North American porcupines are the second largest rodent in North America. They weigh between 5 and 14 kg, and are 600 to 900 mm long. Only beavers are larger. Porcupines have a stout, slow, lumbering form and a spiny coat. They usually appear dark brown to black, although some of the hairs and spines on the backs of these animals are yellow.
Spines called quills are found on the backs of porcupines from the head to the tail. Many of their quills are fringed with white, which makes them stand out against the dark fur underneath. This white on black marking pattern warns even color-blind predators that porcupines are dangerous. Quills are usually 75 mm long and 2 mm wide. They have microscopic barbs on the tip which help them stick into a porcupine's enemies. Each porcupine has approximately 30,000 quills. (Kurta, 1995; Roze, 1989; Sweitzer and Berger, 1997; Vaughn, et al., 2000)
Porcupines have special adaptations to help them chew. They do not have canine teeth. This allows an animal to draw in its lips while gnawing. They also have a special arrangment of their jaw muscles which helps them to chew more efficiently. (Vaughn, et al., 2000)
All porcupines have long claws. These claws help them climb on both large tree trunks and surprisingly small branches. The palms and soles of porcupines have a pebbly surface and no fur. Along with their keen sense of touch, this special texture on the hands and feet improves a porcupine's grip. Because they are so good at gripping trees, porcupines can even stay in trees using only their hind feet to hold on. This frees their forelimbs for use in eating. (Roze, 1989)
North American porcupines are special among their close relatives, because they are able to withstand cold. No other porcupines are able to tolerate such cold temperatures. (Roze, 1989)
Male porcupines are larger than females. This is because bigger males are able to have more mates. Because of this, they have more offspring than smaller males do. (Sweitzer and Berger, 1997)
North American porcupines, Erethizon dorsatum, have the northern most range of all porcupines. They are found throughout most of Alaska and Canada, in the northern part of the Great Lakes region, as well as throughout the west and northeast regions of the United States. Porupines in the forests of New York and Massachusetts, in the Great Basin Desert, and in the woodlands of Texas have been well studied. (Griesemer, et al., 1998; Ilse and Hellgren, 2001; Roze, 1989; Sweitzer, et al., 1997)
Porcupines use a variety of habitats. Because they are found throughout the continent of North America, porcupines can be found in many different climates, and at many elevations.
Porcupine habitat varies geographically. Porcupines live in open tundra, deciduous forest, and desert chaparral. In the Pacific Northwest, they spend most of their time on the ground. In New York, porcupines are found mostly in trees. In Massachusetts, they are found to spend 13% of their time on the ground.
The amount of time porcupines spend on the ground depends on how much groundcover there is for foraging and for protection from predators. When ground cover is scarce, porcupines spend more time in trees. Because most predators of porcupines live on the ground, porcupines also spend a lot of time in trees where predator populations are large.
Porcupines prefer to spend their winter rest time in in rock dens on the ground. When ground dens are not available, porcupines will choose trees as their resting positions. Porcupines usually choose different trees for feeding and for resting. In eastern habitats, hemlock trees are often chosen for both resting and feeding. Hemlocks are preferred because they have very thick foliage, which helps the porcupine to stay hidden. Hemlocks are also sturdy trees with high nutritional value.
In southwestern Texas, porcupines both feed and rest in papershell pinyon pines, as well as in oaks and other hardwood species. Porcupines in the Rocky Mountains feed primarily on ponderosa pines, and rest in dens on the ground. Similarly, porcupines in the Great Basin use dens in rock outcroppings and juniper trees for cover in the winter. They travel between dens and the areas near rivers to forage on tree bark. This travel makes them susceptible to predators. (Griesemer, et al., 1998; Ilse and Hellgren, 2001; Roze, 1989; Sweitzer and Berger, 1992)
The mating system of porcupines is considered female defense polygyny. Successful males mate with many different females, but unsuccessful males don't mate at all. Successful males defend a female from 1 to 4 days so that other males can't mate with her.
Porcupines breed only once a year. Female porcupines attract many males. These males compete with each other to determine dominance. The dominant males are the ones who succeed in defending females from their rivals. These are the males who father offspring.
Females maintain a territory, and defend it against other females. However, male territories usually overlap those of several females. The territories of dominant males rarely overlap. Females all have similar sized territories. A male's territory usually gets bigger throughout his life. (Griesemer and DeGraaf, 1996; Roze, 1989; Sweitzer and Berger, 1997)
Females attract potential mates through scents and vocalizations. Several males converge on an advertising female. Successful males have to compete for, and then defend, the female. Mating will only happen after a female has chosen a male and is receptive to him.
Breeding occurs in October and November. Gestation in this species is 210 days, after which a female gives birth to a single offspring. Newborns weigh between 400 and 530 g. Young are nursed for about 127 days. They become independent of their mothers at approximately 5 months of age, but are not sexual mature until the age of 25 months for females, and 29 months for males. (Roze, 1989; Sweitzer and Berger, 1998)
Parental care is provided by the mother. Mainy, a mother provides her baby with food. For the first six weeks of a porcupine's life, its mother is always close by. They meet only at night. During the day the baby is hidden on the ground, while the mother sleeps in the trees. After six weeks, the baby porcupine follows the mother to feeding trees and waits for her at the bottom. Over the next couple months, resting positions and foraging distances show increasing separation between the young porcupine and its mother. The mother continues to travel to the position of the baby every night, following landmarks and not scent trails back to the infant. By mid-October the baby completely loses contact with the mother and is left to survive its first winter alone. The father spends no energy in the rearing of or caring for the offspring. Males have little to no contact with their offspring. (Roze, 1989; Sweitzer and Berger, 1998)
Porcupines are relatively long-lived animals that can live up to 18 years in the wild. Porcupine longevity is probably limited by the life of their grinding teeth. Porcupines over 12 years show diminished feeding and are usually smaller in size. (Kurta, 1995; Roze, 1989)
Porcupines are not very social creatures. They spend most of their time alone. Pporcupines are more social during the winter, when they may share dens and forage in groups. Because of their well developed defenses, porcupines don't need to live in groups to defend against predators. They are safe and comfortable alone.
Both males and females are territorial, although males denfend their territories more than do females. An individual knows its territory quite well and usually does not venture too far from it; the main exceptions are excursions to get salt or apples.
Unlike most polygynous mammals, dispersal of young porcupines is female biased. Young female porcupines leave their natal range and travel farther than do males. Because dominant males maintain established breeding territories for up to 3 breeding seasons, daughters who don't leave home run the risk of mating with their fathers. Sons, however, are not likely to mate with their mothers because they are not large enough to compete with other males to defend a potential mate. (Roze, 1989; Sweitzer and Berger, 1998)
Home range size varies depending on sex and status. Females have smaller home ranges than dominant males. Young males have smaller home ranges than dominant males. In the Great Basin Desert, dominant males ranged over a larger area (20.7 ha) than subordinate males (12.9 ha) and females (8.2 ha). Male home ranges during breeding periods were found to overlap parts of between 3 to 10 female ranges. (Sweitzer, 2003)
Porcupines use a combination of acoustic, chemical, visual, and tactile communication. Females communicate their readiness to mate by vaginal secretions, urine marking, and high pitched vocalizations. When threatened, a porcupine will chatter its teeth and produce a chemical odor, intended to warn off any predator. Males compete using fierce vocalizations. Visually, the porcupine communicates the presents of its weaponry by displaying the white on black markings on its back and tail. Tactile communication occurs when physical aggression erupts, as well as between mates, and between mothers and their young. (Roze, 1989; Roze, 2002)
Porcupines are generalist herbivores, however their diet changes because of changes in plant chemistry. Feeding rates also change seasonally. In the spring and summer months feeding rates are reduced because plants contain a lot of protein. However, as the protein content of plants decreases in the autumn, feeding rates increase.
Different porcupine populations have different diets. Porcupines in the Rocky Mountains mostly eat ponderosa pine phloem. Porcupines in eastern forests feed on many different trees. In Massachussettes, porcupines rarely eat bark, but in Texas, they forage largely on bark. (Roze, 1989; Snyder and Linhart, 1997)
The crucial nutritional resource for porcupines is nitrogen. In winter, porcupines get nitrogen from bark, twigs, and evergreen needles. Because these are not good sources of nitrogen, porcupines constantly loose weight throughout the winter.
Porcupines can forage on low nitrogen foods because they have a unique ability to retain nitrogen from their food. Porcupines can digest very high fiber foods because food stays in their digestive tract for a very long time, allowing them to extract nutrients from this food. (Felicetti, et al., 2000; Fournier and Thomas, 1997; Roze, 1989)
The diets of porcupines living in eastern deciduous forests have been well studied. In the spring, porcupines eat the buds of sugar maple trees, which are a rich source of protein. As soon as the leaves flush out, the sugar maple is abandoned because the leaves contain high amounts of tannins (chemicals toxic to porcupines). Porcupines begin to eat the cambium of basswood, aspen, and sapling beech trees. These contain a lot of protein and only a little tannin. Also ash trees are fed upon heavily, because they are relatively easy to climb (compared to the smooth bark of beeches), and because they have relatively low tannin levels. In the autumn when oak acorns and beech nuts become available, porcupines begin to feed on these. Porcupines are best a getting these nuts out of trees, and after the nuts have fallen to the ground, most of them are eaten by deer and squirrels. Winter foraging is focused on the phloem of hemlock and sugar maple trees.
Other foods utilized by porcupines include raspberry stems, grasses, flowering herbs, and a large amount of apples. Herbivory has an effect on the sodium metabolism of porcupines, which results in a lust for salt. Porcupines will chew on the wooden handles of human tools, other human-made wood structures, and areas of collected roadside salt runoff. (Roze, 1989)
Porcupine feeding happens primarily at night. This is because of changes in plant and leaf chemistry at night. Porcupines take advantage of the added nutrients available during the night-time metabolic processes of plants. (Roze, 1989)
Porcupines defend themselves from predators in a variety of ways. Even without trying, a porcupine warns its enemies that it is a dangerous animal through its coloration. The white-tipped quills on a black band on the tail and back stand out, and warn possible predators of danger. If their coloration pattern does not deter predators, porcupines often avoid danger by climbing a tree to escape. They can also clatter their teeth, which is another way that they warn predators. If these warnings fail, a porcupine will erect its quills, and release a nasty scent. Quills are only used for denfense if the threat has not been deterred by these other means. (Roze, 1989; Roze, 2002; Sweitzer and Berger, 1992)
Porcupines use their quills in two ways. Quills function a a shield made of barbs. They can also be driven into the predator. Once a porcupine has stuck its quills into an enemy, the porcupine needs to quickly separate from the quills. To do this, these animals have evolved unique quill-release systems. Erect quills release easier from the porcupine after they have been pushed into the would-be predator's body. The force to help the release of the quills is supplied by the contact with the would-be predator. Relaxed quills show no difference in release energy required.
Quills have a design that promotes their movement deeper into a predator once they have been embedded. The quills are not hollow, but are filled with a spongy matrix, which makes them very rigid and light. (Roze, 1989; Roze, 2002)
Quills are dangerous for porcupines, too. It is common for porcupines to fall out of trees, and when they do, they can poke themselves with their own quills. (Roze, 2002)
Even with all their defenses, porcupines are preyed upon by a several predators. The list includes lynx, bobcats, coyotes, wolves, wolverines, and great horned owls. Important predators include mountain lions and fishers.
Mountain lions and fishers have different stratgies for attacking porcupines. Fishers attack repeatedly from the front, avoiding the dangerous tail quills, until they are able to flip the porcupine on its back and attack the unprotected belly. Mountain lions supposedly make no attempt to avoid the quills of porcupines; instead they attack at will and deal with the consequences.
Several factors have led to stresses on porcupine populations. In the Great Basin, near extinction of a population of porcupines was discovered to be a direct result of increased mountain lion predation. Increased predation may have been a result of low mule deer populations in the area. Increased predation on porcupines can also be a consequence of predator shifting when snowshoe hare numbers decline. Increased stress from natural predators signifies the fragile dynamics between porcupines and their environment. Historical studies have indicated a very cyclical fluctuation in populations of porcupines. (Keith and Cary, 1991; Sweitzer, et al., 1997)
In northern Michigan, Fishers Martes pennanti were reintroduced to limit the porcupine population growth and nearly eliminated the species from the area. Limited den sites (standing hollow snags), brought about by logging practices, increased porcupine exposure to fishers. In the northeast, where hemlock plays a major role in porcupine winter foraging, pests such as the hemlock wooly adelgid, Adelges tsugae, along with increased hemlock logging may pose problems for future porcupine habitat. (Griesemer, et al., 1998)
Porcupines of the Rocky Mountains share the habitat of wood ticks. Wood ticks, Dermacentor andersoni, are the host of Colorado Tick Fever virus. It is unlikely that porcupines spread this virus, since only adult ticks attach to porcupines. Adult ticks do not spread the virus. Porcupines still play a significant role in the life cycle of these ticks, and infestation may be severe. In one study, 18 porcupines yielded a total of 448 ticks. (McLean, et al., 1993)
Porcupines sometimes creat stresses on other species, especially plants. In Texas, where they feed largely upon the bark of pinyon pines, it has been suggested that the trees have been made more vulnerable to the infestation of bark beetles. (Ilse and Hellgren, 2001; Roze, 1989)
Porcupines can be an important prey species, and populations of porcupines can be stressed by predators. In the Great Basin, one population of porcupines nearly became extinct because of increased mountain lion predation. Increased predation on porcupines may occur when other prey species, such as mule deer and rabbits, decrease in numbers. Historical studies indicate a cyclical fluctuation in porcupine populations, inidcating that they exist in a fragile balance in their ecosystems. (Keith and Cary, 1991; Sweitzer and Berger, 1992)
In other areas, predators can also have a big impact on porcupine populations. Fishers, Martes pennanti, were reintroduced in Northern Michigan in order to limit the porcupine population growth. The plan was overly successful, and nearly eliminated porcupines from the area. Limited den sites (standing hollow snags), brought about by logging practices, increased porcupine exposure to fishers.
In the northeast, where hemlock plays a major role in porcupine winter foraging, pests that affect the hemlock trees can affect porcupine populations. Pest such as hemlock wooly adelgids, Adelges tsugae, along with increased hemlock logging may pose problems for future porcupine habitat. (Griesemer, et al., 1998; Sweitzer, et al., 1997)
Porcupines have two areas of conflict with humans. Their salt cravings often lead them to chew on housing structures, automobiles, and anything made of plywood or with salt residue (usually from road de-icing salt). They also have a negative impact on the timber industry. Trees that have been fed on by porcupines tend to have stunted growth and twisted evil looking crowns, usually making the tree unsuitable for use as lumber. (Ilse and Hellgren, 2001; Roze, 1989)
Porcupines were once revered by Native American cultures throughout the continent as a food source, a source of quills for decoration, and legendary status. Today, however, they are mostly considered a pest. Bounties, large poisoning efforts and unregulated killing have only recently been discontinued. Some would argue that porcupines’ inherent value is the ease and accessibility of the species to research and study. (Roze, 1989)
This species is not a special conservation concern.
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