Winter Is The Enemy

2013 December 28
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When people talk about natural selection, it generally gets couched in terms of organism versus organism, “Nature, Red in Tooth and Claw”, with direct competition for space, or food, or to avoid being food, or for mates, or for any number of other things. This is all well and good in places with an equitable, or at least relatively unvarying, climate. In the warmer parts of the world there may be occasional mortality from storms or volcanos or droughts or other natural occurrences, but mostly life really is all about the eternal struggle with other living things[1].

As we head towards the poles, however, things change. In the summertime, the livin’ is easy, with plenty of food, and water, and mates, and a climate that is not actively trying to kill you. But, the winter becomes something else again. Heading north[2], the environment spends longer and longer portions of the year being progressively more hostile to life. Water freezes and the cold of winter kills directly; food vanishes, because photosynthesis becomes impossible and scavengeable material gets buried in snow and ice; and then those that the cold does not kill risk death from lack of food. By the time you get up here to the Upper Peninsula of Michigan and parts north, the other organisms become almost incidental to your survival. Winter is the killer, the enemy that you must fight first, because if you can’t survive Winter, then you’re done, no matter how good you are at the conflict of life against life. All the venoms, spines, armor, camouflage, strength, speed, agility, or fighting tricks in the world won’t help you when the ice comes.

So, what are your options for getting through the winter, anyway?

1. Run for it (migration)

Migration to a warmer place is an obvious strategy, provided you can move far enough and fast enough to do some good. The evolutionary pressure is for features that allow you to cover a lot of ground without getting eaten or otherwise killed. In order to get good speed, you pretty much need to fly. And it is easier if you are larger. So only a few insects, like Monarch butterflies, go in for the migration route. One thing that helps them is that they are toxic, so they don’t get heavily preyed on while they are exposed and flying south. The big bonus of migration is that you don’t have to muck with your body’s basic operation, you just need to get out of there.

2. Dig below the frost line

This could be considered a variant on “migration”, except that instead of migrating south where it is warmer, they migrate down where it is warmer. Usually the ground doesn’t freeze more than about a foot down, and in places with heavy snow cover (like here in the Keewenaw Peninsula, where we routinely get around 16 feet of snow) the ground may not freeze at all. So any creature that can dig down (like ants, soil grubs, and the like) just need to get down a bit deeper and weather it out. The downside is that there may be a shortage of food down there. So unless you are one of the animals that eats plant roots, you either have to stash food all summer to tide you over, or you need to live off of your fat reserves. So the evolutionary pressure is for improved digging ability, an ability to live in a fairly low-oxygen underground environment, and a tendency to build up a lot of body fat when times are good.

3. Winterize and find shelter

This is probably the most common way to get through the winter. While it is difficult to dig deep enough to avoid freezing altogether, it is much easier to just find a sheltered spot where it doesn’t get much below freezing, and then adjust one’s body chemistry until you can survive a light freeze. Either by preventing ice crystals from forming (allowing you to cool down to about -20 C or so before freezing to death), or by containing the damage from ice crystal formation in your tissues and freezing more or less solid (which is harder to do, but if done right can let you survive freezing to arbitrarily low temperatures). Then you go to sleep and hope for the best[3].

This can be done as Eggs

or as Larvae

as Pupae buried in the leaf litter

or even as an Adult

The big danger here, though, is that one is pretty defenseless against the few animals that are still roving around looking for food. So the shelter spot also has to be a good hiding place. The creatures that do this generally have to adjust their biochemistry enough that it becomes difficult or impossible for them to function, so they get pretty lethargic just before hibernating.

4. Go aquatic

Ponds and streams rarely freeze up completely. And by definition, as long as you stay where the water is liquid, you don’t have to worry about freezing. So all the aquatic creatures can just hang out underwater. Most of them will hibernate

but some, particularly detrivores that live in streams, will stay actively feeding and growing all winter

5. Fill yourself with antifreeze and tough it out in the snow

And then, there are the hard-core cases. The ones that actually can function in the winter, not just survive. Some of them do this by just hanging out in the “subnivean environment”, where it is cold but not quite freezing. And while they can’t move around all that fast themselves, they can easily prey on the ones that are hibernating and therefore aren’t moving at all. Although they do have to watch what they eat: while their own body might be full of antifreeze and therefore able to supercool below the normal freezing point, the things that they eat might not be, in which case their food turns into a ball of ice in their stomachs. Awkward.

If you push the antifreeze hard enough, you can even survive up on top of the snow, although the thing you have to watch out for here is whether you can survive when it gets warm again.

Doing any of these things, though, is an arduous evolutionary process. Which means that you have to start with easy winters, and then work towards more difficult ones. So initially there is a tropical species that has a tiny difference, allowing them to live a little bit further north than their compatriots. Maybe where it gets a bit cold, but doesn’t quite freeze. Then their descendants work their way a bit further north, where maybe it freezes for a day or two. The ones that can take that, move further north still. Finally, after many generations, they can survive seriously freezing temperatures for significant amounts of time, and their descendants then move rapidly into the temperate regions, where weathering the nasty winters gives them a crack at the largely un-utilized lush growth of the summer. In many ways, life in the temperate zone in the summer is easier than life in the tropics, because of the winters thinning out the competition and predators so much that pretty much all one has to do all summer is eat and grow fat and make lots of babies. Which is why so many temperate-zone animals are very successful, but only as long as they don’t lose sight of dealing the real enemy.

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[1] Even in the desert, while the environment is hostile due to the daily temperature swings and the lack of water, it is at least consistently hostile, what with the desert being a desert pretty much all the time. Everything there has already adapted to deal with the daytime heat, nighttime chill, and low moisture. And after doing that, the threats to one’s existence are mostly from other animals.

[2] This is also true heading south, but not to the same extent. This is because, once you get beyond about 40 degrees south latitude, the landmasses kind of peter out. Pretty much all there is, is the southern parts of Argentina and Chile, New Zealand, Tasmania, and some isolated smaller islands. And what land there is, is close enough to the ocean that their climate is pretty constant: they never really get all that cold, or particularly warm. Of course, that all changes once you make the hop to Antarctica, but that isn’t a gradual change like it is in the north. It is more of a sudden jump from “kind of cool but not quite freezing”, to “bitter, unsurvivable cold that never really ends“.

[3] Hibernation can be a big risk, because you never know what’s going to happen while you are unconscious. A while back, Sandy was reading about turtle fossils in Michigan that had apparently been caught hibernating when the last ice age started. She said, “That’s terrible! They went to sleep through the winter, and then winter lasted for eighty thousand years!”
/em

3 Responses
  1. December 28, 2013

    Great post! I’ve been wondering about the spiders that keep on hunting through the winter; what do they eat? I never thought of the hibernating critters; of course; they’re all through the duff and under tree bark, and hiding under my frozen plant pots*, where a spider can find them.

    *When they were frozen last month; right now, we’re in a probably temporary spring mode. This is BC, on the coast. The weather never stays put.

  2. December 28, 2013

    A fantastic post. Thanks for sharing!

  3. Katbird permalink
    December 28, 2013

    Very nice! Without winter, there would be no spring….

    Happy New Year to creatures great and small.

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