Plodding -- that's the only way to describe them. Deep snow blankets the winter landscape of the bison in Yellowstone National Park and plod they must through both the winter and the landscape. These mighty beasts don't waste a calorie of energy if they can avoid it.

Their apparent lethargy is deceptive; it doesn't imply weakness or slowness, but essential energy conservation in the harsh winters in that scenic swath of the United States straddling northwest Wyoming and parts of southern Montana and eastern Idaho.

Content to slowly and rhythmically push at the snow, a bison uses its massive head like a bulldozer blade; gentle sideways swings pile up mounds of snow, leaving grass forage exposed and their faces frosted. Their pace seems sluggish, gentle, but I watched, impressed, as a small herd disturbed by a snowmobile broke from standing into a fast trot -- suddenly they were an unstoppable, awesome mass in motion.

Conflict in paradise

Interactions with buzzing, roaring snowmobiles are a significant issue in Yellowstone -- and a controversial one, as recreational users of the groomed roads disturb the animals' peace, so putting themselves in conflict with wildlife watchers in that winter paradise. I heard tell of one witless snowmobiler who playfully sidled up to a bison and slapped it on the rump. Energy conservation not withstanding, the bison was miffed at being manhandled. With a casual toss of its bulldozer head, it lobbed man and machine off the road, across a ditch and into the forest. My sympathies lay with the bison. Winter is a stressful enough season without such stupid human behavior to boot.

Energy is at a premium in winter. The low sun provides little warmth, and much of its energy reflects right back off the snow, though the dark coats of animals such as the bison help them absorb heat to some extent. Thermal areas such as Yellowstone's geysers and hot springs are important, too, and around them -- as around outdoor onsen in Japan -- insects remain active all year, in turn helping small birds eke out a winter living. In Yellowstone, bison are naturally drawn to the thermal areas, where they bed down for the night, though in the morning they still awaken rimed with hoarfrost.

A little warmth here, a little sun there, and perhaps some grazing in between -- whichever way you look at it though, winter is a tough season and Yellowstone's wolf packs howl their willingness to snap up any animal that runs short of life-giving energy. The wolves won't waste energy, either, and are quick to recycle weakening elk, deer or bison.

It was with memories of how life plods through those Yellowstone winters and the challenge such conditions pose that I pitched my tent recently in a similar winter landscape beside Lake Kussharo in Akan National Park.

The predawn temperature plunged to minus 18.5, and when I woke in the morning it was to whiteness -- inside my tent, where my breath had condensed on the fabric and everywhere was thickly frosted except, thankfully, my face. I lay for a while there in my high-tech, insulated fabrics, enjoying the cozy warmth and reveling in the challenge and pleasure of winter camping. But I, of course, could escape the elements at any time, unlike the creatures I was there to watch.

Minutes from death

Eventually, I crawled out of my thick, down sleeping bag, shed my ultrawarm Finnish parka, stripped off my merino wool leggings, shed my long-sleeved, roll-neck shirt and took off my thick socks. For just a few moments I was completely exposed to the full blast of the frigid morning air -- and I knew that a matter of minutes would have been all I could have survived.

Clutching my towel, I hastened to the warming waters of the lakeside hot pool, and as I did so I heard a chattering. No, it wasn't my teeth; it was the hard call of a wren -- a small dark-brown cock-tailed bird that was already fossicking about among old tree roots near the pool. The sun wasn't yet up, but the wren was already busily foraging.

I had weathered the night comfortably cocooned on a luxuriously thick air mat. The wren, so tiny I could have hidden it in my closed fist, had survived the night in its own plumage alone -- though it may have huddled with others of its kind in a tree cavity for a little extra warmth.

The metabolism of a small bird like that is fast. To maintain its high body temperature overnight it must consume its body reserves, and to maintain those reserves it must feed almost nonstop throughout the short winter days. In late December, there isn't a moment to lose -- hence the wren's predawn start. This is serious fossicking, with a terminal deadline if the food supply runs short.

Small birds like this may only have enough body fat to carry them through one or two days of poor feeding. So, if freezing rain coats branches, twigs and buds with ice, and they can no longer access their food of insect eggs and larvae, they are dead in a day or so.

For those species inhabiting seasonal, temperate regions such as much of North America, Europe and Japan, weathering winter is a major challenge. Strategies vary enormously. Some birds migrate and avoid the issue completely; some mammals accumulate fat in autumn and hibernate, or store food and live off it during winter. Among the insects, some overwinter as adults, freezing and then thawing back to life in spring, though many overwinter in the egg stage -- as tiny genetic packets capable of sustaining incredible desiccation and extremely low temperatures.

Among the larger species, there are those -- such as the bison -- that seem to make it through from autumn to spring on a combination of their body reserves and what little food is available during winter. Though they eat daily, in the extreme cold of midwinter they cannot consume enough to maintain their body temperatures without cutting into their fat reserves. So even as they eat they lose weight. If they failed to accumulate sufficient reserves in autumn, and food runs out in winter, their weight will dwindle to a point where they cannot survive. Once their energy reserves are gone, they must rely, day to day, on the food available, and a few days of extreme conditions preventing access to food will finish them off.

As you don a hat, pull on warm gloves, put on an overcoat or slurp a steaming bowl of noodles to keep warm, imagine what it must be like to be outdoors day and night with only one set of "clothes" to wear. It's amazing that such small creatures as birds and rodents can make it through winter at all -- something that makes it all the more enjoyable to see these survivors again come spring.