On Evolution

3. The Pocket Gopher

Molecular techniques are powerful enough to identify populationgenetic subdivisions within and among closely related species. This paper describes an application involving pocket gophers of the southeastern United States. In the late s, the U.S. Department of Defense proposed building a Naval Submarine Base in southeastern Georgia, exactly where an endangered species of pocket gopher lived that had been described in the late s. This created a problem for Georgia’s Department of Natural Resources (DNR), which is charged with managing and protecting Georgia’s wildlife. So, DNR officials approached John Avise and Joshua Laerm (who later became the director of the Georgia Natural History Museum) to conduct a detailed genetic assessment of this endangered species. In the course of the study, the two spent many an afternoon capturing pocket gophers under the hot Georgia sun. The process itself was interesting , as were the scientific findings that emerged. This article, written for a popular audience, explains why. “There is a large ground rat, more than twice the size of the Norway rat. In the night time, it throws out the earth, forming little mounds, or hillocks.” With these words, written in , the great American naturalist William Bartram introduced us to a common rodent whose existence is closely interwoven with the soil it inhabits. The rodent is Geomys pinetis, the southeastern pocket gopher. The word Geomys is derived from the Greek roots Geo, meaning “earth,” and mys, meaning “mouse.” The word pinetis derives from the fact that the species is frequently found in stands of longleaf and slash pine, trees that are associated with the sandy soils that characterize typical habitats of pocket gophers throughout much of Florida, Georgia, and Alabama. The Pocket Gopher 3 Bartram was not the only prominent historical figure to comment on the curious mound-building activities of these gophers. Sir Charles Lyell (whose treatise Principles of Geology had a profound influence on the thinking of Charles Darwin) visited the United States in  and wrote: “We also saw small hillocks, such as thrown up by our (British) moles, made by a very singular animal, which they call a salamander.” (The word “salamander” probably originated as a mispronunciation of “sandy-mounder,” the name used earlier by plantation slaves.) To add to the confusion, “gopher” throughout the southeast is also used in reference to the burrowing tortoise, Gopherus polyphemus. Most southerners know the trademark of Geomys: clusters of earthen mounds, each mound about  inches high and – feet across. These mounds are merely the tip of a burrow-system iceberg. Pocket gophers are fossorial and they spend most of their lives beneath the earth’s surface. Their extensive burrows, often more than  feet in length, lie more or less horizontal to the surface and are from one-half to  feet deep (although in very soft sand they may be as deep as  feet). The surface mounds are the soil excavated from the burrow system . Loosened dirt accumulates in the burrow as the gopher digs. Periodically , the gopher bulldozes the soil outward to the growing mound on the surface. A single burrow system usually has six to twelve or more associated mounds spaced at intervals of several feet. The mounds are connected to the burrow system by diagonal tubes. These tubes are usually plugged with dirt by the wary gopher, so no openings are visible above ground. The only time a burrow system is open is when a gopher makes a brief excursion to the surface to forage for grass, which the animal stuffs in external, fur-lined cheek pouches—its “pockets.” Once the pockets are full, a gopher returns to the burrow where some of its food may be stored in special caches against leaner times in winter. It is not always necessary for gophers to leave their burrow systems to forage. Frequently they burrow scant inches beneath the surface collecting edible roots and bulbs. Pocket gophers in their burrows tend to be territorial and very aggressive. Despite their solitary nature, go-  On Evolution phers do aggregate in colonies. These may include a few individuals to as many as a hundred. Male and female gophers maintain separate burrow systems. Females tend to dig localized burrow networks, whereas males dig a more linear burrow system that may extend for hundreds of feet. Southeastern pocket gophers breed throughout the year, with peak activity in February through March and June through August. Litter size is commonly two...