Give birth to a baby quietly during hibernation and surprise all the bears?

Yesterday was Jingzhe. According to tradition, at this solar term, God will awaken hibernating animals by thundering.

"Hibernation" literally means that animals sleep for the entire winter (Monday's office worker: so envious!). But in fact, animals in hibernation do not sleep all the time , and some even carry out some "big activities" - for example, bears may give birth to babies.

These little bears are usually born when their mothers are hibernating｜Pixabay

Hibernation: Not Sleeping

In order to cope with the adverse conditions of low temperature and less food in winter, some warm-blooded animals have evolved the solution of " hibernation ".

When most animals hibernate, their body temperature drops to 2-10 degrees, their metabolic rate can drop to 2%-4% of normal, and their breathing, heartbeat and other activities also drop to extremely low levels. This can greatly reduce the energy they need to maintain normal life activities and help them survive the cold winter.

Among mammals, there are at least eight groups that have the ability to hibernate. In addition to bears, other well-known ones include groundhogs, hedgehogs, hamsters, etc.

Some people think that hamsters’ hibernation means they have passed away. Hamsters: No! I can still be saved! | Pixabay

These characteristics of hibernation are completely different from our "sleep". Moreover, during hibernation, animals do not sleep all the time, but wake up frequently and then fall asleep again . During the awakening period, their body temperature will rise back to normal levels, and they will use the food stored in autumn to fill their stomachs.

Some hibernating animals do not store food in advance , such as bears, which often eat a large meal before hibernation and convert the food they eat into fat. The body temperature of hibernating bears only drops by 3 to 5 degrees Celsius. Although their heart rate and metabolic rate also decrease, they remain awake and even give birth during hibernation - although the baby has been "hidden" in the mother bear's belly for a long time.

Bear hibernation: giving birth to a baby

Bears generally complete mating during the breeding season from May to August, but the fertilized egg will always remain in the blastocyst stage ; the embryo will not implant until the bears begin hibernation in November to early December, and the real pregnancy period begins at this time.

About 60 days later, in late January or early February, the mother bear begins giving birth to cubs. In the chilly winter and early spring, the mother bear needs to stay in the cave to nurse the cubs, usually not coming out of the cave to look for food until April . But in recent years, due to warming weather, bears tend to end hibernation early, and mother bears may come out of the cave with their cubs in March, making their lives more difficult.

Polar bear mother and cub｜Itsmine

Unlike other terrestrial mammals, mother bears have no opportunity to eat during the entire reproductive process .

Whether it is providing nutrition for the development of the fetus in the body, or secreting milk for the cubs after birth and maintaining the cubs' body temperature, the mother needs to pay a lot of energy - but all this energy comes from the mother bear's own reserves. The large amount of fat and even some muscles they stored before hibernation in autumn are used to provide the energy needed for reproduction.

Having a baby is not easy｜Pixabay

Therefore, the mother bear who has spent the hibernation and breeding period may lose a quarter of her weight. The huge energy consumption also reduces the frequency of bear reproduction. Black bears generally reproduce every other year, while brown bears will give birth to cubs again after 2-4 years.

Beyond hibernation: dormancy and hibernation

Homeothermic animals can regulate their body temperature autonomously, and when the external temperature changes, they can still maintain a relatively stable body temperature. But poikilothermic animals cannot regulate their body temperature autonomously, so how do they face the cold winter?

When winter comes, the body temperature of cold-blooded animals will drop sharply with the ambient temperature, thus entering a " dormancy " state, also known as brumation.

Wood frog (Rana sylvatica) | DDauri Daniel D'Auria / Wikimedia Commons

For example, in the winter, wood frogs in North America freeze completely in temperatures ranging from -3 to -6 °C, and their life activities are basically suspended; when spring comes, they gradually melt and wake up. Normally, water forms ice crystals at low temperatures, destroying cell membranes and causing cells to rupture and die; but wood frogs synthesize a large amount of glucose and urea, which lowers the freezing point of water and prevents the growth of ice crystals, protecting cells from damage . In addition, they have evolved unique protective measures to cope with the lack of oxygen caused by the inability of blood to flow when frozen.

Red-eared sliders also have this ability to resist severe cold. They will enter hibernation when the water temperature drops to 10°C, and their heart rate and metabolic rate will be greatly reduced, which can help them endure the lowest temperature of 3°C.

Black-capped chickadee (Poecile atricapillus) | Alan D. Wilson / Wikimedia Commons

Some animals also hibernate, but not in a certain period of time. This phenomenon is generally called " torpor ." For example, some birds no longer actively maintain a high body temperature at low temperatures at night, but instead lower their body temperature and metabolism to save energy .

Some resident birds in temperate regions also save energy reserves in winter and reduce fat consumption. For example, the black-capped chickadee in North America has a body temperature about 12°C lower than normal in winter, and its metabolic rate also decreases accordingly; the body temperature of the North American nighthawk may drop from over 37°C to more than 10 degrees in winter nights.

Little Nightjar (Caprimulgus tristigma) | Charles J. Sharp / Wikimedia Commons

We compare hibernation to human sleep, and this process of simplifying knowledge is beyond reproach, but it also limits our vision. In fact, even plants - the fallen leaves on the ground in late autumn and the bare branches in winter are also strategies for plants to reduce water loss and slow down metabolism in low temperatures. Isn't it also a kind of "hibernation"?

Faced with severe cold environments, different species have evolved various coping mechanisms, creating a colorful nature. Only by protecting these lives that are different from us can we have the opportunity to explore the unknown miracles of life in more depth.

References

[1] Carey, HV, Andrews, MT, & Martin, SL (2003). Mammalian hibernation: cellular and molecular responses to depressed metabolism and low temperature. Physiological reviews, 83(4), 1153–1181.

[2] SHIMOZURU, MICHITO, IIBUCHI, RURIKO, YOSHIMOTO, & TAKURO, et al. (2013). Pregnancy during hibernation in japanese black bears: effects on body temperature and blood biochemical profiles. Journal of Mammalogy.

[3] Claudia López-Alfaro, Charles T. Robbins, Andreas Zedrosser, Scott E. Nielsen. (2013). Energetics of hibernation and reproductive trade-offs in brown bears. Ecological Modelling. Volume 270, 1-10,

[4] Wilson, D., S. Ruff. (1999). The Smithsonian Book of North American Mammals. Washington: Smithsonian Institution Press.

[5] Mommsen, TP, & Storey, KB (1992). Hormonal effects on glycogen metabolism in isolated hepatocytes of a freeze-tolerant frog. General and comparative endocrinology, 87(1), 44–53.

[6] Costanzo, JP, Lee, RE, Jr, and Lortz, PH (1993). Physiological responses of freeze-tolerant and -intolerant frogs: clues to evolution of anuran freeze tolerance. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 265:4, 721-725.

[7] Hicks, JM, Farrell, AP (2000). The cardiovascular responses of the red-eared slider (Trachemys scripta) acclimated to either 22 or 5 degrees CI Effects of anoxic exposure on in vivo cardiac performance. J Exp Biol. 203 (24), 3765–3774.

[8] Edmund C. Jaeger. (1949). Further Observations on the Hibernation of the Poor-Will, The Condor, Volume 51, Issue 3, 105–109.

[9] McKechnie, AE, Ashdown, RAM, Christian, MB and Brigham, RM (2007), Torpor in an African caprimulgid, the freckled nightjar Caprimulgus tristigma. Journal of Avian Biology, 38: 261-266.

Author: Michael Ma

Editor: Mai Mai

Cover image source: Pixabay

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