Overview
In Warm Blood
The ability of mammals to adapt to the extreme environments of Earth is largely due to their warm-bloodedness—that is, their ability to maintain a stable, or nearly stable, active body temperature of about from 107 to 86 degrees Fahrenheit (42 to 30°C). Birds also possess this ability, but fish, amphibians, and reptiles do not; these vertebrates rely on a variety of external sources of heat to keep warm, and so are described as cold-blooded, or ectothermic.
Although fossil evidence reveals nothing about how or when early mammals became warm-blooded, or endothermic, the mechanism of staying warm is clear. A high metabolic rate (for smaller mammals this can measure eight times that of reptiles) allows mammals to burn fat reserves in their body and to generate heat in their muscles and other tissues. This enables them to function when it's cold.
Warm-bloodedness has helped mammals not only to adapt to extreme environments, but also to become nocturnal, and thus avoid becoming prey for some of the larger diurnal reptiles. The ability to hunt and forage during the colder night also diminished competition with reptiles for the same food sources.
Despite its advantages, there is a high cost attached to being warm-blooded. In general, a mammal consumes five to ten times as much energy as a reptile of equal size, and when the temperature drops, it may expend twenty to thirty times as much energy. Mammals, like birds, therefore need plenty of reliable food resources in order to stay alive.
When warm-bloodedness alone is not enough to handle cold temperatures, mammals may take more drastic measures to ensure their survival. Some marine mammals, such as large filter-feeding whales, migrate to warm waters to breed or to wait out the cold. During this time, they live off their body fat.
If migrating to avoid cold weather or an insufficiency of food is not an option, mammals may resort to shallow torpor, a self-induced lowering of the body temperature, or deep torpor, a comatose condition where the body temperature may drop to within 1.5 degrees Fahrenheit (1°C) or less of the ambient temperature. This comatose condition may also include prolonged periods of apnea, where breathing ceases entirely. In the winter, deep torpor is referred to as hibernation, and it is seasonally practiced in northern climates by small mammals, especially rodents and bats. Body size and torpor are related. The bigger the mammal, the more energy that is used up to arouse from a period of dormancy. The largest mammals to enter deep torpor are marmots, which weigh about twenty-two pounds (5 kg) and take hours to arouse. Bears would need a day's energy expenditure to warm up from deep torpor, so instead the black bear (Ursus americanus), for instance, spends winter in shallow torpor, its temperature hovering at about eighty-eight degrees Fahrenheit (31°C), only about twelve degrees Fahrenheit (7°C) below its normal level.
Some mammals resort to shallow torpor for only brief periods. For example, rodents such as the California pocket mouse (Chaetodipus californicus) that dwell in extreme climates can lower their heart rate for a few hours at a time to conserve energy when food is hard to come by.
Next >>Coping with the Heat.
