A critical review of metabolic rate determinations for pinnipeds (seals, sea lions, fur seals, and walrus) and cetaceans (whales, dolphins, and porpoises) does not support the widely accepted generalization that they have higher metabolic rates than terrestrial mammals of similar size. This finding necessitates a rethinking of the thermoregulatory adaptations of these marine mammals for an aquatic existence and has important implications in comparative studies of mammals, which frequently omit marine forms because they are perceived to be "different" from other mammals. It also suggests that numerous studies have overestimated food consumption by marine mammal populations.

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Male grey seals are larger at birth, grow faster during the nursing period, and are weaned at a heavier mass than females. Estimated mean energy expenditure by females from conception to the time of weaning (maternal investment), including the heat increment of gestation, cost of foetal tissue, and energy contained in milk acquired during the nursing period, was greater for male than for female offspring. This pattern of energy investment by female grey seals in their young is consistent with Maynard Smith's model of sexual investment (J. Maynard Smith. Behav. Ecol. Sociobiol. 7: 247–251. 1980).

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Growth and organ allometry of neonatal harp seals (Phoca groenlandica) were monitored during the first 6 weeks of life from 1982 to 1984. At birth, pup mass was 9.9 ± 1.7 (1 SD) kg. After their 1st day of relatively slow growth, pups gained mass rapidly, increasing 2 kg/day throughout the remainder of the ~12-day nursing period. Two-thirds of this mass gain was accumulated as a layer of subcutaneous blubber. Pups lost mass at a rate of ~0.5 kg/day during the postweaning fast, utilizing energy stores from the viscera, muscles, and limited amounts of blubber. In neonates, liver mass fluctuated in conjunction with total body mass gain and loss. The liver of adult harp seals was large relative to terrestrial mammals of similar size, but relatively small compared with other pinnipeds. The heart of harp seals grew slowly in pups and did not lose mass during fasting, and in adults it was of similar size relative to other mammals. The spleen of neonates was large and grew quickly during nursing. Spleen mass was quite variable among postweaning animals. As would be expected for a large-sized, precocially born, relatively advanced mammalian neonate, pups are born with large brains that grow very slowly. Adult brain mass, in relation to body mass, was similar to that of other mammals.

Thirteen captive phocid seals (10 grey seals (Halichoerus grypus), 2 harp seals (Phoca groenlandica), and 1 ringed seal (P. hispida)) were fed Atlantic herring (Clupea harengus harengus) in experiments designed to examine the use of otoliths recovered in stomach contents to interpret food consumption of wild seals. The percentage of ingested otoliths recovered in the stomach contents decreased with the time elapsed after feeding; 100% of otoliths were recovered between 0 and 3 h after feeding and 0% were recovered by 12.9 h after feeding. Absence of otoliths in the large intestine of seals having fed 3 to 6 h previous indicated that unrecovered otoliths had been digested (i.e., a complete disappearance of whole otoliths) while in the stomach. A significant relationship was also evident between the state of digestion of a seal's stomach contents, as measured by the proportion of otoliths remaining in skull cases (skull-recovered otoliths), and the duration of time since it had fed. These relationships have potential application in the estimation of daily fish consumption of seals feeding in the wild.