Geomyidaepocket gophers

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The family Geomyidae includes 5 genera and around 35 species. They are found only in North and Central America.

A number of groups of rodents have become fossorial (living most of their lives underground). The geomyids are one of them. Their bodies are strikingly modified for digging and living in burrows. They are stout without a distinct neck and with short, powerful legs. Their feet are broad and the toes are tipped with enlarged claws; the claws on the forefeet are especially well developed. Forefeet and hindfeet each have 5 digits, and the surface area of the forefeet is increased by the addition of a fringe of stiff hairs around the periphery. The tail is short, sparsely haired, and usually well supplied with nerves, muscles, and blood vessels. These animals move backwards almost as readily as forwards, and their tails probably provide important sensory information. Eyes and ears are small, but the eyes have enlarged lacrimal glands, presumably for providing fluid to wash out dirt. The lips can be closed behind the incisors, so that pocket gophers can use their incisors for loosening dirt or roots without filling their mouths with dirt.

The name "pocket gopher" comes from the unusual fur-lined cheek pouches of these animals. These pouches, which extend from the side of the mouth well back onto the shoulders, are used for transporting food. They can be turned inside out.

Most pocket gophers are moderately large, weighing a few hundred grams. A few species of Central American forms reach very large size, almost 1 kg. The pelage lacks underfur and is usually some shade of brown. It often closely matches the color of the soil in which the animal lives.

The skulls of pocket gophers also reflect their fossorial habits. They are massively built, flattened in profile and angular in overall appearance. The zygomatic arches flare widely. At their anterior end is a large zygomatic plate and very small infraorbital canal, sunken into the skull and opening on the side of the rostrum; these animals are sciuromorphous. The temporal ridges are enlarged and help support large temporalis muscles; frequently, the ridges join in the middle of the skull to form a sagittal crest. On the ventral surface of the skull, there is a deep pit on each side of the rear of the palate. The incisive foramina are small and enclosed by the premaxillae, and the bullae are moderately large but not inflated. The lower jaws are sciurognathus.

Pocket gophers have massive incisors. The dental formula is 1/1, 0/0, 1/1, 3/3 = 20. The premolars are large and 8-shaped (i.e., with deep indentations in the middle), and the molars are smaller and ring-shaped, sometimes with a posterior heel. All teeth are ever-growing.

Geomyids are accomplished burrowers. Their burrow systems include both long, shallow tunnels used for foraging, and deep tunnels used for nesting, food storage, and as latrines. They dig by loosening the soil with their incisors and foreclaws, then pushing it with chest and forefeet to the surface or into empty burrows. The area of a burrow system is marked by numerous mounds where soil has been brought to the surface. During the winter, soil is packed into tunnels through the snow, and snowmelt leaves long "gopher cores" of earth lying on the surface. Pocket gophers usually close the mouths of their burrows, and opening a closed burrow will often bring the burrower hurrying to renew the plug.

Pocket gophers feed primarily on roots and tubers, although they sometimes eat other parts of plants as well. Food is packed into the cheek pouches and carried to underground storage chambers. In some communities where gophers are abundant, they consume an amazing fraction of the underground productivity of plants.

Pocket gophers are generally solitary and pugnacious, coming together only to reproduce. Their burrows, however, are used by a remarkable variety of other species of animals.

The geologic record of this group extends to the early Miocene. They are almost certainly closely related to heteromyids. Pocket gophers tend to be found in small populations, probably due to the patchy distribution of appropriate soils. This, combined with their tendency to specialize to very local soil conditions in color, size, and other characteristics, has resulted in a large number of recognizable taxa (and some fascinating research on microevolutionary processes).


References and literature cited:

Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.

McLaughlin, C. A. 1984. Protrogomorph, sciuromorph, castorimorph, myomorph (geomyoid, anomaluroid, pedetoid, and ctenodactyloid) rodents. Pp. 267-288 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.

Paradiso, J. L. 1975. Walker's Mammals of the World, Third Edition. Johns Hopkins University Press, Baltimore.

Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.

Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.

Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.

Contributors

Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.

Glossary

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.

chemical

uses smells or other chemicals to communicate

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.

motile

having the capacity to move from one place to another.

sexual

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

tactile

uses touch to communicate