black bear

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.

Trail Cam

/ Mike Fitz / 10 Comments

In February, I discovered wolverine tracks in the snow. After mentioning the tracks to a resource management specialist at North Cascades National Park, we decided to set up one of the park’s motion sensor trail cams hoping to capture photos of wolverines or any other animals that might wander by. The cam didn’t capture images of any wolverines, but it did reveal the presence of other large critters. (In this post, all photos from the trail cam are courtesy of North Cascades National Park Service Complex).

On March 20, I mounted the cam on a tree with a clear view of an unplowed, snow covered bridge over Stehekin River, surmising that the relative ease of crossing the bridge would funnel wildlife across it.

Trail cam mounted on Douglas-fir tree

After setting up the cam, the first photos capture me skiing away.

GIF of person skiing across bridge

Nothing appears on the cam until March 30 when a mountain lion waltzes by!

Mountain lion walking on snow. 2017-03-20, 9:52:47 AM, 33˚F

GIF of mountain lion walking across snow

I was surprised the mountain lion was in this habitat during late March. Many deer, the primary prey for Washington state’s cougars, inhabit the lower Stehekin River valley, but I had seen no tracks of those animals anywhere within five miles of the bridge. The four feet of snow in this location was still prohibitively deep for deer. Perhaps this was a male seeking females or simply one of the resident lions moving through its territory. I’m not sure, although I found a pair of lion tracks a couple of miles north along the Pacific Crest Trail three days afterward.

After swapping out the cam’s memory card on March 23…

hand and part of person's face in front of camera, snow in background

…I left the cam alone until April 3. When I returned I noticed some large, semi-fresh tracks post-holed across the bridge. Backtracking the animal’s movement, I found it walked right under the cam before crossing the bridge. Interestingly, the cam took many photos of an empty bridge the evening before I arrived. Something had nudged the cam, triggering it for 13 seconds according to the photos’ time stamps. One photo in the sequence is completely black as if something briefly covered the lens.

GIF of trail cam sequence. Cam is nudged by animal out of frame.

The culprit was revealed a few seconds later.

black bear walking on snow with nose to the ground. 2017-04-02, 7:24:44 PM, 36˚F

 

GIF of black bear walking across snow

Bears are extremely curious. This one found the cam and investigated it seconds before walking across the bridge. Since black bears in the northeastern corner of the North Cascades begin to emerge from hibernation in early April, it may have been fresh out of the den. On the other side of the bridge it’s tracks continued straight up the hillside disappearing into the forest.

I know these animals live here with me. I see their tracks and scat somewhat often, but I encounter the living individuals less frequently. For me, the cam provided an enjoyable, albeit brief, glimpse into their lives.

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.

The Best Thing You’ll Read About Bark Today (If You Don’t Read Anything Else about Bark)

/ Mike Fitz / 6 Comments

Black bear claw marks on quaking aspen, Beaver Ponds Trail.JPG

This post is about bark.

I know what you’re thinking. “Bark—an enthralling topic!” I couldn’t agree more, but bark is often overlooked and ignored by most people. Yet bark records many events in a tree’s history.

Thin and smooth barked trees like quaking aspen (Populus tremuloides) are especially good at recording animal sign. Aspen is the most widely distributed native tree in North America. Its bark becomes thick and furrowed only on old trees, and usually only near the ground. Most of the time aspen bark is smooth, colored white to gray (even greenish on young trees) with dark chevrons where self-pruned branches fell to the ground.

Since aspen bark is thin-skinned, it’s easily scarred. Along hiking trails, you’ll commonly find names and initials carved into it. (Don’t do this. No one cares if you were there with your “true love,” who you probably dumped the week after, and it opens the tree to possible infection.) Aspen bark records more than human impulses though. In bear country, you can often find evidence of bears climbing the trees.

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Black bears have five toes each equipped with claws superbly designed for climbing. Scars on trees from climbing bears usually come in sets of five.

Black bears are particularly adept at climbing trees. Their strength and relatively short, sharply curved claws help them gain purchase even on smooth barked trees like aspen. If I’m in an area where black bears live, I almost always look for bear claw marks on aspen. In the Stehekin Valley, bear claw marks are easily seen on aspen along the Stehekin River Trail and Agnes Gorge Trail.

The claw marks represent a moment in time. Under what circumstances were they made? Was a bear startled by a person? Another bear? Was it simply playing or exploring? Black bears are omnivorous, but I have read no records or seen any signs of them eating any part of aspen trees, so they probably weren’t climbing the tree for food. In the eastern U.S. though, black bears often climb another smooth barked tree, American beech (Fagus grandifolia), to feast on beechnuts.

The next time you find an aspen, take a closer look at its bark. Bark isn’t as static as its outward appearance suggests. You might find a story there.

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