Saturday, January 25, 2014

Animal Winter Adaptations

Note: This program first aired January 18, 2014.

December was brutal this year, with sub zero temperatures, deep snow and ice storms. But now, merely a few weeks later, it feels like early spring. On my walks around various neighborhoods its not just my eyes and my skin that are fooled by the warm moist air. My nose has sensed a creature roused by the spring like conditions the past few days; my nose has been smelling skunks.

Skunks are in the family Mustilidae, or the weasel family. They are unusual for weasels in that their strategy for dealing with winter is to store food as body fat and while away winter in a semi dormant state, usually in a communal den that allows them to share body heat with other skunks. It is not uncommon for them to rouse periodically and even emerge from their dens if the weather is nice, that can explain the skunkiness of a warm winter day. They don’t have to come out and eat, if they are able to store enough fall food as body fat, there is no real reason for them to forage mid winter. They may just want some fresh air.

That body fat issue actually describes a line of demarcation in mammals’ winter coping strategies. The accumulation of body fat and the subsequent living off of it all winter long is but one winter strategy that Maine mammals can utilize. Carrying around a lot of body fat is a burden, animals that partake in this strategy generally don’t put on the weight until the fall, triggered by some kind of environmental cue (most likely shortening day length) that synches their circannual rhythm and initiates some kind of horomonal or physiologic change that increases body fat storage. Its worth noting that fat storage is the strategy of another group of animals that uses this to deal with winter in an entirely different way. Animals that migrate away from Maine in the fall spend all summer eating and fattening up to fuel their fall escape. It is common for them to travel hundreds of miles or more, and not eat during the journey instead burning the fuel accumulated over the summer.

Animals that live off fat for the winter are just caching food in a different form. Instead of stashing it in their burrows, they are stashing it on their bodies. It gives them the most flexibility in terms of whether or not they need to wake up and eat. Contrast the semi dormant fatty skunk or bear or the fully hibernating ground hog with the chipmunk. Chipmunks are often held up as “true hibernators”, but really that term doesn’t mean any one thing exactly. Chipmunks are different from skunks, bears and ground hogs in that chipmunks don’t lay on heavy stores of body fat. They build huge underground burrows and store as much food as they possibly can in them. If you watch squirrels and chipmunks at your bird feeder, you will notice a difference in their behavior. Red and gray squirrels will eat each seed one by one on the spot, as they find it. Chipmunks collect seeds, filling their cheek pouches and returning to their burrows to lay the seeds up for winter. They will spend all summer doing this. Then in the winter, they enter a torpid state, cued by cold temperatures. Torpidity means they have a lowered body temperature and much reduced metabolic function. They wake up periodically to eat. The more food they have stored, the more time they can stay awake, and the advantage of being awake is that they are less vulnerable to predators like weasels when they are awake. A torpid chipmunk can rewarm and arouse fairly quickly, in an hour or less, but that is still to long to ward off a predator’s attack. Awake or asleep, they will spend most or all of their time in a subterranean burrow, waiting out winter.

The last major category of mammal strategy is a bit counter intuitive. The animals that are the most active in winter, usually have little to no body fat. They may grow an extra thick insulating layer of fur, but body fat is a burden that would simply slow them down. Physiologically, fat is food stored on the body for a time when food resources are scarce. Animals that remain active all winter are animals who’s food sources are abundant in winter, so they have no need to store body fat. Red and Gray squirrels, weasels, and snow show hares all fall into this category. If you spend any time in the woods after a snowfall, you will quickly understand that these are the vast majority of the tracks you see, because these animals must constantly be searching for the food that will feed their internal fires.

And how about us? How do we deal with winter? We are tropical animals, evolved at low latitude with the sun high in the sky. But we are also animals who biologically made their last big leap in evolution during a period of glaciation, so lets not rest too heavily on that tropical animal gambit. It is true though that we do have temperature thresholds, above and below which we do not function effectively, we have to create a microclimate around us if the external environment does not match up with our operating parameters. This is especially true in the winter. Physiologically we are all active mammals in the winter, none of us enters torpor or hibernation, we maintain our body temperature, we eat, drink and eliminate. Yet, I see reflections of each of the animals’ winter strategies in the ways we live this time of year. Some of us spend a lot of time in our burrows, eating through our summer stores, others of us flit around on the landscape seemingly impervious to the temperature, and eating as much as we can. Two ends of the spectrum of the body’s reaction to winter, where do you fit in?


I’ve been reading Bernd Heinrich’s Winter World. Most of the material for this week’s show originates there.