What is blubber?
Blubber is the thick layer of fatty tissue directly under the skin of marine mammals. It is not a loose layer of fat but a firm tissue of fat cells in a network of connective tissue (collagen) and blood vessels. In a healthy harbour or grey seal it is several centimetres thick and wraps the whole body like a mantle, except the flippers and head — giving the seal its smooth torpedo shape.
Thickness varies strongly with season, age and condition. A well-fed grey seal pup at weaning is almost half fat by weight. In adults the proportion is lower, and it drops sharply in periods when they eat little.
Four functions of blubber
1. Insulation
Water draws heat from a body far faster than air. A seal keeps a core temperature of about 37 °C in North Sea water that is only a few degrees in winter. Blubber conducts heat poorly, and a seal can constrict the blood vessels running through it so little warm blood reaches the skin. On a warm day on land it does the opposite, sending more blood to the skin and flippers to shed heat.
2. Energy store
Seals eat little or nothing at certain times: mothers while nursing, males in the breeding season, all animals during the moult, and pups in the weeks after weaning while they learn to hunt. They live off their blubber. A nursing harbour seal mother loses a large part of her weight while her pup grows fast on the fatty milk.
3. Buoyancy
Fat is lighter than water, so blubber helps a seal float — for instance when it rests upright in the water ("bottling"). Because thickness changes through the year, so does buoyancy: a thin seal sinks more easily than a fat one.
4. Streamlining
Blubber fills the space between muscle and skin, giving a smooth shape with little drag, so a seal can swim fast for little energy.
Fur and blubber
True seals also have a short, dense coat, but in water it traps hardly any air and insulates poorly. Fur seals (a group of eared seals) are different: they rely much more on very dense fur with trapped air. A newborn grey seal has almost no blubber and a white woolly coat that is not waterproof, so it stays on land until it has built up enough fat and grown its adult coat.
A fat seal is a healthy seal
Beach visitors sometimes think a seal is "bloated" or ill because it is so round. Usually the opposite is true. Signs of malnutrition are a clearly visible neck (head looks narrower than the body), visible ribs, hips or spine, and an angular body with folding skin. If you see such an animal in the Netherlands, keep your distance and call 144; see seal pup found.
Blubber and pollution
Fat-soluble chemicals such as PCBs accumulate in blubber. In the 1970s Wadden Sea seals carried PCB levels high enough to impair reproduction; after PCBs were banned the population recovered. More on threats, and on the rest of the body on seal anatomy.
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