Despite obstacles, Alaska weasels thrive

Posted: Friday, February 05, 2010

The body plan of weasels and many of their relatives in the family Mustelidae is basically a long, slender cylinder with four short legs. Their body sizes vary, and some are more chunkily built than others, but the basic plan remains. And that body plan is not well suited to cold climates. The long, narrow body has a high surface area to volume ratio: a relatively large surface area from which body heat can be lost but a relatively small volume of tissue (muscle and organs) that generates heat. The differential between surface area and volume is especially marked in animals whose total body size is small.

Mary Willson / For The Juneau Empire
Mary Willson / For The Juneau Empire

In addition, when these animals move quickly, they bend that long body to lengthen the stride, using body length to compensate for the short legs. This style of locomotion makes it impossible to add lots of insulative fat-a fat weasel just couldn't bound over the ground, because its short legs couldn't raise a fat body off the ground.

So how do northern weasel-like creatures deal with a cold environment? They do so in various ways, but there are at least as many unanswered questions as there are answers, leaving us with lots of food for thought.

All of the weasels in Alaska eat a lot, to fuel a high metabolic rate and heat production. They are carnivores, so their food provides protein and fat, which have more calories and are more readily digestible than many plant foods. Most of them can erect their fur, creating a thicker layer that traps air, which helps insulate the body.

The smallest mustelids in Alaska are the ermine or short-tailed weasel, weighing up to seven ounces or so, and the least weasel (living in the Interior), weighing up to about three ounces. Because they are so small, they have a big surface/volume problem. These two species are the only ones in the region that turn white in winter. One consequence of white fur is better insulation: white hairs are hollow, the enclosed air spaces help insulate the small bodies.

In addition, the white winter fur may provide camouflage in a snowy landscape. A friend watched an ermine run across a local road, and the only readily visible part of the animal was the black tip of its tail, waving gently behind the running animal. When the ermine stopped on the far side of the road, and stood up on its hind legs, it was more visible as a living thing. The camouflage probably works to protect the small weasels from predators such as owls or hawks.

But the picture is more complex than that. Sometimes these weasels turn white even if there is no snow, whereupon they are highly conspicuous against a background of brown leaf litter and twigs. The change from summer brown to winter white may be triggered by shorter day lengths or lower temperatures. But if snow does not accompany those triggers, the triggers are not well suited to improving the animal's ability to elude predators. Presumably, natural selection works toward improving those triggers!

The biggest weasel-like animal in Alaska is the sea otter. Weighing an average of 60 pounds (females) to 100 pounds (males), the surface/volume problem is less than for the tiny weasels. The sea otter is fully aquatic, so it doesn't have to deal with super low air temperatures. But, on the other hand, water conducts heat away from a warm body much faster than air does. To keep cold water away from the skin, sea otters have exceptionally dense fur, the densest fur of any mammal. There are hundreds of thousands of hairs per square inch, even as many as a million in some parts of the body. This wonderful fur coat historically led to their over-hunting and nearextinction.

Sea otters lack the ability to erect their hairs, which would increase the ability of the fur to trap air. The down side of trapping air, from the perspective of the sea otter, is that the trapped air would increase buoyancy and make it harder to dive. Even though they don't run around on land, they retain the slim build of the more terrestrial relatives, and do not put on much fat. They rely on that dense fur coat for warmth, but that coat requires a lot of grooming. Grooming occupies as much as 10% of the sea otter's day and raises the metabolic rate over 50%, so there is an energetic cost to maintaining a beautiful coat.

River otters weigh up to about 35 pounds (for males); females are smaller. Unlike sea otters, they have to deal with low air temperatures when traveling overland in search of open water and also with cold-water immersion while hunting. River otter fur is about half as dense as that of sea otters and can be erected to increase insulation when out of water. The under-fur is somewhat wavy, so that air is trapped among the hairs. This provides insulation and also helps keep water from contacting the skin. What little fat accumulates is mostly in the tail. They use dens for shelter, often occupying old beaver lodges and burrows.

Wolverines are similar in size to river otters. The guard hairs of wolverine fur are known for their ability to shed frost, hence their use as ruffs on parka hoods. The specific benefit to the original owner of those guard hairs is not clear. Mink and marten are much smaller. Their surface/volume problem would be significant, because they are quite small. They use dens for shelter. But I have not found specific information about how they keep warm in the cold.

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

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