As I sit bundled up on the couch drinking my hot coffee and writing this blog, blizzard Jonas rages outside my apartment in north Jersey.As the wind blows the snow off the roof in gusts, Newark, normally bustling, is silent, no signs of life. Everyone must be holed up inside, hopefully enjoying a leisurely morning, drinking something hot and staying warm!
Which makes me wonder about all of my non-human neighbors; where are the squirrels and birds? How do the trees survive the piles of snow that decorate their branches?
Some animals avoid winter weather completely through migration, leaving for warmer climates during winter months and returning during summer.
Others animals reduce their activity during winter often referred to as hibernation. These animals prepare ahead of time, finding or building a den or burrow, and foraging for enough food to build up fat stores that will last them the duration of their sleep. Hibernation triggers vary by organism but include both exogenous and endogenous factors. Exogenous factors come from the environment of the organism, including abiotic conditions like the changes in day length, a drop in temperature, or reduced food availability. Endogenous factors are those found within an organism (biotic), including hormonal changes or an internal circadian rhythm.
However, only very few animals truly hibernate all winter. Instead many go into torpor or a light hibernation with periodic waking, in order to eat (sometimes from the food stores they prepared during summer and fall), drink and excrete their wastes. This sleep can last anywhere from a few hours to extended periods of time and generally allows organisms to avoid the worst of winter weather events.
Regardless of whether organisms are in hibernation or torpor, they have fantastic physiological abilities that allow them to remain in a state that can appear very much like death to observers. During hibernation animals must reduce their metabolism to incredibly low levels in order slow the use of valuable fat (energy) stores, allowing them to go months without eating. Many organisms, as a means to reducing metabolism, also drastically reduce their body temperature, sometimes, as with bats, to barely above air temperature.
Black bears, the prototype of hibernating species in many people’s minds, can go 6-7 months without eating and during this time they slow their breathing to one breath every 45 seconds and their heart rate drops to 8-19 beats per minute (as compared to 6-10 breaths every minute and a heart-rate of 40-50 beats per minute when not in hibernation. They are also able to prevent loss of muscle and bone mass.
It’s not just mammals that hibernate. Reptiles and amphibians are ectotherms requiring external heat sources to maintain their body temperature. Therefore, in winter they have their own version of hibernation, brumation, in which they have extended periods of dormancy with punctuated (occasional) activity. Reptiles and amphibians also prepare for brumation by increasing their fat stores, but they also increase the level of glycogen (a type of sugar) in their muscles which can be used for energy. However, these organisms wait out the winter in the mud, sometimes under water, and these stores of glycogen allows them tolerate these low oxygen environments. In fact dehydration is a bigger concern for reptiles and amphibians than lack of oxygen and water is one reason reptiles and amphibians break dormancy.
Insects, also ectotherms, also have their own version of hibernation known as diapause. Similar to hibernation, diapause is initiated by abiotic factors including changes in day length and temperature and biotic factors including life-stage. Diapause also has a preparation stage where the organism amasses energy and while in diapause insects reduce their metabolism in order to make these energy stores last. One difference between diapause and hibernation is that diapause is life-stage specific. Whereas animals that hibernate do so annually, no matter what their age, insects only diapause at specific life-stages.
For example, I work with cavity-nesting mason bees and the species I work with diapause as pre-emerged adults and with when spring brings increasing temperatures they emerge. However, other cavity nesting bees that emerge later in the year diapause as pupa and only become adults after diapause ends.
Birds rarely undergo full hibernation, but they do go through torpor, often in groups to share body heat.
Cohabitation during is a common behavioral adaptation in many different organisms. Bat roost in colonies to stay warm.
As will rattlesnakes.
Then during warmer months, both skunks and snakes will disperse and lead solitary lives.
Some animals are active throughout winter. These animals are adapted to cold weather and as the temperature drops, they develop layers of fat as well as thick pelts of fur,
or downy layers of feathers.
Some of these same animals have seasonal color changes to their pelts or plumage. These animals are then better camouflaged to avoid detection by predators
In addition, organisms active in the cold often have specialized circulatory systems that prevent freezing of the extremities. For example, arteries in penguin legs adjust blood flow to prevent their feet from icing over.
Aquatic species including some fish, use glycogen in the blood stream as an anti-freeze agent. While others, like the wood frog, are actually able to survive freezing while in periods of winter dormancy! See here and here.
Some other trends have been observed in polar species, including larger body sizes than their temperate relatives. This is thought to help retain heat and is known as Bergmann’s Rule. For example, polar bears are often thought to be the largest bear species in the world. Although there is much debate of the scientific legitimacy of this trend.
Another such trend, known as Allen’s Rule, observes that endothermic (sometimes called warm-blooded) animals, also tend to have shorter or thicker extremities, like ears or snouts, when compared with their temperate and tropical relatives. This is thought to reduce the surface area to volume ratio and assist in retaining heat. A larger surface-area to volume ratio allows heat to dissipate more quickly (hence the large thin ears of a fennec fox that lives in the Saharan desert and the small thick ears of the Arctic fox).
What about organisms that can’t migrate or hide underground?
Plants also have adaptations that allow them to survive the cold months. For plants, winter brings a lack of water since the cold temperatures mean that most water is in a solid icy form that trees cannot use. Deciduous trees and shrubs essentially hibernate, going dormant after losing their leaves in the fall. Whereas evergreen plants keep their leaves, but the leaves have a waxy protective coating that prevents water loss.
Plants in perpetually cold environments, like polar regions and mountain tops, have additional adaptations including having small leaves and growing low to the ground and close together, like cushion plants.
At the cellular level, plants are able to avoid freezing due to dissolved solutes and anti-freeze proteins. And some plants in the tundra have specialized physiology that allows them to perform photosynthesis, even in cold conditions.
If you are looking for ideas on how to thrive in winter weather, you could do worse that to study up on these lessons from mother nature. Personally, I intend to make like a skunk and curl up with a den-mate, preferably in a warm apartment with a heavy comforter, a good book and some hot chocolate! But not before playing out in the fresh snow!