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Summary 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. 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. 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.
Hårding et al. (Tue,) studied this question.