Animals hibernate in order to survive the harshness and famine of winter.
True hibernators they say are measly creatures like squirrels and lemurs, they suppress many physiological activities to the core, from breathing rate, heart rate to internal body temperature and metabolism.
And then, there are a group of animals called the “super hibernators”, super because the hibernation title must not be conferred to them in any manner whatsoever.
They look nothing like true hibernators and physiologically, their mode of hibernation bitterly contrast with that of the true hibernators.
They are members of the urasidae family and are generally referred to as bears. This article is all about the vastly contrasting but super riveting approach to winter hibernation in bears.
Hibernation in bears
Bears are “super” hibernators. For clarity sake, this means that something about their hibernation strikes great dissimilarity with those of true hibernators.
While most avid hibernators like squirrels and lemurs are naturally small and measly, bears are relatively large and compact creatures.
While true hibernators significantly suppress key physiological processes like internal body temperature and metabolic rate, bears maintain these activities at a relatively high level.
While true hibernators enter into a state of hibernation to avoid both food scarcity and the torment of the winter climate, bears strictly hibernate to overcome winter famine.
So bears are unlike true hibernators by a great margin. Many scientific literatures prefer to call the urasidae experience an “induced-torpor” or a “state of dormancy” rather than winter hibernation.
But we are not here to debate whether beats are hibernators or not and isolate facts from fallacies. In this article, all bears are hibernators and not just any kind of hibernators, but super hibernators for that matter.
How do bears hibernate?
In this section, we are going to discuss extensively on the bear hibernation process. You are going to learn how bears prepare for their winter hibernation, how long they hibernate for and the period when they emerge out from this dormant state.
Bears prepare to enter into hibernation with the onset of external cues— an internal monitoring system identifies these set of external cues and begins to flood their systems with lethargy inducing chemicals called Hibernation indiction Trigger. This chemical basically pass on the crucial message to organs and many biological processes that they should begin turning down activities gradually because very soon it’ll be time for hibernation.
When climate begins to change, fishes start to decay, fruits become scarce, preys begin to migrate and trees gradually begin to bear a starking contrast to the understory directly underneath them, bears know through this genetic internal clock that it’s time for a long winter nap. It’s time to gather as much body fats as they can and excavate dens or look for them.
This simply means to develop extremely voracious appetite for food. It happens to all hibernating bears before the onset of winter.
Their appetite grows voraciously and they consume large amount of food: berries, fishes, or seals and deposit the energy as body fat. They grow extraordinarily large and rely solely on this fat reserve to derive energy and maintain hibernation conditions during their period of dormancy. This fat also craters for their normal water requirements when it metabolizes.
Denning homes for bears can be anything, so far it offers predatory shelter and shelter from environmental factors such as rain snow, and cold . Bears den in hollowed-out tree trunks, (no longer a viable option since trees are occasionally cut down in the forests), in high way culverts, under thick brambles, brushes, windfalls, caves, under-stories, or in excavated areas of hillsides or previously occupied dens.
Dens can be well insulated and retain heat better when dug out with small entrances and at an upwards angle rather than straight or downwards. The upward angle and small space significantly reduces heat loss to the outside surroundings while larger spaces and a downward slope allow heat to easily escape to the outside.
Females with cubs usually excavate the largest dens while pregnant females excavate dens at higher elevation (perhaps for the safety and protection of their newborns). Generally, bears try to dig dens that are only slightly larger them for better heat retention and maneuvering possibilities.
A typical bear den usually has a small opening or entrance, a short tunnel that terminates into a chamber, and the bedding chamber itself lined with heat retaining bedding materials like spruce bough and duff. What bears choose to line their chambers with often depends on what is available at the denning site.
Building dens is an arduous process for bears and it can take anywhere from 3 to 7 days. Some bears begin excavating dens months before the onset of winter while others prefer to start at the dying minute.
Polar bears that live in the arctic regions dig dens in the snow. Other Northern bears like grizzly’s and black bears choose areas with sandy loam soil, clay loam soil or rocky silt soils.
Entry into dens
Entry into dens correlates with weather and snow conditions. This happens around late October to mid November; when situations are being to turn frenetic and cold.
Entry can also be affected by latitude, where bears located in the norther latitudes den earlier than those located in the southern latitudes.
This still falls back to the first point since the bears living in the more northern latitudes experience the savage conditions of winter earlier and more severely than those occupying the more southern positions on the lines of latitudes.
Few weeks before den entry, bears often retire back to their denning area and spend most of their time there resting and sleeping.
The chemical released into their systems must have acted greatly upon many organs and processes (as at then) to suppress physiological activities and thus render the bears lethargic and lazy.
They avoid foraging, as they have done so extensively the previous weeks and have developed enough body fat reserves that can last them throughout winter.
Pregnant females usually enter into their dens first, followed by females with cubs, lone females, subadults and lastly adult males.
Trials and tracks left by bears are often covered by fresh snow fall. This effectively conceals the location of their dens for the rest of winter.
Hibernating in dens
Bears begin experiencing periods of reduced physiological activities right before denning, around few weeks or so. They gradually become lethargic and loose interest for food and water. When bears finally den, their physiological activities reduce even further but not instantly. It may take several months for these phycological processes to hit their lowest point or activity, often around early January or so.
During hibernation, the metabolic rate of bears decreases to about 50- 60% the original value. Breathing reduces from a high 50 breaths per minute to low 4 to 5 breaths per minute. Heart rate also drops from 50 beats per minute in November to about half the value or less in January.
Bears also record a slight decrease in body temperature, often fluctuating 12F off the average value: at 100 to 101F. This low reduction in body temperature which greatly contrasts with most other hibernators is as a result of the huge body mass of the bear.
A significant amount of energy is required to rise the temperature of a body like theirs when they go full force and hit the near freezing conditions and similarly, a very long period of time and huge amount of energy would be required to take a normally heated body from 37 to near zero conditions or below.
Because of these restrictions, bears only maintain a slight fluctuations in their internal core temperature, and because of this, they are able to respond faster to threats than true hibernators mid-way hibernating. So don’t go into hibernating dens because you could still actually get mauled.
All these reduced physiological activities enable bears to feel the impact of hunger less avidly.
They live mainly off their fat reserves which substitutes in for their food and water requirements. While bears may occasionally rouse to bumble around and stretch their bodies during hibernation, they actually don’t forage, defecate, or urinate during these periods the way most other hibernators, the true ones do. Bears can effectively go upto 100 days without observing all this biological necessities.
Their butts develop plugs with keep foreign material out and feaces in. The urea present in their urines are recycled to make proteins which improves their lean-body mass. These proteins also allows them to maintain their muscle mass and organ tissues.
While hibernating, dens along with the lower surface area to body ratio and high insulating pelts of bears help keep them warm and cosy.
Bears remain in this on and off switching system for a whole season or until winter is over and temperatures warm up and food becomes available. This period coincides with early spring, from early to mid-march.
Hibernation in pregnant females
Pregnant female bears delay implantation of their embryo until they excavate and actually settle down to den.
Mating happens from may through July but actually implantation happens around early December. Cubs are often born around late January or early February while naked, blind and virtually helpless.
They suckle milk from their mothers teats while the mothers hibernate but rest on either of the sides of their bodies to expose their underside, but the actual birth happens when the mothers wake up for their periodic arousal on a frequent schedule.
After birth, cubs do not hibernate but rather stick close to their mothers underside to remain warm and cosy all the time. They sleep, wake and suckle milk from their hibernating mothers.
Cubs grow rapidly during denning and emerge out of their maternal dens weighing between 1.8 to 3.6 kg and able to wandering around with their mothers in search for food. They emerge out fully by not densely furred having little but sufficient fat reserves to serves as a manageable insulation against the biting cold.
Emerging out from dens
Bears emerge out from their dens around early spring. In polar bears, this coincides with the emergence of productive sea ice and the pupping season of seals; their primary preys.
The sea ice are the platforms where mother seals nurture and tend for their newly born pups, but are unfortunately the mediums used by polar bears to pounce on the family. In other bears, this coincides with the period where productive vegetation have sprung up in their habitats and waters have arrived. Berries and fishes are also now in abundance.
Male bears emerge out first, followed by solitary females, females with yearlings, and then finally females with newly born cubs.
All other bears with the exception of those with newly born cubs leave the vicinity of their dens within a week of emergence. New mothers leave only after a few more weeks.
Physiological extremes of hibernating bears
Hibernation in bears is truly remarkable and mysterious. Hibernating bears are able to sustain many physiological extremes that are deemed impossible or hugely detrimental to the human health. Below are some of these physiological extremes and their effects to the human health.
1) They get profoundly fat
Bears grow voracious appetite and accumulate a chunk fatty tissues before hibernation. It’s not possible for any human or animal to attempt such feat without suffering irreparable damage to the entire body. A biologically confounding feat native to only the bears and a few other animals.
2) The cholesterol problem and the mysterious solution
During hibernation and body fat metabolism, bears build cholesterol levels twice as high as the levels observed in most humans. And yet somehow, they are able to get away with this rise and not suffer any hardening of the arteries or gallstones as observed in human with higher levels of cholesterol.
This feat is made possible by a secretion from the bears liver that dissolves the gallstones that are formed.
3) Bone mass
Bears do not loose bone mass during hibernation. And they sit for several days on end without moving around in their dens. If humans and many other mammals were to try this, to maintain a non-weight bearing position for an extended period of time, they would end up with osteoporosis: a disease condition which cause weakening the bones.
Somehow again, bears are able to dodge this one by perhaps one or two secretion which could be useful in the treatment of weak bones when they eventually are discovered.