Shenandoah National Park carries roughly 1.5 black bears per square mile in good habitat, some of the densest bear country in the East, and Virginia holds an estimated 18,000 to 20,000 statewide. From about mid-November to late February a mountain bear is in a den, and park biologists put the rest plainly: it does not eat, drink, urinate or defecate, for as long as 100 days. None of that is possible without spending stored macromolecules, and spending them in the right order.

Fat as the winter store

Through autumn the bear is in hyperphagia, taking in up to 20,000 calories a day against roughly 5,000 in summer, and storing the surplus as lipid. Table 1 shows why lipid and not starch. A gram of fat yields 9 kilocalories; a gram of carbohydrate or protein yields 4.

The reason sits in the covalent bonds. A carbohydrate's carbons already carry oxygen, so much of their energy has been spent before the animal ever touches them. A fatty acid chain is almost entirely carbon and hydrogen, and those C–H bonds are the fuel. Because a bear must carry its whole winter supply as body mass, yield per gram decides everything: an animal storing the same energy as carbohydrate would haul more than twice the weight into the den.

Water made, not stored

A bear goes months without drinking and does not dehydrate. The right-hand column of Table 1 explains it. Breaking fat down produces water as a product — about 1.1 grams for every gram burned. Carbohydrate returns 0.6, protein about 0.4. The molecule that stores energy best also supplies water best, and for the same reason: all that hydrogen has somewhere to go, and where it goes is H₂O.

Building a polymer removes water at every bond. Breaking one apart by hydrolysis adds water back to split the bond, and enzymes catalyze each step. The bear spends the winter taking its own macromolecules to pieces, one monomer at a time.

Why the muscle survives

A denning male loses 15 to 30 percent of his body weight, a nursing female as much as 40, and nearly all of it is fat. After 110 days a bear's thigh muscle has given up only about 10 percent of its protein, with strength down about 29 percent. A human held to 90 days of bed rest loses 54 percent of muscle strength. The bear is fasting and beating us.

Nitrogen is a large part of the answer. Every amino acid carries a nitrogen atom, and breaking protein down turns that nitrogen into urea, the waste that normally leaves the body in urine. A bear that does not urinate has nowhere to put it. So it takes the urea apart and sends the nitrogen back into building new protein instead, using the same atoms over again. Fat contains no nitrogen at all, which is one more reason it is the cheaper thing to burn.