From ice to ocean: changes in the thermal function of harp seal pelt with ontogeny.

Many animals exhibit ontogenetic changes associated with adaptations for survival. Harp seals (Pagophilus groenlandicus) live in the Arctic and rely on thick insulation to maintain thermal homeostasis. Adult harp seals primarily use blubber for insulation, but newborn harp seals rely on a lanugo pelt while nursing, as their blubber layer develops and their first-year pelage grows. This study compared ontogenetic changes in the thermal properties of harp seal pelts in water and in air. Thermal conductivity, pelt thickness, and thermal resistance were measured in water for pelts of harp seal neonates (1 day old), thin whitecoats (4 day old), fat whitecoats (9 day old), ragged jackets (2 week old), beaters (3 week old), and adults and compared to previously published measurements made on the same pelts in air. Pelt conductivity was significantly higher in water than air for pre-molt and molting pups (P ≤ 0.031). Unlike adult pelage, which flattened underwater, lanugo hairs lifted underwater, a phenomenon that has not been reported previously. Thermal resistance of the pelt was significantly reduced in water compared to air for neonates and thin whitecoats (P ≤ 0.0001). A mathematical model of conductive heat transfer for an ellipsoid body showed volume-specific heat loss in water decreased and then stabilized as harp seals aged (P = 0.0321) and was significantly higher for neonates, thin whitecoats, and ragged jackets in water than in air (P ≤ 0.0089). Overall, pelt function is reduced in water for harp seal pups with lanugo, and this renders neonates and thin whitecoats particularly vulnerable to heat loss if submerged.