What takes up 50-70 percent of a whale's weight?

Blubber takes up roughly 50–70 percent of a whale’s weight. It is a thick, specialized layer of fat beneath the skin and is one of the defining adaptations of whales and other marine mammals.

Blubber helps whales stay warm in cold ocean water by slowing heat loss. It also stores energy for long migrations and fasting periods, when whales may travel thousands of kilometres or feed very little. Because blubber is less dense than muscle and bone, it also contributes to buoyancy.

The exact percentage varies by species, age, season, reproductive condition, and feeding success. Bowhead and right whales are especially well known for carrying large fat reserves; mature bowhead whales may have blubber representing about 40–50 percent of body mass. Some educational sources give higher estimates, especially for well-fed individuals or certain life stages.

A common mix-up is confusing blubber with ordinary body fat. Blubber is not simply loose fat: it is an organized connective tissue containing fat cells, blood vessels, collagen, and other structures. It should also not be confused with whale oil, which is produced by processing blubber and other tissues.