December 6, 2022

What Drives Maine’s Black Bears to Hibernate?

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There have been some interesting discussions over the past couple of weeks or so about Maine’s black bear hunting season and the fact that hunters seems to be having good success and large bears are being taken. The Maine Department of Inland Fisheries and Wildlife (MDIFW) predicted a higher success rate for hunters this season and they attributed it to the lack of natural food. To go along with this claim, the same predictors said that bears would eat early, hibernate early and exit their dens early come spring.

If interested you can read the articles I wrote about this topic here and here.

It made little sense to me that black bears would show up in record-breaking sizes in a year when there was no natural food, as claimed by MDIFW scientists. I was also troubled by statements made that what drives bears into hibernation is lack of food. This prompted me to go on a multiple day search and rescue mission to see what I could see as it pertained to scientific studies, available to anyone with a computer (and a few extra dollars). What I discovered is that some of my suspicions were confirmed and some of what MDIFW scientists presented was confirmed.

The first major thing I discovered is that most all the studies on bears and black bears specifically, all deal with the physiological affects on bears when hibernating. Not surprisingly, it also appeared these studies were motivated by a desire to learn more so that one day humans can choose (or be forced) to hibernate.

The short of it is that little exists that specifically addresses why bears hibernate, i.e. is there something physiological that takes place or is it as MDIFW biologists state, that it’s all about the food supply? The answer is both.

In some studies, like this one on “The Comparative Anatomy of Eating“, I found statements not unlike several other studies not specific to the forces driving bears to hibernation:

Their diet is dominated by primarily succulent lent herbage, tubers and berries. Many scientists believe the reason bears hibernate is because their chief food (succulent vegetation) not available in the cold northern winters.

It appears as though what actually goes on with a bear in hibernation, if I can put this in words most people, including myself, will understand, is that a bear changes its metabolism. The effects are a myriad of things and the timing and degree to which such changes takes place seem to a product of diet and length and depth of hibernation, among other things.

In another study about how glucose responses by the bears with natural and manipulated amounts, still seem to be regulated by the bear:

Furthermore, the apparent increase in glucose utilization at the end of hibernation when fat stores are nearly exhausted suggests a continuum of metabolic activity from early to late hibernation with a transition to the active phase by the end of hibernation.

All very interesting but what drives the bear to head for the den? It was difficult, at best, to find anything definitive but I think the general consensus was that “something” triggers a black bear’s natural physiological response to increase fat supplies. During this time period, called the hyperphagic stage (transitioning from normal activity to hibernation), the bear naturally begins a gradual metabolic (if that be the correct term) change that will eventually lead them to their favorite winter hibernation local.

It also appears that the time in which a bear decides to actually head into the den can be influenced by whether or not there remains any food to eat. This is part of the equation but not all of it.

In a study titled, “Environmental Relationships and the Denning Period of Black Bears in Tennessee“, we get a glimpse at perhaps what that “something” is that begins to transition toward hibernation:

Den entry and strong fidelity to dens by all instrumented bears indicated that the intensity of dormancy did not differ from that in northern regions; however, duration of dormancy was considerably shorter. Cumulative effects of increased precipitation and lower maximum and higher minimum temperatures, which correspond to passage of a low pressure weather front, provided a proximate stimulus to enter dens. Food supply also appeared to affect denning in a proximal manner because bears denned earlier in years with fair to poor mast yields than in years with excellent mast yields.

The study further explains what determines the timing of denning:

Emergence dates were less strongly correlated with environmental factors. Ultimate synchronization of denning behavior with the environment is best explained by a circannual (endogenous) rhythm; this rhythm is easily shortened or lengthened allowing flexibility depending on environmental variation and the ecology of a species. Such a rhythm encompasses the observed variation in environmental factors affecting the denning period of bears over their broad geographic range and diverse ecological conditions.

It does appear that it’s not just food that determines when denning will occur but a myriad of environmental factors.

But why are Maine’s bears so fat when there’s a poor supply of natural food available? Generally speaking I am not convinced that they are. As the study suggests, it may be proximal, in that the reason we are seeing more bigger, fatter bears is because they got fat from eating bait put out by hunters and guides.

Maine allows baiting to begin approximately one month before opening of the hunting season. One MDIFW biologist told John Holyoke, at the Bangor Daily News, that one bear he was aware of gained 65 pounds in 16 days. If that’s true, then for 30 days of feasting at a bait station, one can imagine the amount of weight a hungry, greedy and dominating bear can put on.

So, are when then to conclude that what we are seeing at the tagging stations is not indicative to what the rest of the bear population, that is those without access to bait barrels, is like? The data being collected by biologists on the bears at tagging stations, is this good, usable and representative data of the general condition of all of Maine’s bears?

With the information I have gathered, some of which I have shared, I can concur that the timing of when black bears decide to go to sleep is partially driven by food supply. I do have concerns about whether big, fat bears is a real representation of the condition of the population in general.