Friday, November 14, 2008

When Winter Comes: Strategies for Survival

Our world is surprisingly full of animals, even in our heavily developed areas. How does nature ensure that fullness? By paying a large price: Excess. This annual tax often comes due in winter. Every student of nature stumbles upon and must come to terms with the necessity for such excess. Charles Darwin once remarked upon the "clumsy, wasteful, blundering, low and horribly cruel works of nature." Henry David Thoreau sounded a somewhat more optimistic note: "I love to see that Nature is so rife with life that myriads can be afforded to be sacrificed and suffered to prey upon one another." Whatever we may think of this system, it works.

In the plant world, we find it useful to describe life cycles by their duration. For example, we understand some plants to be annuals that go from seed to seed in a single season, investing all their energy in the next generation. Others are perennials that take several seasons (or decades) to grow, reproduce, and die. The concept is equally useful when applied to animals. Some animals, especially among the insects, could be considered annuals, going from egg to egg in a single season or year. Even those who could live for years—potential perennials—often don't. Winter is one of the reasons for shorter lifespans.

Many insects invest any hopes they have for the future in an egg or pupa that is dormant during the cold months; most butterflies use this approach. Others, for example hornets, go from the abundance of a large "city"—the paper nest with its thousands of inhabitants—to a few adult queens, stocked with sperm for the following spring. At least one must make it through winter to begin again. On the average, one does. Ladybird beetles also go through winter as adults, coming together by the thousands each fall to hide in crevices and other sheltered places on mountaintops.

For animals large and small, winter success is often a matter of survival of the fattest. Stocking up enough reserve energy to get through the winter is especially important to those who will not look for food again until spring: bears, snakes and lizards, frogs and toads, hibernating ground squirrels, and many more. They sleep, gambling that the fat they've stored will last longer than the winter ahead.

Others remain active, using hidden food caches as pine squirrels and scrub jays do, or searching for food all winter as deer and elk do. Stocking up is still important, though. The more energy they've been able to store internally during summer's abundance, the better their chances of finding enough external food sources to get by.

Among birds, many escape the rigors of winter by migrating, but there is no escaping the annual tax, and no way the world can hold all the young produced each year. In 1991, volunteers for Hawkwatch International counted a thousand Sharp-shinned hawks migrating over one mountain ridge in Utah; almost half were immature birds making their first trip south. Only about one-third of those young birds will live to make the return trip. By our standards, this reflects an oppressive tax indeed; by nature's standards, it is a necessary one.

Our smallest winter-resident bird, the chickadee, lives all winter on a nutritional and energetic edge. In ten years, Aldo Leopold banded 97 chickadees on his Sand County farm. Only one survived five winters; 67 didn't make it past their first. But survival isn't just a lottery; much can depend on the decisions the animals themselves make. Read the chapter on chickadees that ends his Sand County Almanac—it's one of his best.

It seems likely that weather is the only killer so devoid of both humor and dimension as to kill a chickadee....To the chickadee, winter wind is the boundary of the habitable world....Books on nature seldom mention wind; they are written behind stoves.
—Aldo Leopold

It's no wonder, then, that animals do whatever they can to reduce the demands winter places on them, to increase their chances of being here come spring. Deer invade your yard to eat fall apples or early spring tulips; mice and squirrels, along with wasps and spiders, invade your house in search of warm spots where their limited stored energy will not be drained by cold. It's going to be a tough time to be outdoors, and somehow the animals know it. That wasp wedged under the bark in your woodpile may be the queen of a new city; the spider in the corner of your porch could found a new dynasty; the mouse in your basement is the matriarch of next summer's owl food. All are just doing the job nature assigned them at a time when she's not about to make that job easy.

Copyright, S.L. White, 2008. Illustration copyright Jan Ratcliffe.
Originally published in Upbeat, November 1994.


Crafty Green Poet said...

very interesting post,

Crafty Green Poet said...

You'll probably be interested in reading Winter World by Bernd Heinreich if you haven't already read it. I'll be reviewing it on Crafty Green Poet in a week or so.

Crafty Green Poet said...

btw, thanks for your note on my blog saying you were looking for the review, it's at: