Secretive pocket gophers

By Jeff Mitton

I sat with family and friends at a scenic overlook, enjoying the view. Then someone observed the furtive activity at our feet. Several small mammals were sticking their heads out of burrows, grabbing stalks of grass and ducking back in. I had never seen pocket gophers "in the wild" before.

The northern pocket gopher,Thomomys talpoides, is a small mammal, six to 10 inches long with brown fur, a short tail, small ears and small eyes.

Perhaps their most distinctive feature is their large yellowish-orange incisors, two upper and two lower, that are always visible and always growing. When the incisors first develop, they pierce the upper and lower lips, so the lips close behind their incisors, allowing gophers to carry rocks with their incisors while keeping their lips sealed.

They are stocky animals, with short, powerful limbs and large front paws and claws used for digging. Pocket gophers have cheek pouches that open externally, in which they carry food and nesting materials.

Pocket gophers are fossorial, meaning that that they live underground, in burrows excavated in the soil. Their burrow systems are elaborate, multilayered and may be 500 feet long.

A shallow level, approximately one foot below the surface, is used to store feces. A deeper layer, two feet or more below the surface, is used for the nest and food storage. A third layer is constructed in winter, at ground level but beneath the snow. Pocket gophers fill their snow tunnels with soil excavated in winter, and these tubes of soil, called eskers, remain behind when the snow melts, leaving ubiquitous evidence of gophers in mountain meadows.

Northern pocket gophers have an immense range, from New Mexico to Manitoba and from northern California to eastern North Dakota. Their range of habitats is also broad, including sagebrush, meadows embedded in forests and alpine tundra. Their immense geographic range, great variability in color and size, and large ecological amplitude inspired biologists to describe more than 50 subspecies of northern pocket gophers, six of them in Colorado.

One of the subspecies,T. talpoides macrotis, has a tiny geographic range due south of Denver in northern Douglas County. In recent years, while one group of citizens was pressing for protection forT. t. macrotis, a developer was seeking permission to place a new residential subdivision right on top of them. The Colorado Division of Wildlife, seeking information to inform the management of the gophers, asked my laboratory to contribute genetic data to determine if this subspecies was unique and worthy of protection.

Renee Culver earned a master's degree comparing DNA sequences of six subspecies of northern pocket gophers. Decades before, the chromosomes of the subspecies had been studied by professor C. S. Thaeler. The numbers of the chromosomes varied among geographic regions from 40 to 48, but there was little or no correspondence between chromosome numbers and subspecies. Chromosomal differences interfere with development of offspring and thus impose reproductive isolation, one of the definitions of species.

Culver found thatmacrotishad the same DNA sequences as its geographically adjacent subspeciesretrorsusandrostralis. That is, DNA sequences did not provide justification for recognizing these three subspecies.

Culver noted excellent correspondence between DNA sequences and chromosome counts. For example,T. talpoides kaibabensis, isolated on the Kaibab Plateau of northern Arizona, was so distinctly different for chromosome numbers and DNA sequences that it is surely a distinct species. Similarly, correspondence between chromosomes and DNA sequences will likely lead to splitting of the northern pocket gopher into four distinct species in Colorado.

Jeff Mitton (mitton@colorado.edu) is chair of the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.


July 19, 2010