Investigators who study hibernators, like Nicholas Mrosovsky, a University of Toronto zoologist, point out that weight gain, maintenance, and loss in these animals, and so perhaps in all species, is genetically pre-programmed and particularly resilient to variations in food availability. This program is characterized by its ability to adjust readily to changing circumstances and the unpredictability of the environment. Ground squirrels will gain weight through the summer at the same rate whether they’re in the wild or in the laboratory. They will lose it at the same rate during the winter whether they are kept awake in a warm laboratory or are in full hibernation, eating not a bite, and surviving solely off their fat supplies. “It is very hard to prevent them gaining and losing weight” on schedule, explains Mrosovsky, who did much of the original research in this area. When researchers surgically remove a sizable portion of fat from experimental animals—a procedure known as a lipectomy—the animals will restore the lost fat so that within months of the surgery they will be just as fat as they would have been without the surgery.*74

Even the type of fat found in animals and humans is regulated in a way that accommodates differing internal and external environments. The fat in our limbs, for instance, is less saturated than the fat around our organs, and so is less likely to stiffen in cold weather. We will also change the fatty-acid composition of our subcutaneous fat with temperature—the colder it gets, the more unsaturated the fats. This same phenomenon, independent of the type of fat consumed, has been observed in pigs, rats, and hibernators. Another example of the evolutionary specificity of fat deposits can be seen in those desert animals that do not store fat subcutaneously, as humans and most animals do, apparently because it would inhibit heat loss and cooling. So there are fat-rumped and fat-tailed sheep, and fat-tailed marsupial mice, all desert-dwellers that carry their fat almost exclusively in the so-named locations.

The storage of fat, it seems clear, like all evolutionary adaptations, tends to be exquisitely well suited to the environment—both internal and external—in a way that maximizes benefits while minimizing risks. This is why most investigators who considered these issues in the 1970s and 1980s assumed that a tendency to gain any excessive weight during periods of abundance would be the kind of obvious liability that evolution would work to select out of the species rather than select in. The thrifty-gene hypothesis does not hold up. But without a thrifty gene, rendered detrimental by the abundance of food in modern societies and the absence of physical labor needed to procure it, how do we explain why gaining weight in modern societies still seems so much easier than losing it?

Chapter 15

Chapter Fifteen

HUNGER