Lipid-rich, collagen fiber-laced blubber comprises the hypodermis and covers the whole body, except for parts of the appendages. It is strongly attached to the musculature and skeleton by highly organized, fan-shaped networks of tendons and ligaments, can comprise up to 50% of the body mass of some marine mammals during some points in their lives, and can range from 2 inches (5 cm) thick in dolphins and smaller whales, to more than 12 inches (30 cm) thick in some bigger whales, such as right and bowhead whales. However, this is not indicative of larger whales' ability to retain heat better, as the thickness of a whale's blubber does not significantly affect heat loss. More indicative of a whale's ability to retain heat is the water and lipid concentration in blubber, as water reduces heat-retaining capacities, and lipid increases them.
Blubber is the primary fat storage on some mammals, specifically those that live in water. It is particularly important for species that feed and breed in different parts of the ocean. During these periods, the animals metabolize fat. Blubber may save energy for marine mammals, such as dolphins, in that it adds buoyancy while swimming.
Blubber differs from other forms of adipose tissue in its extra thickness, which provides an efficient thermal insulator, making blubber essential for thermoregulation. Blubber is more vascularized—rich in blood vessels—than other adipose tissue.
Blubber has advantages over fur (as in sea otters) in that, though fur retains heat by holding pockets of air, the air expels under pressure (i.e., when the animal dives). Blubber, however, does not compress under pressure. It is effective enough that some whales can dwell in temperatures as low as 40 °F (4 °C). While diving in cold water, blood vessels covering the blubber constrict and decrease blood flow, thus increasing blubber's efficiency as an insulator.
Blubber aids buoyancy and streamlines the body, because the organized, complex collagenous network supports the noncircular cross sections characteristic of cetaceans.