Winter musing about weather, hibernating and opossums

by Linda Martinson, Blue Ridge Naturalist

We often have interesting weather in Western North Carolina. One day this month, during about an hour and a half period mid-morning we experienced; fog, sudden sunshine, rain turning to sleet, gusty winds, sunshine again, and then, a lovely rainbow over the mountains. The last few days have been so cold that the rhododendron leaves are calling for help. It’s sometimes hard to get a grip on the weather in January and in our area in general.

Rhododendron leaves in January
When rhododendron leaves are pointed straight down and curled like cigars, the temperature has dropped to 20 degrees or below. Linda Martin photo

North Carolina is longest state east of the Mississippi River at 503 miles in length; and it has the greatest range of altitude of any state east of Mississippi. The elevation ranges from sea level along the coast to 6,684 feet at the summit of Mount Mitchell in Western North Carolina. It is the highest peak of the Appalachian Mountains, right in the heart of the Blue Ridge mountain range. Western North Carolina has some of the most variable average precipitation in the southeastern United States, and the differences in precipitation within the WNC mountain gradients can vary from up to 100 inches to 36 inches per year. 

The area we live, near Lake Toxaway in Transylvania County, has the wettest recorded weather in the state with an average annual precipitation of around 92 inches. Lake Toxaway is at the southern edge of the Blue Ridge Escarpment, and moist air lifting over the mountains dumps lots of rain here making parts of the area wet enough to be considered rain forests. There are 250+ waterfalls nearby giving the area the description of ‘Land of Waterfalls.’ By contrast, the driest area in the state is only 50 miles away in Asheville/Buncombe County with an average annual precipitation of around 37 inches. The Asheville area sits in the French Broad River basin, where the prevailing moist winds and rain are frequently either squeezed out or blocked from the south by the Balsam Range and from the west by the Great Smoky Mountains.

Considering the weather in general findings released jointly by NASA and the National Oceanic and Atmospheric Administration on  January 15 illustrate a trajectory that should scare us all. Each of the past five years was among the five hottest years since record-keeping began in the 1850s, and 2019 was the second-hottest year ever recorded just behind 2016. Also, 19 of the hottest 20 years ever recorded occurred during the past two decades. Scientists quoted in a Washington Post article state that this warming trend “bears the unmistakable sign of human activity, which emits tens of billions of tons of carbon dioxide into the atmosphere each year”.

The Washington Post, “2019 capped world’s hottest decade in recorded history” Brady Dennis, Andrew Freedman and John Muyskens January 15, 2020.

“No individual hot year — or hot day or hot season, for that matter — is by itself evidence for climate change. But this hot year is just one of many hot years in this decade,” said Kate Marvel, a research scientist at both NASA and Columbia University. “The planet is statistically and detectably warmer than before the Industrial Revolution. We know why. We know what it means. And we can do something about it.”

Reading this article, and the news, and the month of January in general, made me want to hibernate, and then I began to wonder if this warming trend affects migrating and hibernating animals. Apparently, migratory birds are affected more by global warming than hibernating animals. Research indicates that climate change has a triple whammy effect on migratory birds — it disrupts migration times and successful breeding as well as causing food shortages for both birds and their chicks. For example, changes in the length of daylight trigger bird migration, but warming temperatures result in earlier hatching of caterpillars and of other insects at their breeding locations. Migrating songbirds may arrive on time according to the length of daylight, but the caterpillars they feed to their chicks have hatched early because of warming temperatures and are already moths which their chicks can’t eat. Since fewer chicks live, the songbird population immediately begins to decline. Also, because of the changing climate, migrating birds have to travel farther and search harder to find food when they migrate. This tragic scenario exists for many bird species and climate change has become the top threat to migratory birds. For several reasons the number of birds worldwide is declining precipitously.  For example, a report in the journal “Science” in September 2019 states that North American bird populations have declined by nearly 3 billion birds since 1970, a loss of nearly one in four birds.

Note: Cathy Walsh, Board member of Elisha Mitchell Audubon Society, will speak to Blue Ridge Naturalist Network members and other guests at the West Asheville Library on Tuesday, February 4, from 5:30 to 7:00 pm on ‘Native Plants, Birds, and the Climate Crisis’.

Another adaptation to winter is hibernation, which is not a very precise term. A general definition is that hibernation is a period during which some animals stop self-regulation of their body temperature and submit to being physically affected by the surrounding environment, usually by the cold temperatures of winter. Generally, hibernation is characterized by low body temperature, slow breathing and heart rate, and a low metabolic rate. Some researchers and writers use a term “true” hibernation to describe a state of “suspended animation” during which breathing and heart rates slow considerably and body temperature drops to the air temperature of the animal’s burrow and sometimes even below freezing. “True” hibernators also stop eating and drinking and excreting, too; and they wake up only once the outside temperature warms sufficiently and then very slowly. Of course, they need to eat furiously and store significant fat reserves before they enter hibernation.

Among the mammals of the Appalachians, only the ground hog or woodchuck, and all native Appalachian bats are “true” hibernators. The heart rate of ground hogs during hibernation goes from about 80 beats per minute when active to only 4 to 5 beats per minute. In contrast to “true” hibernators, for example, Eastern chipmunks could be characterized as “deep” hibernators. They spend the winter with reduced body temperatures but in intermittent deep sleeps from which they awaken occasionally to eat their cached seeds and to eliminate. Then they return to their deep sleep until spring.

Bears are unique hibernators and their denning for the winter has been referred to as “winter torpor” or “winter sleep,” but it is also considered in other physical descriptions to be a special case of hibernation. Their heart rate, breathing rate, and metabolic rate do slow down but not as much as other hibernating animals, and a bear’s body temperature is only slightly reduced by just 10 degrees or so. In contrast to other hibernators who awaken slowly at the end of a constant deep sleep, a bear can wake up fairly easily during its denning period, and sometimes frequently. They do not eat, drink, defecate or urinate during the entire time in their dens, however, which makes bear hibernation a period of amazing physiology.

During hibernation, a bear’s fat cells break down to provide water and calories and their muscles and organ tissues break down to supply protein. The same process would happen with any animal deprived of food and water, but it would be the beginning of the process of starvation if it wasn’t a hibernating bear. Humans, for example, can’t restore muscle and organ tissue that breaks down to supply protein to the body, but bears can. Hibernating bears can also utilize the nitrogen in their stored urea, a component of urine that is produced during tissue breakdown, to build new proteins. These new proteins can then be used to maintain organ and muscle tissue.  

Female bears develop fertilized eggs during mating in the summer, but they delay attachment of the eggs until just before they den. If a female bear’s condition is not strong enough, she will abort the eggs because lactation is so energy intensive. However, if it has been a good foraging season, for example during a mast year, bears can have up to six cubs. The cubs are born usually in late January, and they are blind, bald, and about the size of a guinea pig. The cubs grow rapidly in their dens with their lactating mother because bear’s milk is about 35% fat. By the time they leave the den in March/April, the cubs are the size of a large puppy and ready to move around and to follow their mothers. They leave their mothers at about 18 months, which means that they typically spend a second denning season with her. An aside: the number of black bears in North Carolina has increased significantly from about 4,000 in 1971 to estimates as high as 20,000 now.

Hibernating Bears, National Geographic photograph

And now a leap to opossums, a common nocturnal omnivore about as big as a cat. I’ve been musing about opossums because I’ve noticed quite a few dead ones along the roads this winter; and because their physiology, although different, is as amazing as that of hibernating bears. I’m quite fond of opossums, partly because I had one as a pet for a week or two when I was about 16. I was riding my horse through the woods one day when I saw that my dog had found a critter. It was a dead opossum with only one surviving baby. I stuck the baby opossum in my pocket to take home thinking (I swear this is true) that I could impress my high school friends by driving them around in my parents’s car with a baby opossum swinging by its tail from the rear view mirror. 

I named her Daisy and kept her alive with condensed milk fed by an eye dropper for a week or so, and she did make an attempt to swing from the rear view mirror, in terror I’m sure. What I didn’t know then was that opossums can’t swing by their tails for long periods of time, although they have been known to use their tails to pick up and carry bunches of dry grass and leaves. 

Opossums are best described as marsupial mammals, because they are not fully developed at birth and have to spend a significant amount of time in a pouch on the mother’s belly. There are over 65, and some sources say over 100, species of opossum, but the only marsupial native to the Appalachians is the Virginia opossum (Didelphis virginiana). Opossums are prolific breeders and can have up to three litters in a year. Their gestation period is a little less than two weeks, and they can have as many as 20 small honeybee-sized babies called joeys, just like kangaroo babies. The newborn joeys crawl into their mother’s pouch, which is a perilous journey. Inside the pouch there are only 13 nipples (neatly arranged in a circle with one in the middle). Usually fewer than half make it, and those fortunate little joeys each latch onto a nipple and continue developing for about 50 to 70 days. Opossums are devoted mothers and even after the joeys crawl out of their mother’s pouch, she will stay with them and carry them around on her back for about another 100 days. Opossums are sexually mature at six to eight months and live for two to four years.

Another reason I am fond of opossums, and that I believe we all should be, is that they are super efficient tick-killers. According to Richard Ostfeld, a research scientist at the Cary Institute for Ecosystem Studies studying Lyme disease, opossums can kill and eat as many as 5,000 ticks in a single season. They also eat cockroaches and kill rats, and I’m cheering for them to add stinkbugs to their menu. As omnivores, they will eat just about anything dead or alive: rats, mice, snails, slugs, birds, eggs, fruits, grain, frogs, pet food, table scraps, carrion, etc. Opossums have an unusually high need for calcium, and they are often seen on or near roads crunching the bones of roadkill. 

Opossums prefer arboreal living, but they also seem to enjoy swimming. They are strong swimmers and seem to prefer living in woods and swampy areas that get plenty of rainfall. They are, however, exceptionally adaptable and will live happily among humans in open country, in the suburbs and even in cities. Opossums often den in tree holes or in ground burrows, but will also move into sheds, garages, barns and under houses. They are harmless to humans; they rarely contract rabies; and they have a natural immunity to poisonous snake bites. Although they don’t hibernate, opossums do slow down during the winter. They spend more time in burrows that they fill with dry leaves and draw down their fat reserves.

Opossums are also fastidious groomers like cats, and they are quite intelligent. They score as high as dogs on intelligence tests, particularly in one category: they have a remarkable ability to both find food and to remember where it is. When tested for the ability to remember where food is, opossums scored better than rats, rabbits, cats, and dogs but not as well as humans. They also can find their way through a maze more quickly than many animals. Also, opossums have opposable “thumbs” although they are on their hind feet so technically they are opposable first toes. They share this trait with only one other mammal species, primates, and it’s the main reason why they are such excellent climbers. 

Another interesting fact about opossums is that they have more teeth than any other mammal; they have 50 teeth and in comparison, dogs have 42, humans have 32 and cats only have 26. They also have excellent night vision partly because they have giant pupils with just a small rim of iris which makes their eyes seem black. Unfortunately, if spotted during the day opossums look stunned and rather slow-witted because they are semi-blinded by the bright light.

So what’s not to love about opossums? One reason they are not more popular is that when threatened, they turn ugly. They try to escape, growl, hiss, belch, urinate and defecate. And if they have joeys with them, they will attack and bite, too. As a last resort, they act as if they were dead. They fall over, stiffen up and either close their eyes or assume a blank stare. Also, they bare their teeth and foul-smelling saliva foams from their mouth. This ability to “play ‘possum” by feigning death is not a conscious act; it is an involuntary response like fainting. Opossums can remain in this catatonic state for up to four hours, so it is a very effective deterrent to possible predators. Reading, thinking and writing about opossums, I’m in awe of their intelligence and adaptability and no longer convinced that Daisy’s mother was really dead.    

“Playing ‘possum”

References: State Climate Office of North Carolina  

          The Washington Post January 15, 2020 

          Frick-Ruppert, Jennifer Mountain Nature The University of North Carolina                                   Press 2010 

          Florida Fish and Wildlife Conservation Commission 

          San Diego zoo website 

          Richard Ostfeld Cary Institute for Ecosystem Studies Millbrook NY