The polar bear is not white. Its skin is black, and every strand of its fur is transparent and hollow, scattering visible light the way crushed ice does while channeling ultraviolet energy down to the skin. This episode looks past the holiday image at Ursus maritimus as a machine built for one habitat, the frozen ocean. Genetic evidence shows the split from brown bears was no clean break: the two interbred through the Pleistocene, and extinct Irish brown bears carried up to 21.5 percent polar bear DNA. Along the way polar bears shed genes for digesting starch and smelling plants and gained extra copies for breaking down fat.

That specialization shapes everything. A bear wrapped in up to 10 centimeters of fat overheats if it runs, so it hunts by waiting motionless at seal breathing holes for hours or days. Females pause their pregnancies until autumn and nurse cubs through winter without eating. Every one of these strategies needs sea ice, and the ice is melting earlier and freezing later, stretching the summer fast and locking the species into a diet it cannot replace on land.

• Males weigh 300 to 800 kilograms. The record was a 1,002 kilogram bear found in Alaska in 1960.
• Through delayed implantation, a fertilized embryo stops developing until fall, and the mother’s body reabsorbs it if she lacks the fat reserves to survive denning.
• Cubs are born blind, hairless, and about 600 grams, then grow to 10 to 15 kilograms on fat-rich milk before leaving the den in spring.
• Fat-binding industrial chemicals called PCBs concentrate up the food chain and reach polar bears in doses that damage immunity and reproduction.
• In 1773 a young Horatio Nelson left his post on an Arctic expedition to shoot a bear for its fur, and barely survived when his musket misfired.