Lactation performance and nutrient deposition in pups of the harp seal, phoca groenlandica, on ice floes off southeast labrador

Harp seals (Phoca groenlandica) give birth on pack ice and nurse their pups for about 13 d. We hypothesized that lactating females would transfer milk and energy to pups at a high rate to compensate for the brevity of lactation. To test this hypothesis, the milk intake of five pups was measured over a 6-8-d period after administration of deuterium oxide, the initial age of pups was estimated to range from 0 to 5 d postpartum. Body water content of 17 pups was also measured by isotope dilution, and the body composition of 12 pups was determined by chemical analysis. The proximate composition of harp seal milk (n = 21) was also assayed. Harp seal pups contained little fat (3%) at birth, but 61% of the increase in body mass (average 2.3 kg/d) was fat, with the result that fat accounted for 47% of body mass at weaning. Body water estimated by isotope dilution was not significantly different from that measured directly (n = 9 paired samples); water composed 72% of lean boyd mass. From birth to weaning, milk lipid increased from 36% to 57%, and milk water declined from 51% to 32%. Pup milk intake averaged 3.7 kg/d. Mass gain of pups was highly correlated with both milk intake and energy intake (r(2)>0.98 for both). On average, pups consumed about 75 MJ/d, which is equivalent to a maternal energy yield of 1.9 MJ/(kg(0.75) . d). This rate of energy output is higher than that measured in most other phocids. Daily energy output is inversely related to lactation duration in phocids: a polynomial regression relating energy output to lactation duration explained 99% of the interspecific variation.