Brown adipose tissue and non‐shivering thermogenesis aid harp seals ( Pagophilus groenlandicus ) but not hooded seals ( Cystophora cristata ) or Weddell seals ( Leptonychotes weddelli ) at birth

Harp, hooded and Weddell seal neonates face harsh environmental conditions being born on ice in polar environments. But, because harp and Weddell seals are born with a thinner blubber layer than hooded seals (1.2±0.2 and 0.5±0.1vs 2.0±0.2cm) and have a wetable lanugo pelage, each species may use different tactics to maintain euthermia. To test whether neonates rely on non‐shivering thermogenesis (NST) or increased metabolic heat production, brown adipose‐like tissue (BAT‐lt) was collected from the neck region of neonatal harp (n=5) and hooded seals (n= 6) and examined histologically and biochemically. BAT‐lt from both species consisted of multilocular cells filled with lipid droplets. BAT‐lt from harp seals contained more lipid droplets/100um 2 (p=0.02), however, these lipid droplets were only slightly smaller than those in hooded seals (65.8um 2 vs 91.1um 2 , p=0.16). Uncoupling protein 1 was only detected BAT‐lt from neonatal harp seals. Activities of citrate synthase, cytochrome c oxidase, and β‐hydroxyacyl CoA dehydrogenase were all significantly higher in harp seals. No such tissue was found in Weddell seal neonates (n=5). Together, these findings suggest lipids in BAT‐lt tissue of harp seals are oxidized to generate heat via NST prior to the establishment of a thick blubber layer, while the neonatal hooded seals' thicker blubber layer and dense pelage are sufficient for maintaining euthermia without NST. Weddell seals seem to rely on increased metabolic heat production through an increased metabolic flux in muscle. This project is funded with support from Alaska EPSCoR (NSF EPS‐0346770), DFO Canada and a UAF CGC Student Research Award.