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Book Summary: Winter World by Bernd Heinrich

My Personal Summary

This book is about how different types of animals survive during the winter.

Different species depend on different strategies.

Some animals, such as bears, hibernate through the entire winter, dropping their heart rate as low as 8 beats per minute for nearly 7 months.

Other animals, such as many species of birds, deploy several strategies including building wind-resistant nests and huddling together for extra warmth and reducing energy expenditure.

Other animals, such as hummingbirds, simply migrate south to more hospitable locations.

Animals that live primarily in the water also use several fascinating tricks to survive the winter. For example, beavers build dams with such thick walls that they can survive comfortably inside even if temperatures outside are sub-zero.

Muskrats carry more oxygen in their blood during the winter by increasing their red blood cell count. This allows them to make longer dives underwater to forage for mussels, shrimp and small fish.

The ingenuity of animal survival during cold months is truly fascinating.

Book Notes

  • “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.”
  • Snow provides shelter at night for many kinds of birds, ranging from Siberian tits, ptarmigan, and ruffed grouse that burrow in and create igloolike snow caves.
  • Within this space, temperatures are physically “regulated” within a degree or two of the freezing point of water, all winter long. Several factors are involved. First, as already mentioned, snow affords remarkable insulation and, even at-50 ° C, heat rising from the earth generally keeps the temperatures near the ground close to 0 ° C. When both ice and water are balanced at near 0 ° C in the subnivian zone, the temperature is stabilized since whenever heat is lost through the snowpack to cool this space slightly below 0 ° C, there is then a water-to-ice-crystal conversion, which releases heat.
  • The subnivian zone is the environment between the ground and fallen snow. It is in this area where many animals like voles, shrews, birds, and other small rodents often hibernate as it provides protection from predators and a temperature controlled climate.
  • Snowshoe hares are famously lightweight yet have massive feet, which help them bound across the surface of the snow in the winter. Their hair also turns from brown to white when winter begins so that they’re camouflaged from predators.
  • Many predators such as weasels, foxes, coyotes and owls have phenomenal hearing that allows them to hear prey in the subnivian zone and pounce on them through the snow.
  • Great gray owls also have acute hearing and can detect a meadow vole’s movements under snow from thirty meters away. Drawing near one, they plunge from twenty-five feet in the air, and with their balled-up feet can punch through crust thick enough for a person to walk on.
  • Smaller animals often walk in the tracks of larger animals, such as moose, because it’s easier to traverse the snow in snow that has already been compacted.
  • Chipmunks are known for building complex tunnel systems underground and for storing up plenty of food like nuts and seeds during warmer months within these tunnels that can last them the winter. This is why chipmunks can often be seen with chubby cheeks filled with food that they’re carrying to their tunnel system.
  • Weasels have several behavioral traits that help them thrive in the winter. First, their coat turns from brown to white in the winter for better camouflage, but the tip of their tail remains black because this has been shown to confuse predators like hawks and makes them more difficult to catch. Second, they have narrow, flexible bodies that allow them to “weasel” their way into holes made by squirrels or chipmunks and capture these prey. They then curl up inside the hole made by the prey to conserve energy so they never have to spend energy creating a permanent den of their own to live in.
  • One of the most common ways that animals stay warm during winter months is by building nests and dens. Birds use twigs, bark, moss, grass, and animal fur to build nests.
  • Many birds also use woodpecker holes as shelter in the winter. For added warmth, birds also huddle together in holes or nests.
    Beavers build dams at the edge or directly in ponds using sticks and mud. When the mud freezes during the winter, it provides a structure with excellent warmth and protection from wolves and other predators. The entrance to the dam is through a water lock in the bottom of the dam, which only allows beavers to enter it.
    Bears rely mainly on their fur for insulation, which doubles its insulative capacity in winter. Once settled in, they don’t feed for seven months, living off their body fat until late spring.
  • A bear’s heart rate during the day is normally 50 to 90 beats per minute, although the heart rate of a sleeping bear in winter may decline to as low as 8 beats per minute.
  • The circadian clock has many potential uses. It allows hibernating ground squirrels, for example, to measure the daily light-dark durations, and from that data the squirrel can derive information about the changing seasons. Correct seasonal responses are crucial for winter survival. Indeed, the circadian clock mechanisms are necessary for all organisms that must prepare for winter, whether by pupating (insects), migrating (insects, birds, some mammals), or hibernating and physiologically preparing (most northern organisms).
  • Chipmunks are “true hibernators.” Like other ground squirrels, they spend most or all of the winter in a subterranean nest where they curl up, cool down, and become torpid.
  • Like chipmunks, northern flying squirrels as already mentioned also do not fatten up for winter, nor do they put on a thick insulating fur as red squirrels do. Nor do they lay up stores of food. Instead, they solve the energy problem by huddling in groups in snug nests. Even at-5 ° C outside the nest, the temperature within the nest is not yet low enough for them to have to shiver to keep warm.
  • Obesity has its advantages, such as when the animal can be safely inactive in its den. For the rest of the time obesity makes the animal a considerably more attractive meal to predators, all the while compromising its speed and agility. To minimize its duration of obesity, the groundhog must maximize the speed and extent of becoming obese. To be successful in this endeavor, it delays fattening until near the end of the summer.
  • One of the chickadees’ remarkable winter adaptations is their plumage, which is denser than that of other birds their size. Heat loss is mainly from the area around the eye and bill, and when the birds fluff out and then ball up to sleep, they are reducing specifically that area of heat loss by tucking their heads under their scapular (shoulder) feathers of the wing.
  • The Anna hummingbird found from northern California to Baja California, regulates its daily energy budget less by nocturnal torpor than by daily gain of energy stores, increasing its body mass by over 16 percent during the course of the day.
  • Snapping turtles are air-breathing reptiles that lay buried in the mud at the bottom of frozen-over ponds for six months of the year, without ever once coming up for air.
  • Turtles reduce their energy expenditure in winter to zero to extend their oxygen and energy reserves.
  • Muskrats carry more oxygen in their blood during the winter by increasing their red blood cell count. This allows for longer dives underwater when foraging for food such as mussels, shrimp and small fish.
  • Just prior to diving into ice water in winter (in contrast to summer), a muskrat increases its body temperature on average by 1.2 ° C (MacArthur 1979). Then, after returning to the lodge, the rat shivers and expends energy at a high rate to heat itself back up to 37 ° C (MacArthur 1984b).
  • When the first ice crystals begin to form on or in the skin of a wood frog, it sets off an alarm reaction. Skin receptors relay the message of freezing to the central nervous system (CNS), and the CNS activates the adrenal medulla to release adrenaline into the bloodstream. When the adrenaline circulates to the liver, it there activates the enzymes that convert the liver’s stores of glycogen to glucose. As a result, the frog responds with a quick rise in blood glucose. In the wood frog, this response is massive and before the ice reaches the cells they become packed with glucose that acts as an antifreeze. Precisely the opposite occurs outside, between the cells, where special proteins act as ice-nucleating agents to promote ice crystal formation in areas of dilute fluid. As a result, pockets of concentrated fluid are created, and these act to osmotically withdraw water from the cells, making them even more resistant to ice formation. In about fifteen hours, the frog is frozen solid except for the insides of its cells. Its heart stops. No more blood flows. It no longer breathes. By most definitions, it is dead. But it is prepared to again revive at a later date.
  • Frozen bodies that can revive upon thawing out have long been a pipe dream of cryobiologists. The frogs that hibernate in the forest floor do it routinely, coming out of their frozen state at the first flush of spring when it is time to mate. They, like the hibernating bears I’ll discuss
  • Most land frogs burrow deep into soil below the frost line, where they hibernate for the winter.
  • Deer mice often build nests in woodpecker holes or other tree holes, drop their body temperature during the day to conserve energy, and huddle together for additional warmth.
  • To survive the winter, monarch butterflies migrate south to Mexico where they spend their time in torpor in a specific location that has high relative humidity, cool temperatures and low wind. This allows them to remain dormant and not lose much fat.
  • Many bird species join flocks for the winter because it allows them to find food easier and it’s easier to avoid predators because only one bird must notice the predator before alerting the group.
  • Some species of snakes are known to huddle together in the winter because of energy savings.
  • Many birds require berry crops for fattening up on their long fall migratory flights, putting on as much as 10 percent body weight per day. That impressive fattening feat involves adjustments of gut length and other digestive adaptations for berries that allow for rapid food processing
  • Once in hibernation, bears will stay in their winter dens for as many as five months at a time, conserving their hard-won energy resources (fat) that they have accumulated during their feeding frenzies of the previous fall.
  • Most bears engage in a feeding frenzy in late summer and early fall, in which they down about five times their normal food intake, putting on a five-inch layer of fat. By late fall they slowly lose their appetites until they eat nothing and when they leave their dens in the spring, they are no more hungry than before entering. Instead, provided they retain some fat, it takes them a long time to regain their appetite. Appetite suppression during hibernation is probably under the control of leptin, a “satiety” hormone secreted by fat cells that circulates in the blood and affects the appetite centers in the brain (Ormseth et al. 1996). In spring, leptin levels decrease and appetite increases.
  • Since hibernating bears metabolize mostly fat, they do not accumulate huge amounts of urea in their blood. What small amounts that they do produce they convert into creatine, which is nontoxic. Additionally, instead of becoming a toxic waste, the nitrogen wastes in hibernating bears are biochemically recycled back into protein; hence no loss of muscle mass is experienced even as they don’t exercise. Thus a hibernating bear never needs to get up to take a drink or go take a leak all winter. Water is conserved because none is needed to flush out toxic wastes, and the animals stay in shape.
  • Many species solve the problem of lack of food in the winter in one of three ways: (1) store up food, (2) hibernate so they don’t need food or (3) migrate to a more hospitable location that has food.
  • In late summer, the pikas, a small relative of the hare that live in the high mountains of the West, collect grass, dry it in the sun, and then pack the hay into dry cavities under rocks for winter food.
  • After the leaves turn and fall in the autumn, beavers all over North America begin felling trees and saplings, and dragging them into the water to make huge underwater food caches near their lodges. The icy water keeps the bark fresh throughout the winter, and the beavers live by feeding on it for about six months. Some squirrels and many other rodents, including deer mice, pocket mice, kangaroo rats, and hamsters, stockpile seeds that reduce or eliminate their need for torpor.
  • Gray jays live in northern regions like Maine, Canada and Alaska. When their saliva hits the air, it turns into a sticky glue-like substance that they can use to stick nuts and seeds to trees where it can be kept safely from ground animals and so they don’t have to spend energy burying it in the snow.
  • Unlike the blue jay’s flimsy see-through nests of bare twigs and rootlets, those of the gray jay are bulky, deep, and well-insulated cups lined with fur and feathers that cradle and keep warm the clutch of three or four grayish, olive-brown-spotted eggs.

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