| Summary: | I examined the effects of body size on the size of the small intestine in phocid seals, and on the retention times and digestibilities of food in harbour ('Phoca vitulina') and grey seals (' Halichoerus grypus'), and I investigated selection by harbour and grey seals offered different sizes and species of fish. The interspecific scaling of length, circumference, and surface area of the small intestine with body mass in phocid seals was similar to scaling relationships for other mammalian taxa. Body size explained far more of the variation in the surface area of the small intestine than in length and circumference. Surface areas of phocid seal small intestines scaled with body mass in a similar way to the scaling of metabolic and feeding rates in seals, and were similar to terrestrial mammals of similar size. These results were consistent with the hypothesis that the scaling of intestinal surface area in phocid seals is correlated with energetic requirements, but were inconsistent with the hypothesis that the small intestines of phocid seals have greater surface areas than expected for mammals of similar size. Concomitant measurements of retention times and assimilation efficiencies in harbour and grey seals revealed no effect of body size or seal species on assimilation efficiency, but older, larger grey seals had longer retention tunes than younger, smaller grey and harbour seals. The effects of body size on retention times, however, were not evident in comparisons of harbour and grey seals of similar age, but dissimilar mass. Size-independent analysis, expressing individual values relative to group means, revealed that assimilation efficiency increased with mean retention time in grey seals. I used operant conditioning and an underwater feeding apparatus to examine the selections of grey and harbour seals under conditions of standardized prey availability (paired presentations). Within a prey species, all grey and harbour seals selected the larger fish in paired presentations; when offered a choice between ...
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