Plants such as roses and devil’s club aren’t the only prickly ones — lots of mammals have gone in for defensive spines in a big way.
There are the echidnas, four species endemic to Australia and New Guinea; they share with platypuses the habit of laying eggs, having branched off from the main mammalian radiation long ago. Endemic to Madagascar are the tenrecs, over 30 species of them (the actual count depends on what source you read); they originated from some shrew-like critters native to Africa. Hedgehogs, about 17 species of them, are native to Eurasia and Africa. The large mammalian group called rodents holds an assortment of spiny mice and rats as well as a group known as porcupines .
Porcupines are classified into two distinct evolutionary lineages represented in two taxonomic families; although both types are spiny, they are thought to have evolved those spines independently, by convergent evolution (not by inheritance from an original ancestor). The Old World porcupines (about 11 species in the taxonomic family Hystricidae) tend to live in open areas and don’t climb much. New World porcupines (about 18 species in the family Erethizontidae) tend to live in more wooded areas and are often good climbers (one species even has a prehensile tail). About 30 million years ago, some African porcupines rafted over the Atlantic to Brazil, where they prospered and eventually radiated into numerous species in South America. Then, some of those began to move north about three million years ago, when South and North America got connected via the Panamanian isthmus. One of them gave rise to our North American porcupine, which has expanded over much of the continent, now occupying a range from northern Mexico to northern Alaska.
Courtesy Photo / Amy Sherwin A porcupine shreds cottonwood leaves.
North American porcupines typically occupy wooded areas, but they also roam out into grasslands and tundra. In some regions, they usually have home ranges less that about thirty acres, but in the Alaskan Interior, they seem to be considerably larger. And in the fall mating season, the males roam more widely. When a male finds a female, the ensuing courtship can be rather noisy — a pair outside my window, years ago, kept me awake part of the night with their whines and screeches and groans. If two males want the same female, there can be a fight, with more noise. As a courtship proceeds, eventually the male squirts urine on the female and, if she is ready, they mate — very carefully. The male is positioned behind the female, facing an array of quills, but she raises her tail over her back, covering at least some of them, and the underside of the tail has hairy bristles but no quills.
About seven months later, if all went well, a porcupette is born in spring. Its quills are all in place but they are soft; they harden up soon after birth. Their incisors are white, initially, but gradually the front surface turns orange and hardens, as iron is taken up from the diet (mostly milk at first, but with increasing amounts of foliage). The softer back sides of the incisors wear away, leaving sharp chisel-teeth. Young ones stay with their mothers through the summer and early fall, and then become independent (while she goes off to find a male). The young ones continue to grow and are mature by the following fall.
A porcupine wears thousands of quills over most of its body. They detach readily upon contact with an importunate dog or a would-be predator. No, the quills are not shot out or thrown! Wily predators know to attack the vulnerable belly, flipping over their prey to expose the unprotected underparts. Or they grasp a porcupine by its nose and toss it around until it weakens; fishers are reported to be very good at this technique. Porcupines are rather slow-moving critters, so running away from a perceived threat is not a good escape option. A threatened porcupine may scramble up a tree, which may work for dogs but not fishers. Or they hide forequarters between tree roots or in other crannies, so that the attacker is faced with a backside of quills and a muscular, thrashing tail loaded with quills on top.
Porcupines are herbivores, eating chiefly vegetation but sometimes adding fruit or fungi. They have a large cecum attached to the intestine, and that’s where much of the fiber digestion takes place.
In spring and summer, they focus primarily on fresh vegetation, which has relatively few protective chemicals and is easy to digest. They are good swimmers and can forage on succulent aquatic vegetation. And they readily climb up cottonwood trees to shred the new leaves and eat catkins, sometimes staying up there to snooze on a branch that (to me) seems too small to be safe and secure.
Courtesy Photo / Mary F. Willson This photo edited by Bob Armstrong shows new growth (indicated by an arrow) that can be seen at the top of a tall spruce that was trimmed by a porcupine.
In winter, the diet includes mostly inner tree bark and conifer needles, which are fibrous, tough, poor in nutrients (such as nitrogen), and laced with defensive chemicals that need to be de-toxified (which costs energy). The metabolic rate of porcupines is low, as is typical for herbivores of their size, and may decrease somewhat in winter, but not too much, because winter environmental temperatures are often very low, and some energy is needed to maintain body temperatures at normal levels. They can conserve some body heat by denning and the winter fur coat provides some insulation. Although their home range in an Alaskan winter may be enlarged, they also spend some time just sitting in a tree and conserving energy. Porcupines depend on using stored body fat in winter and reportedly can tolerate some seasonal nutrient imbalances and low food intake, but they nevertheless often lose weight on the poor winter diet, and starvation deaths sometimes occur. In most cases, however, the lost weight can be regained rapidly from the good spring forage.
There is evidence that porcupines are choosy about what trees they favor for food. Some local hemlock trees have clearly had their bark chewed off repeatedly, in some cases even in different years, while other seemingly similar hemlocks remain unscathed. Down south in ponderosa pine country, porcupines are moderately selective of which individual pines they chew on, tending to favor those with less resin (but Abert’s squirrels are even more selective of specific individuals).
Spruce needles are a favorite food—we often see porcupines up in a spruce, nibbling off needles. I suspect that they are being somewhat selective in their choice of needles, because young needles have fewer defenses and are more digestible than old needles. I have seen porcupine gnawing on the recent growth of very small spruces, leaving the older needles that were within easy reach. It seems that they are willing to work for this selectivity too—one day, a couple of years ago, I looked out my front window and saw a lump at the very top of a tall spruce (I’ll hazard a guess at about a hundred feet tall). That lump was a porcupine, systematically demolishing all the young needles at the top of the tree (except the leader). Did it climb all the way up there for those particular needles on that particular tree, by-passing young growth at the ends of branches where hanging on might have been chancy? The tree continued to grow, putting out new twigs just below the leader (photo).
I have to hope that it got down safely—it was a long climb up and would have been even more awkward and difficult coming down backwards (although the bristles under the strong tail help provide bracing and balance). Despite the fact that they spend a lot of time in trees, falling out of trees is a fairly regular and sometimes lethal sort of accident.
• Mary F. Willson is a retired professor of ecology. “On The Trails” is a weekly column that appears every Wednesday.
