Camas pocket gopher

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Camas pocket gopher
A defensively postured dull brown gopher, bearing large protuberant incisors
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Rodentia
Family: Geomyidae
Genus: Thomomys
Subgenus: Megascapheus
Species: T. bulbivorus
Binomial name
Thomomys bulbivorus
(Richardson, 1829)
See text.
Distribution of the Camas pocket gopher in the Willamette Valley of northwest Oregon
Synonyms[2]

The Camas pocket gopher (Thomomys bulbivorus), also known as the Camas rat or Willamette Valley gopher, is a rodent in the genus Thomomys of the family Geomyidae. First collected in 1829, it is endemic to the Willamette Valley in the northwest part of Oregon in the northwestern United States. It is the largest member of the genus, commonly known as smooth-toothed or western pocket gophers. The herbivorous gopher forages for vegetable and plant matter collected in large, fur-lined, external cheek pouches. Surplus food is hoarded in an extensive system of underground tunnels. The dull brown to lead-gray coat changes color and texture over the year. The mammal's characteristically large, protuberant incisors are well adapted for use in tunnel construction, particularly in the hard clay soils of the Willamette Valley. The young are born toothless, blind, and hairless. They grow rapidly and are weaned around six weeks of age. The females have four mammary glands. The gophers make chattering sounds with their teeth. Males and females make purring or crooning sounds when together, while the young may make twittering sounds.

The Camas pocket gopher is fiercely defensive when cornered, yet may become tame in captivity. While population trends are generally stable, threats to the species' survival include urbanization, habitat conversion for agricultural uses, and active attempts at eradication through trapping and poisons. In addition, it may fall prey to raptors or carnivorous mammals, or become host to a number of parasitic arthropods and worms. Scientists believe that the evolutionary history of the animal was disrupted when the cataclysmic Bretz floods washed over the Willamette Valley at the end of the last ice age. The floods nearly entirely covered the gopher's geographic range, which may have caused a genetic bottleneck as survivors repopulated the region after the waters receded.

Taxonomy[edit]

Audubon print of four gophers beside a burrow, near a river bank
Naturalists in the 19th century made reference to the "Camas Rat", as in this print by James Audubon

The Camas pocket gopher (Thomomys bulbivorous) is a rodent in the family of pocket gophers, Geomyidae. There are three genera of North American pocket gophers, Cratogeomys, Geomys, and Thomomys.[3] They are differentiated by the number of grooves on the surfaces of the anterior aspects of their upper incisors.[3] The Camas pocket gopher is a smooth toothed pocket gopher of the genus Thomomys.[4] In 1855, Johann Friedrich von Brandt first referred to the Camas pocket gopher as Thomomys bulbivorous.[5][6] The name Thomomys derives from the Greek words σωρός 'heap' + μῦς 'mouse', probably in reference to the heaping mounds of excavated soil produced by the burrowing gopher.[7] In Latin, bulbus translates as 'bulb', while the Latin word for 'devour' is voro.[7] Naturalist David Douglas had reported that the gopher was known to consume bulbs of the Camas lily.[8] Later, Vernon Bailey also attributed the lack of Camas lilies in areas where the gopher lived to the bulbs having been eaten. However, another naturalist of the time, H. M. Wight, was not convinced.[9]

There is a specimen of a quadruped in the Hudson's Bay Museum, which Mr David Douglas informs me is the animal known on the banks of the Columbia by the name of the Camas-rat, because the bulbous root of the Quamash or Camas plant (Scilla esculenta) forms its favourite food. The scull is wanting, and the animal, therefore, cannot be with certainty referred to a genus, but the form of its exterior cheek-pouches leads me to think that it may belong to the diplostoma of M Rafinesque-Smaltz.[8]

John Richardson, Fauna boreali-americana, 1829

The taxonomy of the Camas pocket gopher and the genus Thomomys have a complicated history.[10] The early confusion arises from writings by John Richardson during the years 1828 to 1839.[11] He describes six species in the group, although later critics state that he was not familiar with all specimens.[10] Richardson's descriptions of the animals and the figures in the text were also criticized.[10] In his 1829 work, Fauna boreali-americana,Richardson describes the type specimen of the Camas pocket gopher, obtained from along the "banks of the Columbia River, Oregon," which defines the northern limit of the gophers geographic range.[8][12] Most likely, this was in Portland, which is at the confluence of the Willamette River and the Columbia River, and the only place on the Columbia River from which subsequent specimens have been taken.[2] It is uncertain where this initial specimen now resides.[2] Although reportedly stored in the Hudson Bay Museum, in 1915 it could not be located.[2] At the time that Richardson made his first examination, it seems that this specimen was incomplete.[10] Later, Joel Asaph Allen writes in 1893 that Richardson's initial specimen consisted only of the skin.[13] However, Richardson offers detailed descriptions of skull and facial features.[14]

Woodcut of large gopher from 1829 book
Diplostoma douglasii from Fauna boreali-americana, 1829

In Fauna boreali-americana, Richardson assigns the animal to the now defunct genus Diplostoma, described by Rafinesque-Schmaltz in 1817.[8] He names it Diplostoma ? bulbivorum, which is an archaic name for the Camas pocket gopher.[10] Errors with labeling of illustrations in Richardson's book confounded subsequent taxonomists further still, as the plate was labeled Diplostoma douglasii.[10] The confusion over taxonomy and identification of the species deepened when renowned naturalist Spencer Fullerton Baird interpreted Richardson's reports.[13]

The large size of the Camas pocket gopher somehow led Baird to conclude that the measurements of the animal described by Richardson were an artifact of taxidermy preparation.[15] Baird was also apparently mistaken as to the location from which the specimen was taken.[13] He thus attributed the name Thomomys bulbivorus to a set of previously collected specimens, all subsequently referred to as the "California Gopher."[13] This confusion resulted in name being taken up by subsequent writers.[13] The gopher article of the 1879 edition of The American Cyclopædia provides an illustration captioned "California Gopher (Thomomys bulbivorus)."[16] The Encyclopædia Britannica, Ninth Edition from the late 1800s also mistakenly reports Thomomys bulbivorus as abundant in the state of California, along the central coast.[17]

Taxidermied specimen, Museo Civico di Storia Naturale di Genova, mislabeled California

Both Baird and Elliott Coues had been involved in the early assessments of the genus, but Allen reports that neither of them actually ever saw a specimen of the Camas pocket gopher (T. bulbivorus).[13] Allen was finally able to obtain and examine two large adults (male and female) collected in Beaverton, Oregon, in May 1890.[13] These specimens were found to be much larger and darker than previously examined specimens.[18] The characterization of white markings around the mouth and anus was different, as were certain skull features.[18] These findings, along with the location of specimen collection helped differentiate the Camas pocket gopher as a separate species from the gophers of California.[18] The California specimens were classified by Eydoux and Gervais as Oryctomys bottae now known as Thomomys bottae (Botta's pocket gopher).[18] They had been taken from around Monterey, California, which is over 1,000 kilometers (620 mi) south of the extent of the now recognized range of the Camas pocket gopher.[18]

It is one of five species in the subgenus Megascapheus.[19] The subgenus was established in 1903, at that time for the Camas pocket gopher alone.[5] The distribution of Elliot's "Great pocket gopher" as it was referred to, extended along the California coast, "north of San Francisco."[20] In the mid-1800s, James Audubon referred to the species as the "Camas rat", while combining what at the time was thought two separate species Pseudostoma borealis and Diplostoma bulbivorum. Based on observations of taxidermy specimens while in Europe, he suggested that Townsend's pocket gopher (Geomys (Thomomys) townsendii) was also the same species.[21] In 1875, it was reported as a sub-species of the Northern pocket gopher (Thomomys talpoides).[22] In the 1920s, H. M. Wight referred to it as the "Willamette Valley Gopher.[23]

Description[edit]

Camas pocket gopher at habitat restoration site

Gophers are burrowing rodents of the family Geomyidae. They are characterized by fur-lined external cheek pouches, which they employ when gathering and transporting food.[24] The Camas pocket gopher is the largest member of its genus Thomomys, though not distinctly so.[25] As with other gophers, they have small eyes and ears and a nearly hairless tail. The shoulders are broader than the hips. They are pentadactyl, with five claws on each foot.[5] The claws on the forefeet are longer than those on the hindfeet and the middle claws are longest.[5] The front claws of the Camas pocket gopher are relatively short and weak for its size.[26] They exercise plantigrade locomotion.[5] The males are larger, measuring on average 300 mm (12 in) in length.[5] Large males weigh around 500 g (18 oz).[5] One male specimen measured 321 mm (12.6 in) long and weighed 633.8 g (22.36 oz).[25] Females are approximately 271 mm (10.7 in) long.[5] The tail measures 90 mm (3.5 in) in males and 81 mm (3.2 in) in females.[5] The hind feet of adult males measure 40–43 mm (1.6–1.7 in), while the average female hind foot measured 39 mm (1.5 in).[5] There are 4 separate mammary glands, each supporting a pair of nipples. There are two pairs of nipples in the inguinal region and another two pairs in the pectoral region.[2] Morphologically, they most closely resemble Botta's pocket gopher.[27] Differentiation can be made based on concavities on the inner surfaces of the pterygoids, small claws, more uniform fur coloring, and exoccipital grooves of the Camas pocket gopher.[28][29]

Dorsal and ventral views of a Camas gopher skull
Skulls of the Camas pocket gopher (Bailey, 1915)

The fur coat of the Camas pocket gopher is a flat, dull brown.[3] The underside is a dark leaden gray.[3] In many cases, there are patches of white on the chin, throat, and around the anus.[27] They have blackish ear and nose markings.[5] The external ear consists merely of a thickened rim of tissue.[27] The large upper incisors are distinctive and give the animal a buck-toothed appearance.[3] They are sharply procumbent, meaning they tilt sharply forward. During the warmer summer months, the coat is short and fibers are coarse.[5] Winter pelage is longer and furrier.[5] The coats of the young are similar to adult summer coats, but more sparsely distributed, such that the skin may be apparent on the abdomen.[27]

Additional views of the skull (Brandt, 1855)

The skulls of the Camas pocket gopher are sturdily proportioned.[28] Smooth toothed pocket gophers with a more robust snout, including the Camas pocket gopher and four others, are placed in the subgenus Megascapheus.[19][25] Male skulls measure 52 mm (2.0 in) in length across the base and 57 mm (2.2 in) if the incisors are included.[5] The short, wide skulls have relatively short nasal passages.[5] In breadth, the skulls measure 19 mm (0.75 in) across the nasals passages, 30.5 mm (1.20 in) across the mastoids, 36.5 mm (1.44 in) at the zygomatic arches.[5] The slender incisors are prominent and distinctive, smooth surfaced, with yellow surface enamel. However, the incisors are white-tipped due to soil abrasion.[27] Neither the upper nor lower incisors are covered by the lips.[5] There are very faintly visible grooves on the inner aspects of the upper incisors, which are more pronounced in other members of the genus, such as the Mazama pocket gopher (T. mazama).[5] The upper molars have an alveolar length of 10 mm (0.39 in).[5] The external auditory meatus is broad and open, although the auditory bullae are more confined.[27]

The cheek pouches of geomyids such as the Camas pocket gopher are controlled by a set of muscles.[30] A sphincter controls the opening and closing of the pouch. A pair of muscles, attaching to the premaxilla, pull the pouches towards the opening. Paired retractor muscles pull the pouches back.[30] These extend back and up from the cheek surfaces, forming a band 7–10 cm (2.8–3.9 in) long and around 2 cm (0.79 in) wide.[5] These attach to tendinous coverings of the latissimus dorsi muscle.[30]

As with many placental mammals, the penis of the Camas pocket gopher contains a bone, the baculum.[5] The baculum of this species was initially reported as being smaller than in other gophers, measuring 1.5 mm (0.059 in) in height and 1.8 mm (0.071 in) width at the base and 8.5 mm (0.33 in) in length.[31] However, the examiner was uncertain if the specimen had reached full maturity.[5] Subsequent reports averaged around 2.1 mm (0.083 in) in height and 2.2 mm (0.087 in) width at the base and 10.1 mm (0.40 in) in length.[32] The phallus' total length averaged 13.5 mm (0.53 in).[5] Distally, the tract is long and narrow and the glans covers more than half the length.[5]

Distribution and habitat[edit]

Prairie land in the Willamette Valley, now plowed over for agricultural use
Prairie in the Willamette Valley, converted for agricultural use.

The Camas pocket gopher is found in the Willamette Valley.[5] It is also found in the associated drainage areas of the Yamhill River and other tributaries of the Willamette River.[5] The extent of the range extends north from Eugene to Portland and Forest Grove. It extends west to the Grand Ronde.[2] There was a report in 1920 of recovery of a Pleistocene fossil in Fort Rock, Oregon.[5] Subsequent experts question the validity of this specimen, however, since it lies far outside the current geographic range.[5] As of 1987, the specimen was not able to be located for further evaluation.[5]

The clay-rich Willamette Valley soils are hard in the dry season, and the gopher's protuberant incisors seem well adapted to working in these conditions.[27] Adequate drainage of the soil and the presence of suitable plant food are key components of an ideal habitat for the gopher.[33] The gophers are typically not found in wetland areas, where their tunnels might easily flood.[33] More commonly, they are found among seral communities, where grasses and shrubs may prevail.[33] They have been found in fields of alfalfa, wheat, and oats.[33] They will also select cultivated habitats, such as the vast expanses of land throughout the valley now converted to agricultural purposes.[33] They are found also in areas of ecological disturbance with similar terrain features.[33]

Topographic map of the Missoula floodwaters

The patterns of genetic variation in the Camas pocket gopher have been studied.[34] While there are no subspecies of the Camas pocket gopher, there is a substantial degree of genetic diversity within the species.[34] The genetic patterns are consistent with limited inbreeding within specific populations of Camas pocket gophers.[34] The degree of diversity in the genetics of the Camas pocket gopher is similar to that of Townsend's pocket gopher, which is distributed across a much greater geographical area.[34] Meanwhile, there is a great deal of differentiation between separate populations of Camas pocket gophers.[34] This degree of genetic variability does not carry a corresponding variation in the gross appearance of the animal.[34] By studying effects of genetic changes modeled over time, a pattern was found indicating that the species was affected by a cataclysmic event around 13,000 years ago, across their entire geographic area of distribution.[34] Such an event would cause a population bottleneck and lead to scattered, isolated populations. Over a geologic timescale, the Willamette Valley has been the site of massive floods.[34] During the late Wisconsin glaciation, a series of floods known as the Missoula Floods occurred.[34] The last of this series of floods, the Bretz flood, occurred around 13,000 years ago. This was a massive flood, with an estimated 1,693 km3 (406 cu mi) of water flowing at a rate of 42 km3 per hour (412 million ft3 per second) over a 40-hour period.[34] The flood covered the Willamette Valley up to around 122 m (400 ft), in an almost perfect overlay of the range of the Camas pocket gopher.[34] The species has been collected above this elevation, though not commonly.[33] A temporary lake was formed, known as Lake Allison.[34] It is assumed that the gopher lived in the valley prior to the flood, although no fossils have been recovered from sites in the valley.[34] The Chehalem Mountains—highest point 497 m (1,631 ft)—probably provided refuge for survivor populations.[34] The survivors subsequently would have repopulated in isolated pockets, once the waters receded.[34] Before and since the floods, the mountains have probably served to limit gene flow between populations.[34] The Willamette River does not appear to present an obstacle to genetic flow patterns within populations of gophers, possibly because it is a relatively narrow and sluggish river.[34]

Behavior[edit]

The gopher has been credited with being one of the most vicious animals known for its size. It has a great deal of courage and fights a man savagely until an opportunity for escape is offered, then it turns and runs as rapidly as possible, attempting to hide from its pursuer.[35]

— H. M. Wight

Multiple mounds of excavated earth from gopher activity, scattered over dry grass field
Camas pocket gopher mounds

The Camas pocket gopher is a mostly solitary herbivore that is active throughout the year and does not hibernate.[36] The gopher spends most of its time excavating tunnels in search of food.[37] The hard clay soils of the Willamette Valley create a challenge.[9] The front claws are too weak to dig through the clay, particularly during dry seasons, when it can be especially hard to penetrate.[9] The large incisors of the gopher and their strongly "procumbent" orientation are well adapted for this purpose.[9] The system of tunnels constructed by the Camas pocket gopher can be complicated. Some tunnels exceed 240 m (260 yd) in length. They are around 90 mm (3.5 in) in diameter and up to 0.91 m (3.0 ft) deep. There are conflicting reports that the range of the Camas pocket gopher and the Mazama pocket gopher overlap. If so, it is a departure from prior reports that ranges of gophers in Oregon do not overlap. There is some suggestion that in areas where the ranges do overlap, the Mazama pocket gopher is found at higher elevations. Another small rodent endemic to the Willamette Valley, the gray-tailed vole, (Microtus canicaudus) uses the tunnels of the Camas pocket gopher.[9] When soils are especially damp, the gopher is known to construct what appear to be ventilation ducts or chimney mounds, thought to increase ventilation.[9] These chimney mounds make be unique to the Camas pocket gopher.[37] They rise vertically 15–25 cm (5.9–9.8 in) and are open at the top. They are thought to ventilate the burrows through Bernoulli effects.[37] It not known if burrow systems of gophers adjacent to each other interconnect.[37]

Camas pocket gopher burrow

They are primarily fossorial, but occasionally gather food near the entrance of the tunnel.[9] Dandelions appear to be the most favored food of the Camas pocket gopher and are also used as nest material.[38] During breeding season, males will enter the tunnels of females. The males and females may make purr or make cooing when together.[39] In addition, the mother seems to comfort the young with soft vocalizations[39] The young reportedly will twitter in response.[23] They may have an aggressive demeanor when forced into a defensive position. Noted mammalogist Vernon Orlando Bailey described them as "morose and savage."[9] However, they can be easily tamed in captivity.[37] The females are more readily tamed than the males.[40] Mammals sharing the range of the Camas pocket gopher, which may also share the tunnels include: the vagrant shrew, Townsend's mole, the brush rabbit, the eastern cottontail rabbit, Townsend's chipmunk, the California ground squirrel, the dusky-footed woodrat, the North American deermouse, the gray-tailed vole, the creeping vole, Townsend's vole, the pacific jumping mouse, the long-tailed weasel, and the striped skunk.[9]

Ecology[edit]

Thomomys bulbivorus pups
Growth and development[31]
Age (weeks) Weight Length Description
Birth 6.1 g (0.22 oz) 50 mm (2.0 in) no hair, no teeth, no cheek pouches
2 23 g (0.81 oz) 90 mm (3.5 in) developing hair
3 35.5 g (1.25 oz) 108 mm (4.3 in) crawling, eat solid food
4 53.6 g (1.89 oz) 123.5 mm (4.86 in) pockets developed
5 70.5 g (2.49 oz) 153 mm (6.0 in) eyes open
6 86 g (3.0 oz) 164 mm (6.5 in) weaned

Different times of onset and duration of the breeding season of the Camas pocket gopher are reported.[32] Early reports suggested an onset in early April, extending through June.[32] Others reported "evidently pregnant" females in late March.[32] In areas of intensive irrigation, the breeding season may be further extended. The season may extend into early September.[33] Around four young are born with each litter, though this can be as many as nine.[32] The blind, hairless, and toothless offspring weigh around 6.1 g (0.22 oz), measuring 50 mm (2.0 in) in length.[32] In the first six weeks, they will begin to crawl, develop cheek pouches, open their eyes, and convert from milk to solid food.[32] They then weigh around 86 g (3.0 oz) and measure 164 mm (6.5 in) in length.[32] At weeks 8, 10, and 17 they will weigh 101 g (3.6 oz), 160 g (5.6 oz), and 167 g (5.9 oz).[32] There are variable reports that more than one litter can be born in a season.[33] Sexual maturity likely develops by the breeding season of the subsequent year.[32] Males are fully grown by that time, though females may continue to accumulate mass.[33]

There is little data on the longevity and mortality of the Camas pocket gopher.[33] Presumably, carnivorous mammals prey upon them and their bones have been identified in the regurgitated pellets leavings of raptors, such as the great horned owls.[9] Parasites include mites, lice, fleas, roundworms, and flatworms.[9] The tougher skin of the Camas pocket gopher may protect it from some fleas known to infest Botta's pocket gopher and the Mazama pocket gopher.[9] Mites known to parasitize the Camas pocket gopher include Androlaelaps fahrenholzi and Echinonyssus.[9] The chewing louse Geomydoecus oregonus has also been reported.[9] There are two parasitic worms, which were first discovered in the gastrointestinal tracts of Camas pocket gophers.[9] They are the nematode Heligmosomoides thomomyos and the cestode Hymenolepis tualatinensis.[9] Other worms include two nematodes and the cestode Hymenolepis horrida.[9]

1879 anatomical illustration

Human interactions[edit]

Due to economic impacts related to crop damage and destruction of grazing grounds, the Camas pocket gopher is treated as an agricultural pest. Crops damaged include clover, alfalfa, and vetch. These may be eaten or damaged from subterranean activities, which can injure and dry out the roots. Fruit trees may be damaged in the same manner. Root crops are particularly susceptible to damage and consumption. Potatoes, carrots, parsnips and other crops may be eaten on site, or dragged off by the gopher for storage in the burrows. Additional damage can result from the sheer volume of soil excavated covering up the grass to the point that grazing grounds become unusable. Freshly sprouted grains may be covered up and damaged in similar fashion. Poisoning dandelions has been a method for controlling populations in agricultural settings.[38] Strychnine laced clover, carrots, sweet potatoes, and parsnips are also suggested. Due to its position as the largest member of the group, conventional gopher traps are often too small to be effective for catching and killing.[9] Toxic baits and fumigants may serve inadequate in their purpose, as the gophers will sometimes simply wall off that segment of the burrow.[9] Gophers in general may also be implicated in causing local flooding, if dikes are damaged by their tunneling activities.[41] On a positive note, gopher activity aerates the soils.[41] Similarly, soil retention of water following rains or snow melts is enhanced.[41] Buried vegetation can compost to further enrich soil.[41]

Conservation status[edit]

Skeleton model of the Camas pocket gopher in a museum display case
Skeleton at the Osteology Museum of Oklahoma

The conservation status of the Camas pocket gopher is listed as least concern by the IUCN (International Union for Conservation of Nature) Species Programme.[1] The population trend is listed as stable by the IUCN.[1] Citing concerns of urbanization, habitat loss, and active attempts at eradication, NatureServe assesses the conservation status as "G3 – Vulnerable."[36] However, they list the overall degree of threat impact as medium.[36] They also project long-term population declines may reach 50% due to habitat loss.[36] The IUCN notes that the area of species distribution is less than 20,000 km2 (7,700 sq mi).[1] The Willamette Valley contains 70% of the human population in the state.[42] The IUCN and other express concern regarding degradation of habitat through urbanization and expansion of territory dedicated to agricultural production.[1][43] The IUCN notes that the range likely provides a few protected areas. However, many of the territories set aside as preserves within the valley are more geared towards waterfowl protection for hunters.[43] Such wetland areas are not well suited for the Camas pocket gopher, as tunnels are prone to flooding. The gophers can do well in disturbed habitats and pastoral farmland settings.[33] In those same areas, they may be considered pests and subjected to active programs geared towards eradication through poisoning and trapping.[1] The animal is common within its range and studies demonstrate that populations can recover rapidly after traps are removed from the area.[1][33] It is also noted that the gopher adapts well to changes in its environment.[1]

References[edit]

Footnotes:

  1. ^ a b c d e f g h IUCN Red List 2008.
  2. ^ a b c d e f Bailey 1915, p. 40.
  3. ^ a b c d e Kays Wilson2010, p. 82.
  4. ^ Elbroch 2006, p. 296.
  5. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac Verts & Carraway 1987, p. 1.
  6. ^ Brandt 1855, p. 188.
  7. ^ a b Verts & Carraway 1987, p. 4.
  8. ^ a b c d Richardson 1829, p. 206.
  9. ^ a b c d e f g h i j k l m n o p q r s Verts & Carraway 1987, p. 3.
  10. ^ a b c d e f Allen 1893, p. 53.
  11. ^ Allen 1893, pp. 53–64.
  12. ^ Allen 1893, p. 55.
  13. ^ a b c d e f g Allen 1893, p. 56.
  14. ^ Richardson 1829, pp. 206–207.
  15. ^ Allen 1893, pp. 53–56.
  16. ^ "Wikisource link to Gopher". The American Cyclopædia (1879). Wikisource. 1879.
  17. ^ "Wikisource link to California". Encyclopædia Britannica, Ninth Edition. Wikisource.
  18. ^ a b c d e Allen 1893, p. 57.
  19. ^ a b "Thomomys (Megascapheus)". Integrated Taxonomic Information System. Retrieved 14 November 2014. 
  20. ^ Elliot 1905, p. 272.
  21. ^ Audubon, Audubon & Bachman 1851.
  22. ^ Coues 1875, pp. 136–137.
  23. ^ a b Wight 1922.
  24. ^ Vaughan, Ryan & Czaplewski 2011, p. 207.
  25. ^ a b c Verts & Carraway 1998, p. 229.
  26. ^ Bailey 1915, p. 42.
  27. ^ a b c d e f g Bailey 1915, p. 41.
  28. ^ a b Bailey 1915, p. 32.
  29. ^ Bailey 1915, p. 36.
  30. ^ a b c Merriam 1895, p. 101.
  31. ^ a b Verts & Carraway 1987, pp. 1–2.
  32. ^ a b c d e f g h i j Verts & Carraway 1987, p. 2.
  33. ^ a b c d e f g h i j k l m Carraway, B.J. Verts, Leslie N. (1998). Land mammals of Oregon. Berkeley: University of California Press. pp. 229–231. ISBN 9780520211995. Retrieved 15 November 2014. 
  34. ^ a b c d e f g h i j k l m n o p q Carraway & Kennedy 1993.
  35. ^ Wight 1918, p. 16.
  36. ^ a b c d NatureServe 2014.
  37. ^ a b c d e Verts & Carraway 1998, p. 230.
  38. ^ a b Experiment Station Record 1920.
  39. ^ a b Verts & Carraway 1987, pp. 3–4.
  40. ^ Wight 1918, p. 3.
  41. ^ a b c d Nowak 1999.
  42. ^ Jewell, Judy; McRae, W. C. (31 May 2011). Moon Spotlight Eugene, Salem, & the Willamette Valley. Avalon Travel. p. 9. ISBN 1-61238-094-8. 
  43. ^ a b Ricketts, Taylor H. (1999). Terrestrial Ecoregions of North America: A Conservation Assessment. Island Press. p. 155. ISBN 978-1-55963-722-0. 

Sources:

External links[edit]