Chapter 11: Food, Energy, and Temperature: The Lives of Mammals in Frigid Places

Food, Energy, and Temperature: The Lives of Mammals in Frigid Places begins with a dramatic portrait of newborn reindeer calves, which must immediately thermoregulate in Arctic conditions without maternal warmth, relying on their woolly pelage and rapid development. The physiology of adult reindeer is then analyzed in detail, emphasizing their dense, air-filled fur, low metabolic cost in frigid air, and use of regional heterothermy to conserve energy. A key focus is the composition of limb lipids, which shift chemically from core to extremities to remain fluid and functional at subzero temperatures. Reindeer nutrition is equally specialized: they consume a highly diverse array of northern flora, including lichens, which they digest more efficiently than other ruminants due to unique rumen microbiota that shift seasonally in response to dietary changes. The chapter explains how these adaptations support their survival despite nutrient-poor winter diets and explores the ecological relationships that tie reindeer to predators and Indigenous cultures. The second half of the chapter moves into broader themes of thermoregulation and hibernation across mammals. It contrasts large-bodied species like reindeer—which rely on pelage, large body mass, and behavioral strategies like migration—with small-bodied mammals like white-footed mice, which depend on insulated nests and huddling. It also discusses the thermogenic role of brown fat in newborns, especially the rapid postnatal decline of brown fat in reindeer and its replacement by shivering thermogenesis. A deep dive into hibernation follows, examining how species like Arctic ground squirrels and alpine marmots supercool, periodically arouse, and use brown fat to rewarm. The composition of white fat, especially polyunsaturated fatty acids (PUFAs), is shown to affect hibernation efficiency, linking dietary lipids to physiological performance. Finally, the chapter explores sociobiological interactions during hibernation—such as arousal synchrony in marmots, survival tradeoffs between adults and juveniles, and the effect of pregnancy on hibernation. This integrative chapter reveals the intricate balance between energy metabolism, environmental stress, and social behavior in cold-dwelling mammals.