by Christine Wren

Lately, I’ve been watching the sun, paying attention to its path across the foothill canyons near my home. It turns out that changing daylight is the answer to a question that’s needled me for awhile—just how do certain animals know when to change color?

short-tailed weasel standing on hind legs in grass

Short-tailed weasel. Photo by Steve Hillebrand, USFWS, public domain.

I started pondering this question last fall during a hike along a small creek in the Bitterroot Mountains near the Montana/Idaho border. Late fall, but no snow on the ground. Instead, a brown understory of rotting leaves, bare shrubs, and downed timber. Out of this earthy landscape a small, white shape caught my eye. Then it moved!  A short-tailed weasel—called an ermine when winter white—tiny, but long, with its signature black-tipped tail. It skirted the creek bank, darting between bushes, and then, amazingly, moved toward me as it crossed a fallen log that spanned the creek. Mid-way across it stopped, raised up on hind legs, eyes fixed in my direction. Just as quickly, it disappeared under a rotting wood cabin along the creek bank.

I was dumbfounded. Without the color contrast, I wouldn’t have seen it. The benefit of camouflages, and hazards without it, were clear. Ermine have the ability to change the color of their fur to match different seasonal environments. Switching from brown to white in winter, then back again to brown in spring, this animal survives by blending into both landscapes. But why had it turned white prematurely, before the first snow arrived? Changing too soon, it becomes easy prey for a fox. So just how does an ermine know when to change color?

white snowshoe hare in brown landscape

Day length triggers seasonal coat change in snowshoe hares, regardless of snow cover. Photo by Lance Schelvan.

Animals respond to seasonal change in a variety of ways in order to survive. Some hibernate, some migrate, some stick around and grow thicker fur. A few special animals change the color of their fur or feathers to match changing camouflage needs. It turns out that ermine, like ptarmigan and snowshoe hare, change color because of a physical reaction to the photoperiod, or number of daylight hours. For these animals, decreasing amounts of daylight in late fall trigger hormone reactions that cause changes in the production of natural pigments in their bodies. Reduced daylight causes a chemical reaction that decreases levels of dark, melanin-containing pigments. As new fur or feathers grow in, a white coat gradually replaces the brown one. In spring, the reverse happens. Increased daylight hours cause dark pigment levels to rise, meaning brown fur starts to grow again. Temperature shifts play a role, but the amount of daylight is key to this amazing color transformation.

So, I want to retrace my footsteps along that creek this spring, keeping an eye out for a brown short-tailed weasel. Will the increased daylight coincide with a complete snowmelt? Maybe yes, maybe no. Snowfall patterns vary from year to year, but nature is constant in the time she keeps, in her annual cycles of darkness and light.

Christine Wren is a Teaching Naturalist at MNHC as well as an educator with 20 years of experience teaching in elementary public schools and outdoor environmental education programs. She earned a B.A. in English-Creative Writing and M.A. in Bilingual-ESL Education from the University of Colorado. She credits her husband, Richard, for first awakening her naturalist tendencies and their many miles of hiking together for nurturing that identity. The Visiting Naturalist in the Schools program at MNHC provides a great place for her efforts to integrate academic content with hands-on, inquiry-based learning.


This article was originally published in the Winter 2009-2010 issue of Montana Naturalist magazine, and may not be reproduced in part or in whole without the written consent of the Montana Natural History Center. ©2009 The Montana Natural History Center.

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