On the Trails: High tides and swimming voles

A vole is swims for its life as the high tide came in.

Across their geographic range, long-tailed voles live in a variety of habitats, including meadows and marshes, forests and thickets, alpine tundra and rocky mountain slopes, and around streams and lakes. But those that live in salt marshes and tidal sloughs face a special hazard. A big high tide floods their living space several times a year. Then, they have to run — or swim — for their lives.


Mid November brought some 20 foot high tides to Juneau. That’s high enough to cover parts of the airport dike trail with a few inches of salt water. I strolled out there, with a couple of friends, to see what might happen when one of those monster tides came in.

As the water approached the trail, we began to see voles hastening toward shore. A small, dark form popped up out of the water and scuttled quickly into a heap of bent-over grass on dry land. Another one shot across the trail up into the edge of the woods. Yet another dithered back and forth across the trail from one snow-covered stack of grass to another, finally burrowing through the snow and disappearing.

Not all of the voles were so fortunate. We watched one struggle across a patch of slushy ice, going ever more slowly, until it came to a full stop. It lay on the slush, almost still, and gradually sank down out of sight. Another one swam vigorously across some open water, began to climb out on some slush, but couldn’t get its frantically paddling hind legs up on the surface. After a few minutes, its little snow-capped head slid down into the dark water. I wished I had a long-handled net with which to rescue them!

When high tides force the voles out of their homes, they would commonly face a major risk of becoming prey for some raptor. But no predators appeared, to snatch the doomed voles before they sank down. Our presence so near may have deterred the short-eared owl that was hunting out over the floods. Short-eared owls love to capture long-tailed voles in the salt marshes of Juneau. A Juneau biologist once observed a short-eared owl diving completely under the water while hunting a vole. Bones of this vole commonly appear in the regurgitated pellets of the owl. Our presence may also have deterred the eagle that was flying low and scaring the ducks and the raven that arrived just too late for lunch. Something else will no doubt scavenge the little carcasses.

We watched about eight voles trying to get to shore, and of these, two didn’t make it. The slushy ice became a trap of sorts. Perhaps warmer weather, with no slush on the surface of the water, would have favored better passages to shore. Moreover, in general, voles have a high metabolic rate for their body size, and the rate increases as ambient temperatures drop toward freezing (effects below freezing unknown). So the voles can keep the body warm at cool ambient temperatures, but at the cost of using up energy. In water that is close to freezing, swimming voles would presumably expend a lot of energy both for warming (a bigger problem because they lose heat faster when they are wet) and for locomotion. At some point, they might simply run out of energy to stay warm and continue toward shore.

Long-tailed voles have not been studied as intensively as some other species of vole. However, they are reported to have an average lifespan of less than a year, and a female may only produce two litters of young in her short life. One study in forested habitats in Southeast found that females survived better than males throughout the year. It would be interesting to compare the life histories of the long-tailed voles living in the salt marshes and sloughs with those from other habitats.

A big question, in my mind, concerns the behavior of the successfully escaping voles near the dike trail. After a big high tide, do they try to go back out into the area that was flooded? If they had nests out there, the nests would be soaking wet. And what if there are two or three really high tides on successive days, as happened in November. Do the displaced voles give up and just stay on the margins of the marsh?

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


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