In yesterday’s blog post, I summarized the challenges of living in (and surviving) the harsh weather of northern latitude winters and described a few of the solutions to those challenges. But there are more solutions available to animals — and humans.
4. Turn up the heat: For those animals (and humans) that can afford to do so, the best solution to surviving cold temperature extremes is to fire up the metabolic furnace. No one enjoys standing out in the cold shivering intensely, but that and physical exertion are the first line of defense in staying warm. The trick is to preserve the heat inside the body by improving insulation.
Flying around looking for food generates body heat for this Red-bellied Woodpecker, but the bird must fluff out those feathers when sitting still to retain the heat produced by activity.
Mammals, like this Gray Squirrel, fight the cold by increasing both shivering and non-shivering heat production, using an extra source of heat production from their “brown fat”. It doesn’t hurt to have a furry tail to keep your back warm either.
Brown fat (more vascularized than regular white fat) is more prevalent in mammals acclimated to cold (even humans!) and especially in young mammals and in hibernators that undergo dormant sleep for most of the winter. Localized in the trunk and back, brown fat heat production preferentially warms the spinal cord and brain.
5. Don’t spend what you don’t have: In cases where food is limited or costly to obtain (i.e., resulting in a net loss of energy), the opposite strategy from #4 above is to be more conservative in expending energy by turning down the metabolic furnace when resting, decreasing activity, sleeping more, etc. A variety of mammals, some birds, and even some humans employ this strategy in the winter. Hibernation, or winter sleep, is key to survival in many rodent species (except tree squirrels), because there is little energy wasted on heating up their body that is essentially the same temperature as their burrow.
Hibernating chipmunks store food in their underground burrow, and rouse every couple of weeks from their torpid sleep to snack a little before becoming dormant again. Photo from sni.scholastic.com
The British Medical Journal described a case of “human hibernation” in a group of Russian peasants, living in an impoverished area with inadequate food, who typically slept through the winter, rousing only once a day to eat a little bread, drink some water, and add fuel to their fireplace.
Just turning down the furnace and lowering body temperature a few degrees at night can make the difference between survival and succumbing to the cold. Birds as small as Black-capped Chickadees and as large as Red-tailed Hawks save 30-40% of their overnight energy expenses by cooling off a few degrees.
It’s not unusual to see Red-tailed Hawks perched on telephone or light poles along highways in the winter, where they can get a good view of potential prey moving around below them. But if these hawks miss a few meals, they may not have enough energy reserves to make it through the night. Better to reduce night-time costs and save energy. Photo by Allan Block
6. Tolerating net energy loss: This is kind of a last-ditch effort to survive winter, but may be a viable strategy in larger-bodied, well-fed animals. For example, White-tailed Deer may not find enough forage to sustain themselves over an entire winter, so they put on weight by eating a lot in the Fall and coast through the winter, using up their reserves.
Winter cold must be especially tough on smaller-bodied fawns with less energy reserves than the adults.
Longer winters with more extreme temperatures may mean lower survival rates, and may even compromise an animal’s ability to recover in the spring.
This White-tailed buck looks pretty emaciated after a long, cold winter. He may not be able to rebuild his muscle mass over the summer in time for the Fall rut season.
A more atypical illustration of this strategy is that of hibernating bears. They aren’t really hibernating in the true sense, since their body temperatures are only a few degrees lower than normal, but they purposely fatten up in the fall, and then metabolize that fat over the months of winter sleep, losing 25-40% of their body weight before they emerge from the den in the spring. Females use an additional portion of energy reserve to nurse cubs born during the winter sleep.
Hibernating Black Bear and cub; photo from blackbearreserve.com
In summary, animals use a variety of strategies to offset the cost of surviving winter cold; it’s not really mysterious or magical, but is a product of selecting what works best in a particular situation. Animals using the wrong strategy are quickly removed from the breeding pool, and thus solutions get better and better over time.