Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)
FIRE AND ICE
Microscopic life evolved some 3.5 billion years ago in the Precambrian period during the first and longest chapter of life that covers about 90 percent of geological time. No one knows exactly what the earth was like when microbial life began, but we do know that at some time the earth was a hot and hellish place with an atmosphere that lacked oxygen. Early microbes, probably blue-green algae or bacterialike organisms, invented photosynthesis to harness sunlight as a source of energy. They took carbon dioxide out of the air as their food, and they generated oxygen as a waste product that further transformed the atmosphere and hence the climate. They developed DNA for storing information, invented sex, which produced variation for natural selection, and evolution took off on its unending and largely unpredictable course.
Molecular fingerprinting suggests that every life-form on earth today originated from the same bacterialike ancestor. That ancestor eventually led to the three main surviving branches of life, the archaea, bacteria, and the eukaryotes (the organisms made of cells with a nucleus that include algae, plants, fungi, and animals).
Remnants of the first ancient pre-oxygen-using life may still exist little-changed today. They are thought to be sulphur-consuming bacteria now living only in the few remaining places where the ancient and to us hellish conditions still remain. These habitats include hot springs and deep oceanic thermal vents where water at 300°C (that stays liquid there rather than turning to steam because it is under intense pressure in depths of some 3,600 meters) issues up from the ocean floor. One of the species living at the edge of these hot water vents is Pyrolobus fumarii, which can’t grow unless heated to at least 90°C, and which it tolerates 113°C. As the earth cooled new environments became available and new single-celled and then multicelled organisms evolved from these or similar species to invade ever-new and cooler environments.
Some cells much later also escaped their ancestral conditions by invading other cells, finding that environment conducive for survival and adapting to it. Such initially parasitic organisms ultimately evolved into cooperative or symbiotic relationships with their hosts. Perhaps the most fateful of these eventually mutually beneficial associations occurred when some Precambrian green algae successfully grew inside other cells, to ultimately become chloroplasts, while their hosts then became green plants.
The ability to capture solar energy that ushered in the multicellular life and the fantastic diversity of life we see today was followed by or concurrent with one other critical parasitic-turned-symbiotic cellular invasion. The availability of oxygen from plants led to energy and oxygen-guzzling bacteria, and when some of these invaded other cells they became mitochondria and their hosts became animals.
Mitochondria are the cell’s source of power or energy-use, and having mitochondria with access to oxygen allowed vastly greater rates of energy expenditure. It made the evolution of multicellular animals possible. One of the ultimate expressions of the high-energy way of life that is powered by the use of mitochondria is, of course, animals like the kinglets that maintain a liveness at an, to us, almost unimaginably high and sustained rate through a northern winter.
The metabolic fires generated by the mitochondria can be fanned to run on high, given the availability of much oxygen, or they may be turned down low. Life is the process that harnesses, and more importantly, controls that fire. It produces heat, and heat is often synonymous with life.
Temperature is, to us, a sensation measured on a scale of hot to cold. Physically, it is molecular motion, and we can measure it with a thermometer because the greater the motion of the molecules of a substance, say mercury, the farther apart they are spaced. We measure this molecular expansion as mercury (or some other liquid) in a column is displaced up a calibrated scale. The molecular motion, as such, is not life but a prerequisite for it.
Heat, on the other hand, is the energy that goes in or out of the system to change temperature. Some substances must absorb more energy (from the sun for example) before their molecules are set into motion, raising the temperature. One calorie is the unit of energy defined to raise one gram of water one degree Celsius. Substances, like rock, heat up with much less energy than that required to heat water. Again, energy is not life, but a prerequisite for it, and life is insatiable for it. What is truly miraculous, therefore, is that life continues and even thrives in winter, when the sun is low.
There is no upper limit of temperature. Within our solar system, the surface temperature of the sun is about 6, 000°C; the center is about 3, 000 times higher, or 18, 000, 000°C. The lower temperature limit in the universe, on the other hand, is finite. It’s the point at which all molecular motion stops and the heat energy content is zero. That temperature precludes living, but from adaptations to the winter world that I will discuss, it need not destroy life. Life can, at least theoretically, persist on hold at the lowest temperature in the universe.
Our Celsius scale is defined as a 100-arbitrary-unit division of heat energy content of water, between when water molecules leave the crystal structure to become liquid (0°C) and 100°C when the liquid water boils at sea level. The zero energy content of matter, or lowest temperature limit in the universe, is defined as 0°K on the Kelvin scale and it corresponds to -273.15°C or -459.7° on the Fahrenheit scale. Since life as we know it is water-based, the active cellular life that most of us are familiar with is restricted to the very narrow temperature range between the freezing and boiling points of water (which vary somewhat depending on pressure and presence of dissolved solutes) where the controlled rates of energy use become possible. We are composed mostly of water, and when the water in our cells freezes, i.e., turns into ice, it shreds membranes and kills.
Water influences life as profoundly at the ecological level as it does at the cellular level. Every fall in the North Temperate Zone we can observe the ecological effects of the various physical properties of water. Most of the creatures of the earth experience water as a transparent liquid that runs downhill and that can only be contained by barriers. For part of the year some of us also see water as a white powdery matter that sticks to the trees and the sides of the hills and that makes the woods look like a fairyland. This substance can be stacked into piles, tunneled into, and made into dwellings for man and beast. It can accumulate and become so dense and deep that we can’t walk through it. It can shut out the light to plants and may crush them. In northern areas, when the tilt of the earth is appropriate, it may collect over long periods of time to create glaciers that transform the landscape, grinding down mountains and valleys. With a difference of just one degree Celsius, or less, water also can become a clear, glasslike substance that seals over the surface of lakes and allows us to walk across them with impunity.
The fate of almost everything in the winter world is ultimately determined by the crystallization of water. In a matter of a few hours that crystallization can change the physical surface of the earth, and in the course of millions of years it has profoundly changed the physiological, morphological, and behavioral characterizations of all organisms that have to contend with that magic transformation of liquid to crystal.
Every fall the winter world creeps up gradually and inexorably onto those inhabiting the Northern Hemisphere. As it does, the nights get progressively longer and colder. Less energy from the sun reaches the ground. First the water in topsoil freezes to form a solid cover (unless it’s already snow-covered). The swiftest-flowing streams and brooks are the last to freeze over because the cold air-water interface constantly mixes. The cold that causes the water to freeze comes from the air just above the water. The water is at least slightly warmer. When the water is stirred (as in swift-flowing streams), the surface doesn’t cool down to 0°C so quickly.
One night the inevitable happens: the bodies of water freeze solid. The temperature drops enough for water molecules on marsh grass stems, twigs, and leaves floating at pond’s edge to slow their molecular momentum enough that they drop into stable crystalline positions. The stems, twigs, and leaves then serve as nucleation sites for ice-crystal formation. Like billiard balls rolling into pockets, the water molecules lock into position, first indiscriminately on any object they encounter, then on other molecules that have come to rest, forming an ice lattice. What little energy these molecules had left is now released as heat, the heat of fusion, 76.7 calories for every gram of liquid water turned to ice. (This heat is not enough to cause any appreciable temperature rise in the pond or lake because it is so quickly absorbed by the much larger mass of water. However, the sudden freezing of a small droplet isolated from others often causes an appreciable “exotherm” of several degrees Celsius.)
The ice crystals being formed reach out like sharp fingers over the surface of the water. They meet, interlock, and by morning the whole pond may be covered with a transparent pane of ice that physically separates the water denizens from those on land. Quite overnight, one can literally walk on water, a capacity obviously less a function of supernatural abilities than of the physical properties of water at temperatures below 0°C.
There is something quite remarkable, simple, and yet profoundly important that happens when water turns to ice in a pond. Compare this with what transpires when water turns to ice in a cloud. In a cloud, the ice crystals fall because water and ice are heavier than air and the gas phase of water. However, water becomes lighter when it transforms from a liquid to a solid state. If this were otherwise, then ice crystals would sink as soon as they formed on the surface of a pond. Heat near the bottom of the water would at first continually melt the ice crystals coming down, but at some point temperatures near the bottom would reach 0°C and lower. The water would then freeze from the bottom up, rather than from the top down. The ecological consequence of this phenomenon would be that there would be no bodies of water in the north. Sunshine in the summer would melt only the upper layers of ice, and any aspiring body of water would soon become a huge permafrosted ice lens.
Another ecologically important aspect of the behavior of water is that its density changes with changing temperature. Cold water is denser than hot water, and so cold water sinks as hot rises. As is also true for air. But, in water, the change is not so uniform. Water becomes densest at 4°C. As a result, when lakes warm up in the springtime from 0° to 4°C, as the ice melts, the surface water sinks. This denser water displaces the colder bottom water and its nutrients, which then rise toward the surface and feed the life above.
Geologically, the earth has experienced regularly recurring ice ages that are dependent on an astronomical cycle of the earth’s tilt (Imbrie and Imbrie 1979). This, the Milankovich cycle (named for its discoverer, Milutin Milankovich), is currently in a cooling period that began seven thousand years ago. But at the present time we are experiencing global warming instead, because the cooling effect of the astronomical cycle is being overridden by a human-induced climate change. The burning of fossil fuels produces carbon dioxide gas that is accumulating in the atmosphere at a greater rate than it is being absorbed by forest trees and other plants. The carbon dioxide acts as a thermal blanket, trapping solar heat. Unlike the astronomical cycle, which is gradual and permits evolutionary adaptations, this new phenomenon in the history of planet earth is sudden. It will affect kinglets, and us.