Hibernation

Living beings, especially animals like mammals, have (plural) a state named ‘hibernation’. It can be described as a state or natural tolerance with minimal action and metabolic activity¹. Commonly; it has several symptoms like low heart rate and temperature, slow metabolism, and minimum activity.¹ We commonly see this state in wintertime, continuing for several months and seemingly in mammals like bears, bats, squirrels, etc.¹. In the beginning, heat production for the body ends in slow-wave sleep to be able to save energy for basal metabolism^1,2. It is also essential to know the difference between hypothermia and hibernation³. Hypothermia is a severe reaction that the body responds to surgical purposes with mental confusion and shivering³. But why is this happening, and why do animals need to do that? To understand more clearly, we need to know that there are two types of hibernation: obligate and facultative hibernation^1,4.

Obligate hibernation is the one that happens in some animals, like ground squirrels and chipmunks, automatically and yearly regardless of access to food and conditions^1,2. It happens in natural conditions and spontaneously^1-3. The Winter season for obligate hibernations is characterized by periods of torpor¸a periodic euthermic arousal in hibernation¹. Moreover, torpor is functionally distinguished from hypothermia when metabolic suppression precedes a fall in core T_b (T_b: body temperature)². The reason or purpose of arousals and why hibernators return periodically to their temperature is still unknown, but there are multi hypotheses for his topic’s explanation¹. One of the hypotheses says that the body builds a sleep debt, and it has been supported by evidence, but of course, it is most likely not the same for every animal¹.

The second type of hibernation is facultative hibernation this hibernation occurs when hibernators like Syrian hamsters are cold-stressed,^1–3 lack food, or maybe both. In this situation, torpor is increased by food restrictions and low temperatures^1–35.

During the entry of hibernation, oxygen consumption, heartbeat rate, and respiratory rate drop to a more gradual decline in core T_b^1,5. Active inhibition is shown by metabolic rate reduction that occurs before T_b change, and these metabolic changes exceed those expected by just temperature⁵. To sum up, oxygen consumption in larger hibernations during a steady state of torpor does not connect with the change of T_b over the range of temperatures above 0^oC⁵. For large hibernations, active metabolism pressure happens initially and maintains low torpid metabolic rates, specifically when T_b is high⁵.

As an example, there is a table that shows how the temperature changes during hibernation in Syrian hamsters:

Torpor is the most effective way to conserve energy for mammals and birds. It is often a state with minimum activity, and recent studies have shown that some torpid individuals express their behaviors differently. For example, they can eat, move, and even mate while torpid⁷.At this point, we can say that even though torpids tend to act with minimum activity, some express different behaviors⁷.

The topic is still very confusing, although we know that there are two types of hibernation, even though we don’t know exactly why. We can say that this is mostly a natural annual behavior, and it helps hibernators survive in wildlife. The increase in temperature, heartbeat rate, and minimal activity is very logical when we think about the purpose of this action. However, I believe some parts need to be discussed and found to understand this behavior better. So I strongly believe that research that is still in process will help it more efficiently than previous ones. For example recent and potentially ongoing research about the neural origins of hibernation by Madeleine S. Junkins, Sviatoslav N. Bagriantsev, and Elena O. Gracheva. It includes canonical and physiological behaviors during hibernation⁸. So we can say that hibernation is a very wide topic to research. In the future, I believe that we can understand and have the necessary information about hibernation.

References:

1. Hibernation – Wikipedia. Accessed May 14, 2022. https://en.wikipedia.org/wiki/Hibernation
2. Srere HK, Wang LCH, Martin SL. Central role for differential gene expression in mammalian hibernation. Proc Natl Acad Sci U S A. 1992;89(15):7119-7123. doi:10.1073/PNAS.89.15.7119
3. LYMAN CP, CHATFIELD PO. Physiology of Hibernation in Mammals. https://doi.org/101152/physrev1955352403. 1955;35(2):403-425. doi:10.1152/PHYSREV.1955.35.2.403
4. Nedergaard J, Cannon B. Mammalian hibernation. Philos Trans R Soc London B, Biol Sci. 1990;326(1237):669-686. doi:10.1098/RSTB.1990.0038
5. Central nervous system regulation of mammalian hibernation: implications for metabolic suppression and ischemia tolerance. Accessed May 14, 2022. https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1471-4159.2007.04675.x
6. Chayama Y, Ando L, Tamura Y, Miura M, Yamaguchi Y. Decreases in body temperature and body mass constitute pre-hibernation remodeling in the Syrian golden hamster, a facultative mammalian hibernator. R Soc Open Sci. 2016;3(4). doi:10.1098/RSOS.160002
7. Geiser F. Hibernation, daily torpor and estivation in mammals and birds: Behavioral aspects. Encycl Anim Behav. Published online January 1, 2019:571-578. doi:10.1016/B978-0-12-809633-8.20755-3
8. Junkins MS, Bagriantsev SN, Gracheva EO. Towards understanding the neural origins of hibernation. J Exp Biol. 2022;225(1). doi:10.1242/JEB.229542/273864

Figure References: Chayama Y, Ando L, Tamura Y, Miura M, Yamaguchi Y. Decreases in body temperature, and body mass constitutes pre-hibernation remodeling in the Syrian golden hamster, a facultative mammalian hibernator. R Soc Open Sci. 2016;3(4). doi:10.1098/RSOS.160002

 Inspector: Süleyman ŞAHİN