by Geoffrey Carpentier
In my recent columns, we’ve been exploring how birds deal with bitter-cold weather. Let’s next look at how some mammals face the challenges of winter. So, pausing for a second - this was supposed to be an ‘easy’ column to write, as I was simply going to talk about fur and denning options. Then I started to explore all the different terms which are used to describe how mammals “sleep” during the winter. I thought, maybe I should spend some time explaining those terms before I get around to how other mammals stay active all winter. To complicate it a bit more, I’m going to throw reptiles and insects into the mix, for comparative purposes!
The most widely known and understood term is hibernation which deals with situations where the mammal goes into a temporary deep sleep. This can be a months long process, where huge amounts of body weight and fat are lost, just keeping the animal alive. It is a stage when the heart rate, respiration and metabolic activity drop drastically, and the core body temperature is maintained, at a level just above a life-sustaining plateau. Before the animals enter this stage of their lives, they must gorge themselves for weeks, putting on fat, to tide them over through this period of deprivation. When we think of hibernation, we usually think of bears as the most well-known hibernating mammals – wrong! Bears don’t hibernate – they go into a state of torpor. Chipmunks, some frogs, many snakes, some pollinators (e.g. queen bumblebees) and bats generally hibernate.
One example, of how extreme these body processes are, can be demonstrated by the fact, chipmunks reduce their heart rate from the usual 350 beats per minute to an incredible four beats per minute, during hibernation! Note, if a hibernating animal is disturbed during this period, it may be fatal, as it may not have sufficient energy reserves to maintain its needs, if awakened.
So, back to the bears, which don’t hibernate but enter a stage of torpor. This is different than hibernation, in the sense, the animals go into a state of thermoregulation, where the focus of survival is to maintain the body temperature and physical processes at a critically low level. Animals in this state can be awakened and often do so, even if they don’t leave their dens. They can still forage, if necessary.
Torpor may be short or long-lived but it is never as long as true hibernation. In bears, for example, they give birth, while in the winter den, and, produce milk and allow the young bears to suckle. This would never happen in hibernating animals. Bears can sleep more than 100 days without eating, drinking or passing waste, and they can turn their pee into protein!
Many birds, daily, enter a state of torpor, to conserve energy or deal with cold (e.g. some hummingbirds). Reptiles and amphibians enter a state called brumation which, again, is very similar, but here, the animal goes into a state where body processes are halted or suppressed temporarily. Cold is one stimulus but changes in light intensity can also trigger brumation, lasting a night, many nights or even weeks. Unlike hibernating animals, brumating ones need to find water, so will periodically emerge from this state and seek water, but not necessarily food.
The simplest way to differentiate between torpor and hibernation is to think of hibernation as a voluntary process, in response to hormonal changes and daylight reduction. In this state, the animal cannot awake quickly and consumes a great deal of energy when forced to do so, while torpor is involuntary, and the animal easily can awaken and forage, as the need or opportunity allows.
Then there is ‘estivation’ which is similar to all of these but is generally entered into in response to hot or dry periods, when survival becomes more challenging. Some turtles, frogs, earthworms, and even, occasionally, some desert-dwelling mammals can use estivation as a survival technique. Goodness, there are still more of these terms (e.g. stupor, dormancy, cryptobiosis and diapause) but perhaps that’s enough for now! Confusing!
Geoff Carpentier is a published author, expedition guide and environmental consultant. Visit Geoff on-line on LinkedIn, Facebook and Instagram.
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