Winter always ends up coming… And this year, because of the energy crisis, our interiors will be confined to a reasonable 19 degrees Celsius. The unpacking of thick sweaters, soft socks and other gloves has therefore begun… Because being cold is unpleasant – but that is the least of its faults.
Above all, the cold is a real threat to our body. Our vital organs need a temperature of 36,8°C (precisely) to function optimally. Too cool an atmosphere and they may simply stop. A study conducted in 18 French cities between 2000 and 2010 showed that the number of deaths due to cold was three times higher than that due to heat.
Our species, however, is not entirely helpless against frost. What are its strategies in the face of freezing temperatures? And why don't we hibernate to get through the winter like other mammals do?
The technique of the internal boiler
Mammals and birds are endotherms, i.e. "warm-blooded" creatures. The average body temperature of the former is between 36 and 39,5°C, while in the latter it can reach 42 degrees. Features that are the result of a long evolution.
Some 200 million years ago, animals that survived the devastating so-called Permian-Triassic mass extinction had to readapt to a new competitive environment. Back then, the mighty dinosaurs still ruled the planet Earth. They depended mainly on thermal energy from the sun to be active. But gradually, species appeared which adopted new survival strategies.
Brasiltherium riograndensis, one of the first animals that lived at the end of the Triassic and exhibited mammalian characteristics.
Smokeybjb, from Wikimedia
The first mammals were nothing more than small furry quadrupeds (tetrapods) living in the shadow of giant reptiles. Literally. Their chances of survival were much better if they opted for night hunting. In the absence of an external heat source, mechanisms to generate clean body heat capable of continuously fueling their metabolism were selected and developed. A strategy that is also profitable in the unstable climate of the time.
But endothermy has a downside: to power the boiler, fuel is needed and, as we are now realizing, this has a cost. To meet this demand, a wide range of efficient and energy-efficient adaptations have emerged as a result of natural selection. They can fall into two main categories: better store and use energy, and better insulate to prevent unwanted heat loss.
Fat (and hair) is life
We already have a weapon always at hand: fat! It serves both as an insulator (due to its low thermal conductivity) and as an energy resource. Mammals that survive in cold regions or near arctic seas possess a special fat-like tissue called blubber or panniculus. It measures 10 cm under the skin of the polar bear and can reach 50 cm in the bowhead whale!
bowhead whales (Balaena mysticetus) have, under the skin, a fat-like tissue called lard, the thickness of which can reach half a meter. It is essential to reduce heat loss in the icy waters of the Arctic.
Bering Land Bridge National Preserve/Flickr
Many small mammals, including newborn humans, have another type of fat, called "brown fat". Its particularity is that it is able to burn lipids to generate heat (thanks to the mitochondria it contains) in order to maintain body temperature. This is especially important in babies, due to their small size and lack of muscle power to convert movement into energy.
But we are also equipped, somewhat unwittingly, with millions of sensors that are ultra-sensitive to ambient temperature and integrated into our skin. The slightest detected risk of body temperature deviating from its optimum immediately triggers an alarm in the hypothalamus, the region of the brain where temperature control resides. Reflexes that generate more heat and reduce heat loss are activated – often involuntarily. Basically, our body takes control to save our lives!
The anatomy of a hair. Attached to the root, there is a small muscle that can straighten it when it is cold.
OpenStax College, from Wikimedia
Mammals have a structure that is unique in the entire animal kingdom and infinitely precious: the… hair! When it's cold, a small muscle attached to its root pulls it up (the “piloerection”). Like an army, thousands of hairs then stand up and block the airflow together to create an insulating layer. A phenomenon trivially called “goosebumps”. Which hardly pays homage to this ancestral reflex, also controlled by primitive regions of our brain dealing with emotions and danger. Which explains why it also activates when we are taken by a particularly strong movie scene… or when cats intend to show that they are ready to fight.
In this little game, not all animals are equal. The smaller the body, the higher the surface area to volume ratio, and the faster the heat loss. This is the reason for this adorable little down that covers many small newborn mammals: it is necessary, at all costs, to limit the loss of energy.
Another technique: shivering, the involuntary contraction of muscles to generate heat through movement. The shivering increases the metabolic rate approximately fivefold. For this, the muscles burn the fats, proteins and sugars available, the "coal" of the energy factories of our cells (the mitochondria).
The objective is, whatever the cost, to protect the vital organs – heart, brain… In the event of extreme cold, the flow of warm blood to the hands and feet can be cut off and redirected to them. Hence bluish fingers and lips, numbness and ominous white skin.
Loss of body heat in cold weather visualized by a thermal camera.
What about hibernation?
To avoid these risks and inconveniences, some opt for flight. Migratory birds, mammals such as reindeer and moose temporarily leave areas that have become too cold and inhospitable. But not everyone can cover such distances, sometimes on the scale of entire continents. In harsh winter conditions and in the event of a lack of resources, others therefore choose to temporarily reduce their metabolism.
This ability is widespread and found in many orders of mammals, proof that it is an effective survival strategy. But depending on the size of the animal, the cursor is more or less pushed.
The common marmot (Marmota monax) is a mammal capable of hibernation.
Marmots, hedgehogs... don't take half measures and go into hibernation. The phenomenon is striking and is characterized by an almost complete cessation of the vital functions of the organism and a real lethargy (their metabolism can drop by almost 98%). In the marmot, for example, the heart then beats only five times per minute – compared to 80 in normal conditions – and the temperature can drop to 5°C… Enough to give chills! The technique is not without risk, as some animals no longer have enough energy when they wake up to revive their organism.
After 7 months of hibernation, Vancouver Island marmots (Marmota vancouverensis) lost a third of their body weight.
In the bear, on the other hand, if there is indeed a torpor that sets in, it is less deep (its metabolism is reduced by up to 75%) and is discontinuous – we then speak rather of hibernation. . The animal takes shelter, but its temperature does not drop as much and it is still able to wake up.
And our species? Although the genes that underlie this ability are probably still present in us, we are no longer really capable of it and the reason for this is not very clear… question has been raised for our extinct neanderthal cousin. In 2020, a study suggested that, to get through the long cold months in a calorie-starved environment, the bones of a population in southern Spain would have shown signs of slowing down their metabolism.
It must be recognized that, in general, the physiology of hibernation is still poorly understood. Mammals seem to have to wake up from time to time, but we don't know why. Studies showed that hibernation is different from coma, anesthesia and normal sleep – the brain is in a kind of ill-defined deep sleep. Even though the brain appears to be dormant, certain populations of nerve cells are still active and can respond to certain stimuli.
As for the mechanisms that allow the animal to enter and exit this phase, they are also unknown. So that even if one day we manage to put humans in hibernation (called artificial or synthetic), how long before waking them up? And how ?
The applications could however times be numerous. Some think of space travel. If we ever have to go to Mars, a state of hibernation could drastically reduce the energy and therefore food requirements during the trip – and the waiting time… More unexpectedly, animal tests have shown that it can partially protect the body from the harmful effects of space radiation.
Medicine would also be interested. Being able to reduce the metabolism of life-threatening patients (advanced cancer, etc.) could extend the period during which it is possible to fight the disease.
An adaptation over the generations…
Today we have learned to deal with the cold in another way: We wear clothes, build houses that we heat, etc. Over the millennia, we have even lost most of our precious body hair, earning us the nickname "naked ape".
Apart from protective clothing, Inuit living in cold regions have a short stature, small hands and a rather flat face with typical fatty pads at the sinuses.
Ansgar Walk, Wikimedia
And in the less clement latitudes, our species has seen selected, over a long period of time, traits granting better resistance to cold. This is the case in populations that live in cold climates, such as Siberia or the Himalayas. They often have short arms and legs, a stockier physique and more body fat (to better retain their internal heat), flat faces with protective fat pads at the sinuses, and flat noses (more relevant in a icy air). In addition, specific versions of certain genes, linked to a higher metabolism or increased blood pressure, are found more frequently among these peoples of the Far North.
Either the same type of adaptations that the powerful Neanderthals showed to withstand the cold climates of Europe in the Paleolithic.
However, let's not deny it: to protect yourself from the cold and its dangers, having the chance to wrap yourself in warm clothes while drinking hot chocolate around a good fire is a very good option. Better still, as migratory people of a new kind, we now have the luxury of flying to warmer places whenever we want.
But not everyone has these possibilities… So never forget to pay special attention to those most vulnerable to the cold, homeless, elderly or toddlers… you could save lives.
Pieter Vancamp, Post doctoral, National Museum of Natural History (MNHN)
This article is republished from The Conversation under Creative Commons license. Read theoriginal article.