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On the Trails: Wildlife CSI

Posted: November 30, 2012 - 1:00am
This short-tailed weasel, or ermine, has a coat that is in transition between seasons. The black tail tip is present in all seasons.                     Courtesy of Bob Armstrong
Courtesy of Bob Armstrong
This short-tailed weasel, or ermine, has a coat that is in transition between seasons. The black tail tip is present in all seasons.

In early November, there were a few late coho in Steep Creek, and a friend saw a pair that seemed to be close to spawning. They hid out under a ledge of ice, in a little pool near some overhanging branches. That was at 9 a.m. At noon, I happened to visit this location, just checking for anything of interest. A big patch of scarlet stained the snow on my side of the stream. Of course, I had to look more closely. Then I found what was left of a salmon: the spine, picked clean, and the skin, turned inside out and covering the tail. I saw no live fish.

All around the bloody remnants were raven tracks, and ravens still perched in nearby trees. Occasionally one would come down and pick minute bits from the bloody snow. But did ravens catch the salmon, themselves? That seemed unlikely, even if they had worked as a team (which they do, sometimes).

Could a mink have been the hunter? There were mink tracks in the snow a few yards away, and some indeterminate, bloodless drag marks on the bank. A full-size coho would seem to be a challenge for a mink that weighs perhaps half as much, but a mink can carry off a chicken, so we can’t rule it out. So, possibly an otter, accounting for the drag marks on the bank, but there were no definitive footprints. An otter could certainly snag a coho from under the ice.

An adult eagle sat, proprietarily or hopefully, in a cottonwood just upstream. No eagle tracks were visible under the carpet of raven tracks, but an eagle could surely grab a coho — and open it up for the ravens to scavenge whatever the eagle left.

Our detectival skills were inadequate for the case!

A few days later, on the west side of Mendenhall Lake, Parks and Recreaction hikers found a matching scarlet stain, but no carcass, on the snow, and a big fish was stirring the nearby waters where a small stream entered the lake. This could have been another would-be spawner that had just lost its partner. A little crowd of eagles watched over the blood-smeared snow, perhaps hoping to nab the remaining fish. And the ravens had been here too, making tracks all over the site.

As we passed the small, wooded almost-island on the west side of the lake, we spotted an ermine (a.k.a. short-tailed weasel), dashing over the ice from the island to the shore. It was just a blur of motion, marked by the black tip of the tail. Its white fur blended superbly with the snowy background.

The white winter coat of the ermine was fine camouflage on the snow. But that lovely fur would be a visual stand-out against the brown leaf litter of the shoreline forest. The ermine’s seasonal change of coat color was out of sync with the existing background in the forest, making it more likely that an owl or other raptor might make a grab for its slender body. The color change is probably regulated mainly by hormonal responses to daylength and perhaps temperature. But there may be an out-of sync transition time in fall, if snow does not come with the shorter days and lower temperatures. And there may be another transition time in spring, as days grow longer and warmer, if brown fur comes in before the snow is gone.

Are ermine whose fur does not provide camouflage in the transition seasons able to choose habitats with more protective cover, to reduce the risk of predation? The evidence is equivocal. There may be some slight compensation for having a prematurely white coat in fall: The winter coat is thicker and provides better insulation, retarding heat loss (and simultaneously slightly reducing incoming heat gain from solar radiation). In spring, however, prematurely brown coats would mean loss of insulation and possible energy costs just to keep the body warm. And besides, it would be possible to change color without changing the thickness of the fur. In short, the problems encountered by coat color changes that are out-of-sync with the color of the background have apparently not yet been resolved by research.

• Mary F. Willson is a retired professor of ecology.

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