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Too Hot or Too Cold: The Grizzly's Goldilocks Issue

Author: Noah Rosen

Grizzly bears (Ursus arctos) are one of the most famous animals on the planet. We see them as children, usually in a nature book, and cement them in our memories. They live all over the world, usually in frozen forests full of pine cones and fir needles. Overall carnivorous, they feed on animals ranging from bison to salmon. They’re largely solitary, meaning we don’t really see them in groups through most of the year. When we do see them gathering, it’s usually during the summer when they meet to reproduce. After that, they go their separate ways; the females go to get ready to give birth to their cubs. 

In the winter, grizzlies enter a period called hibernation, where they’re in a sleep-like state for long stretches of time. This is what they do to survive the winter, when the weather becomes too cold for them and there’s very little food. When they hibernate, there’s an extreme physiological response; their heart rate and metabolism decrease, they become inactive, and their body temperature goes down. 

Bears aren’t the only hibernators in the world. There are others, like the yellow-bellied marmot (1). What sets bears apart from the rest is that they’re the only large hibernator. This means that because of their lower ratio of surface-area to volume, it takes more time to cool them down and they have different metabolic needs. Therefore, they have to hibernate slightly differently than others and are sensitive to changes in the environment.

What if the winter isn’t the same every year? Climate change is changing weather patterns and annual temperatures, which affects how bears hibernate. It impacts how long they hibernate, which in turn affects how much energy they have when they’re awake and the likelihood that they survive.  

 

How Long They Hibernate

Environmental conditions play an important role in grizzly bear hibernation; a drop in temperature is a cue for bears to begin preparing. To survive the winter, they need great amounts of fat to draw from for energy. The first step of preparation is a phase known as hyperphagia, or to “excessively-eat.” To build up fat, they forage constantly but slowly become less active over time. Then, they enter the predenning phase, when their metabolism, heart rate, and body temperature all decrease (2). Eventually, they enter a den, usually a cave or some kind of structure they build, and begin hibernation. Part of the research that has been done is determining what drives different bears into hibernation earlier than others. It’s been found that pregnant females and females with cubs are the ones who consistently enter the earliest. 

While it is the environment that triggers hibernation, the bear’s own physiology is what ends it. All organisms maintain something called homeostasis, which refers to the body’s internal mechanisms to keep certain factors like temperature or metabolism at a specific set point or range. During hibernation, the grizzly bear’s set point for homeostasis is dropped, letting it get colder than it would when awake (1). However, about 2 months before they awaken, the bear’s body temperature begins to rise, along with its set point. Because of the heat radiating off the bear, the temperature within the den rises with it. Eventually, the temperature will rise above the bear’s set point, causing it to awaken and exit its den to seek out better conditions. As of now, we don’t understand what exactly causes the bear’s internal temperature to rise; it could be a form of biological timekeeping, it could be a response to a lack of energy, it could be a response to something we don’t know about yet. 

The shifting temperature brought on by climate change is affecting how bears hibernate. In areas near the equator where weather is more consistently warm and food is available year-round, bears hibernate for a shorter period of time or even not at all. Further north, we can see how climate change is making the grizzly bears resemble their southern counterparts; they’re entering their dens later and exiting them earlier. Scientists have found that right after they enter and right before they exit is a sensitive period where a change in temperature has the biggest impact (3). By exiting their den too early, bears can face what’s known as a trophic mismatch; they come out before conditions of their environment can provide enough food for them to survive. This not only affects them, but their cubs as well (4).

 

Hibernation and Bear Cubs

Because females give birth to their cubs in their dens, hibernation isn’t just a survival strategy but also an important part of reproduction. As was said before, grizzly bears are usually solitary, meaning the females are alone when they’re taking care of their cubs. They’re the first to enter and the last to exit their dens to give their cubs the most time possible to develop and grow (4). However, that time is being shortened because of the shifts in their behavior brought on by climate change.

Cubs are exiting their dens earlier than they should be; at a smaller size, they’re more exposed to predators and bigger animals that could kill them (4). In addition, with having to feed herself and take care of her cubs, the female needs even more food to sustain herself. With the trophic mismatch, food availability has an increased impact on survival. This can lead to another issue of climate change: den abandonment, where bears leave their dens prematurely to seek out food.

 

Den Abandonment

Another big difference between how bears hibernate and smaller mammals hibernate is that bears can become active for a short time, usually in response to something (5); smaller hibernators stay in a coma-like state (2). As of now, it’s not well understood what exactly causes this, other than food availability has a significant role. In recent years, bears have been prematurely leaving their dens more and more (5). One identified cause of this is food provided by humans, whether it’s litter that was left behind or left on purpose for them. Either way, this is bad for the bears’ health. At first glance, it can appear to be beneficial, because it provides extra food and energy for them. However, weather can change drastically in a moment; bears can be caught outside their dens when foraging for this extra food and be in danger. Any energy they gain from leaving their den is used up just to survive harsh conditions and even use up energy they saved up from the previous autumn, leaving them with less than they started with.

 

In Conclusion

As we’ve demonstrated, climate change is dramatically impacting how bears hibernate. It affects how long they hibernate, which has a domino effect on their survival rate and the survival of their cubs. Unfortunately, there is still not a lot that’s understood about bear hibernation; it’s only been a specific point of interest for research in the past few years. For the most part, studies have been mainly observational, tracking bear movement and sightings. Only a few have done more in-depth research, where they track heart rate and other physiological responses. More studies like this need to be done to better understand the extent of certain factors like temperature and human proximity. Forest managers and conservationists should understand this aspect of grizzly bear physiology to better predict the shifts brought on by climate change and human interaction, giving them better tools to respond in kind.

 


Citations

  1. A.L. Evans, N.J. Singh, A. Friebe, J.M. Arnemo, T.G. Laske, O. Frobert, J.E. Swenson, S. Blanc, Drivers of hibernation in the brown bear. Frontiers in Zoology. 13, (2016).
  2. L.F. Russo, E. Gonzalez-Bernardo, E. Valderrabano, A. Fernandez, V. Penteriani, Denning in brown bears. Ecology and Evolution. 10, 6844-6862 (2020)
  3. M.M. Delgado, G. Tikhonov, E. Meyke, M. Babushkin, T. Bespalova, S. Bondarchuk, A. Esengeldenova, I. Fedchenko, Y. Kalinkin, A. Knorre, G. Kosenkov, V. Kozsheechkin, A. Kuzenetsov, E. Larin, D. Mirsaitov, I. Porkosheva, Y. Rozhkov, A. Rykov, I.V. Seryodkin, S. Shubin, R. Sibgatullin, N. Sikkila, E. Sitnikova, L. Sulangareeva, A. Vasin, L. Yarushina, J. Kurhinen, V. Penteriani, The seasonal phenology in response to climatic variability. Frontiers in Zoology. 15, (2018).
  4. E. Gonzalez-Bernardo, G. Bombieri, M. Mar Delgado, V. Penteriani, The role of spring temperatures in the den exit of female brown bears with cubs in southwestern Europe. Ursus. 31 1-11, (2020).
  5. K. Bojarska, S. Drobniak, Z. Jakubiec, E. Zysk-Gorczynska, Winter insomnia: how weather conditions and supplementary feeding affect the brown bear activity in a long-term study. Global Ecology and Conservation. 17, (2019). 

 

Photo Credits:

Intro Image

"Grizzly Bear" by DenaliNPS is marked under CC PDM 1.0. To view the terms, visit https://creativecommons.org/publicdomain/mark/1.0/

 

Second Image

"Grizzly, Yellowstone" by NPCA Photos is licensed with CC BY-ND 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-nd/2.0/




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