A new year – fresh hope! Spring is on its way, but it’s cold right now, and the trees are bare. How on Earth do living things cope with these harsh conditions- the little birds and delicate plants? And what about us human beings: how has Nature helped us cope? As we can see by considering the animals we know, various strategies have emerged through evolution to help life survive the cold. The simplest is, of course: just get away to somewhere warmer!
Figure 1 Preparing for migration
Migration
Birds are driven to migrate, not so much to avoid the cold but to seek better sources of food. They do it in all manner of ways. Many, but not all, move in flocks – it can save energy. They often follow a kind of ‘flyway’, using geographical features such as mountain ranges or rivers to guide them. Some fly high – even over the Himalayas; others keep low – seabirds over water, for example. Some fly at night to maximise feeding time during the day. and to avoid predators.
For many species, the timing of migration is genetically programmed and is triggered by hormonal changes linked to the length of the day. Changes in temperature can be a cue for some species. Climate change is beginning to upset established distribution patterns, so local bird populations are likely change as average temperatures rise.
The feats of navigation are remarkable. Some species use the angle or position of the Sun, others sense the Earth’s magnetic field and many seem to retain a map of particular territories where they have been before. A chemical pigment in the beak of some birds has been found to respond to the strength of the magnetic field – giving the bird the ability to ‘see’ a magnetic map. There’s more on this in the blog: Singing and navigating – the extraordinary feats of ordinary birds .
Like other warm-blooded animals, birds are sensitive to the cold. For those that stay at home it‘s a matter of coping with the cold. They may fluff up the feathers, to incorporate pockets of air which act as an insulator, slowing down the rate at which heat is lost. A good bout of shivering helps generate a bit of heat, to maintain body temperature. Finding a cosy hole or niche to snuggle down in is another avian strategy.
Figure 2 European Robin
If you can’t get away for the winter, and shivering or sheltering are not enough, what’s the alternative? Shutting down altogether and waiting for Spring, is one option.
Hibernation
If food is short and you need energy to keep your temperature up, but have no wings to fly away, winter is a serious threat. For some species a ‘go slow’ policy is the best option. Cold-blooded animals, lacking an internal heating system, just have to do it – that’s how snakes, frogs and fish cope. Butterflies and other kinds of insect also hibernate. In the UK one or two mammals also do it – notably bats, hedgehogs and dormice.
Hibernation is not simply a kind of deep sleep. In sleep, normal temperature and metabolic rate are maintained, whereas for hibernation the heart rate, metabolism and breathing all slow down dramatically, and body temperature falls.
A dormouse, for example will cool down to the ambient temperature, typically, just a few degrees Celsius, and its breathing and heart rates can drop by as much as 90%.
Figure 3 hibernating dormouse.
To get through this dormancy period, the animal’s body needs to keep going; the chemical processes that constitute its metabolism have to be maintained, albeit at a much-reduced rate. To cope with the continuing low-level energy demands over the winter months, hibernators need an energy storage strategy. The larger species eat large amounts of food in advance, storing the energy in the form of fat within their bodies. Smaller ones may hide or ‘cache’ their food, presumably waking up periodically to eat it. Remarkably, some species, notably black bears, use the opportunity of hibernation to become pregnant, ensuring their offspring enjoy a warm and cosy early environment, thanks to the extra fat.
It was once thought that birds hibernate too. Even the renowned, eighteenth-century naturalist Gilbert White refused to believe swallows would actually leave their homeland in winter. It was not until the nineteenth century that scientific observations proved they did. Birds are able, however, to enter a state known as ‘torpor’ in which their body temperature and metabolic rate are significantly reduced. The do this from time to time, at night for example, in order to conserve energy. It has even been suggested that inducing a state of ‘torpor’ in human beings might be a way of getting astronauts all the way to Mars!
So, animals that aren’t able to fly off to warmer climes can at least get through winter by dramatically reducing their use of energy – moving to a cosy niche helps. But what about those living things that can’t even move to a sheltered spot nearby – trees and plants, in particular.
Plants
Not only are plants unable to move away for any cold spot they happen to be in, they also have an entirely different way of maintaining life than animals. With no lungs to take in air, heart to circulate blood or stomach to absorb food, they get their energy in a very different way – a more direct way.
With energy flooding the Earth all day long from the Sun, plants offer up their green surfaces to capture it. Molecules of the pigment chlorophyll abound in the cells of plants. The structure of these molecules enables them to absorb energy and use it to create energy-rich carbohydrate molecules out of the carbon dioxide in the air and water in the ground.
Figure 4 Photosynthesis
Figure 5 model of a chlorophyll molecule.
(Carbon atoms black; nitrogen blue; oxygen red; hydrogen white; green magnesium).
This works well so long as water flows into the cells, to enable the vital carbohydrate-producing reaction to take place. As we know when water is scarce, plants can dry up and die. In winter the threat to plant life comes, not from the absence of H2O, but from its transformation to solid ice when the temperature falls below zero Celsius. Water ceases to flow and, worse still, expands slightly as it freezes in the cells. This may ruptures cells, potentially causing the plant to die.
Evolution has brought about various solutions to this problem. Perennials have evolved strategies to survive the winter. They may develop tubers, bulbs and rhizomes while its warm, to store up nutrients that provide energy during the lean months.
Figure 6. rhizomes of Jerusalem artichokes
To ward off the threat of destruction by ice formation, they carry sugar and alcohol molecules, dissolved in their watery parts which act as a kind of antifreeze, preventing the H2O molecules from linking together and crystallizing.
For a different kind of plant – ‘annuals’ – the individual dies but the line continues. By making seeds, these plants cunningly make sure their all-important genes are spread far and wide before they die. Encased in a tough outer coat, capable of withstanding all manner of brutal weather, the genes survive, though the plant doesn’t.
Ingenious ways have evolved to get their offspring growing away from, but not too far from, the parent plant. Some are shaped to blow away in in the wind (think dandelion); others to be eaten and excreted by animals; others to flow away on rivers (coconuts) and most remarkably, are fired explosively like a cannonball (Himalayan balsam). Further explanation is given in the blog The perennial question of sex . Seeds remind us that evolution is about the survival and proliferation of genes fitted to the environment rather than the individuals who carry the genes! You could say, we and other living things, are just handy vehicles for genes to get themselves spread around.
Already in January, early signs of leaf formation may be seen on some plants. During the Autumn, following the shortening of the day and decline in average temperatures, plants harden their buds to protect the growing tissue underneath.
Figure 7 Leaf bud
Complex systems of hormones and enzymes, in effect keep track of the hours of daylight and average temperature in order to gauge when to start forming leaves. A tally of the number of cold days that have passed is also kept, and each species requires a minimum number to have passed. This ensures against a plant budding too early on a freak warm day. This is why plants from warmer climates don’t do well in cooler ones – they expect fewer cold days to have passed before risking moving into leaf. As a result, they may come out too soon and get blasted by a subsequent frost. So, yes – plants calculate!
As we’ve seen, evolution has equipped living things with many ingenious strategies for coping with the low temperatures and scarce food supplies of winter. Is this inevitable? Do we really need seasons? Are they part of some Grand Design? Are they essential for life?
Why do we have seasons?
Disappointing for creationists, I know, but it’s just one more aspect of the arbitrariness of celestial mechanics! At some point in Earth’s history, some gigantic chunk of rock smashed into our planet at enormous speed. The collision caused huge pieces of the Earth to fly off in fragments, one of which is thought to have become our Moon. The impact also threw the planet off its ‘upright’ orientation, causing it to tilt.
The axis about which Earth spins makes an angle of approximately 23 degrees with the plane of the rest of the solar system (though it does vary very slightly and slowly over the centuries).
Figure 8 Tilt of the Earth.
The axis of the Earth remains steadfastly pointing in the same direction all the time as it proceeds around the Sun. This means at one point in the year, the top half of the globe gets more overhead sunlight (summer) and the lower half sees the Sun at a more glancing angle (winter).
Figure 9 How sunlight falls on a tilted Earth over a year.
The opposite occurs half a year later, when the Earth has got halfway around its orbit of the Sun.
So, warmer and cooler seasons alternate in the Northern and Southern hemispheres, just because the Earth is tilted. However lifeforms began on Earth, they have had to evolve over the aeons, to take account of these seasonal variations. Hibernation, migration, tubers, and leaves – not to mention fur, hair, stored fat and sweat glands – all owe their origin to the accidental tilt of planet Earth. Come to think of it, the very nature of human beings – with their brain-powered ability to find shelter, make clothes and grow food – owes much to this accidental tilt.
Advice for humans
Given the inescapable logic of the seasons and Nature’s adaptation to them, what can be said to help us humans get through the winter? What do the medical and psychological sciences advise?
First, is the question of light: there’s a lot less of it in winter. We commonly think of dull days and early nightfall as cause for gloomy thoughts; indeed, the incidence of depressive episodes is known to rise during the winter months. The term Seasonal Affective Disorder (SAD) is used to characterise the more extreme version of this condition.
SAD is a type of seasonal depression whose symptoms can include sustained anxious mood; feelings of hopelessness or pessimism; loss of interest in activities; reduced energy; difficulty in making decisions and several others. Shorter daylight hours may lead to decreased levels of the brain chemical serotonin, which helps regulate mood. Changes in levels of serotonin and another body chemical, melatonin, can disrupt normal daily rhythms, affecting sleep, mood, and behaviour. Where these symptoms persist, recommended treatments include: sitting in a very bright light for a half an hour or taking vitamin D supplements (to stimulate melatonin production), as well as the various medical and psychotherapeutic approaches for depression.
Basic advice for coping with ‘winter blues’ is consistent across many organisations promoting health and wellbeing. Getting out and about in the daylight hours helps. The physical act of walking or cycling helps raise mood, and being in natural surroundings, such as woods, meadows and parks, adds to this. Engaging socially with others and being affirmative about oneself can also give a psychological boost. Here’s what the British Association for Counselling and Psychotherapy suggests: Winter wellbeing and mental health tips (bacp.co.uk).
Conclusion
The challenges of the winter months affect all who live far from the equator. It’s often cold, and sometimes icy. Homes are harder to heat and transport liable to disruption. The days are short, and the natural world has lost its colour and vigour. But all is not as it seems; beneath the frosty ground and on the bare branches, the unceasing cycle of growth continues, as plants store up their nutrients and prepare their leaf buds. Our ability to concentrate is actually sharpened by the shifting daylight hours and, statistically, average libidinal levels are raised (attested by condom sales and Autumn births).
There’s even a view that evolution may have favoured lethargy and mild depression in the winter months, as a specific advantage at the time of our early ancestors. Both conditions would have tended to reduce physical activity, helping us conserve energy at a time when food sources were scarce. This may be of little practical relevance today, when food is available all year round, but it strikes me as rather comforting to think of low mood as a positive asset rather than a personal failing!
Whatever we feel about winter as individuals, it always lifts the spirit to think that the days are lengthening and unfurling of the leaves in Spring is just around the corner.
© Andrew Morris 14th January 2024
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