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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
There is no current positive economic importance of Pacific shrews to humans.
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)
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.
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.
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.
uses sound to communicate
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.
an animal that mainly eats meat
uses smells or other chemicals to communicate
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.
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.
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.
parental care is carried out by females
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
an animal that mainly eats seeds
An animal that eats mainly plants or parts of plants.
An animal that eats mainly insects or spiders.
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).
eats mollusks, members of Phylum Mollusca
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
an animal that mainly eats fungus
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
an animal that mainly eats all kinds of things, including plants and animals
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
breeding is confined to a particular season
reproduction that includes combining the genetic contribution of two individuals, a male and a female
places a food item in a special place to be eaten later. Also called "hoarding"
uses touch to communicate
Living on the ground.
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
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
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