Animals at the extremes: Hibernation and torpor
Animals at the extremes: Hibernation and torpor

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Animals at the extremes: Hibernation and torpor

Course Questions

Question 1

Describe three measures of physiological regulation central to hibernation. Using these measures as definitive criteria state why the Svalbard reindeer is not a hibernator.

Answer

The three measures are (a) induction of thermal dormancy, (b) the suppression of behavioural activity and (c) the depression of metabolic activity. Although the Svalbard reindeer becomes behaviourally inactive and ceases feeding in the winter months, its BMR and T b are not depressed as in true hibernators.

Question 2

What reasons might you advance to support the argument that the ability to hibernate might have arisen several times in the evolution of warm-blooded vertebrates?

Answer

Adaptive hypothermia appears to have evolved in birds and in several mammalian orders that are only distantly related. Within single families, the occurrence of torpor is not universal, suggesting adoption of the strategy at species level based on energy cost-benefit considerations. Furthermore, internal and environmentaltriggers for entry to torpor vary considerably between species.

Question 3

From what you have read in Section 3 indicate whether the following statements are true or false. Briefly explain your answer.

  • (a) The laying down of fat deposits is a criterion for identifying an animal as a hibernator.

  • (b) The decline in heart rate on entry to hibernation is due to an increase in the number of skipped beats and a lengthening of the period between beats.

  • (c) The heart and brain are the warmest tissues during arousal.

  • (d) Although blood pressure is lowered during entry to hibernation, there is evidence for vasoconstriction.

  • (e) A drop in temperature below a critical level can lead to an increase in heart and respiratory rates but no change in BMR.

Answer

(a) False. Many non-hibernating (e.g. migrating) animals lay down fat on a seasonal basis. Some hibernators store food as well as fat.

(b) True.

(c) False. Although in some species the heart may be the warmest tissue during arousal, thermographic imaging shows that for most mammals, deposits of BAT, which are sources of thermogenesis, are the warmest.

(d) True.

(e) False. Unless BMR increases, the animal will not warm and thus there would be no effective response to the fall in temperature.

Question 4

List the factors that determine (a) entry to hibernation, and (b) length of periods between arousals.

Answer

(a) The main signals that initiate entry into hibernation are environmental, i.e. food supply, day-length and ambient temperature.

(b) Periodic arousal from torpor is sometimes linked to the time of day, although mechanisms for initiating periodic arousal may be diverse. Early arousal may also be initiated by alarm mechanisms, such as mechanical stimuli or a fall in T a to below 0° C. Final arousal is normally independent of T a and may be triggered by endogenous seasonal changes or reduction in fat stores below a critical level.

Question 5

Draw a flow chart indicating the design and outcomes of an experiment which might identify a new enzyme that controls metabolism in hibernating liver cells.

Answer

Your flow chart should show that most or all of the following steps were followed:

  • Perform subtractive gene expression analysis in an experimental mammalian model using tissue from hibernating and euthermic liver.

  • Identify genes that are expressed at a higher level in hibernating liver.

  • Obtain a partial or complete DNA base sequence of one or more of these genes.

  • Predict an amino-acid sequence for all or part of the encoded protein.

  • Delete the gene to determine the impact of its absence on liver cell metabolism.

  • If you have identified a new gene with no existing sequence homologies, you may want to design experiments to determine the function of the protein.

Question 6

Summarize the main changes occurring prior to and during hibernation at the cellular level.

Answer

Protein synthesis is dramatically reduced; cell cross-sectional area is reduced; changes occur in the structure of the Golgi apparatus; there are changes in the numbers of glycogen granules and lipid droplets; membrane-associated proteins undergo redistribution; metabolic processes designed to neutralize reactive oxygen species are activated; there are metabolic changes relating to use of alternative respiratory fuel sources; there is reduced electron transport and protein-leak in mitochondria.

Question 7

From Sections 3 and 5, describe in one sentence each the changes that occur in the heart rate, breathing patterns, blood PCO2 and BMR in hibernating mammals.

Answer

Heart rate is reduced to less than 10 min−1. Breathing is episodic, punctuated by periods of apnoea. Blood P CO2 is increased up to four-fold. BMR drops rapidly at the onset of hibernation. BMR may increase periodically during hibernation despite a sustained low T b. Heart and respiratory rates may increase if T a falls below a critical level.

Question 8

What factors determine the choice of carbohydrate, lipid and protein as respiratory fuel sources in mammals during torpor and hibernation.

Answer

Lipid is the preferred energy source for long-term hibernation. Glucose is the preferred source of energy in the short-term for animals in which torpor episodes last a day or less. Glucose is also used in species that hibernate at ambient temperatures below freezing. Protein is used to supply amino acids for gluconeogenesis when carbohydrate fuel sources are exhausted.

Question 9

What factors determine whether a species gains an advantage in using torpor as an energy-saving measure? Give explanations for the energy-saving figures for July and November in Table 5.

Answer

The factors include the comparative energy cost of maintaining euthermia over a comparable period and the energy savings made possible for costly physiological processes such as gestation. The savings must take into account: animal size; the energy cost of re-warming during arousal; and the nature of the habitat. In Richardson’s ground squirrel, an obligative hibernator (Table 5), energy is saved through periods of torpor both in July and November, but is greatest in November when the difference between T a and T b is larger and the number of arousal episodes is reduced.

Question 10

List two pieces of evidence each for and against the rheostasis theory of thermoregulation on entry to hibernation.

Answer

There are a number of studies that support the rheostasis theory. Two key ones are the experiments on marmots and ground squirrels in Section 6. Experiments on marmots show that there is a steady shift down in the set point when the hypothalamus is cooled. The work on golden-mantled ground squirrels (Figure 36) shows progressive and continuous resetting of the thermostat.

Studies that do not support the rheostasis theory would be that a gradual, controlled decline in T s is not universal, either in different species or within individuals of the same species entering torpor. The theory does not explain the metabolic response to temperature changes applied to the hypothalamus of Eastern chipmunks (Figure 37).

Question 11

What adaptive changes occur in hibernating neurons? What is the evidence that neurons are ‘prepared’ for hibernation as conditions change?

Answer

Rapid-response genes controlling electrical firing activity of neurons are activated, neuronal structure is altered, the critical temperature of firing is depressed, and there is an increase in the number of cold-sensitive neurons. Biosynthesis of histamine and serotonin is also increased. Rapid-response genes are characterized by their activation very soon after the onset of hibernation – they initiate many of the other changes which hibernating neurons undergo, and can thus be viewed as a preparative signal.

Question 12

From the evidence of brain sleep activity patterns, why is the popular concept that hibernating mammals ‘are asleep’ incorrect?

Answer

Slow-wave sleep activity (SWA) is depressed in the brain of hibernating mammals. This is characteristic not of sleep, but of sleep deprivation. SWA activity cycles do not follow normal sleep activity cycles in torpor but become shortened.

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