More Information

Additional Information

Sorex pacificusPacific shrew

By Ahyana Calloway

Geographic Range

Pacific shrews (Sorex pacificus) are a Nearctic species found along the Pacific coast of the United States. Their range includes nearly the entire western coastline of Oregon inland to national forests. There are 4 disjunct populations: 2 in Oregon (one in the Mt Hood National Forest and one north of Upper Klamath Lake and south of the Calapooya Mountains) and 2 smaller areas in northern California (one west of Siskiyou and one in the Salmon Mountains, north and east of Trinity National Forest). (Carraway, 1985; Carraway, 1988; Galindo-Leal, et al., 1993; Hammerson, 2019; Maser, 1998; Verts and Carraway, 1998)

Habitat

Pacific shrews inhabit forests dominated by redwoods (Sequoia sempervirens) or dense spruce trees (genus Picea). These shrews are commonly found in moist forests that contain a high degree of canopy cover and thick vegetation with fallen, decaying logs. They use grasses, mosses, lichens, or leaves found in the forest to make their nest. Pacific shrews are associated most frequently with alder (Alnus), salmonberry (Rubus spectabilis), or skunk cabbage (Symplocarpus foetidus). Pacific shrews are commonly found at elevations of 150 to 914 m. (Carraway, 1985; Cole, et al., 1998; Galindo-Leal, et al., 1993; Kritzman, 1977; Maser, 1998)

  • Range elevation
    150 to 914 m
    492.13 to 2998.69 ft
  • Average elevation
    396 m
    1299.21 ft

Physical Description

Pacific shrews are insectivores weighing 10 to 18 g. Pacific shrews range in total length from 135 to 160 mm. Their tails range from 52 to 71 mm, averaging around 75% of their body length. Their tails are either unicolored or indistinctly bicolored, getting darker toward the tip. They have traffic-cone shaped heads with long, pointed snouts and reduced eyes. Ear lengths for Pacific shrews range from 7 to 8 mm. Pacific shrews have 5 toes and plantigrade feet that help with grasping food. In summer, their pelage is short and reddish brown or cinnamon-brown color. In winter, their hair is long, dark reddish brown to dark brown. Pacific shrews have incisors that are light and distinct reddish brown. The dental record of Pacific shrews is reported as 3133/1113, with a total of 32 teeth.

Pacific shrews differ from other species in the genus Sorex by the absence of median tines on their first upper incisors. Their first and second unicuspids are nearly equal in length. Their third unicuspids are smaller than the fourth. Not much is known about the differences between male and female Pacific shrews. In dusky shrews (Sorex monticolus), male shrews have enlarged dermal glands along the sides of their bodies that excrete odors during mating season. It is assumed that Pacific shrews are similar.

When first born, Pacific shrews are blind and hairless. They range in size from 2 to 11 mm in length. Pacific shrews have deciduous teeth that are shed before birth and replaced with permanent teeth. (Belk and Smith, 1996; Burt and Grossenheider, 1980; Carraway, 1985; Carraway, 1988; Maser, 1998; Verts and Carraway, 1998)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    10 to 18 g
    0.35 to 0.63 oz
  • Range length
    135 to 160 mm
    5.31 to 6.30 in

Reproduction

Little information is known on how Pacific shrews attract mates. Dusky shrews (Sorex monticolus) have dermal glands along the sides of their bodies that excrete odors. A correlation between the growth of testes in male dusky shrews and the development of side glands suggests they play a role in attracting potential mates. The glands in female dusky shrews do not change during mating season. It is assumed that Pacific shrews have a similar way of attracting mates. During mating season, both male and female Pacific shrews find multiple partners with which to mate. (Belk and Smith, 1996; Carraway, 1988)

The mating season of Pacific shrews starts in March and ends in August. Although sexually mature males are found year-round, their main breeding months are February to August. Given that females are only reproductively active from March to August (with a few cases as late as November), their activity limits the breeding season. Litter sizes can range from 2 to 6 but 4 to 5 is more typical. Many species in the genus Sorex have multiple litters throughout the breeding season and it is believed that Pacific shrews do, as well.

The gestation period of Pacific shrews ranges from 16 to 28 days. At birth, Pacific shrews range in size from 2 to 11 mm in length. Pacific shrews are born blind and without fur. Their first set of teeth is shed before birth and replaced with permanent teeth. Pacific shrews are fed via breastmilk and weaned within 3 weeks after birth.

Juvenile Pacific shrews obtain a winter pelage from September through October. Four to six months after birth, Pacific shrews are sexually mature and able to mate in the breeding season. Once juvenile shrews wean, they are fully independent. (Belk and Smith, 1996; Carraway, 1985; Carraway, 1988; Maser, 1998; Verts and Carraway, 1998)

  • Breeding interval
    Typically 2 to 3 litters per breeding season
  • Breeding season
    February to November
  • Range number of offspring
    2 to 6
  • Average number of offspring
    4 to 5
  • Range gestation period
    16 to 28 days
  • Average gestation period
    21 days
  • Average weaning age
    3 weeks
  • Average time to independence
    3 weeks
  • Range age at sexual or reproductive maturity (female)
    4 to 6 months
  • Range age at sexual or reproductive maturity (male)
    4 to 6 months

Male Pacific shrews have no parental investment outside of mating. Female Pacific shrews provide milk and shelter for juvenile shrews. Because they are still reliant on milk for their main source of food, juvenile shrews stay with their mothers until they are weaned. After about 3 weeks, the juvenile shrews wean and leave to find a territory of their own. (Belk and Smith, 1996; Carraway, 1985; Carraway, 1988)

  • Parental Investment
  • female parental care
  • pre-fertilization
    • protecting
      • female
  • pre-hatching/birth
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

The average lifespan of Pacific shrews is 18 months in the wild. Little is known about the life span of Pacific shrews in captivity. Pacific shrews are difficult to keep in captivity for long periods of time due to their high metabolic demand. Pacific shrews need to feed continually throughout the day. (Belk and Smith, 1996; Carraway, 1988; Maser, 1998)

  • Average lifespan
    Status: wild
    18 months

Behavior

Pacific shrews are nocturnal animals. They use quick and sudden movements to assess their location. Pacific shrews emit a twittering sound and constantly smell the air to locate potential dangers or prey nearby. Pacific shrews are typically inactive during the day but awaken periodically to eat and groom. Pacific shrews awaken throughout the day to consume prey caches near their nests to meet their high metabolic demand.

Pacific shrews groom frequently. They groom both day and night in a crouched position. Pacific shrews groom their body by scratching with their hind feet after stretching their skin by bowing their bodies to spread their fur. They clean their heads by licking their forefeet and rubbing them over their facial area. Pacific shrews clean their tails using their mouths, starting at the tip and licking their way to the base. They clean their genital area with their mouths while they are laying on their side. They also clean their hind feet with their mouths. Once Pacific shrews are fully groomed, they enter their nests.

Shelter is a key component for Pacific shrews, and they become very disturbed if shelter is not available. Nests are composed of natural materials such as grasses, mosses, lichens, or leaves. Pacific shrews carry these natural materials in their mouth and pile them in a specific area. Once enough material is gathered, Pacific shrews push themselves into the middle of the pile and pull the excess material around them, creating a cup shape with an opening at the top. Pacific shrews sleep in a curled position with their heads near their anus and hind feet on their shoulders. This sleeping position allows them to minimize surface area and conserve body heat, which is vital for their survival. Excretory materials are stored in a separate area near the nest. The only time excretion does not occur is while they are hunting. Maser (1998) found that Pacific shrews re-ingested some defecated material, possibly as a way to obtain vitamins B and K. (Belk and Smith, 1996; Carraway, 1988; Maser, 1998; Verts and Carraway, 1998)

Home Range

Little is known about how territorial Pacific shrews are, but juveniles help defend the territories of their mothers, until they reach maturity and find territories of their own. Home range and territory sizes have not been reported for Pacific shrews. Dusky shrews (Sorex monticolus) maintain home ranges of 1,227 m^2 during the non-breeding season and 4,020 m^2 during the breeding season. It is suspected that home ranges of Pacific shrews are similar. (Belk and Smith, 1996; Carraway, 1985)

Communication and Perception

Pacific shrews are known to constantly move their snouts and emit twittering sounds when active at night. It is believed these sounds are a kind of echolocation and are used to communicate between shrews. Like all shrews, their eyes are reduced in size, which forces them to depend on other senses. In captivity, they use their sense of smell and hearing to hunt for prey. These shrews use odor to detect terrestrial prey and sound to find airborne prey. Not much is known about how Pacific shrews attract mates, but male montane shrews (Sorex monticolus) use pheromones to attract females. It is presumed that Pacific shrews do the same. (Belk and Smith, 1996; Carraway, 1985; Mares, 1999; Maser, 1998)

Food Habits

The diet of mainly insectivorous Pacific shrews also consists of other animals. Carraway (1985) reported that amphibian flesh, centipedes, slugs and snails, insect larvae, and additional invertebrates are the most common prey. Less common diet components include insects from the orders Coleoptera, Diptera, and Hemiptera (18% of these insects were larvae). Vegetation (plant seeds, and moss), fungi, and miscellaneous invertebrates (ant eggs, ticks, and earthworms) make up the remaining diet.

Pacific shrews locate prey capable of flying, such as wasps and bees, using sound. Prey that are unable to fly such as ants or beetles are located by smell. Pacific shrews have been reported to kill scorpions, bees, and wasps by biting their heads off, or biting the back of their necks, killing them immediately. Non-stinging prey such as beetles or ants are immobilized, then killed. Prey, if not eaten immediately, are buried in a cache next to, within, or under their nests, to meet their high metabolic demand throughout the day. (Carraway, 1985; Kritzman, 1977; Maser, 1998)

  • Animal Foods
  • amphibians
  • eggs
  • insects
  • terrestrial non-insect arthropods
  • mollusks
  • terrestrial worms
  • Plant Foods
  • seeds, grains, and nuts
  • bryophytes
  • Other Foods
  • fungus

Predation

Little is known about the predators of Pacific shrews. It is believed that spotted owls (Strix occidentalis), Pacific giant salamanders (Family Dicamptodontidae), raptors (genus Buteo), and mammalian carnivores are predators of Pacific shrews. Along the Oregon coast, domestic cats (Felis catus) kill, but seldom eat, Pacific shrews. Pacific shrews are able to move swiftly when disturbed, which allows them to escape predators. Their natural habitat also allows them to escape predators by remaining in confined areas, too small for prey to access. The pelage of Pacific shrews allows them to camouflage with decaying trees and leaf litter. (Belk and Smith, 1996; Carraway, 1985; Maser, 1998; Schmidt, 1994)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Pacific shrews commonly use vegetation such as alder (g.Alnus), salmonberry (Rubus spectabilis), or skunk cabbage (Symplocarpus foetidus) to build their nests. These shrews fall prey to salamanders, raptors, and mammalian carnivores. Endoparasites in their digestive system include tapeworms (Hymendepis pulchra, Hymenolepis kenki, Lineolepis parva). Roundworm parasites include Liniscus maseri. They are also known to have lice (Liga soricis) and mites (Macrocheles praedafimetorum, Pygmephorus mahunkai). (Carraway, 1985; Cole, et al., 1998; Maser, 1998; Schmidt, 1994; Verts and Carraway, 1998; Voge, 1955)

Commensal/Parasitic Species
  • Rat Tapeworm Hymenolepis pulchra
  • Tapeworm Hymenolepis kenki
  • Tapeworm Lineolepis parva
  • Roundworm Liniscus maseri
  • Louse Liga soricis
  • Mite Macrocheles praedafimetorum
  • Mite Pygmephorus mahunkai

Economic Importance for Humans: Positive

There is no current positive economic importance of Pacific shrews to humans.

Economic Importance for Humans: Negative

Pacific shrews could potentially be considered pests to humans. Though uncommon, Pacific shrews make shelter inside the homes of humans, similar to rats (genus Rattus). No scientific study has been done to support this claim. Pacific shrews are considered seed predators. Keyes (2000) found that plant seeds used for natural regeneration of ponderosa pine (Pinus ponderosa) were consumed by Pacific shrews, which lead to failure of the regeneration. (Keyes, 2000)

  • Negative Impacts
  • crop pest

Conservation Status

Pacific shrews are considered a species of “Least Concern” on the IUCN Red List. Pacific shrews are not listed on the US Federal list. They do not have any special status on the CITES list. Pacific shrews are not found in Michigan and thus are not on the State of Michigan list.

Deforestation threatens the habitat of Pacific shrews. Cole et al. (1998) found that the number of Pacific shrews captured decreased after logging within their habitat. Cole et al. believe that this decrease is from due to lack of shelter to hide from predators.

There are no current conservation efforts to protect Pacific shrews because their populations are considered stable. (Cole, et al., 1998; Galindo-Leal, et al., 1993; Hammerson, 2019)

Contributors

Ahyana Calloway (author), Radford University, Lauren Burroughs (editor), Radford University, Layne DiBuono (editor), Radford University, Lindsey Lee (editor), Radford University, Karen Powers (editor), Radford University.

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

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.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

echolocation

The process by which an animal locates itself with respect to other animals and objects by emitting sound waves and sensing the pattern of the reflected sound waves.

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

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.

insectivore

An animal that eats mainly insects or spiders.

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).

molluscivore

eats mollusks, members of Phylum Mollusca

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.

mycophage

an animal that mainly eats fungus

native range

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

nocturnal

active during the night

omnivore

an animal that mainly eats all kinds of things, including plants and animals

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

seasonal breeding

breeding is confined to a particular season

sexual

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

solitary

lives alone

stores or caches food

places a food item in a special place to be eaten later. Also called "hoarding"

tactile

uses touch to communicate

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

viviparous

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

References

Anthony, R., D. Gomez. 1998. Small mammal abundance in riparian and upland areas of five seral stages in western Oregon. Northwest Science, 72/4: 293-302.

Belk, M., M. Smith. 1996. Sorex monticolus. Mammalian Species, 528: 1-5.

Burt, W., R. Grossenheider. 1980. A Field Guide to the Mammals: North America North of Mexico. New York, New York: Houghton Mifflin Company.

Butts, S., W. McComb. 2000. Associations of forest-floor vertebrates with coarse woody debris in managed forests of western Oregon. The Journal of Wildlife Management, 64/1: 95-104.

Carraway, L. 1988. Records of reproduction in Sorex pacificus. The Southwestern Naturalist, 33/4: 478-480.

Carraway, L. 1985. Sorex pacificus. Mammalian Species, 231: 1-5.

Castor, L., H. Manter, C. Ritzi, B. Walters, J. Whitaker Jr.. 2007. Host and distribution lists of mites (Acari), parasitic and phoretic, in the hair or on the skin of north American wild mammals north of Mexico: records since 1974. American Midland Naturalist, 91/1: 1-67.

Cole, E., W. McComb, M. Newton, J. Leeming, C. Chambers. 1998. Response of small mammals to clearcutting, burning, and glyphosate application in the Oregon coast range. The Journal of Wildlife Management, 62/4: 1207-1216.

Evans, A., B. Grzimek, M. Hutchins, J. Jackson, D. Kleiman, J. Murphy, D. Olendorf, N. Schlager, D. Thoney. 2003. Grzimek's Animal Life Encyclopedia. Detroit, MI: Gale.

Galindo-Leal, C., J. Runciman, G. Zuleta. 1993. Patterns of small mammal communities in riparian habitats of wet coniferous forests. Riparian Habitat Management and Research, 6: 733-85.

Hammerson, G. 2019. "Sorex pacificus" (On-line). The IUCN Red List of Threatened Species 2016: e.T41409A22318109. Accessed January 30, 2019 at http://dx.doi.org/10.2305/IUCN.UK.2016-2.RLTS.T41409A22318109.en..

Kays, R., D. Wilson. 2009. Mammals of North America: Second Edition. Princeton, New Jersey: Princeton University Press.

Keyes, C. 2000. Natural regeneration of ponderosa pine: Pest management strategies for seed predators. The Forestry Chronicle, 76/4: 623-626.

Kritzman, E. 1977. Little Mammals of the Pacific Northwest. Seattle, Washington: Pacific Search Press.

Mares, M. 1999. Encyclopedia of Deserts. Norman, Oklahoma: University of Oklahoma Press.

Maser, C. 1998. Mammals of the Pacific Northwest : From the Coast to the High Cascades. Corvallis, Oregon: Oregon State University Press.

Orr, E., W. Orr. 2009. Oregon Fossils. Corvallis, Oregon: Oregon State University Press.

Schmidt, R. 1994. The Handbook: Prevention and Control of Wildlife Damage. Lincoln, Nebraska: University of Nebraska.

Verts, B., L. Carraway. 1998. Land Mammals of Oregon. Berkeley, California: University of California Press.

Voge, M. 1955. A list of cestode parasites from California mammals. The American Midland Naturalist, 54/2: 413-417.

Search

Navigation Links

Information
Specimens
Classification

Classification