As I've mentioned in previous posts, I have several co-located porcupine dens in my yard. There are several dens with active porcupines within the space of 25 feet or so. They may be connected underground, or not. I really can't tell. At any rate, knowing the locations of these dens gives me an opportunity to learn more about these porcupines first-hand.
I've set up both still picture and video trail cams and recorded some good footage and pictures which seem to show at least three different individuals, although I suspect there are more. Despite being generally solitary creatures, porcupines do sometimes form communal dens in the winter. Although they are well insulated with thick fur and quills, each porcupine throws off body heat. Have several of these living heaters in a single den means the air temperature inside the den will be higher and each individual will need to burn fewer calories to stay warm. It also lowers the chance of freezing to death if the weather turns very, very cold.
Since the snow around the dens is tracked with mud from inside the dens, we can determine that either the dens go down deeper than the frost line into unfrozen ground, or the animals' body heat inside the den is sufficient to melt the frozen ground inside the den.
Fresh snowfall also gives me the opportunity to follow the nightly routes travelled by the porcupines. Today, I did just that to find out exactly where they go and what they eat. We know that porcupines eat small tender twigs and the inner bark of trees, especially during the winter, but what species do they prefer? How big is their range? And how many trees does it take to support them?
We had a significant snowfall here five days ago on top of bare ground. That means that for five days, the porcupines from these dens have been laying down tracks and trails. From previous experience, I know that porcupines (at least in winter) tend to be creatures of habit and follow set patterns. That means that rather than dozens of trails radiating out from the dens in all directions, we'd expect one or possibly two frequently used trails going to specific destinations repeatedly.
The sketch labelled figure 1 shows what I found. It's not to scale but shows the well-worn trail laid down by the porcupines from these dens. As I would have bet, there is a single trail leading from Den #1 directly to Den #2 and from there to a beech tree then on to a hemlock tree. From there, the porcupines (since they were not still in the trees) used the same route to return back to their dens. In the five days since the most recent snow at least, the porcupine or porcupines from these dens visited only two trees.
The last time I set out trail cams to view the nocturnal porcupines was earlier this winter. There could be fewer or more living in the dens now than there were then. Although I could see three that looked different enough in the previous pictures for me to believe they were distinct individuals, that is by no means an accurate population count of the dens at the time.
Examining the trees to which the porcupine trails lead gives us more information about them. Although there is inner bark available at the base of the tree, these porcupines climb high into the tree and its smaller branches to eat. Fresh scars left by their gnawing can be seen no lower than about 15 feet up. I can think of three main reasons or theories about why the porcupine should eat only the inner bark from high in the tree even though it takes more energy to climb the tree than to eat from ground level.
The first is that the porcupine is safer from potential predators while high up in the tree than it would be on the ground. Under this theory, those with a predilection for climbing before eating would fall victim to predation less often and have an evolutionary survival advantage over porcupines that stayed on the ground to eat. The porcupine in a high tree would certainly be protected from predators like coyotes which cannot climb at all. Although some porcupine predators can climb trees quite well, the backside of a porcupine climbing a tree is pretty well defended. On the other hand, once it is out on a narrow branch, the porcupine's undefended underside might be exposed. This theory is pretty difficult to prove.
A second possibility is that the outer bark is thicker at the base of the trees than it is higher up on the thinner branches, so it is more difficult for a porcupine (with relatively short chisel-shaped incisors) to gnaw through to the inner bark around the tree's base. It might wear out the rodent's teeth too fast. Most animals tend to select food that offers the highest nutrition for a given amount of effort. For this theory to be workable, the lesser chewing and digestion effort or the lower rate of tooth wear of eating thin upper bark, instead of thicker lower bark would have to outweigh the effort of climbing the tree. Difficult to prove outside a lab setting.
The third choice is similar to the second. There might be a difference in the nutritional content of the upper tree bark compared to the lower tree bark. Its composition might be different so that it contains nutrients not found in the lower bark in sufficient density to support the porcupine's health. Although this is easier to test, we'd still need a lab to analyze bark samples.
Finally, of course, it could be a combination of any of those theories, or something else entirely.
What did we learn?
What we do know is that porcupines eat the inner bark of beech trees and hemlock trees when there is thick snow cover on the ground. We know they chew through the small twigs, near the main branch dropping the twig ends to the forest floor below. They do not appear to eat these small twigs from the forest floor once they have fallen (unless they carry them back to the den, a possibility for which I found no evidence).
We know they visit a small number of trees when there is heavy snow cover, only two in this case over a period of five days. The entire length of the slightly winding trail used by these porcupines extends perhaps 100-175 feet in length. We know they do significant damage to the trees from which they eat during the winter, with the inner bark completely stripped from large sections of the upper trunk and branches, likely killing branches and perhaps even the top of the tree.
From previous experience, I know that a littering of fresh hemlock twig ends on the forest floor is a sure sign of the presence of porcupines all year round, so we know porcupines include hemlock in their diet all year round. I have also seen porcupines, especially very young porcupines, eating clover and grass in the summer.
What we still don't know
What we don't know from the above-reported observations is whether the porcupine's diet or habits change when there is not a thick snow cover. We don't know whether the small twigs are removed because they get in the way of stripping the bark off larger branches, or if the ends simply fall when they are eaten through. We don't know if porcupines eat other tree species. We don't know how many porcupines might eat from a tree at one time, how much time they spend eating, or how long it might take them to strip a tree to the point where diminishing returns necessitate finding a new tree from which to eat. We don't know if their range increases when there is less snow and travelling is easier. We don't know how many porcupines share these communal dens, the familial relationship, if any, between the members of this seasonal group, or their ages.
We don't know if a winter communal group stays in the same group or den all winter or if individuals might leave and move into other dens within a single season. We don't know much, if anything about their behavior within the den. We don't know whether one individual might stay in the same den throughout the following summer or if they all split up to find new dens. While we might find the answers to some of these questions in existing literature about the porcupine, I'd like to do some non-invasive observations of my own to learn about my own resident porcupines directly. I think I'll start by putting a trail cam high in a tree to observe the animals as they eat.
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| Despite several freezes and thaws, the well-used path depression leading from the porcupine dens is easy to find and follow. Not single set of tracks diverged from the path. Photo by Brad Sylvester. Do not copy. |
Since the snow around the dens is tracked with mud from inside the dens, we can determine that either the dens go down deeper than the frost line into unfrozen ground, or the animals' body heat inside the den is sufficient to melt the frozen ground inside the den.
Fresh snowfall also gives me the opportunity to follow the nightly routes travelled by the porcupines. Today, I did just that to find out exactly where they go and what they eat. We know that porcupines eat small tender twigs and the inner bark of trees, especially during the winter, but what species do they prefer? How big is their range? And how many trees does it take to support them?
We had a significant snowfall here five days ago on top of bare ground. That means that for five days, the porcupines from these dens have been laying down tracks and trails. From previous experience, I know that porcupines (at least in winter) tend to be creatures of habit and follow set patterns. That means that rather than dozens of trails radiating out from the dens in all directions, we'd expect one or possibly two frequently used trails going to specific destinations repeatedly.
 |
| Figure 1: Sketch of the path leading from the porcupine dens in my yard. Not to scale. Drawing by Brad Sylvester. Do not copy. |
The last time I set out trail cams to view the nocturnal porcupines was earlier this winter. There could be fewer or more living in the dens now than there were then. Although I could see three that looked different enough in the previous pictures for me to believe they were distinct individuals, that is by no means an accurate population count of the dens at the time.
Examining the trees to which the porcupine trails lead gives us more information about them. Although there is inner bark available at the base of the tree, these porcupines climb high into the tree and its smaller branches to eat. Fresh scars left by their gnawing can be seen no lower than about 15 feet up. I can think of three main reasons or theories about why the porcupine should eat only the inner bark from high in the tree even though it takes more energy to climb the tree than to eat from ground level.
 |
| Beech tree gnawed by a porcupine. The inner bark has been stripped, girdling the upper trunk. This part of the tree will die as a result of porcupine feeding. Photo by Brad Sylvester. Copyright 2012. Do not copy. |
A second possibility is that the outer bark is thicker at the base of the trees than it is higher up on the thinner branches, so it is more difficult for a porcupine (with relatively short chisel-shaped incisors) to gnaw through to the inner bark around the tree's base. It might wear out the rodent's teeth too fast. Most animals tend to select food that offers the highest nutrition for a given amount of effort. For this theory to be workable, the lesser chewing and digestion effort or the lower rate of tooth wear of eating thin upper bark, instead of thicker lower bark would have to outweigh the effort of climbing the tree. Difficult to prove outside a lab setting.
 |
Photo by Brad Sylvester. Copyright 2012. |
Finally, of course, it could be a combination of any of those theories, or something else entirely.
What did we learn?
What we do know is that porcupines eat the inner bark of beech trees and hemlock trees when there is thick snow cover on the ground. We know they chew through the small twigs, near the main branch dropping the twig ends to the forest floor below. They do not appear to eat these small twigs from the forest floor once they have fallen (unless they carry them back to the den, a possibility for which I found no evidence).
We know they visit a small number of trees when there is heavy snow cover, only two in this case over a period of five days. The entire length of the slightly winding trail used by these porcupines extends perhaps 100-175 feet in length. We know they do significant damage to the trees from which they eat during the winter, with the inner bark completely stripped from large sections of the upper trunk and branches, likely killing branches and perhaps even the top of the tree.
 |
| Hemlock twigs littering the forest floor are a sing of porcupine feeding. Photo by Brad Sylvester. Copyright 2012. Do not copy. |
What we still don't know
What we don't know from the above-reported observations is whether the porcupine's diet or habits change when there is not a thick snow cover. We don't know whether the small twigs are removed because they get in the way of stripping the bark off larger branches, or if the ends simply fall when they are eaten through. We don't know if porcupines eat other tree species. We don't know how many porcupines might eat from a tree at one time, how much time they spend eating, or how long it might take them to strip a tree to the point where diminishing returns necessitate finding a new tree from which to eat. We don't know if their range increases when there is less snow and travelling is easier. We don't know how many porcupines share these communal dens, the familial relationship, if any, between the members of this seasonal group, or their ages.
We don't know if a winter communal group stays in the same group or den all winter or if individuals might leave and move into other dens within a single season. We don't know much, if anything about their behavior within the den. We don't know whether one individual might stay in the same den throughout the following summer or if they all split up to find new dens. While we might find the answers to some of these questions in existing literature about the porcupine, I'd like to do some non-invasive observations of my own to learn about my own resident porcupines directly. I think I'll start by putting a trail cam high in a tree to observe the animals as they eat.
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