There’s no doubt fall can be beautiful and the first arrival of cold weather invigorating, but trees don’t lose their leaves because they want to. Energy is scarce in winter, and all living things have been shaped by the timeless evolutionary challenge of how to spend it wisely, even ourselves.
Unlike many, I can’t bring myself to fully embrace the fall season despite its obvious charms. We are fortunate enough to live on a large property surrounded by natural beauty, and there will be a week later this month when the trees lining our yard and the low hills beyond glimmer with an almost unearthly glow. The two trees that overshadow the patio in our backyard usually turn deep red then bright yellow before losing their leaves. The two that sit in the open stretch of lawn beyond our pool were relocated as saplings from South Carolina, but they hold their leaves a little longer, turning yellow first, adding perspective as the hills loom beyond, gentle and rounded, like shoulders rimming the landscape. There is a line of trees forming a little woods on the opposite side, each of which seems to choose a different color, adding up to a wide variety of yellows and golds, reds and oranges, even the hint of purple. The front yard also faces low hills that will seem alight with warm flame, as though we were cupped in two glowing hands. There are already signs of this process unfolding right now, like the turning of the leaves were a symphony of color and the conductor was teasing the opening notes for the audience. The result is undeniably beautiful, albeit short-lived. The same can be said for my enjoyment of the arrival of chillier weather. There is an invigoration to it for a while after the heat and humidity of a New Jersey summer, but one that fades almost as soon as the New Year begins and winter seems like it will last forever, as though the cold had such a grip, it might never let go.
Sometimes, I think to myself the trees don’t really want to lose their leaves and shed their bright green, energy-creating chlorophyll. They’d prefer to grow and thrive forever if they could, rather than come as close to death as living beings get and then undergo another cycle of resurrection. They do so because they have to: However glorious their greenery, the energy their leaves produce in the winter is less than that required to keep them green and glorious in the first place. What are known as deciduous trees lose their leaves every season to solve a classic evolutionary problem. At its core, evolution is about the management of energy. Energy is to evolution what money is to the economy. It is required for everything an organism does, every structure in its body and every action it may take, but energy, like money, can only be spent once. Organisms are therefore incredibly careful about what they spend their energy on. In the northern hemisphere, energy is abundant in the summer. The output of the sun remains equal throughout the year, but the angle at which sunlight hits the Earth varies. When the northern hemisphere is tilted towards the sun by approximately 23.5 degrees, the light is far more intense and contains far more energy. You can see this for yourself by pointing a flashlight at a sheet of paper. When you point it directly, the same amount of light occupies a smaller area, but if you point it at an angle the light is spread out and more diffuse. The same amount of light, but the energy varies significantly. The more concentrated light has less entropy, and is more usable by the chlorophyll in plants. They can produce more energy themselves via the essential process of photosynthesis.
As winter approaches, the lower intensity of the light prompts deciduous trees to quite literally kill their own leaves. The connection between a leaf and a branch is known as the abscission layer. This connection is strengthened by a hormone known as auxin. When the level of the hormone is reduced, the bond becomes weaker and eventually breaks, cutting the leaf off from the tree and preventing it from replenishing nutrients needed to keep it green and healthy. The dying of the leaf causes the changes in color, prompted by the dying of the summer sunlight. Conifer trees, however, never lose their leaves. Known as evergreens, this type of tree has evolved a waxy, pointed leaf more like a needle and it thrives in colder and more arid environments where energy and the other resources to support life are scarce. The waxy coating locks in the available moisture, and the shape allows it to cope with harsher conditions such as snow and ice. The leaves can remain active all year round, slowly but surely transforming the energy from any and all available sunlight. Evolution is all about trade-offs, however. There is no right or wrong answer to any given problem, only an answer that allows the species to propagate itself. In an optimal environment, the needle-like leaves generate less energy for the tree overall because they are not shaped broadly enough to capture as much sunlight, allowing the deciduous trees to grow larger and stronger in more suitable climates, outcompeting their conifer cousins.
Animals are as directly affected by the changes of the seasons as plants. They too need to solve the same energy problem. Black bears are fairly common in my area. Though I have not been fortunate enough to see one myself (my wife would probably say “unfortunate enough”), several were spotted in the neighborhood in the spring and one tried to raid our garbage can in the middle of the night. Black bears feed primarily on berries, fruit, sedges, and insects, none of which are prevalent in the winter. If they were to remain active, there would not be enough energy to survive and they would starve. They hide out the winter months and hibernate instead. Technically, hibernation is not the right term. Hibernation refers to an actual sleep, almost like a coma. Only a few mammals do it, such as woodchucks, ground squirrels, and bats. A woodchuck’s heart rate while active is about 80 beats per minute. While hibernating, it drops to 4-5 beats per minute, reducing energy consumption by nearly twenty fold in the heart alone. Black bears on the other hand enter a “torpor” or go through a process known as “denning.” This is differentiated from hibernation because the bear can be woken up, though their heart rate is reduced and their body temperature as well. They still manage to pass several months without eating, drinking, urinating, or defecating, losing up to 30% of their body mass in the process. Hence, they exit their torpor mad with hunger in the spring, when they have been known to devour deer and even attack dogs. Bears, squirrels, bats, and woodchucks are warm blooded mammals. They need to cope with the energy loss and the more challenging conditions, but cold-blooded animals, from reptiles and amphibians to insects, face yet another obstacle. They cannot regulate their body temperature, and are even more subject to the whims of the environment to the point where their blood can actually freeze. This is why reptiles are also expert hibernators. Snakes, turtles, and other species all sleep through the entire winter. Some amphibians like frogs actually do freeze. Insects cope with the change by having different stages in their lifestyle. The adult insects that bother us all summer long, crawling and flying everywhere you look, die. They lay their eggs first though, and persist in a larval stage under the earth or inside of trees where the temperature change is not as drastic, ready to emerge when the time is right.
Humans, of course, have long been able to exert some control over their environment. While most people tend to be more active in warm weather, we do not undergo any process as drastic as hibernation or torpor, much less shedding a portion of ourselves or outright dying. We heat our homes and wear warm clothes, huddled up more so than the summer, but more or less following our usual routine. This should not imply that our minds and bodies have been shaped any less by the need to economize energy. In fact, almost everything that makes us human has its root cause in an energy related trade off, starting with our brains. Compared to other animals, our brains are massive for our body size. This gives us plenty of neurons to support our higher level intellectual functions, enabling speech, reading, writing, creativity, and analytical thought, but it comes at a massive cost. About 20% of the energy we use goes directly towards sustaining our massive brains. These brains, however, come with a means to cheat the system, solving the evolutionary energy challenge in a novel way reserved almost exclusively for humans: We make stuff with our hands rather than having to produce it directly via our bodies. Fur is a classic example. Most mammals enjoy a rich coat that protects them from the elements far better than human skin ever could. A husky can play outside for hours in the winter naked, even run dog-sled races for days when a human would freeze to death without special gear. Fur, however, is very expensive energy wise to grow and maintain. Cats, for example, spend anywhere between 15% to 50% of their time simply grooming themselves, much less the cost of actually producing the coat. Imagine if you had to spend half your waking hours in the shower. Early humans avoided most of these costs, outsourcing it to warmer clothing, freeing up calories for usage in our brains. The same is true of our teeth and muscles, which are generally significantly weaker than our animal counterparts. A chimpanzee, for example, is our closest cousin. They are estimated to be between three and five times stronger. Each individual muscle can produce more force and power a factor of 1.35. We invented weapons to maintain our ability to hunt while shedding some of this costly muscle mass.
This principle applies inside our bodies as well. Every protein produced by our genetic machinery requires energy. If the body produces one protein, it must do so at the cost of another. A dog’s sense of smell is legendary, orders of magnitude better than ours. The reason why goes back to our genes. A dogs’ body produces 300 million receptors in their noses, compared to a measly 6 million in humans. Further, the smells we can detect and discern are based on dedicated molecules known as olfactory receptors. There is an olfactory receptor for each odor (or small group of odors) we can experience. Humans and dogs both have somewhere over 900 olfactory receptors in principle and many of them are in fact shared between our distinct species, but about 63% of ours are not used at all compared to a mere 18% for a canine. The term to describe this is a pseudogene. The blueprint for the molecule remains in our bodies, but it is currently in the “off” position and never activated because that activation costs energy, and we have evolved to use that energy for something else. The presence of these shared olfactory receptors is also evidence of the shared history of all mammals. It is believed that humans and dogs inherited these genes from the same common ancestor, and many of them are shared across a wide range of mammals. Mice, for example, have a similar set. It is only humans, however, who don’t need them anymore and choose to “spend” the energy somewhere else. In fact, perhaps the only thing humans and our primate cousins don’t scrimp on in this regard is our eyesight. Dogs have two genes that produce color receptors, similar to the olfactory receptors. We have three, giving us better color vision, suggesting that humans relied on sight more so than any other senses. Primates originally started with two like most mammals, but somewhere fairly recent in our evolutionary history, the need to hunt in trees put evolutionary pressure on better color differentiation in the forest. In other words, the additional expenditure of energy to produce three color-sensitive molecules instead of two was worth it. We are not special in this regard: Birds and fish have four unique color receptors.
Ultimately, all life, whether plant, animal, or otherwise, makes Ebenezer Scrooge seem like a big spender. Humans are the only creature in the history of the known universe with access to a surplus of energy, and the rapid rise of obesity suggests we are ill equipped to cope with it. Evolution has never had to solve the too much energy problem before, only too little. Either way, the arrival of fall means less energy for the entire world in my neck of the woods and life of all kinds is busy adapting to the new, harsher conditions. There is beauty there and loss as well, even knowing spring and summer will come again.