduces maintenance energy costs during the fast.
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