6.2 Opting out

Hibernation is a more radical and extended adjustment than torpor. Insect eaters that are able to store sufficient amounts of fat in their bodies can undergo hibernation in response to the low ambient temperatures of winter. For shrews, a combination of high metabolic rate and small body size means that this particular physiological feat seems beyond them. By contrast, although some bats display torpor on almost a daily basis, many other species, especially those living in cool climates, undergo genuine hibernation.

The flying insects preyed upon by bats are scarce or absent in winter, the very time when bats would need extra energy to maintain a high body temperature. Before hibernation, bats build up stores of fat in their body, and some such stores have an important role during arousal, as I'll explain shortly.

Biologists used to believe that hibernating animals simply allowed their body temperatures to drop passively to that of the burrow or cave in which they spend the winter. More recent research has shown that body temperature is tightly controlled during entry into hibernation and during hibernation itself. Data from a much larger insect eater, the hedgehog, is of special interest.

Hedgehogs in Britain hibernate from around November to April, with precise timings dependent on weather conditions. They become aroused from hibernation about every 7-11 days. Temperature and heart rate during hedgehog hibernation have been measured for long periods via miniature sensors implanted into the animal. Figure 8 shows body temperature (T_b) and heart rate readings during a period of deep hibernation (for four hours from 14.00 to 18.00, i.e. 2 p.m. to 6 p.m.), and during a subsequent period of spontaneous arousal (18.00-23.00, i.e. 6-11 p.m.).

Figure 8 Changes in body (T_b) temperature and heart rate during a typical spontaneous arousal from hibernation in the European hedgehog

Compare the very low heart rate during hibernation with the normal heart rate of 250-300 beats per minute when the hedgehog is active. Breathing rate is also reduced sharply in the hibernating hedgehog, down to just one breath per hour, compared to 25-50 breaths per minute in an active individual. Prolonged periods of breath-holding, known as apnoea, have been observed in hibernating hedgehogs, with a 'record' breath-hold of 150 minutes.

A hibernating hedgehog may appear lifeless and not subject to any physiological control, but this is far from the case. When not hibernating, the hedgehog, like all mammals to a greater or lesser extent, maintains its T_b around a set-point. If T_b falls below or rises above 35-36 °C, physiological mechanisms come into play to warm or cool the hedgehog's body as appropriate. You can think of the control mechanisms as being similar to those that control a central heating system to maintain your house at a comfortable temperature. If the temperature of a room falls below that set by the thermostat, the heating is switched on; if the room temperature is too hot, the thermostat switches off the heating. The thermostat of the hedgehog has no known physical basis but it is clear that heating and cooling of the body is controlled by a special area in the brain, the hypothalamus. If a non-hibernating hedgehog gets too cold (T_b dropping below 35-36 °C), the heating is switched on - shivering and increases in metabolic rate (see course S182_1) take place. If the body temperature of the hedgehog exceeds the set T_b, the heating mechanisms are switched off.

During hibernation, the set T_b is reduced to just 4 °C, analogous to turning down the thermostat to 4 °C in your central heating system. If the hedgehog's T_b drops below 4 °C during hibernation, the animal will react by increasing its heart and breathing rate and its rate of metabolism, generating sufficient extra heat to raise its body temperature. Arousals are provoked by chilling but, as you have seen, many arousals occur spontaneously.

Why hedgehogs, and hibernators in general, become aroused periodically is not fully understood. Hibernating hedgehogs spend a total of about 80% of the time with a very reduced T_b, waking up between 15-22 times during a hibernation period.

During arousal, the body is warmed up by fat breakdown in BAT (brown adipose tissue or brown fat), which is very different from the more familiar white fat (white adipose tissue, or WAT). What we commonly think of as fat consists of masses of WAT cells, each loaded with a large round fat vacuole (Figure 9a). Traditionally, WAT has been viewed as a storage tissue that animals draw on during prolonged exercise, or in 'emergencies' when they are short of food. (Recently, WAT has been found to have more complex structures and a wider range of functions than was believed.) Breakdown of the fat contained in WAT into its constituent building blocks (the most significant types are called fatty acids) is the first step to releasing the chemical energy used to fuel metabolism. Prior to hibernation, hedgehogs and bats build up stores of WAT. BAT normally builds up too during this period.

Figure 9 (a) White adipose tissue: (1) A developing cell. The nucleus is round and the main part of the cell that lies outside it (the cytoplasm) contains a single large fat vacuole. (2) A mature cell. The nucleus is compressed against the cell membrane. The large single fat vacuole occupies most of the cell. (3) A fat-depleted cell. (b) Brown adipose tissue. (1) A developing cell. The cytoplasm contains a round nucleus, numerous large mitochondria, vacuoles containing fat, and glycogen granules. (Glycogen is a macromolecule, made up of many linked glucose units, and is the major form of stored carbohydrate in animals.) (2) A mature cell. The cell contains more cytoplasm and more fat than the developing cell. (3) A fat-depleted cell. The cytoplasm is filled with mitochondria and a few small fat-containing vacuoles. Fat cells, especially those of WAT, vary greatly in size, so the scale provides only a rough guide.

BAT consists of masses of somewhat smaller cells, but each has a number of small fat droplets, and also reserves of a 'storage' carbohydrate, glycogen (Figure 9b). What is remarkable about BAT is that its primary function is the generation of heat. In most other types of cell, metabolism is geared in such a way that the breakdown of fuels such as fatty acids and glucose releases energy in a chemically useful form. This 'captured' energy can be used within such cells to maintain all the energy-demanding 'housekeeping' activities, such as synthesising macromolecules, or transporting different chemical building blocks around the cell. But the breakdown of fatty acids in BAT (the final stages of which occur in the mitochondria) releases energy not so much in the form of these neat 'parcels' of chemical energy but as heat. Once BAT becomes switched on in this way, blood circulating through BAT is warmed and the warmed blood circulates throughout the body, raising T_b. Indeed, the endearingly termed 'bat BAT' has a vital role in arousal.

So the use of thermal imaging provides a wonderful insight into the role of BAT in arousal of hibernating bats. But why arousal is necessary remains a mystery, especially since it uses up so much of the precious energy reserves of the body. Little brown bats do not feed during arousal, but as you've seen, they drink water. Perhaps doing so helps them get rid of accumulated toxic wastes via urination, but this explanation is by no means certain. This question is just one further example of just how little is known about bats, which only increases a sense of wonder about these intriguing mammals.