Summary 1.Winter survival rate in Harbour Seal pups is significantly correlated with the autumn body mass of pups. Multi-type mark-recapture statistics were applied to individual re-sighting histories of branded seals, and survival probability was estimated with weight as a covariate. The probability of surviving to an age of 1 year is only 0·63 for the smallest pups at 17 kg, whereas pups at 32 kg have a survival probability of 0·96.2. An energetic model for juvenile Harbour Seals reveals how metabolic rate is related to body mass, skin surface area, blubber thickness and water temperature . There is an increasing thermal stress with decreasing body size of pups. Low winter water temperatures induce a negative energy balance in small pups, which is a probable cause of the observed mass-dependent survival. 3. This study explicitly links a physical property of the environment, sea-water temperature, to energetics and life history. The approach opens possibilities for studying aspects of life-history evolution, such as optimal weaning weight and pupping time, in marine mammals.