Seasonal variation in the metabolic rate of harp seals: unexpected energetic economy in the cold ocean

The metabolic rate of nine harp seals was measured chronically over a 12-month period using indirect calorimetry. The extent to which the seals' oxygen consumption was predicted by the allometric equation relating basal metabolic rate to body mass depended upon how the former was operationally defined and on the breeding status, sex, and age of the animal. There were large seasonal changes in the. oxygen consumption of adult males and reproductive females when metabolic rate was defined as the lowest hourly mean Vo(2) in, typically, 23 h of measurement. From April until August, the males' metabolic rate averaged as much as 83% higher than the allometric prediction from body mass, but for the rest of the year their oxygen consumption was not different from the expected value for mammals. Pregnant and pseudopregnant females showed a brief spring elevation in metabolic rate, but otherwise their oxygen consumption was well below that predicted by allometry. In one female who spontaneously aborted some 7 months after insemination, VO2 increased to the value for mammals shortly thereafter, to a level resembling that of the only female who was not pregnant and who showed no seasonal Variation in oxygen consumption. The immature seals' records are highly variable, showing no clear intra-annual pattern; however, their metabolic rates were lower than expected for young mammals. All seasonal shifts were in the opposite direction to the large changes in body mass exhibited by these seals. The effect of these findings in reducing the calculated impact of harp seals on the North West Atlantic fishery is discussed.