At the beginning of each winter, I snowshoe past a few rock ledges near my house to see if there are any local porcupines in residence. Some winters there is a lot of activity, but during others, I find none. Recent findings by biologists from Quebec University at Rimouski show a correlation between porcupine density and solar cycles. Amazingly enough, it turns out that the sun may set the rhythm of population fluctuations of porcupines in the woods and, perhaps, my chances of finding them each winter.
Porcupines are most at risk during the lean winter months, losing 25 percent of their body weight and dropping from roughly 21 to 15 pounds between November and April. Porcupines are herbivores, and their winter forage is restricted to poor-quality foods like the inner bark of sugar maple or the bark and foliage of hemlock. Hemlock groves are not only good deeryards but also good porcupine winter habitat because they provide both food and a favorable microclimate. Add a rock ledge with an overhang or a large hollow tree for a nearby resting area, and you have found prime winter real estate.
Ilya Klvana, a graduate student from Quebec University, took advantage of the porcupine winter foraging habit to create a 130-year index of porcupine populations on the south shore of the Saint Lawrence River by precisely dating each feeding scar on a tree trunk. She was able to assign an exact age for each scar by using a tool that removes a pencil-sized core of wood when screwed into the tree trunk both directly into the center of the scar and then beside the scar, where the tree grew uninterrupted. Since there was a direct correlation between the number of annual scars and the annual porcupine population, she was able to amass a long record of porcupine population density in the region.
The most critical nutritional element for porcupines is nitrogen. They slowly lose weight over the winter because of very low nitrogen levels in winter forage. They are able to live on such poor foods because of their unusual ability to reduce fecal nitrogen loss, slow their metabolisms, and digest incredibly high-fiber foods over extended periods of time.
With the protection of more than 30,000 sharp quills on their bodies, porcupines can afford to waddle slowly through the woods. In fact, they are nicknamed “dead man’s food,” because their top speed is a mere 2 miles per hour, allowing someone lost and half starved deep in the woods to catch one for survival food. But their biggest predator is the fisher. Fishers specialize in avoiding quills during the kill. A fisher will circle a porcupine, repeatedly biting its face. After it has severely wounded or killed it, the fisher flips the porcupine over so it can feed from the underside, where there are no quills. Coyotes and great horned owls are also major predators of the porcupine.
Perhaps the biggest predators, however, are vehicles. The porcupine’s herbivory affects its salt balance, which causes it to lust for the crystals. Where is a good place to find salt after a long winter of fiber? Along the nearest paved road, of course, and that can spell disaster for the slow walkers.
With all this in mind, how might the sun help regulate porcupine populations? Klvana and her colleagues noticed that the 130-year index of porcupine scars had an unusual pattern: every 22 years or so it peaked, with minor peaks every 11 years. The researchers also knew that every 11 years, the sun has an increased intensity of sun spots and solar radiation, with higher peaks roughly every 22 years. This phenomenon has been linked to precipitation fluctuations, cloud cover, St. Lawrence River flow rates, sea water temperature, and upper atmosphere temperature.
From 1968 through 2000, major peaks in sunspot numbers occurred in 1871, 1893, 1917, 1938, 1958, and 1980. These were followed by major peaks in porcupine feeding scars about five years later, in 1876, 1899, 1922, 1943, 1963 and 1987. Klvana was able to demonstrate that these two cycles, as well as cyclical winter precipitation, were statistically correlated. She speculates that the sunspots affect snowfall amounts, which in turn may affect both birth and death rates of porcupines. However, further evidence and hypothesis testing are needed to help confirm this interesting correlation.
If this turns out to be true throughout the region, what does the future look like for the population I visit near my house? A quick look at the sunspot data on the World Data Centre for Sunspot Index from the Royal Observatory of Belgium shows that there was a peak in activity about two years ago. I’d better get ready to find those sun-grown porcupines a few years from now.