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Bears forgo a lot of activities to conserve  their energy in the winter when food is scarce - even important bodily functions  like eating, and peeing, and pooping. But they aren’t just saving  energy for an extra-long nap - they also give birth during hibernation.

Yeah, bears have their babies while they’re tucked away in their dens for the winter! This seems like it would be the  harshest possible time of year for cubs to be born, especially when  they’re pretty much stuck inside. So how and why does that happen?

Hibernation is a state of inactivity  that many warm-blooded animals, like bears, enter for several  months out of the year. But contrary to popular belief, this  physiological state isn’t just a really long sleep when the temperatures drop. Instead, it’s an extended period of torpor  - a state of reduced activity that is triggered in adverse environments  and during periods of food scarcity.

During torpor, an animal’s metabolism, breathing, and heart rate slow down and  its body decreases temperature. Animals can go in and out of torpor  daily or it can last for several days, weeks, or months, at which point  it’s referred to as hibernation. Some researchers have called bears  super hibernators because their body temperatures only drop a few degrees, in  contrast with smaller hibernators like squirrels, whose body temperatures  drop to near freezing.

Because they don’t have to unthaw themselves, bears can react to any potential  threats really quickly. And even if they look like they’re sleeping, bears are still aware of their surroundings. A paper published in 2011 reported that  black bear heart rates increased as soon as researchers approached their dens, indicating the bears could detect  the potential danger outside.

They also found that pregnant black  bears' heart rates increased as their pregnancy progressed, and the bears  moved even less once the babies arrived, to probably avoid accidentally crushing them. When the babies are first born, they only require warmth and a constant supply of milk. As long as the female has adequately  prepared before hibernation, she can provide for both herself and the cubs.

To do that, bears have to build  up a massive fat store in order to survive several months of hibernation. Their bodies slowly metabolize those fat  stores to use as a food and water source. Their bodies even turn waste  products into proteins to keep their muscles and essential organs functioning properly.

And the success of a female  bear's pregnancy is tightly linked to those fat stores. That’s because bears are one of several mammals whose bodies delay pregnancy  until conditions are right. After mating takes place in  the spring or early summer, the fertilized embryo hangs around, waiting  to implant in the uterine wall in the fall but only if the female has enough stored fat.

For example, researchers have found  that brown bears with less than twenty percent body fat didn’t get  pregnant even when mating had occurred. This may have evolved as a way to protect  female bears from expending energy that they don’t have since they  have to use their fat reserves as both fuel for themselves and their cubs. As weird as it sounds for them to  go to these lengths to give birth during the months when food is hardest to find, there are some good reasons  that this works for them.

This strategy allows bears to  spend all their time foraging for food when it’s most abundant,  without pausing to give birth. And the babies are protected in the den  when they’re most vulnerable to predators. So when a bear emerges from the den with  little cubs who are seeing the sky for the very first time, they weren’t  just lazily hanging out inside - a ton of preparation and energy went  into bringing them safely into the world!

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