Yellowstone Photos and Books

Changes in Patterns of Grizzly Bear Hibernation in Yellowstone

My first winter working in Yellowstone was that of 1978-79, when I was employed in the kitchen of the Old Faithful Snow Lodge for the old Yellowstone Park Company. True to my nature of having a lifelong passion for wildlife, beginning that very first winter I was drawn to the large assemblages of animals that wintered in the geyser basins along the Firehole River. While most of my coworkers preferred to spend their free time on cross-country skis in the higher country above Old Faithful, where the snow quality was better for skiing, I chose to roam the thermal basins north of Old Faithful. There I was enthralled by everything I saw, what with so much geothermal ground completely bare of snow, while just a short distance away on earth that was not geothermally influenced the snow was many feet deep. There were other sections of the valley that were thermally warmed to a lesser extent, to the point where they had a snow cover but its depth was greatly reduced compared to nearby, unwarmed areas. Also amazing to me were the many game trails that connected the various thermal areas. Mostly beaten out by bison, I quickly learned that it was far easier to walk around much of the Firehole valley using only my own two feet (protected by heavy winter boots, of course) rather than using snowshoes or skis. Those options were good only for comparatively short distances across unbroken snow fields between thermal areas, but the snowy expanses invariably led to open ground where either snowshoes or skis were an impediment rather than a benefit. And by following the trails the bison had bulldozed through the snowy sections between thermal areas, I was able to go anywhere the animals did.

As much as I was fascinated by Yellowstone's famous geothermal displays, and by the stark contrast between their extreme heat and the frigid air that surrounded them, it was the wintering animals that most thrilled me with wonder. In those days the Firehole valley, as well as the connected valleys of the Gibbon and Madison rivers, were filled with hundreds and hundreds of elk and bison that had been driven down to the lower, geothermally influenced valleys that were much more hospitable for their winter needs than the higher country where most of them had spent their summers. In addition to the elk and bison I frequently saw coyotes, and in the immediate vicinity of Old Faithful there was also a small herd of wintering mule deer. Also interesting to me were the large numbers of waterfowl that made a living on the rivers in that part of the park, waters that because of geothermal inflow never froze no matter how cold the weather. Even more improbable was the presence of summer birds I found living in the geothermal oases along the Firehole River, birds like great blue herons and common snipes. Somehow these normally winter-averse species had discovered that it was possible to make a living in the thermal areas through Yellowstone's notoriously ferocious winter, while all others of their kind had migrated to warmer climates long before the cold season set in.

It was much more than just the simple sighting of wildlife that appealed to me. Rather, it was the opportunity to watch them at length as they interacted with their environment and others of their own species, as well as individuals of other species, and I spent hours and hours just sitting quietly and watching the interplay. In those days, long before wolves were reintroduced to Yellowstone in 1995 and 1996, there was little predation on adult elk and bison. Instead, population control was mostly caused by winterkill, which in turn was influenced by factors like the condition of the animals' range during the previous summer, the level of competition between the various species of grazers, and perhaps most of all by winter conditions of snow depths and temperatures. As my first winter in Yellowstone progressed, I found the carcasses of more and more elk and bison that had died from the deadly combination of starvation and exposure.

After spending that first winter in the park (I had already spent two summers here), the Yellowstone hook was deeply set, I realized probably for life, and more than anything else I wanted to find a job that would allow me to stay on through the spring until my summer job as a fishing guide on Yellowstone Lake was scheduled to begin in early June. Such off season positions were few and far between at that point in time, but luckily for me I was able to land a job on a heavy labor crew for the Yellowstone Park Company that was then called the Bull Gang. That crew's principal duty in the spring was clearing snow from park buildings to facilitate opening for the summer season. Most importantly for me, the job allowed me to live in the park, beginning in mid March at Old Faithful, and then moving to the Lake Area on the north side of Yellowstone Lake in mid April. I continued my solitary and contemplative outings to the thermal basins on my off time while I worked on the Bull Gang, going out nearly every evening after work and all day long on my days off, so I was out and about when the park's grizzly bears began to emerge from their dens. Even now, after almost 50 years in Yellowstone, it is still an incredible rush for me to find the first bear track of the spring. But that very first spring in Yellowstone, the thrill of finding the first track of the year for the first time in my life was almost indescribable. In addition to my passion for wildlife, I have also been a lifelong student of western history, and so when I found that first track I was moved to think of a quote from Captain Meriwether Lewis that he recorded in the spring of 1805 as he and his partner Captain William Clark were leading their famous expedition up the Missouri River in what is now eastern Montana. Lewis was walking on shore along the river when he came across what he recorded as  "many tracks of the white bear, of enormous size." I have added the emphasis here, believing that Lewis penned that last clause because he was so moved by the signs indicating the presence of the great bears, even though he had yet to see one, and like almost everyone else both before and since his time he was powerfully taken by the awe-inspiring size of those huge, hugely clawed feet.

As that spring of 1979 wore on I found more and more tracks of the big bears, and more and more of the winterkilled buffalo and elk carcasses that they had fed upon. There were no bear management closures in those days, so it was perfectly legal for me to do so, and I continued to roam the thermal basins in subsequent springs while I worked at various jobs that allowed me to live in the park at that time of year. The more I observed and learned about Yellowstone's grizzly bears, the more intrigued I became with them. More than just the image of the bears as being big and powerful, which they are, I was impressed with how in tune they were with their environment. I noticed that they tended to show up in the thermal basins at about the time that the rate of die off of wintering ungulates was peaking. I further noticed that the bears moved between the geothermal areas along the existing trails that had been packed out through the snow by the bison and elk --- as big and as powerful as they were, it was still important for them to save energy where and when they could.

I further saw many places where a individual bear's tracks clearly showed that he or she had walked out of direction to go to the very brink of a thermal feature, to look directly into either a hot spring, a geyser vent or a fumarole, displaying a curiosity in the sounds or smells that were emanating therefrom. In my mind, such curiosity had to be reflective of a high level of intelligence. On occasion I found were bears had deftly used their claws to pull bits of ungulate flesh from the runoff channels of hot springs. Elk and especially bison often fall into lethally hot springs when they crowd around them for the warmth they provide on cold winter nights. It is usually smaller members of the herd that fall victim to such accidents, as they are apparently bumped into the hot water when there is jostling to approach as closely as possible the most favorable spots in proximity to the seething water. But whether large or small, the carcasses of such unfortunate animals often boil apart, after which bits and pieces of meat wash out into the runoff channels of the springs, and it is from there that bears fish out the edible morsels. In my view, the bears were too smart to try to extricate boiling carcasses from open expanses of the deadly springs themselves, but instead waited until the bits of meat washed out to where they were safely accessible. To me, all of it was endlessly fascinating.

I have always been fortunate with landing highly desirable jobs during by time in Yellowstone, and my interest in bears and winterkills led me to a paid position with the Interagency Grizzly Bear Study Team (IGBST), professionally pursuing what I had already been doing as an avocation for years. From the early 1980s through the early 1990s I systematically canvassed the geyser basins of the Firehole valley, almost always alone, looking for winterkills and bear sign. The study was supervised by David Mattson of the IGBST from his office in Bozeman, Montana, and with my field work combined with Dave's general insights and his statistical expertise we reached many academic conclusions. We found that at that time the grizzlies in the Firehole apparently were averse to the presence of human activity --- they almost never utilized carcasses of animals that died within about a quarter mile of the road, for example, and neither did they like to venture within about three or four miles of the Old Faithful developed area. Another bit of information was that the carcasses of larger ungulates, especially those of adult bull bison, tended to remain underutilized by scavengers longer than the carcasses of smaller animals, with yearling elk and mule deer being at the lower end of the size spectrum. In the case of bull bison carcasses, they tended to persist untouched or nearly so for more than a month (on average) before they were entirely consumed, while yearling elk carcasses were usually completely consumed by scavengers within about 24 hours. First of all, Dave and I figured was because the smaller carcasses with lighter hides were easy even for the more numerous small scavengers like coyotes and ravens to tear into, and obviously because they simply amounted to less food. Larger carcasses with larger, tougher skins, on the other hand, were more difficult to access initially and offered much larger volumes of carrion. This phenomenon also told us that carcasses of larger animals were more important as spring food for bears because they were more likely to last until bears found them after they emerged from their dens. It also pointed out that bears themselves were symbiotically important to other scavengers, in that they had the strength (along with their formidable teeth and claws) to tear into the larger carcasses and thus make carrion available to smaller scavengers.

Our study revealed all these phenomena and many more, but for me one of the most fascinating facts was that the average date of the peak of the winter-caused die off was March 25. That was the average for all the carcasses of all species I found during all the years of the study, which amounted to several thousand animals. That late March date may seem late in the season, but it had to do with the severity and the then long lasting nature of Yellowstone's winters. It also had to do with the fact that many ungulates were so worn down by conditions during the heart of the winter that while they might manage to survive the worst of the weather in the short term, they were depleted they could not recover when the snow began to melt and new forage began to sprout as spring came on.

That average date of death was made even more fascinating by the fact that during all the springs I covered the thermal basins for the Interagency Study Team, I always came across the first grizzly bear track of the year within one day of March 25 --- i.e., either March 24, 25, or 26. The bears' activities were apparently that keyed in to conditions, that in common with all their other food sources over the course of the seasons the bears not only knew where to find the foods, in this case winterkilled carrion, they also had some sort of calendar in their heads that told them when to show up to find those foods. Now, I readily admit that there may have been some degree of happenstance involved with the precise nature of me finding the first bear tracks within one day of March 25 over the course of only one decade, but it's also true that I was out almost every single day during my spring carcass surveys --- I almost never took a day off --- and I covered many miles of country on each one of those days. I am confident in stating that I think I found the first bear track of each year not long after it had been made, meaning shortly after the first bear of the spring entered the Firehole valley.

All that said I'd like to point out that many things have changed in Yellowstone since I did that carcass survey, and since Dave Mattson and I drew the conclusions from it that we did. A big one is that wolves reappeared on the Yellowstone scene in 1995 and 1996, when animals captured in Alberta and British Columbia were released in the park. Wolves effected big changes in Yellowstone when they came back. In this context they tended to kill ungulates outright when they became weakened by wintry conditions, and so elk and bison as a rule did not linger as far into the late winter and spring as they formerly did before succumbing to winterkill. The bonanza of carrion that was available to bears emerging from hibernation in the days of my carcass survey was no longer present. That loss was somewhat offset by the chance for bears to scavenge wolf kills, which scavenging was possible in some places and at some times and by some bears (mostly larger individuals). In all, the interaction between bears and wolves is a complicated one, as are all other relations between all other elements of the ecosystem.

What's not complicated, or at least much more demonstrable, is the way the climate in Yellowstone has changed since my early days in the park in the mid 1970s. Conditions are undeniably warmer now, and more subject to extremes, than they were when I first arrived. For one example, winter used to reliably arrive in Yellowstone during the first week of November. By the middle of that month interior park residents had to switch from wheeled vehicles to snowmobiles and snow coaches, while foot travel in non geothermally warmed landscapes required snowshoes or cross-country skis. And that's because snow depths demanded such changes, not because the changes were scheduled or bureaucratically mandated. Now almost all autumns and early winters do not see such conditions develop until mid to late December, and sometimes not even until after the new year. The extended periods of strong cold that used to be a feature of wintry Yellowstone, with temperatures falling far below zero and staying there for days or even weeks at a time, seem to be a phenomenon of the past. Midwinter rains, which were all but unheard of in my own early days, as well as in more distant Yellowstone history, now happen one or more times almost every winter. Snowpack in the past quite reliably developed to considerable, some would say awesome, depths. Now winters are more erratic, with some producing substantial snowpacks within the narrower window of the cold season, while way too often snowpacks are pathetically low. One certain feature of the new climatological conditions is that true winter almost invariably begins later in the year, and that spring almost always ensues earlier than it formerly did.

Almost certainly because of the later onset and earlier conclusion of winter, Yellowstone's grizzly bears seem to have changed their denning habits. Almost all park bears used to den up between mid October and mid November. Now it is not at all uncommon for bears, or at least their tracks, to be seen well into what used to be the winter season. In my own experience in recent years I have seen tracks as late as New Year's Eve. That find was in 2019 on the Craig Pass road between Old Faithful and West Thumb, and I actually saw a big bear on the same road in late November of 2013. Just this year (2024), I found a grizzly track at Mud Volcano in early December, and a friend of mine actually saw (and videoed) a bear at the mouth of Nez Perce Creek just a week or so before Christmas. Additionally during this present winter, tracks have been found immediately outside the park in the town of West Yellowstone, just outside Yellowstone's boundary, as well as in Mill Creek on the east side of Montana's Paradise Valley just north of the park --- again in the days just before Christmas.

At the other end of the effectively shortened winter season, it is common to find bear tracks long before the March 25 date I cited earlier. In many cases bears are actually sighted well before that date, even in the higher elevations of the park outside the Northern Range, with one of the more extreme examples being a big grizzly that showed up at Mud Volcano on February 11, 2015. And that was the day that the bear was first noticed --- it is quite possible that he (and the bear was almost certainly was a male) was out and about for some time before he was seen by park employees and visitors. In my own experience while working as a winter caretaker in the park for 45 winters now, a job that necessitates quite a bit of traveling around Yellowstone on a snowmobile to check on the various properties in my charge, in recent years I have frequently come across bear tracks much in advance of that erstwhile benchmark of March 25. Attached are two photographs, one showing a grizzly bear track in the road next to my snowmobile near West Thumb that was shot during the winter proper, after the point in time when park roads had been restricted to over snow vehicles, and the other showing a track of enormous size, to borrow that phrase from the great Meriwether Lewis, right outside the Old Faithful Inn. Significantly, that latter photograph was shot very early in the spring. The picture is telling not only for the fact that the image features a grizzly bear track in front of the most iconic building in Yellowstone, but also because the nature of the snow is wet with springlike slushiness at a date well before that now outdated temporal touchstone of March 25.

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