Erethizon dorsatumNorth American porcupine

Geographic Range

North American porcupines, Erethizon dorsatum, have the northern most range of all porcupines. They inhabit much of North America between the Arctic Ocean and northern Mexico. Porcupines are found throughout most of Alaska and Canada, in the northern part of the Great Lakes region, all throughout the west and northeast regions of the United States. Populations have been studied extensively in the eastern deciduous forests of New York and Massachusetts, the Great Basin Desert, and the woodlands of Texas. (Griesemer, et al., 1998; Ilse and Hellgren, 2001; Roze, 1989; Sweitzer, et al., 1997)

Habitat

Erethizon dorsatum uses a broad variety of habitats. With an extensive range, porcupines can be found in varied climates, and at varied elevations. Porcupines live in open tundra, deciduous forests, and desert chaparral. Porcupine habitat varies geographically. In the Pacific Northwest, these animals are primarily ground dwelling. In New York, porcupines are mainly tree dwellers. In Massachusetts, they spend 13% of their time on the ground. The time porcupines spend on the ground is related to the amount of ground cover that exists for foraging and protection from predators. In places of deer overpopulation, ground cover can be scarce, thus keeping porcupines in the trees. Density of predators also determines time spent on the ground, because most porcupine predators are non-arboreal species. Porcupines will spend winter in dens, usually rock dens where available. When ground dens are not available, porcupines choose trees to rest in. Different trees are chosen for resting than for feeding. In eastern habitats, hemlocks are usually chosen over other conifers for both resting and feeding. These trees have superior thermal protection, sight protection (hemlocks have thick foliage), are stronger, and have higher nutritional value.

Southern populations of porcupines exhibit no seasonal use of trees. In southwestern Texas, porcupine populations both rest and feed in papershell pinyon pines (Pinus remota), 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 have been observed to use dens in rock outcroppings and juniper trees for cover during the winter. They travel between dens and small riparian areas to forage on tree bark, making them susceptible to predators. (Griesemer, et al., 1998; Ilse and Hellgren, 2001; Roze, 1989; Sweitzer and Berger, 1992)

Physical Description

Erethizon dorsatum is the second largest rodent in North America, outsized only by beavers. Individuals range in length from 600 to 900 mm, and weigh between 5 and 14 kg.

It is distinguished by its stout, slow, lumbering form and by its spiny coat. On the whole, porcupines appear dark brown to black, with dorsal guard hairs and spines that contain bands of yellow. Spines called quills extend from head to tail on the dorsal surface. The middle of the tail and lower back are marked by a black line. Quills on the black area are fringed with white. Because porcupines are nocturnal, the white on black markings stand out, alerting their mostly color blind nocturnal predators of the danger they present. This pattern is visible after porcupines are three months old.

Porcupine quills have microscopic barbs on the tip. They are usually around 75 mm long and 2 mm wide. Each animal has approximately 30,000 quills. If a quill becomes lodged in the tissues of a would-be attacker, the barbs act to pull the quill further into the tissues with the normal muscle movements of the attacker, moving up to several millimeters in a day. Predators have been known to die as a result of quill penetration and infection. (Kurta, 1995; Roze, 1989; Sweitzer and Berger, 1997; Vaughn, et al., 2000)

Skull characteristics include a lack of canine teeth and a dental pattern of 1/1, 0/0, 1/1, 3/3. The prominent diastema allows the lips to be drawn in while gnawing. Like other hystricomorphs, porcupines have unique chewing muscles. Efficient chewing movements are made possible by an arm of the masseter muscle, which passes through the infraorbital foramen. (Vaughn, et al., 2000)

Like the porcupines found in Central and South America, North American porcupines have arboreal adaptations including long claws (four in front with a vestigial thumb, and five in back). These claws, along with rather unique palms, allow for unproblematic navigation in trees. Porcupines can easily climb large trunks and surprisingly minute branches. The palms and soles of porcupines are naked with a pebbly surface. This texture increases the surface area and thus the friction while in contact with a branch. Associated with this adaptation, porcupines have a keenly developed sense of touch. This further aids them in their nocturnal navigation. It also allows porcupines to secure themselves in trees with their hind feet while manipulating food with their front appendages. Finally, the quills of porcupines prevent downward sliding while animals are grasping trees with their hind feet. The quills on the tail are used to stab the tree, further increasing fiction and stabilizing the animal in the tree. (Roze, 1989)

A characteristic unique to E. dorsatum, compared to its Central and South American relatives, is adaptation to cold. No other hystricomorphs can withstand such high variance in temperature as North American porcupines can tolerate. (Roze, 1989; Vaughn, et al., 2000)

North American porcupines are sexually dimorphic, with males being larger than females. The size dimorphism is explained proximately by the faster growth rates and longer periods of growth which males experience relative to females. Because males with larger size have higher reproductive success than do smaller males, sexual selection promotes increased male size in this species. (Sweitzer and Berger, 1997)

  • Sexual Dimorphism
  • male larger
  • Range mass
    5.000 to 14.000 kg
    11.01 to 30.84 lb
  • Range length
    600 to 900 mm
    23.62 to 35.43 in
  • Average basal metabolic rate
    13.675 W
    AnAge

Reproduction

The mating system of E. dorsatum is considered female defense polygyny. Males defend a pre-estrous female from 1 to 4 days prior to copulation. Porcupines breed only once a year. Female porcupines advertise their 8 to 12 hour estrous period well ahead of time through vaginal secretions, urine marking, and high pitched vocalizations. In doing this, females attract many males who compete with each other to determine dominance. A dominant male breeds with a number of different females, but only when the females are willing. This ensures that the "best" male fathers a female's offspring.

Females maintain a territory, and defend it against other females; however male territories typically overlap those of several females. The territories of dominant males rarely overlap. Females all maintain similar sized territories, but male territory size varies with age and dominance status. Juvenile males settle as permanent residents in their natal area. They have smaller territories than do adults, but expand their territory size as they mature. Females disperse from their natal area before reaching maturity.

Male/female pairs sometimes share winter dens, although this den sharing is not exclusive to mated pairs. Den sharing is not necessarily considered part of the mating system of porcupines. (Griesemer and DeGraaf, 1996; Roze, 1989; Sweitzer and Berger, 1997)

Females advertise their readiness to mate by vaginal secretions, urine marking, and high pitched vocalizations, well before the time of ovulation. This means that several males converging on an advertising female will 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.

Males compete with each other using loud vocalizations, violent biting, and each uses his quills as weapons. Although the competition happens in trees, mating exclusively happens on the ground. The pair will mate for several hours until a vaginal plug is formed, which then stops the copulation, and prevents further copulation with other males. This plug is formed by enzymatic action in the semen.

Male porcupines display a strange courtship ritual, which involves dousing of the female with his urine. The urine showers are continued until the female is receptive to both the shower and the male. According to Roze (1989), "Everything suggests the urine is fired by ejaculation, not released by normal bladder pressure. Porcupines with everyday full bladders don't squirt their urine, don't have erections, and don't aim at females."

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; Woods, 1999)

  • Breeding interval
    North American porcupines breed only once a year.
  • Breeding season
    Mating occurs in the months of October and November.
  • Range number of offspring
    1 to 2
  • Average number of offspring
    1
  • Average number of offspring
    1
    AnAge
  • Range gestation period
    205 to 217 days
  • Average gestation period
    210.25 days
  • Range weaning age
    127 (low) days
  • Range time to independence
    5 (low) months
  • Range age at sexual or reproductive maturity (female)
    25 (low) months
  • Range age at sexual or reproductive maturity (male)
    29 (low) months

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)

  • Parental Investment
  • no parental involvement
  • precocial
  • female parental care
  • pre-fertilization
    • protecting
      • male
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

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)

Behavior

Porcupines are not very social creatures, although their social structure is well defined. They spend most of their time alone. There is some sharing of dens in the winter (up to eight in a den). Also, some porcupines forage in groups of up to twenty during the winter months. Herbivores which live in groups usually do so as a response to predators, but with the acute defenses of porcupines, a solitary existence is entirely tolerable.

Both males and females defend their territories, the former more so than the latter. An individual knows its territory quite well and usually does not venture too far from it; the main exceptions being salt or apple excursions.

Young porcupines display an unusual characteristic with respect to dispersal. Normally in polygynous mammals, females are philopatric, because of the amount of resources used in offspring production and care. In porcupines, dispersal is female biased. Dominant males maintain established breeding territories for up to 3 breeding seasons. Philopatric daughters, who are usually capable of maing by the age of 22 months, run the risk of mating with their fathers. Selection would favor female dispersal in order to ensure viable offspring. Because males have no contact with their offspring, the offspring and the father have no way of recognizing one another. For male offspring, the chance of becoming dominant enough to effectively guard an advertising female (his mother) is quite remote, since it will take years to reach that social rank. (Roze, 1989; Sweitzer and Berger, 1998)

Home Range

In the Great Basin Desert, home range size in dominant males was found to relate to increased mating possibilities and not metabolic increases due to the larger size. 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 3 to 10 female ranges. (Sweitzer, 2003)

Communication and Perception

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)

Food Habits

Porcupines are generalist herbivores. Diets vary throughout the year in response to minute changes in plant chemistry. Feeding rates also change seasonally. Throughout the spring and summer months, when high protein foods are readily available, feeding rates are reduced. However, in the autumn, feeding rates increase, probably in preparation for the winter scarcity of high quality forage.

Although typically a generalist, research has shown some selectivity between plants with higher nutrient content. However the amount of selectivity does not approach that shown by other specialized herbivores.

Selectivity varies between populations, with porcupines in some habitats showing more selectivity. Porcupines in the Rocky Mountains feed exclusively on ponderosa pine phloem, probably because this species dominates the ecosystem. Porcupines in eastern deciduous forests, however, feed on a number of different species of trees. In Massachussettes, porcupines were found rarely to feed on bark, whereas in Texas they forage largely on bark. (Roze, 1989; Snyder and Linhart, 1997)

The crucial nutritional resource for porcupines is nitrogen. Winter sources of nitrogen include bark, twigs, and evergreen needles. These are relatively poor sources of nitrogen, so porcupines move continuously towards starvation, constantly loosing weight throughout the winter.

Porcupines are able to forage on low nitrogen, high fiber foods because of their unique ability to retain nitrogen from their food. This is done by reducing fecal nitrogen losses. The ability of porcupines to digest very high fiber foods better than some hindgut fermenters and ruminants may be due to the extended time food matter stays in the digestive system. (Felicetti, et al., 2000; Fournier and Thomas, 1997; Roze, 1989)

Dietary patterns of porcupines have been extensively studied in eastern deciduous forests. In the spring, they focus their feeding energy on the buds of sugar maple trees, a rich source of protein. As soon as the leaves flush out, the sugar maple is abandoned because leaves contain high amounts of tannins (chemicals toxic to porcupines). Porcupines move on to the cambium of basswood, aspen, and sapling beech trees. Understory beech trees contain much less tannin than adult trees, a trait a porcupine can easily ascertain. Aspen catkins are utilized for their high level of protein. Also ashes 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. Fall feeding abruptly changes when oak acorns and beech nuts become available. Porcupines feed on these nutrient packed meals before they have fallen to the ground. However, after the nuts have fallen, porcupines are out-competed by deer and squirrels. Mast years in nut producing trees have a direct effect on these herbivores. Winter foraging is focused on the phloem of hemlock, sugar maple, primarily within the canopy of these stands.

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 related to 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)

  • Plant Foods
  • leaves
  • roots and tubers
  • wood, bark, or stems
  • seeds, grains, and nuts
  • fruit
  • flowers

Predation

Porcupines possess a very unique defense system. A porcupine’s first line of defense is escaping from danger by climbing up a tree. However, if such an escape is not possible, the porcupine has many options. Porcupines are the only mammal in North America to use quills to deter predators. Around 30,000 quills are present on the dorsal side of a porcupine. Because loosing quills is very expensive to a porcupine, these animals have developed several warning signs to precede their use of their ultimate weapon.

First are the aposematic colorations on the animal. The white-tipped quills on a black band on the tail and back stand out, and warn possible assailants of danger. Second, porcupines emit an auditory warning: a quiet clattering of the teeth. If both visual and accoustic warnings fail, a porcupine will erect its quills, and simultaneously release a nasty scent. Quills are only used 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, defensively (as a shield made of barbs) and offensively (when they are driven into the predator). Upon contact, the porcupine needs to quickly separate from the quills, and thus separate from the enemy, so they have evolved unique quill-release systems. Erect quills release easier from the porcupine after the quill has been pushed into the would-be assailant's body. The force 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 present some dangers to porcupines. Falling out of trees is quite common for porcupines, and self-impalement is definitely a hazard. Also, the force needed to remove the quills from the porcupine, after they are embedded in the assailant, may be larger than the weight of the porcupine, so separating may be problematic. (Roze, 2002)

Even with this elaborate defense mechanism, porcupines are preyed upon by a couple of co-adapted predators. Several predators exist that at least have been known to kill a porcupine. The list includes lynx, bobcats, coyotes, wolves, wolverines, and great horned owls. Important predators include mountain lions and fishers Fishers will attack from the front repeatedly, avoiding the tail quills, until they are able to flip a porcupine on its back and attack the unprotected ventral surface. Mountain lions supposedly make no attempt to avoid the quills of porcupines; instead they attack at will and deal with the consequences. Predators tend to hunt and kill porcupines mostly in open habitats. (Sweitzer and Berger, 1992; Sweitzer, et al., 1997)

Ecosystem Roles

Porcupines of the Rocky Mountains prefer habitats on rocky, south facing slopes. This brings them into contact with wood ticks that share the same habitat. Wood ticks, Dermacentor andersoni, are the host of Colorado Tick Fever virus; however porcupines do little to spread this virus. This is because only adult ticks attach to porcupines, and adult ticks do not spread the virus. In one study, 18 porcupines yielded a total of 448 ticks. (McLean, et al., 1993)

Porcupines can generate stress on their environment, although their overall detrimental effect is usually largely over-emphasized (usually focused on their intrepid salt excursions). One example of porcupines adding a stress to their environment is in Texas, where they feed largely upon the bark of pinyon pines. It has been suggested that because of porcupine foraging, these trees have been made more vulnerable to the infestation of bark beetles. (Ilse and Hellgren, 2001; Roze, 1989)

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)

Economic Importance for Humans: Positive

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)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

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)

  • Negative Impacts
  • crop pest
  • household pest

Conservation Status

This species is not a special conservation concern.

Contributors

Nancy Shefferly (editor), Animal Diversity Web.

Christopher Weber (author), University of Michigan-Ann Arbor, Phil Myers (editor, instructor), Museum of Zoology, University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

aposematic

having coloration that serves a protective function for the animal, usually used to refer to animals with colors that warn predators of their toxicity. For example: animals with bright red or yellow coloration are often toxic or distasteful.

arboreal

Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

granivore

an animal that mainly eats seeds

herbivore

An animal that eats mainly plants or parts of plants.

induced ovulation

ovulation is stimulated by the act of copulation (does not occur spontaneously)

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

polygynous

having more than one female as a mate at one time

scent marks

communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

solitary

lives alone

tactile

uses touch to communicate

taiga

Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5? N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

tundra

A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

young precocial

young are relatively well-developed when born

References

Felicetti, L., L. Shipley, G. Witmer, C. Robbins. 2000. Digestibility, nitrogen excretion, and mean retention time by North American porcupines (Erithizon dorsatum) consuming natural forages. Physiological and Biochemical Zoology, 73/6: 772-780.

Fournier, F., D. Thomas. 1997. Nitrogen and energy requirements of the North American porcupine. Physoilogical Zoology, 70/6: 615-620.

Griesemer, S., R. DeGraaf. 1996. Denning pattern of porcupines, Erithizon dorsatum . Canadian Field Naturalist, 110/4: 634-637.

Griesemer, S., T. Fuller, R. Degraaf. 1998. Habitat use by porcupines (Erethizon dorsatum) in central Massachusetts: effects of topography and forest composition. The American Midland Naturalist, 140/2: 271-279.

Ilse, L., E. Hellgren. 2001. Demographic and behavioral characteristics of North American porcupines (Erethizon dorsatum) in pinyon-juniper woodlands of Texas. The American Midland Naturalist, 146/2: 329-338.

Keith, L., J. Cary. 1991. Mustelid, squirrel, and porcupine population trends during a snowshoe hare cycle. Journal of Mammalogy, 72/2: 373-378.

Kurta, A. 1995. Mammals of the Great Lakes Region. Ann Arbor: University of Michigan Press.

McLean, R., A. Carey, L. Kirk, D. Francy. 1993. Ecology of porcupines (Erethizon dorsatum) and Colorado tick fever virus in Rocky Mountain National Park, 1975-1977. Journal of Medical Entomology, 30/1: 236-238.

Morin, P., D. Berteaux. 2003. Immobilization of North American porcupines (Erethizon dorsatum) using Ketamine and Xylazine. Journal of Wildlife Diseases, 39/3: 675-682.

Roze, U. 1989. The North American Porcupine. Washington, D.C.: Smithsonian Institution Press.

Roze, U. 2002. A facilitated release mechanism for quills of the North American porcupine (Erethizon dorsatum). Journal of Mammalogy, 83/2: 381-385.

Silva, M., J. Downing. 1995. CRC handbook of mammalian body masses. Boca Raton: CRC Press.

Snyder, M., Y. Linhart. 1997. Porcupine feeding patterns: Selectivity by a generalist herbivore?. Canadian Journal of Zoology, 75: 2107-2111.

Sweitzer, R. 2003. Breeding movements and reproductive activities of porcupines in the Great Basin Desert. Western North American Naturalist, 63/1: 1-10.

Sweitzer, R., J. Berger. 1998. Evidence for female-biased dispersal in North American pocupines (Erithizon dorsatum). Jounal of Zoology, 244: 159-166.

Sweitzer, R., S. Jenkins, J. Berger. 1997. Near-Extinction of porcupines by mountain lions and consequences of ecosystem change in the Great Basin Desert. Conservation Biology, 11/6: 1407-1417.

Sweitzer, R., J. Berger. 1997. Sexual dimorphism and evidence for intrasexual selection form quill impalements, injuries and mate guarding in porcupines (Erethizon dorsatum). Canadian Journal of Zoology, 75: 847-854.

Sweitzer, R., J. Berger. 1992. Size-related effects of predation on habitat use and befavior of porcupines. Ecology, 73: 567-875.

Vaughn, T., J. Ryan, N. Czaplewski. 2000. Mammalogy, Fourth Edition. USA: Brooks/Cole.

Woods, C. 1999. North American Porcupine| Erethizon dorsatum . Pp. 671-673 in D Wilson, S Ruff, eds. The Smithsonian Book of North American Mammals. Washington and London: The Smithsonian Institution Press.