Still, as daunting and wide-ranging as Mills' project is, some preliminary answers are already emerging. Unpublished results suggest that the hare population is shifting its cycles from one year to the next to stay in sync with the weather -- an environmental response, rather than an evolutionary one. Mills and Marketa Zimova, a master's student from the Czech Republic who is studying the specifics of the hares' coat changes -- as she put it, "How do they change, when, how long does it take, and what is the cost of mismatch" -- have graphed two years' worth of information on the animals' seasonal color alongside data on snow accumulation.
On the graph, a gray line indicates the snow and a black line shows the average whiteness of the Seeley Lake hares. The lines track each other. Last year was a big snow year; autumn was fairly normal, but the spring snow stuck around much longer than usual. "The amazing thing," said Mills, tracing the lines on the chart with his finger, "is that the hares shifted remarkably." They began to change back to brown at the same time they normally do, but they stayed mostly white for about two weeks longer than they did in 2009, when there was far less snow in the spring. At Mills' second field site, near Gardiner, Mont., the snow persisted even longer. And the hares stayed white even longer, too.
Last winter, the region had the greatest number of days with snow on the ground in the past 40 years. Meanwhile, the previous winter ranks among the lowest for that period. "So in those two years, we got a window on the kind of drastic change we might expect to see over the next 80 years," Mills said. "And the result is that hares, at least to a large extent, were able to adjust the (pace of their) coat change to match the snow."
Based on other seasonal phenomena that are better understood, such as fattening up and growing thicker fur, Mills thinks temperature might control the rate of change. He's scattering temperature monitors -- little metal buttons -- around his field sites and attaching them to the hares' radio collars, to determine whether the animals have some sort of thermal regulator that helps them change quickly or slowly. The buttons measure the outside temperature, and will ideally show whether hares choose to hang out in warmer or cooler places to speed or slow their change. (If a warmer temperature could help a hare stay brown longer in the fall, for example, and if the ground was still brown around him, it might seek out a nice south-facing slope.) "Maybe," said Mills, "there's not the mismatch you'd expect intuitively."
On the bleak November morning in the Seeley-Swan Valley, we scrambled down a dangerously steep slope on the trail of a female hare. We found it in a cozy hollow under a juniper bush surrounded by a sprinkling of snow. Its fur, visible when you came up close, was predominantly white, though here and there on its small body -- on its face, its neck, its enormous front feet -- remnants of its brown summer garb persisted. Still, it was far more white than the first hare we saw.
Studying hares in the wild is a bittersweet endeavor. Biologists who study larger, long-living mammals, such as bears, might track the same individual animal for years. With hares, though, you're never sure if each encounter will be the last. "You really kind of get attached to them," Mills said. (After he finished his Ph.D., Mills couldn't bear to part with some voles from his dissertation research; he brought them along when he moved from Santa Cruz to Moscow, Idaho, where he briefly worked before coming to Missoula.)
Seitz snapped some photos and flushed the hare from its juniper hideout so the team could see it in full view. Sultaire recorded the coat color and snow cover data, while Mills got down on all fours to inspect the hollow. An excellent home for a hare, he concluded. Only a very wily -- or very hungry -- coyote would venture onto a hillside this steep.
And then it was time to head back to the truck. "See you next week," Seitz called to the hare, as we climbed back up the hill. "Hopefully."
Hillary Rosner is an Alicia Patterson Fellow. Her last story for High Country News, "One Tough Sucker," won an AAAS Kavli Science Journalism Award.
This story was funded with reader donations to the High Country News Research Fund