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What porcupines do in winter (besides eat and sleep)

Posted: January 20, 2012 - 10:42am  |  Updated: January 20, 2012 - 10:49am
The unique tracks of porcupines in snow.  Rob Fontaine
Rob Fontaine
The unique tracks of porcupines in snow.

Have you ever seen a porcupine? Compared to other Alaskan wildlife, the porcupine seems to be ignored, but talk to any dog owner and you'll likely hear some stories. Porcupines can weigh up to 30 pounds, making them the second largest rodent in North America (only the beaver is larger). Covering the porcupine's back and tail are quills, which are actually modified hairs. Porcupines do not throw their quills as some believe. These quills easily detach, but only through physical contact with a porcupine.

Fun Fact 1: The porcupine's scientific name, Erethizon dorsatum, means "the animal with an irritating back." The common name "porcupine" comes from two Latin words: porcus and spina, giving porcupines the nickname "thorny pig."

Fun Fact 2: Porcupines often impale themselves with their own quills. To prevent infection, porcupine quills are coated with an antibiotic.

Porcupines are found throughout most of Alaska including the Kenai National Wildlife Refuge. They live in conifers or mixed forested areas but can also be found in deciduous forests, scrubby tundra areas, and rocky slopes. Porcupines are most often seen in trees, but you can encounter them almost anywhere at any time of the year, even in winter.

During winter, most smaller rodents conserve body heat and reduce energy loss by hibernating, burrowing under the ground or snow, or by eating cached food. Porcupines, on the other hand, don't use any of these strategies, spending most of their time in trees, exposed to below freezing temperatures and harsh winds. They choose to stay active throughout the winter, if moving slowly and sleeping a lot can be considered "active."

Fun Fact 3: Despite spending time in trees, porcupines are not graceful climbers. Many biologists have found porcupines with previous bone damage, suggesting they fall out of trees often.

Surviving winter conditions is simple. Porcupines eat their way through winter. Porcupines are herbivores, meaning they only eat vegetation matter, but the vegetation they eat depends on the season. During spring and summer, porcupines eat mostly green leaves and buds from deciduous trees such as willow, birch, aspen and cottonwood, foods high in protein. As summer progresses, tannin builds up in these trees to levels that porcupines have trouble digesting. As high quality food decreases, porcupines then have to eat more.

Winter foraging behavior isn't well known for porcupines. What biologists do know is that the nutritional value of the food porcupines eat during winter is only slightly better than cardboard. Research conducted in the Anchorage area found that porcupines mostly eat the inner bark of predominately white spruce and paper birch. In addition, they eat white spruce needles, which are filled with chemical compounds (essentially toxins) that deter other wildlife from eating them. However, porcupines are able to consume these low-nitrogen high-fiber foods because of their ability to absorb what little nitrogen is available. These physiological adaptations that allow porcupines to survive winter here on the Kenai also allow them to inhabit harsh environments ranging from tundra to deserts where forage is also nutritionally poor.

Beneath the trees that porcupines are in or have been in, you'll often find small twigs and branches that a porcupine drops after breaking it off and chewing on it. Snowshoe hares often come to eat the porcupine's "leftovers," which then attracts lynx and coyotes. In a way, these trees that porcupines favor become feeding stations.

To handle a sparse diet like this throughout the winter, porcupines start their winter with a lot of body fat. Like other northern herbivores, such as moose and mountain goats, porcupines spend summer and autumn putting on weight. A recent study by Jessica Coltrane and her colleagues found that porcupines averaged around 60 percent body fat at the start of winter. By April, porcupines lost roughly 35 percent of their body weight, mostly body fat. They lost almost no lean body mass during this study period, meaning that despite a poor diet, they fared through the winter pretty well. This behavior of putting on weight isn't typically seen in other rodents. Other northern non-hibernating rodents, such as muskrats, rely on food caches to get them through the winter. Unlike porcupines, muskrats won't use body fat until their cached food has almost run out.

Fun Fact 4: To put it into perspective, the body fat of most non-hibernating rodents makes up only 3-8 percent of their body mass, compared to the 60 percent found in porcupines at the beginning of winter.

Another way porcupines get through the winter is by denning when they aren't eating. Den sites aren't very elaborate and usually just consist of any shelter, such as a hollowed log or even a rock crevice.

While it may be hard to actually find a porcupine, their winter eating habits make it easy to see where they have been. In order to reach the inner bark of trees, porcupines scratch and bite away at the bark of trees.

Porcupines are also slow moving animals, often "waddling" more so than walking, so watch in deep snow for tracks that plow through the snow, leaving a winding trail (see photo).

So the next time you're taking a walk through the refuge, watch for the telltale signs that a porcupine has come before you!

Bethany "Bennie" Johnson is a recent graduate in Wildlife Science from Virginia Tech. She is currently working as a biological intern at the Kenai National Wildlife Refuge. You can find more information about the Refuge at http://kenai.fws.gov or http://www.facebook.com/kenainationalwildliferefuge.

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