Hole in the ground becomes winter den

This hole goes as deep as I can see, twisting around
underground boulders. Now it appears that porcupines are
using it for a winter den.
Photo by Brad Sylvester. Copyright 2012. Do not copy.

During the summer, I found a deep hole in the ground in my backyard forest. It's right at the base of a tree and if you peer into it, you can see about six feet down although there are numerous very large stones (two foot diameter or more) causing the hole to twist and turn as it descends. It seems to go much deeper. At any rate, it's well below the frost line for New England which is somewhere just under three feet as I recall from my study of architecture in high school.

Two days ago, we had about three inches of snow and I happened to walk through the part of my yard where the hole is. There was a very well-trod path coming out of the hole and proceeding about 15 feet to another similar, but smaller hole, in the ground. From there, the path continued on and split. One fork went to the base of a nearby pine tree, straight to the trunk. From the tree, it appeared as though it came straight away from the trunk and continued into the forest.

The tracks seemed to show that the path went up the tree and came back down before continuing. The paths were very well traveled as I mentioned. So, it was impossible to make out individual prints to see what creature or creatures had made the trail. There was no scat or other evidence to help determine what might be using the dens.

Today, I went back to install my trail cam in a position to catch photographs of whatever it is. There was new evidence. Urine and scat right outside the holes in the ground suggested that it might, indeed, be what I suspected. A communal den of a fairly common New England mammal that does not hibernate in the winter: the porcupine.

Communal Porcupine Dens

Second suspected porcupine den located just 15 feet from
the previous one. One trail leads to the other opening,
and one leads further into the forest.
Photo by Brad Sylvester. Copyright 2012. Do not copy.

Porcupines generally have solitary or family dens, but in periods of severe winter weather, they may group together in a single den which benefits them all by the sharing of the communal body heat. I believe these two holes in the ground are communal porcupine dens. Today, I placed my trail cam in a position where it will almost certainly get some good identification photos of whatever is spending its days in these holes.

Which brings us to the question of why don't porcupines hibernate or go into a winter torpor like so many other small mammals. I would guess that it has to do with the preferred diet of porcupines. They eat the small, tender twigs of evergreen trees, green bark, and the young evergreen needles. They especially like hemlock. Of course, as the name says, evergreen trees stay green and full of porcupine nutrition throughout the winter. There is no need, therefore, for a porcupine to shut down its metabolism and wait for the new growth of spring. They will also eat fallen fruit when it is available.

Signs of Porcupines

As an aside, there are two sure signs of porcupines. First, lots of bits of the ends of hemlock branches littering the forest floor under the tree. The porcupine climbs out on the branches to eat the small twigs and to reach the newest needle growth. It eats the needles from the small twigs, but as it does so, the branch tips are discarded and fall to the ground. Porcupines, being both lazy and creatures of habit, will revisit the same few trees over and over again around its den, relying on the growth of those trees, especially during the spring and summer months to keep the porcupine's plate full.

Porcupine Droppings
The second sure sign of porcupines, is an excess of droppings outside of a ground level tree hollow, small cave or other suitable den entrance. Porcupine's are not meticulous about the placement of their droppings. Very large piles of droppings of varying ages will, therefore, accumulate outside the entrance to a porcupine's den over time. As they walk through it several times a day, trailing their dragging tails and quills, they will form a stained trail into and out of the den. On the positive side, the scat is made up almost entirely of undigested cellulose lacking in strong odor. Procupine droppings are a bit like rounded, oblong deer droppings, but tend to have a pronounced curve, making them almost "C" shaped. Other than outside the den where they accumulate, they'll be present in much smaller piles than deer droppings, or even individual pellets.

A trail leading straight into a tree trunk and then away
indicates an animal that climbed up the trunk such as
a squirrel or porcupine. These tracks are too big for
a squirrel.
Photo by Brad Sylvester. Copyright 2012. Do not copy.

Porcupine Winter Behavior

As long as a porcupine can withstand the cold temperatures or winter, they can find plenty of food to sustain them even in the deepest snows of winter. By grouping together during the coldest weather body heat of many porcupines is combined to raise the ambient temperature of a den, which may be underground or in a large tree hollow. Together they can keep the temperature in the den much higher than any individual could by itself. That means fewer calories need to be burned to stay warm. In extreme conditions, it means the difference between surviving and freezing to death.

Of course, when it comes to the "whys" of animal behavior, we can only make assumptions based on the available evidence. There may be some other reason for communal dens during the harshest part of the winter, but Occam's Razor says that we should first assume the simplest explanation is correct until additional evidence disproves it.

I'm actually very excited to find dens of any mammal in my yard because it offers the possibility of regular and close observation of the den's occupants to learn more about their behavior than I might learn by simply catching pictures of them visiting feeding stations or walking along a common game trail.

Hopefully, one or more of the porcupines, assuming I am right in my guess of the occupants of these dens, will choose to raise offspring here in the spring. That opens up the possibility of me putting a remote camera inside the dens in addition to a camera outside the den to observe the kits as they develop.

I can hardly wait.

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Update: My new YouTube channel: @WhatLivesinmyYard talks about the porcupine in Episode 1.