Harp seals are born on the drifting ice of the North Atlantic Ocean during arctic winter when temperatures of -20 degrees C, occasionally in combination with wind of 10 m/s, might prevail for days. At birth the pups lack subcutaneous blubber and the wet infantile fur has a conductance value of 30.0 W . m-2 . degrees C-1, as compared with only 2.0 W . m-2 . degrees C-1 when dry. While still wet immediately after birth the pups are nevertheless able to retain body core temperature by shivering. This activity leads to reduction of muscle fat and glycogen stores. Nonshivering thermogenesis commences in thermogenic adipose tissue by virtue of loosely coupled mitochondria. Thermogenic adipose tissue is found at birth both as a subcutaneous layer along the back and as internal deposits around venous plexuses in the neck, on the pericardium, on the kidneys, and the abdominal walls. After about 3 days of suckling the subcutaneous adipose tissue loses its thermogenic function being gradually transformed into blubber, whereas the internal deposits persist at least until the pups venture into water at the age of 3-4 wk.