FOR ANIMALS, dealing with the dramatic variations in habitat brought about by changing seasons can be a big problem. There are obvious concerns, including coping with heat, cold, and fluctuating food supplies. But these are not the only challenges. Something as basic as colouration can become a deadly detriment. The brown that blended perfectly with the grasses of summer will stick out like a sore thumb when the landscape becomes white with snow. This is not a good thing if you are a predator such as a weasel, and it's an even worse thing if, like the snowshoe hare, you are the daily special on everyone's menu. Through evolution, many birds and mammals have coped with this challenge by changing colour throughout the year to preserve camouflage. The keratin cells in feathers and fur are dead, just like those in our hair and fingernails. So, unlike chameleons and octopi, which can alter their colour through the action of living cells, birds and mammals must grow an entirely new coat of fur or feathers if they are to transform their appearances. And, to make the change effective, the timing must be accurate. A wardrobe malfunction can prove deadly: turning white too early or brown too late can be as dangerous as not changing at all. The key to getting it right is provided by changes in day length and temperature. These factors trigger hormonal changes in animals that cause their bodies to start producing different pigments, and also to shed the old coat and produce a new one. The change of coats may provide benefits in addition to better camouflage. A ptarmigan's winter feathers are white because of air bubbles within the material of the feathers. This plumage provides both extremely good insulation and an amazing ability to blend into the landscape. I've watched a clearly visible group of ptarmigan in flight vanish upon landing on hard-packed snow. I knew they were there, but I could not pick them out - even through binoculars. In spring, they grow a mottled brown plumage every bit as effective for that season. The hare family, including the widespread snowshoe hare, offers many of us a first-hand look at these seasonal wardrobe adjustments. In our neighbourhood lives a great, gangly white-tailed jackrabbit. In summer, he often sleeps and grazes in our shady front yard. His brown coat makes him obvious against the green grass, but let him hop into a natural, treed and weedy area and it is difficult to see him, despite his strapping size. In autumn, his coat begins to change. By mid-winter, he wears a coat of brightest white, except for his dashing black ear tips. Blustery days may find him behind our house, rump dug into a snow bank, hunkered down out of the wind. He blends into the scene so effectively that if I'm not actively looking for him, I might walk right by. Only wary, golden eyes give him away. If the wind dies down and the sun comes out to shine on him, those, too, will disappear, leaving only a drowsy lump of winter white. - REBECCA L. GRAMBO