Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [43]
Although energy economy helps to explain the squirrel’s low body temperature, far from behaving in a manner strictly in accord with just saving energy, the arctic ground squirrels appeared to squander energy by warming up to 37°C from subzero temperatures about a dozen times throughout their winter hibernation. Each time they spent about a day being fully warmed and required another to cool back down. These periodic warmings are calculated to cost the animals over half of the fat reserves they had built up during the summer. Why do they bother? It had previously been shown that this behavior is not unique to arctic ground squirrels. Indeed, no mammal hibernator avoids such periodic bouts of normal body temperature during a winter’s hibernation. Therefore, given the high energetic costs of warming up and staying warm for a day or so, the behavior seemed most peculiar. It must buy something precious. And this is where Barnes and his colleagues’ fourth remarkable discovery comes in. Although still controversial, the hypothesis is that the animals warm up to sleep!
Since the early 1950s two different kinds of sleep have been defined. One is “rapid eye movement” (REM) sleep, also called “dreaming sleep” or “deep sleep.” The other is defined as “nonrapid eye movement” (NREM) sleep, or “light” or “ordinary sleep.” These two types of sleep have been studied in humans by taking voltage measurements from the scalp surface. Brain electrical activity records, called electroencephalograms (EEGS), are plotted against time. In humans, apes, and cats, where the EEG brain-wave patterns have been most studied, there is a progression of very different patterns from awake to four arbitrary stages presumed to represent depth of sleep, with the awake state showing very low voltage amplitude and high frequency (8–13 waves per second) and deep sleep showing a high-voltage amplitude, low-frequency wave pattern. However, when the EEG sleep patterns are followed throughout the night, periods occur in which low amplitude brain-wave patterns reappear that resemble those during wakefulness. It is during these, the REM sleep periods, that the rapid eye movements, and often increased heart rate, changes the breathing pattern, and muscle twitching occurs. The animal is then dreaming.
Animals in hibernation torpor do not show the EEG sleep patterns. Instead, as their body temperature drops and they enter torpor, there is a gradual reduction of voltage until brain electrical activity eventually disappears, as if they were dead. However, although there is no spontaneous brain activity (Lyman and Chatfield 1953), the animals must still be able to generate at least some electrical activity in the nervous system, or else they could never arouse.
Teaming up with neurobiologists H. Craig Heller and Serge Daan, Barnes recorded electroencephalograms of squirrels going into and coming out of hibernation. Squirrels entering hibernation showed typical sleep patterns, and then as they cooled down their brain waves disappeared and their EEGs then resembled those in humans who would be considered brain dead. However, once rewarmed by shivering, after having been in hibernation torpor for about a month, the squirrels spent most of the day showing the brain patterns associated with rapid eye movements (REM) during dreaming in humans. Had they heated up to sleep, or to dream? If so, why do they sleep or dream? Why do we? It’s one of the large remaining biological mysteries that probably relates in some way to how the brain works to consolidate, edit, delete, and store memory.
It seems ironic that a hibernator has evolved the astounding capacity to arouse from subzero temperatures normally only encountered in winter in order to sleep. If the animals did not need to arouse they could stay torpid until spring and save much energy. Since they don’t stay continually torpid despite the obvious energy economy they would experience by doing so, there is apparently some great cost to long torpor or sleep deprivation.
The hibernating arctic