How do animals hibernate?

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Understanding How Animals Hibernate

Physiological Preparations for Hibernation

Endocrine and Metabolic Changes

Before entering hibernation, animals undergo significant physiological changes. These include polyglandular endocrine involution, fattening, and changes in the saturation of depot fat. These adaptations are crucial for surviving the prolonged periods of inactivity and low temperatures during hibernation.

Pre-Hibernation Remodeling

In some species, such as the Syrian golden hamster, pre-hibernation remodeling involves a decrease in body temperature and body mass. This remodeling is essential for the initiation of hibernation and is regulated by endogenous circannual rhythms.

Entering Hibernation

Controlled Physiological Downregulation

The process of entering hibernation is under precise physiological control. Heart rate, respiratory rate, and oxygen consumption slow down before a decline in body temperature occurs. This controlled downregulation ensures that the animal can safely enter a state of torpor.

Molecular and Cellular Responses

Hibernation involves a phenotypic switch that includes profound changes in physiology, morphology, and behavior. These changes are regulated at the molecular and cellular levels, with differential expression of genes common to all mammals. This allows hibernators to survive extreme conditions by reducing metabolism and maintaining essential functions even at near-freezing body temperatures .

During Hibernation

Metabolic and Temperature Regulation

During hibernation, animals experience extended bouts of torpor, where body temperature can drop significantly, and metabolism can be reduced to as low as 1% of euthermic rates. Despite the low temperatures, hibernators maintain homeostasis, evidenced by normal blood pH and sensitivity to CO2.

Energy Conservation

Hibernation allows animals to save energy by reducing the need for heat production. The body temperature approaches that of the surroundings, eliminating the need for metabolic heat production to compensate for heat loss. This energy conservation is critical for surviving periods of food scarcity.

Arousal from Hibernation

Coordinated Physiological Event

Arousal from hibernation is a coordinated event where the anterior part of the body is rapidly warmed by shivering and other heat-generating mechanisms. Warmed blood is shunted from the posterior to the anterior until the body temperature reaches nearly 37°C. This process allows the animal to return to a normothermic state without suffering from reperfusion injury.

Role of Brown Adipose Tissue

During arousal, brown adipose tissue plays a crucial role in heat production. It provides the necessary substrate for thermogenesis, enabling the animal to quickly regain normal body temperature.

Unique Adaptations in Different Species

Flexible Thermal Responses

Some species, like the Madagascan fat-tailed dwarf lemur, exhibit flexible thermal responses depending on their hibernaculum. In poorly insulated tree holes, their body temperature fluctuates with the ambient temperature, while in well-insulated holes, it remains more constant. This flexibility helps them adapt to varying environmental conditions.

Sleep and Arousal Patterns

In hibernating primates, such as the dwarf lemur, sleep-like states occur during arousals when body temperatures passively rise above 30°C. This contrasts with other hibernators that do not exhibit sleep during torpor but only during arousals.

Conclusion

Hibernation is a complex and highly regulated physiological state that allows animals to survive extreme environmental conditions. It involves significant pre-hibernation preparations, controlled entry into torpor, maintenance of homeostasis during hibernation, and coordinated arousal processes. Understanding these mechanisms provides insights into the remarkable adaptations that enable hibernators to thrive in inhospitable environments.