By Caitlin Kennedy
It’s a strange thought that for many months of the season, only a meter or so beneath your skis or snowboard lie a multitude of marmots lasting out the winter in the protection of their burrows. Now is approximately the middle of the big sleep (a misnomer that we’ll come to later), so we thought we’d plunge into this fascinating lifestyle choice that is actually very prevalent in the animal kingdom.
So what is hibernation? This deceptively simple question is actually incredibly controversial in the world of Ecology. You may well have heard that bears do not truly hibernate.
This is argued to be the case in certain scientific spheres because their body temperatures only drop a little in winter and it is still relatively easy to wake them from their state of reduced activity. In fact, female bears usually awaken to give birth and then return to their state of torpor, whilst her cubs nurse and wait for her to return to the land of the living. The general consensus these days is that the different forms of prolonged inactivity found in the animal kingdom exist on a spectrum and utilise similar physiological processes. For the purposes of this article, hibernation is a state of energy conservation which features reduced metabolic activity. Some animals have to regularly emerge from their state of rest to eat, to poo or to check for danger. Others are able to stay submerged in a coma-like state for upwards of 11 months, such as the pygmy possum which holds the record for longest hibernation at 367 days.
General changes that occur are:
A drop in heart rate to as little as 2.5% of the usual level. For example, a chipmunk’s heart rate slows to just five beats per minute down from 200 whilst awake.
Breathing reduces or stops entirely. Yup, some reptiles go their entire hibernation period without breathing at all!
Consciousness diminishes. This hugely varies between species but for some organisms, they are nearly impossible to wake up. If you did manage it, you’d effectively kill them as it would require so much energy to warm them up enough for normal wakeful bodily functions to resume that they would have no chance of making it through to spring.
Excretion is either reduced or ceases altogether. In order to stay alive, some processes must keep ticking over, which requires energy. This is achieved by burning body fat or by regularly coming out of torpor and tucking into stored food supplies. Some animals like bears use the urea that would normally be released as waste and turn it into its constituent amino acids in an incredibly effective recycling loop.
So how does hibernation work? Well the difficulty in answering that lies in the variety of reasons that animals hibernate. Marmots survive solely on body fat due to the scarcity of food available when their usual nosh is covered in snow.
Desert tortoises, crocodiles and salamanders go into a state of inactivity in summer, called aestivation rather than hibernation, which specifically refers to dozing through winter. For desert dwelling animals, conditions regularly become too hot to handle and so they burrow beneath the surface of the ground to stay cool and conserve water. The lungfish can hibernate in a waterproof mucus envelope if their lake dries up, breathing through a mucus tube. What these processes have in common though is that the organism responds to certain cues to trigger its state of reduced activity. In the case of our friends the marmots, they are responding to the external temperature- when it drops below a certain level on successive days, it’s time to start hibernating. And once warm enough outside, they wake up. This means that the length of hibernation is weather dependant. Other animals detect the photoperiod (length of day) or quantity of food supply, or have inbuilt circuannual rhythms, which don’t depend on external conditions at all.
Once in a state of inactivity, the process is mostly controlled by the endocrine or hormonal system. For mammals and birds, they have a special heat furnace in the form of brown adipose tissue (fat cells), which are able to burn fat directly in the mitochondria, a chemical reaction which produces heat directly. The alternative waking method of heat production is via shivering, but this is far too energy intensive.
In terms of brain activity, hibernation is actually more similar to wakefulness. Sleep is defined by very particular patterns of muscle and brain activity (eg REM (Rapid Eye Movement) sleep in mammals and birds). And in fact, upon ending their hibernation, most animals are in a state of severe sleep deprivation that requires long periods of sleep to recover from. The exception to this is the only primate that hibernates, Madagascar’s fat-tailed dwarf lemur. These animals experience long periods of REM sleep, and are therefore dreaming during their hibernation.
If you’re reading this in the hope that one day, you too will be able to lie dormant through winter (or summer if you’re a real powder hound), there’s good news and bad news. Research has been done to apply a state of dormancy to organs in order to preserve them prior to transplants, with reasonable success. But we’re a long way off putting humans into the level of inactivity predicted by Sci-Fi movies to get us into galaxies far, far away. In the meantime, spare a thought for the marmots and their sleepless slumber.