Seals are fur bearing mammals that are highly adapted to living in an aquatic environment, however arctic seals, like the Hooded seal, have to adapt to the cold as well as an aquatic lifestyle (Kvadsheim and Aarseth, 2002). Thermoregulation can be difficult for marine mammals living in cold aquatic environments, and so it is managed through a combination of behavioral and physical mechanisms (Pearson et al., 2014).
When born, the young Hooded seals do not have all the adaptions to keep warm that the adults do, however they are born with blubber which is considered to be an advanced marine mammalian trait. The young seals must however use some methods to keep warm that the adults do not. One of these methods is to use brown adipose tissue (Cannon and Nedergaard, 2004) which they are born with. Although this brown adipose tissue is present, it is actually far less functional in Hooded seals than in other polar phocid seals. In addition to this, the pups also have low muscle enzyme activity and so Pearson et al (2014) suggests that the Hooded seal pups low surface area to volume ratio and their thick blubber layer are sufficient to meet most of their thermoregulatory challenges. This is likely essential because any additional need for thermogenisis in the extremely short nursing period would be detrimental to the energy savings needed for the post weaning fast.
Fur is a very good insulator and is far more effective than blubber in air, which is why many species of seal pup have a fluffy lanugo coat when born to help keep them warm before they develop blubber. The problem however, is that once in water the fur compresses and the warm air is replaced by cold water that readily conducts heat away from the body, causing the fur to lose more than 90% of its insulating capacity (Kvadsheim and Aarseth, 2002). This loss of insulating capacity is not practical for the Hooded seals that enter the water early in life and spend much of their time there (Oftedal, 1991), and is why the Hooded seal pups moult the lanugo in utero and are born with a coat which is more similar to the adults (Blix, 2005). The fur is only really important for Hooded seal pups at birth because the blubber layer they are born with is only thin, however this rapidly increases in thickness and after just 4 days they have thermal protection in water similar to that of the adults (Kvadsheim and Aarseth, 2002). Kvadsheim and Aarseth (2002) found that although fur is of no use to adult phocid seals in water, it makes a considerable contribution to their theroregulation in air.
Once fully developed, a thick subcutaneous lipid layer provides insulation and greatly contributes to their overall thermal resistance. This blubber layer protects against the cold and is effective against the increased conductivity of water as, unlike fur, the water cannot penetrate it (Kvadsheim and Aarseth, 2002). This is also helped by having a small surface area to volume ratio which reduces heat loss from the surface of the seals and helps keep their core temperature up (Pearson et al., 2014). If the seals become exposed to temperatures below their lower critical temperature, they can also shiver to increase heat production.