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Geomys bursarius - Plains Pocket Gopher

Description:

Geomys bursarius is a stout, thick bodied gopher with little evidence of a neck.  It has a short sparsely haired tail sensitive to the touch.  It has both small eyes and ears, while well developed lachrymal glands supply a thick fluid that cleans the cornea free of dirt and other debris.  The lips can also be closed behind the curved incisors, which are used for gnawing dirt, and prevent dirt from entering the mouth.  Geomys bursarius is equipped with powerful forearms for digging with five strong digits on each arm and five digits on each hind foot.  They have loose, flexible skin that is thick around the head and throat that is advantageous for fighting.  Two long external fur lined pockets extend from the facial regions back to the shoulders that are used in transporting food.  The pockets can be turned inside out for cleaning and pulled back into place by a specialized muscle (Nowak 1999).

A baculum is present that is simple and rodlike, being bulbous at the proximal end and pointed at the distal end (Anderson and Jones 1984).

Kurta (1999) provides the ranges of four external characters in inches: total length, 9.3-12.2; tail length, 2.5-3.5; hind foot length 1.2-1.5; and ear height, 0.2-0.4.  The tail is generally one-third of the body length.  The weight generally ranges from 4.5-12.5 ounces.  The actual size of Geomys bursarius may vary.  The back is usually a dark brown, black or gray with under parts slightly lighter.  The top of the feet is covered with whitish hairs (Kurta 1999).  Geomys bursarius is noted for the color of its pelage varying greatly over its range.  Color of dorsal fur generally matches the color of local soils (Krupa and Geluso 2000).  The fur should match the color of the moist soil being excavated.

The skull is modified for fossorial life being massive, angular and flattened with strong and wide zygomatic arches.  The infraorbital canal is long and narrow.  It is sunken in the skull to provide protection from muscle pressure.  The opening to the canal is anterior to the zygomatic arch (Nowak 1999).  The bullae are moderately large with the nasals usually narrow.  Temporal ridges are strong and frequently form a sagittal crest.  A deep pit exists on each side of the palate at the level of the third molar.  The palatines unite with the pterygoids behind the pits to form strap-shaped pterygoid plate on the sides of the posterior nares (Anderson and Jones 1984).

The dental formula of Geomys bursarius is as follows: 1/1,0/0,1/1,3/3=20.  The premolar is 8-shaped with an indentation on each side and it is the largest grinding tooth.  All teeth have reduced enamel while cheek teeth grow throughout life.  The upper incisors are broad and chisel-like (Anderson and Jones 1984).

 Distribution:

Geomys bursarius  is found mainly in the western parts of Wisconsin (Figure 1).  Geomys bursarius is widely distributed across much of the central United States from Texas north to southern Manitoba and from eastern Indiana west to Wyoming, Colorado, and New Mexico (Krupa and Geluso 2000).  The average density of Geomys bursarius is 10-12 gophers per hectare or 4-5 per Acre (Kurta 1999).

Figure1.  Geographic distribution of Geomys bursarius.

 

Source:  http://www.dnr.state.il.us/

Geomys bursarius is a common inhabitant of the Great Plains of North America.  There are five distinct species of Geomys; Geomys bursarius, G. attwateri, G. breviceps, G. texensis and G. knoxjunes (Wilson 1999).  Densities are often locally high, ranging to more than 200 per hectare and frequently are 50-100 per hectare.

Ontogeny and Reproduction:

Geomys bursarius is a highly solitary animal throughout its life, except during the breeding season.  They are most active during the winter, spring and early summer (Anderson and Jones 1984).  Although females are generally monestrous, having one single annual litter, some individuals have two active breeding periods in a year.  The gestation periods are currently unknown (Kurta 1999).  Females may have one to two litters of one to eight offspring produced between March and August.  The newborn weigh and average of five grams and are a length of 1.6 inches.  The eyes of the young open after three weeks, weaning occurs at five weeks and dispersal occurs two to three months after birth (Kurta 1999).

The testes are abdominal except during the breeding season when they become enlarged and scrotal (Anderson and Jones 1984).  Breeding season is the one exception when individuals do not fight vigorously.

Ecology and Behavior:

Geomys bursarius is a highly fossorial animal, rarely coming to the surface from their complex system of burrows.  Their burrows consist of a central gallery (~140m long) with numerous storerooms for food and a single nest chamber at approximately 60-90cm below the surface (Anderson and Jones 1984).  Caches provide stores for food for getting through times when fresh vegetation is not readily available.  The energy cost of burrowing is estimated to be 360 to 3400 times that of aboveground travel (Huntly and Inouye 1988).  Individuals can almost run as fast backwards as forwards in their burrows.  They do so by arching their tails when traveling backwards, with the tip almost being in contact with the ground (Nowak 1999).  Geomys bursarius doesn’t travel far and occurs in localized, isolated areas of suitable habitat.

The construction of these tunnels makes up a high proportion of its life.  It loosens the hard-packed soil with its claws, pulls the dirt under its belly, and then kicks the pile backwards with its rear feet.  It also uses its incisors to loosen soil, rocks and cut roots.  After a large accumulation of dirt has occurred, it proceeds to push the load of dirt down the tunnel using its head and front feet.  The dirt is then distributed to the surface making crescent-shaped mounds up to 30cm high and 60cm across.  These mounds mark each gophers burrow system.  Mound building generally occurs at sunrise or sunset, with one to three mounds built per day (Kurta 1999).  Mound production provides channels for deep penetration of water, reducing erosion (Anderson and Jones 1984).  Huntly and Inouye (1988) state mounds have been assumed to affect plant community diversity and species composition simply by providing space for colonization by so-called fugitive species, plants that are competitively eliminated over time by other plants.  Also, mounds of nitrogen-poor subsurface soil are deposited on the ground surface.  Soil nitrogen is then redistributed, creating patches of surface soil with lower than average nitrogen content.  Pocket gophers appear to be able to locate areas of higher biomass suggests that these animals have the potential to influence patterns of plant biomass production in old fields.  In addition, the disturbance caused by digging activities, coupled with an increased cycling of organic matter and nutrients, influence vegetative composition (Behrend and Tester 1988).

Mound building by Geomys bursarius increases the risk of predation because of exposure at the surface while shoving soil from their burrows.  These predators have a wide range of visual perception ranging from highly developed color vision to monochromatic vision.  Predators include badgers (Taxidea taxus), weasels (Mustela), coyotes (Canis latrans), foxes (Vulpes, Urocyon), skunks (Mephitis, Spilogale), domestic cats, hawks (Accipiter, Buteo), owls (Bubo, Tyto), and bull snakes (Pituophis melanoleucus) (Krupa and Geluso 2000).

Geomys bursarius is a strict herbivore with a diet of roots, rhizomes and bulbs of plants that are encountered in its tunnels during digging.  Stems and leaves are sometimes consumed, but to a lesser extent, such as clover, alfalfa, grass, dandelion, plantain, mullein and dock.  If abundant food is encountered, the cheek pouches will be filled with that food source and then taken to an underground food chamber.  The cheek pouches may hold up to 50 corn kernels and plant stem lengths up to three inches long (Kurta 1999).  Not all of the food stored in these chambers is consumed, as some of it goes to waste. As many as one percent of young pines and oaks at the margins of old-fields are killed each year by pocket gophers that graze off their roots, found by Huntly and Inouye (1988), and more than 25% of young trees may be killed by gophers.

Literature Cited:

Anderson, S. and J.K. Jones Jr.  Orders and Families of Recent Mammals of the World.  1984.  John Wiley and Sons, Inc.

Behrend, A.F. and J.R. Tester. 1988. Feeding ecology of the plains pocket gopher in east central Minnesota. The Prairie Naturalist 20(2):99-107.

Huntly, N. and R. Inouye. 1988. Pocket gophers in ecosystems: patterns and mechanisms.  Bioscience 38(11):786-793.

Illinois Department of Natural Resources Website.  <www.dnr.state.il.us>.  Accessed 24 November 2003).

Krupa, J.J. and K.N. Geluso. 2000. Matching the color of excavated soil: cryptic coloration in the plains pocket gopher (Geomys bursarius). Journal of Mammalogy 81(1):86-96.

Kurta, Allen.  Mammals of the Great Lakes Region.  1995. University of Michigan Press. 138-141.

Nowak, M. Walker’s Mammals of the World. 6th Edition, Volume 2. 1999. John Hopkins University Press. 1309-1311.

Wilson, D.E. and S. Ruff. The Smithsonian Book of North American Mammals. 1999.  The Smithsonian Institution. 485.

Reference written by Kevin Pankow, Biol 378: Edited by Chris Yahnke. Page last updated 4-23-04.

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