<figure id="attachment_453057" aria-describedby="caption-attachment-453057" style="width: 4252px" class="wp-caption alignnone"><a href="#" data-ident_hash="135d7d69"><img src="https://res.cloudinary.com/fridaydigital/image/private/t_article_image/wpmedia/2026/01/18c8a2a4e33a93d22b37beb0e442ac3a.jpg" alt=""></a><figcaption id="caption-attachment-453057" class="wp-caption-text">The average number of bears giving birth is 1.8. Bears share the same habitat with their mothers from the age of 1.5 to 2.5 years old, learning how to survive.</figcaption></figure>
<h3> Fertilized eggs are held &#8220;on the verge&#8221; for six months.</h3>
 The Chinese character for 2013 is &#8220;bear. Even in December, some bears still come down to human settlements, and it is said that the number of bears that do not hibernate may increase.
 Bears give birth during hibernation. But can they give birth without hibernating?
 &#8220;Bears need a calm place to give birth, so I think it would be difficult for them to give birth unless they are in a hibernating burrow,&#8221; said Dr. Yoshihiko Nakamura, a graduate student at Hokkaido University.
 Professor Toshio Tsubota, who studies bear hibernation and reproduction at the Hokkaido University Graduate School and is also the president of the Brown Bear Society, explains, &#8220;Bears give birth during hibernation.
 Since the bears give birth during hibernation, we thought they would mate in the fall,
 The mating season for bears is from May to June, but the fertilized eggs (or more precisely, embryos) remain in the uterus. The fertilized egg (or embryo, to be more precise) remains in the uterus and does not implant. This is called &#8220;delayed implantation. Implantation occurs when the embryo goes into hibernation. Then, after a two-month gestation period (embryonic development), the baby is born.
 How did bears come to adopt such a method of &#8220;delayed implantation&#8221;?
 Professor Tsubota: &#8220;We don&#8217;t know exactly because it happened during the course of evolution. If I had to guess, I would say it is because they mate at the best time and give birth at the best time.
 Bears hibernate for about five months, from late November to late April. The gestation period is two months, and it takes three months after birth for the bears to be able to walk and feed on their own.
 When they are on their own, they will die if they do not have food. Bears give birth from late January to early February, just in time for spring, when feeding conditions are better. Since the gestation period is two months, they usually mate in late November.
 However, at that time of year, they have to hoard food and store a lot of fat in preparation for hibernation. This is not the time to use energy for reproduction. I think this is why they adopted the &#8220;delayed implantation&#8221; strategy. 
 In addition to bears, delayed implantation has also been observed in mink, weasels, seals, and bats, all of which may have acquired the &#8220;delayed implantation&#8221; strategy during the course of evolution in order to mate at the optimal time and give birth at the optimal time.
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<h3> Sleeping &#8220;one-upper</h3>
 When born, bear cubs weigh only a few hundred grams. They are small enough to fit in the palm of your hand. They can&#8217;t even open their eyes or walk properly yet.
 Herbivores try to have as many mature cubs as possible because of the danger of being attacked by predators,&#8221; says the professor. However, carnivores such as bears, dogs, and cats are at less risk of being attacked, and it is easier to give birth to smaller cubs. Therefore, they give birth to small cubs and raise them as large as possible.
 A bear cub born weighing only a few hundred grams can walk within three months, which is quite a speedy growth rate.
 When we checked the composition of the bears&#8217; milk, we found it to be high in fat and protein. We feed them highly nutritious milk and raise them all at once.
 During this time, the mother bears remain asleep. She may sleep more lightly than a female bear that has not given birth, but she does not urinate or defecate.
 After five months of hibernation, the bears wake up and go out on foot to look for food. 
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<div data-friday-nextpage-lead=" In Hokkaido, 99% of bears go into hibernation..."></div>
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<figure id="attachment_453058" aria-describedby="caption-attachment-453058" style="width: 4252px" class="wp-caption alignnone"><a href="#" data-ident_hash="6356d0b6"><img src="https://res.cloudinary.com/fridaydigital/image/private/t_article_image/wpmedia/2026/01/21e192ffd6836e0572a8aa8f0e1dacdf.jpg" alt=""></a><figcaption id="caption-attachment-453058" class="wp-caption-text">Once they are on their own, the female bears stay close to their mothers, while the males sometimes move several hundred kilometers away from their birthplace in order to avoid inbreeding. Many young bears stray into human habitat at this time, says Prof. Tsubota.</figcaption></figure>
<h3> Bedridden and &#8220;muscle maintenance</h3>
 Prof. Tsubota says that there are still many things we don&#8217;t know about bear reproduction and hibernation.
 For example, how can an unimplanted embryo survive? We don&#8217;t even know how implantation occurs in late November. We don&#8217;t even know how bears hibernate in the first place.&#8221;
 Don&#8217;t bears hibernate because they run out of food?
 That is the reason, not the mechanism. Hibernation is a special physiological mechanism. So all the organs in the body change their physiological functions. We know very little about how such things are done. 
 To figure these things out, we have to do experimental things, such as taking blood samples, examining brain waves, and taking pieces of muscle tissue and bone tissue. We can&#8217;t find out anything just by observing.&#8221;
 In December, 99% of the bears in Hokkaido went into hibernation. What is going on inside the bears&#8217; bodies? There are so many mysteries.
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Toshio Tsubota is a professor at the Graduate School of Veterinary Medicine, Hokkaido University. He conducts research on the behavior, reproduction, genetics, infectious diseases, and ecology of large mammals, bears, deer, and seals living in Hokkaido. He is particularly interested in the perspectives of conservation medicine and conservation management for the conservation of biodiversity. In particular, he is focusing on the physiology and ecology of bears, and his immediate research agenda is to elucidate their unique mechanisms, such as hibernation and reproduction, which are not found in any other animals. He is also the chairman of the &#8220;Brown Bear Society,&#8221; a group of researchers, government officials, hunters, and members of the general public working toward the coexistence of humans and brown bears. 
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 Izumi Nakagawa

 Afro

 Fasting to give birth and maintain muscle strength even when bedridden...Hibernating bears defy medical conventional wisdom! What is "delayed implantation," in which a fertilized egg waits for six months? Professor Toshio Tsubota of Hokkaido University thoroughly explains the mysterious ecology and amazing mechanism.

May Mating and Pregnancy in Winter? Fasting births and &#8220;maintaining muscle strength while bedridden&#8221;&#8230;hibernating bears defy medical convention.

May Mating and Pregnancy in Winter? Fasting births and “maintaining muscle strength while bedridden”…hibernating bears defy medical convention.

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