hibernation

Happy Birthday Bear

/ Mike Fitz / 8 Comments

Across much of North America, tucked within isolated dens, a new generation of bears is beginning their lives.

Mother bears spent much of the last year preparing for this event. Although the timing varies among species and individuals, North America’s bears mate in late spring and early summer. The fertilized eggs, however, do not immediately implant in the uterus, undergoing only a few cell divisions before they enter a state of arrested development. During this process of delayed implantation, the female goes about her business while embryos remain in suspended animation. Implantation and fetal growth renew only close to the time she enters her winter den. Afterward, bear fetuses gestate for 6 – 8 weeks.

The gestation time is remarkably short for such a large mammal, and it produces especially tiny and helpless cubs. Brown bear cubs, for example, weigh a scant pound and measure only 8 – 9 inches long at birth, about the size of a beagle puppy. They are also born blind, lightly furred, and nearly immobile. Their ears are closed and their muzzles are short with a round, toothless mouth. Newborn cubs are so underdeveloped and small that they cannot maintain their own body heat in the den and must remain in contact with their mother to stay warm. About the only thing they can do is scream, which, not unlike human newborns, they employ frequently to gain their mother’s attention. It’s hard to imagine large adult bears so helpless, but they all start life this way.

Three small cubs held in a person's hands.

Newborn black bear cubs. U.S. Fish and Wildlife Service photo.

The small size of newborn cubs is surprising for animals that weigh several hundred pounds when fully grown. Generally, larger mammal species have longer gestation periods and give birth to larger offspring than smaller mammal species. African elephant calves gestate for nearly two years and are born bigger than elk calves; elk calves gestate for about eight months and are born bigger than deer fawns; deer fawns gestate for seven months and are born bigger than fox kits; etc. But, bears break the rule by a considerable margin. Bears give birth to the smallest offspring in comparison to adult female body size of any mammal.

Cubs are only 1/200th the size of even the smallest reproducing female grizzlies and commonly 1/500th or less for large adult brown and polar bears. In contrast, newborn human babies are an order of magnitude larger than bear cubs. A 10 pound child born from a 150 pound woman is 1/15th the size of its mother (yeah, I know that’s a big baby but the math was easy). Additionally, offspring born to large mammals are generally precocial, i.e. they are at least somewhat and sometimes highly mobile soon after birth. Bear cubs, however, are more akin to helpless hatchling birds or pinky mice. There is no parallel among placental mammals—only marsupials give birth to offspring as undersized as bears.

But why are bear cubs born purposefully premature? Why not just have a longer gestation time and birth larger, more independent cubs? The short gestation period and the relatively small size of bear cubs at birth both appear to be an adaptation to maximize the use of fat.

Bears are the only mammals that give birth while hibernating, a time when they do not eat, drink, urinate, or defecate. Survival during this time is dependent on stored body fat, but the paradigm poses a problem for expectant female bears. A developing mammal fetus cannot metabolize free-fatty acids, perhaps because these substances do not cross the placenta as readily as sugars and protein. So, as long as a bear tries to sustain fetal growth through her placenta, she needs to draw energy from her own body protein. Fetuses also produce bodily waste, which is transferred to the mother and adds to her physiological challenges. To cope, bears evolved an alternative strategy, one that allows her to give birth while hibernating, support the continued growth of cubs, and keep the family safe.

Unlike in the womb, baby mammals can metabolize fat shortly after birth and milk is the vector to deliver it. Bear milk is a particularly rich and nourishing substance. Brown bear milk, for example, is about 22% fat by volume. Polar bear milk is even richer, a whipping cream composed of over 30% fat. By shortening the gestation period, mother bears trade placental nourishment (mostly protein and sugar) for mammary nourishment (mostly fat) and tap into the one resource they have in abundance.

fat brown bear exiting water

Female bears utilize their fat reserves to support the growth and nourishment of their cubs.

On a diet of fatty milk, a brown bear cub can gain about a 1/5 of a pound of body mass per day, weighing about 5 pounds when one month old and 15 – 25 pounds by 90 days. Not coincidentally, this is about big as they would be if gestation was of an “expected” length like other placental mammals. The den, therefore, becomes a surrogate womb, protecting the family during the most vulnerable time in their lives.

Two polar bear cubs standing at the entrance to a snow den.

Polar bears play at the entrance to their mother’s den. These cubs are probably several weeks old. U.S. Fish and Wildlife Service photo.

Bears face many obstacles to survive and reproduce, not the least of which is winter famine. Hibernation provides bears with the ability to outwit winter by surviving on accumulated fat, but during this time a female bear must support the growth of her cubs with nothing more than the energy stored in her body. Given the challenges posed by gestation, hibernation, and winter famine, the birth of a bear represents a remarkable and unparalleled feat of mammalian adaptation.

So, happy birthday brown bear.

Hibernation Hangover

/ Mike Fitz / 4 Comments

In Glacier National Park, Montana, a black bear has emerged from hibernation, but hasn’t left his tree cavity den.

According to the park website, this bear was first seen on March 23. Since then, the black bear, who is male, has mostly rested in the tree cavity. After a long winter of hibernation, you might assume a bear would be eager to get moving and find something to eat, but bears often don’t leave their denning site for days, sometimes weeks, after they emerge in the spring.

A bear fresh out of the den isn’t the same bear it will be in May. Immediately after emerging from their dens, bears are active but neither hungry nor particularly thirsty. In one of the first studies on the physiology of hibernating bears, researchers found captive bears ignored food and water for up to two weeks and some bears didn’t begin to eat and drink normally for three weeks after they emerged from their dens. One grizzly bear didn’t even urinate for two days after it emerged. (In contrast, during another study a black bear in the fall urinated copiously, producing eight to sixteen liters of urine per day.)

This annual life stage of springtime bears has been described as “walking hibernation.” Compared to summer and, especially, early fall, bears in walking hibernation are hypophagic. They actively ignore food and drink little water while still surviving on body fat. During walking hibernation, bears experience an internal transition from full hibernation to a more active physiology. Research on brown bears in Sweden, which I wrote about previously, has found the body temperature and metabolic rate of brown bears doesn’t stabilize until 10 and 15 days, respectively, after den emergence and their heart rate doesn’t stabilize for another month.

Graph that shows the timing of several variables affecting the start and end of hibernation in bears.

These graphs chart the relationship between physiological parameters of brown bears in Sweden. Den entry (left column) and exit (right column) are indicated by time zero (the green vertical line) to determine the sequence of physiological events. SDANN is the standard deviation of heart rate variability over five minute intervals. It was used a proxy measure of metabolic activity. A red line denotes when a variable was decreasing, while a blue line indicates when a variable was increasing, with the number of days from the entry/exit indicated. From Drivers of Hibernation in the Brown Bear and reposted under the Creative Commons Attribution 4.0
International License.

bear feet sticking out of hole in tree trunk

The transition from hibernation to fully active includes lots of resting. Screen shots from the Glacier National Park bear den live stream.

black bear in tree cavity

Possibly because their metabolism and heart rate remain somewhat low, many bears seem to loathe leave their dens, at least right away. So, it’s not uncommon for bears to remain near their denning site while their bodies transition back to more active levels.

The bear at Glacier will leave its tree cavity den relatively soon. His hunger will grow as his metabolism returns to active levels. His libido will increase too, and he’ll begin to prowl the land for females in estrous (the mating season for black and grizzly bears peaks in late spring). Compared to other stages in their annual cycle, less is known about the first few weeks of life for bears after they emergence from hibernation. It is rare for us to witness a bear’s life at this time. With webcams and other digital tools like GPS collars, we’re gaining a greater depth of knowledge about many wild animals. Glacier’s webcam provides a rare opportunity to observe a bear shortly after it has emerged from hibernation. Like most bears right now, it remains in a bit of a hibernative hangover.

Drivers of Hibernation

/ Mike Fitz / 10 Comments

Brown and black bears hibernate to avoid winter famine. For five to seven months, they do not eat, drink, urinate, or defecate, a strategy quite unlike other mammalian hibernators. Chipmunks, for example, cache food to eat in between bouts of torpor. Marmots and arctic ground squirrels don’t eat during winter and survive off of their fat stores like bears, but they activate their metabolism periodically to wake and urinate.

I recently spent about 40 hours reviewing studies related to hibernation and denning in brown bears for a chapter in my book on Brooks River’s bears and salmon, which reminded me just how remarkable this process is. While in the den, bears spend about 98% of their time not moving. Their heart rate declines dramatically from 50-60 beats per minute during summer to 10-20 per minute in hibernation. During this time, they hardly breathe, taking 1.5 breaths per minute on average. Their body temperature drops several degrees entering them into a state of hypothermia. Finally, the metabolic rate of a hibernating bear is 70-75% less than its summer peak. To survive, bears subsist on their body fat, catabolizing it into energy and water.

brown bear sitting on rock surrounded by water

All brown bears, like this adult male known as 89 Backpack, get fat to survive.

Despite their lack of physical activity, hibernating bears maintain muscle strength and bone health. Even if immobilization didn’t cause starvation, osteoporosis, and atrophy in people, we would die of dehydration if placed in an equivalent situation. Hibernating bears, however, are nearly completely self-supporting. The only input they need from the outside world during hibernation is oxygen.

The physiology of bear hibernation is complicated and not fully understood. Scientists are still elucidating basic details about this remarkable process. For example, what causes bears to enter and exit the den? How long do bears need to switch their metabolism from to hibernating mode? As it turns out, the switch is a long process.

Researchers in Sweden used implanted heart rate monitors and GPS-enabled tracking collars on fourteen brown bears. The devices recorded the movement, heart rate, heart rate variability, and body temperature as well as ambient temperature and snow depth. The results, published last year in “Drivers of hibernation in the brown bear,” are insightful because it allowed the researchers to develop correlations between the variables that drive and trigger hibernation.

In fall, well before hibernation begins, body temperature and heart rate of bears began to decrease. Heart rate started to slow, on average, 24 days before den entry, and body temperature began to drop 13 days before den entry. Overall activity lessened 25 days before entry, but metabolic activity declined steeply just as the bears entered their dens. It took an additional 20 days after for heart rate and metabolic activity to bottom out.

The transition back to a more active physiology started long before bears left their dens. Heart and metabolic rate began to rise one month and 20 days, respectively, before den exit. Body temperature began to rise even earlier, a full two months before den exit when winter still locked the landscape in ice and snow. All bears left the den when their body temperature was 36.7˚C (98˚F) ± 0.15 °C, the active-state body temperature for brown bears. As the researchers note, the narrow temperature range at this time suggests bears exit the den when their body temperature reaches a specific point. Body temperature and metabolic rate stabilized 10 and 15 days, respectively, after den exit, but heart rate didn’t stabilize for another month.

Graph that shows the timing of several variables affecting the start and end of hibernation in bears.

These graphs chart the relationship between physiological parameters of brown bears in Sweden. Den entry (left column) and exit (right column) are indicated by time zero (the green vertical line) to determine the sequence of physiological events. SDANN is the standard deviation of heart rate variability over five minute intervals. It was used a proxy measure of metabolic activity. A red line denotes when a variable was decreasing, while a blue line indicates when a variable was increasing, with the number of days from the entry/exit indicated. From Drivers of Hibernation in the Brown Bear and reposted under the Creative Commons Attribution 4.0
International License.

Even though the bears’ physiology initiates the ultimate beginning and end of hibernation, climate plays a role in this process too. Changes to body temperature before den entry were affected by ambient air temperature, but bears largely relied upon a physiologic slowdown to cool themselves. In spring, bears left the den when the weather was right, exiting when air temperature rose to above 3.7˚C ± 1.5 ˚C (38.7˚F ± 2.7˚F).  Some biologists have suggested that food availability drives the timing of den entry, but this study did not attempt to test the hypothesis.

As a survival strategy, bear hibernation is remarkably efficient, and no other animal attains the same physiologic feats. Small mammal hibernators wake to pee; bears don’t even need to do that. Changing from an active metabolism to one of hibernation and back again takes a lot of time. If you are fortunate enough to see a bear in the middle of fall or the middle of spring, that bear is likely living in a transitional body equipped to handle two worlds—one with food and one without.

 

Late Season Bears on Dumpling Mountain

/ Mike Fitz / 8 Comments

Dumpling Mountain, in west-central Katmai National Park, rises gently between Naknek Lake and Lake Brooks. Overridden repeatedly by glaciers during the last ice age, its slopes contour less abruptly than taller mountains to the east. About half the mountain’s topographical prominence lies above timberline. The upper mountain is a chilly, wind-swept place (especially in mid October) where only hardy, ground-hugging shrubs and forbs grow.

tundra and view of low mountain

Tundra on upper Dumpling Mountain on August 22, 2015

snowy tundra

Tundra on upper Dumpling Mountain on September 30, 2015.

The Dumpling Mountain Cam recently captured footage of a mother bear and three yearling cubs there.

The Dumpling Mountain Cam is located about 2,150 feet above sea level on the mountain’s dry alpine tundra, just under 300 feet below and .75 miles distant from the mountain’s 2,440-foot high summit. I hiked up Dumpling Mountain dozens of times, mostly to escape the relative hustle and noise of Brooks Camp, but I rarely saw bears on the mountain. Tracks, sure. Scat, definitely. But bears? Almost never. They don’t use the mountaintop as frequently as other areas. So why would bears venture nearly to the summit of Dumpling now? Are they migrating to a denning site?

Last fall, in a blog post for explore.org, I discussed what is known about the denning habits of Brooks River’s bears. From limited radio tracking studies done in the 1970s, we know these bears probably den on steep, well-vegetated slopes that collect a lot of snow. The same study determined Katmai’s bears denned, on average, at 1,300 feet in elevation.

Dumpling Mountain offers much suitable denning habitat. Although none of the bears radio-collared at Brooks River in the 1970s were tracked to it, I found at least three areas with bear dens in my explorations of the mountain. None are visible within the Dumpling Cam’s viewshed, but they aren’t very far away either.

Screen shot from Google Earth. Purple polygon is viewshed of Dumpling Mountain Cam. Text reads: "Dumpling Mountain Cam" "Bear Dens" "Bear Dens" "Bear Den in Video"

All the dens I found on Dumpling Mountain were around the 2,000 foot elevation line or lower. The purple area represents the Dumpling Mountain Cam viewshed.

Person squatting in entrance to bear den.

Yours truly sits at the entrance of a bear den on Dumpling Mountain.

Bear dens are cozy places. An entrance tunnel leads to a sleeping chamber, which is usually just large enough for the bear crawl into and turn around. Brown bears have the strength and endurance to dig their dens quickly, but den excavation typically takes place over several days. They may also make several excavations near their denning site, perhaps aborting these first attempts due to poor soil conditions.

Bear abundance at Brooks River peaks in late September and early October then decreases coincident with fewer spawning salmon. The bears’ migration away from the river doesn’t necessarily mean they’ll immediately head to their denning site. Bears can still find opportunities to feed elsewhere, even on Dumpling.

These bears on Dumpling may not have been moving to a denning site. Instead, they could’ve been there to eat. Their time on camera showed them traveling, playing, and grazing. Crowberry (Empetrum nigrum), alpine or bog blueberry (Vaccinium uliginosum), and lingonberry (Vaccinium vitus-vitae) all grown on the mountain’s tundra and can be important, and easily accessible foods for bear. Wild berries in Katmai are a fickle crop though. Some years, berry plants produce bumper crops, while in others I was hard pressed to find many berries at all. When one or all are abundant, however, berry-filled scat reveals the bears’ motivation on the mountain. In October, all three species can linger on the bush, but lingonberries are most likely to remain abundant into fall.

Lingonberry (Vaccinium vitis-idaea)
This dark purple bear scat is filled with black crowberry (Empetrum nigrum).
Bog or alpine blueberry (Vaccinium uliginosum)

 

Dumpling Mountain offers several things bears need—food in the form of seasonally abundant berries, open space relatively free of human disturbance, and pockets of prime denning habitat. Bears using the mountain, especially in the fall, could be there to locate a denning site, to graze frozen berries, or simply on their way from one place to another.

Addendum:

Some bearcam viewers have speculated the bear family recently seen on Dumpling Mountain was 854 Divot and her three yearlings. While the video evidence is inconclusive I saw Divot on Dumpling Mountain in the spring of 2015, so the mountain is part of her home range.

What is Hyperphagia?

/ Mike Fitz / 3 Comments
Side-by-side comparison of bear in late spring and late summer. Text reads, "747 June 13, 2107" "747 September 11, 2017"

Photos of bear 747 from late spring and late summer illustrate this bear’s substantial weight gain. Photos courtesy of Katmai National Park.

As we’re in the midst of Fat Bear Week, it’s a good time to ponder some of the mechanisms that allow bears to gain enough weight to survive hibernation. Bears get fat to survive and hyperphagia is how they do it. In the bearcam week in review for October 5, I briefly explained hyperphagia. Here it is in case you missed it.

Bears experience hunger in ways humans do not. They need to eat a year’s worth of food in six months or less to survive winter hibernation and the lean months of spring. To do this they eat A LOT, especially at this time of the year. 

In bears, hyperphagia is the period of excessive eating which takes place in late summer and fall. During this time black bears will eat 20,000 calories of food per day. Katmai’s brown bears can easily eat even more by catching calorie-rich salmon. Even though their calorie intake is extremely high, hyperphagic bears don’t feel full.

Satiety is the feeling of fullness after a meal. During hyperphagia, a bear’s body temporarily suppresses the normal mechanisms that balance food intake with weight gain. The body says, “You aren’t full. You don’t have enough fat reserves and need to eat more.” In short, hyperphagic bears don’t feel sated. This contrasts with the excessive eating of bears in June and July when they are eating a lot, but their bodies probably still respond to a feeling of fullness (which is sometimes hard to believe when you watch Otis or 747 catch fish after fish after fish). Hyperphagia, therefore, is a physiological state of bears in late summer and fall, as much as it is a behavior we can see. It’s how they prepare to survive the famine ahead.

747 should be your choice for Fat Bear Week

/ Mike Fitz / 18 Comments

There are small and fat bears, old and fat bears, young and fat bears, just plain fat bears. But none, NONE I say, are as fat as 747 in 2017. He has earned my official endorsement in the 2017 Fat Bear Week tournament.

fat bear walking in shallow water near grass

747 displays his massive silhouette near Brooks Falls on September 6, 2017.

747 is a mature adult male in the prime of his life. He has gained at least as much and probably more weight than all others. In my opinion, 747 is the biggest and fattest at Brook River.

Compare 747’s overall size in late spring…

Large brown bear

747 in mid June 2017. Photo courtesy of David Kopshever.

…with his fatness in early September.

Fat bear walking in grass

747 is so fat, his belly almost touches the ground.

Still not convinced? Then watch this video of 747 from September 6, 2017.

Since then, 747 has gained even more weight.

Too much fat is unhealthy for humans, but fat is essential to the survival of brown bears. It is a savings account against famine. Without ample fat, bears do not survive hibernation. In spring, often a season of starvation for bears, females with cubs will metabolize fat into milk to nurse their growing cubs, and adult males will use their fat to fuel their pursuit of mates.

747 won’t be rearing any cubs next spring as male brown bears play no role in raising offspring. During a season when almost no high calorie foods are available to bears, 747 will use his fat to roam the landscape for mates instead.

747 faces some tough competitors in this year’s tournament, but don’t fall for any other fat bear propaganda from the fake news mainstream leftwing socialist progressive liberal media. 747 is larger and fatter than any other bear at Brooks River. He’s huge, tremendous, and will win “bigly.”

2017 Fat Bear Week bracket with 747 as champ

This is my 2017 #FatBearWeek bracket. I look forward to seeing your bracket and campaign posters in the bearcam chat on explore.org.

 

 

A January Bear

/ Mike Fitz / 5 Comments

It was late January, but I enjoyed nearly perfect hiking weather in Big Bend National Park. The sky was clear, the wind was calm, and the temperatures hovered in the hiking Goldilocks range (for me, that’s the low 60˚s F). I had spotted a few piles of bear scat earlier that day, but all were dry and desiccated. Then in the late afternoon, I found one particularly fresh pile of crap.

This scat was soft and pliable and hadn’t been exposed to the dry desert air for very long. (I poked it with a stick to gain a very scientific measure of its age.) Was there a bear nearby? I hoped to find out.

The previous day, I stopped in the Chisos Basin Visitor Center to purchase a book to help me search for the park’s endemic oaks. A map on the visitor center wall was marked with sticky notes identifying when and where people had spotted black bears. At least a dozen had been seen over the past two weeks. I made a mental note to watch carefully for bear sign. Maybe, just maybe, I would be lucky enough to see one for myself.

Mountain rising above pine and juniper forest

The pinyon-oak-juniper habitat near Emory Peak (center) is preferred habitat for Big Bend’s black bears.

Although I spent considerable time searching for the endemic oaks (and found at least a couple, plus some species rarely found in the U.S.), bears were never far from my mind. Backcountry campsites all had bear-resistant food storage boxes, and signs clearly informed people that bears will take your unattended pack.

metal sign. Text says, "Bear Country Do Not Leave Back Packs Unattended"

Occasionally, I’d find old piles of bear scat or a marking tree.

scratch marks on bark of tree

Black bears used this Arizona cypress (Hesperocyparis arizonica) near Boot Spring as a marking tree.

No long after I photographed the marking tree, I stumbled on the aforementioned scat. Its freshness caught my attention, but it wasn’t steaming so I couldn’t be sure if a bear was close or not. I only knew it was there earlier in the day. As I proceeded up the trail, motivated to pick up my pace and return to the campground before dark, two hikers traveling in the opposite direction told me they had just seen a bear not far from the trail. This was their first wild black bear sighting, and they spoke excitedly about their experience. I thanked them for the info and continued on, now even more alert.

The hikers said the bear was near a switchback in the trail, not far from a backcountry campsite. I slowed my pace as I approached that location, not wanting to startle the animal. A moment later, through some thick vegetation, I heard cracking branches and there it was—a black bear.

black bear ears seen through thick vegetation

My soon-to-be award winning wildlife photo of a black bear in Big Bend National Park. Move over Tom Mangelsen.

What would a bear be doing out in January? Since bears are omnivorous and eat a wide variety of food, their scat reveals a world of information about where they’ve been and what they’ve been up to. The fresh bear scat I found 10 minutes before, like the older scat on the trails, was filled with fragments of pinyon nuts and shells. Pine nuts are exceptionally nutritious, containing almost 700 calories per 100 grams. The pinyons pines in the Chisos Mountains seemed to have produced a sizable cone crop in 2016, one which helped sustain the bears into mid winter.

pile of black bear scat in grass

Pinyon pine nut shells and fragments fill this fresh pile of bear scat. I found this scat just moments before seeing an active bear.

The density of the shrubs made it difficult for me to se exactly what the actual bear was doing, but it appeared to have its nose to the ground and it wasn’t moving far. Perhaps it was still feeding on pine nuts.

pine cones on the ground

These Mexican pinyon pine (Pinus cembroides) cones still hold their fatty nuts.

Black bears in the Chisos Mountains rely heavily on habitats with pinyon, oak, juniper, and grassy talus slopes, although male bears will make more frequent use of low elevation areas. Even so, this was January 26. Shouldn’t the bear be inside a den?

Black bears in more northernly locations hibernate well before January. However, bears in Big Bend don’t typically enter their dens until late January or February, and when they do many don’t seem to fully enter hibernation. Male bears, especially, are more likely to remain active. Pregnant females in Big Bend, like other bear populations in North America, have the longest average denning period, beginning in mid to late December and ending in late April.

This winter activity isn’t unique to Big Bend’s bears. Black bears in Florida have similar winter dormancy patterns. Mild weather and the prospect of food, especially, can keep bears active for longer time spans. After all, bears are avoiding winter famine more than winter weather when they hibernate. The bear I saw probably wasn’t doing anything abnormal for a Big Bend black bear. It was just another bear doing bear things like eating and shitting in the woods.