Harbor seals have a metabolic rate somewhat higher than land mammals of the same size. This helps them generate body heat for warmth. A thick layer of blubber insulates the harbor seal, reducing heat loss. The blubber of an Atlantic harbor seal during winter may account for 27% to 30% of its total body mass. Because seals are so well insulated, they can even overheat if their environment gets too warm!  

Blubber also streamlines the body and functions as an energy reserve from which the harbor seal can draw energy during periods of fasting. A harbor seal’s hair provides only a small amount of insulation. In cold water, blood is shunted inward as blood vessels in the skin constrict, reducing heat loss to the environment. When hauled out on land, blood vessels in the skin may dilate, allowing heat to be exchanged with the environment. 

Project Oceanology students are more likely to see seals when they are hauled out on the rocks.  Seals haul out on the rocks to rest, but they do it more frequently and for longer when they also get a thermal benefit.  During the time of year the seals are in Fishers Island Sound, both the air and water temperatures vary a lot. Water temperatures range from less than 5˚C (~40˚F), to more than 15˚C (~60˚F), and air temperatures vary even more: from below freezing (<0˚C/<32˚F), to more than 20˚C (68˚F)!