Morphological constraints in the digging apparatus of pocket gophers (Mammalia: Geomyidae)

The digging apparatus of pocket gophers offers a unique opportunity to examine morphological constraints within a historical context because relationships among extant taxa are well resolved and the features enhancing digging performance are relatively well understood. Structural and functional considerations suggest that the muscles associated with tooth- and claw-digging in pocket gophers are subjected to contrasting levels of morphological constraints. To assess this hypothesis, we analysed the bones and muscles of the jaws and forelimbs in four genera comprising five species of pocket gophers. Morphometric analyses were performed on 12 osteological measurements selected to reflect overall skull size, variation in rostral shape and procumbency, differences in overall length of the forelimbs and processes relating to the function of lever systems used in claw-digging. In addition, dissections were made of the jaw, hyoid, neck and all of the forelimb muscles excluding the intrinsic muscles of the manus. Results of our morphometric analyses corroborate the recent suggestion that pocket gophers encompass a wide range of morphological variation extending from claw-diggers to tooth-diggers. Myologically, however, we found structural variation only in the forelimb muscles, some of which may be advantageous for digging. No changes in jaw, neck and hyoid muscles, other than differences in muscle mass or those concordant with differences in rostral shape, were noted. These results support our hypothesis that constrasting levels of morphological constraint exist between the jaw and forelimb muscles of pocket gophers. We present a discussion of the structural and functional constraints on jaws and forelimbs in gophers as well as an analysis of historical constraints acting on this group, and perhaps on mammals in general.     

Patterns of size change in late Neogene pocket gophers from the Meade Basin of Kansas and Oklahoma

A dense late Neogene history of pocket gophers from the Meade Basin of south-western Kansas and north-western Oklahoma preserves a detailed record of size change, as indicated by mean length of the lower fourth premolar. A ‘time for space substitution’ interpretation results in the tentative recognition of a pattern analogous to character displacement, in which the small to medium-sized Pliogeomys buisi, P. louderbachi, Geomys minor and G. floralindae are considered ecological analogues and treated as a single ecological entity, Avatar A. The larger G. jacobi and G. quinni are combined as Avatar B. Statistical analyses confirm that populations of Avatar A are of equal size when allochronic from Avatar B, and significantly smaller when synchronic (and sympatric) with Avatar B. However, this pattern is complicated by the observation that Avatar A began dwarfing before the appearance of Avatar B. This historical perspective suggests that static size patterns among modern taxa may have a complex history. Occasional diminutive geomyid transients (G. adamsi, Thomomys cf talpoides) are always paired with significantly larger, medium-sized Geomys. Although a random walk explanation for the pattern of size changes is unlikely, we are currently unable to distinguish between environmental change and competition as the likely forcing mechanisms.     

Pocket gophers and chewing lice: a test of the maternal transmission hypothesis

The life-history traits of pocket gophers and their chewing lice suggest that there is little opportunity for transmission of parasites among pocket gophers, with the exception of transmission from mother to offspring. Herein, we test the hypothesis that lice are transmitted maternally by using an indirect approach that compares the distribution of louse populations to the distribution of mitochondrial DNA haplotypes in the pocket gophers. Comparison of the chewing louse distributions to the distribution of mtDNA haplotypes for the gophers revealed no significant concordance, and thus falsifies the maternal transmission hypothesis.     

Soil translocation by pocket gophers in a Mima moundfield

Summary We measured soil translocation due to the tunneling of valley pocket gophers (Thomomys bottae) in a Mima moundfield at Miramar Mounds National Landmark, San Diego, California, from December, 1984 through December, 1985. We placed 1-l soil plugs containing 20 11-g iron pellets into pocket gopher tunnels at locations between mound tops and points about one mound radius beyond mound edges. After about 4–10 d, sites to which the marker-containing soil had been translocated were located with a metal detector and the horizontal and vertical displacements measured. Between 1 October and 15 May (the cooler, wetter portion of the year), pocket gophers removed an average of 63% of the experimental plugs and moved an average of 38% of the markers that we recovered. From 15 May through 1 October (the hotter, drier portion of the year), only 32% of plugs were cleared and 12% of the recovered markers were moved. On average, markers that were moved were displaced 41 cm moundward and 4.9 cm upward in elevation. The intensity of moundward translocation increased with distance from the mound center. At a distance of 0.5–1.0 mound radius beyond the edge of the mound, the moundward translocation tendency averaged 71 cm. The intensity of moundward translocation was also inversely related to maximum mound height. These observations provide strong support for the fossorial rodent hypothesis of Mima mound origin, and constitute a first step in development of a mathematical model of mound formation.     

DNA Data Support a Rapid Radiation of Pocket Gopher Genera (Rodentia: Geomyidae)

In this study, we address the question of phylogenetic relationships in the Geomyidae, focusing primarily on intergeneric relationships within the tribe Geomyini. Our study makes use of DNA sequences from two mitochondrial and two nuclear genes, and we use model-based methods of phylogenetic analysis to infer relationships and determine the level of support for each proposed relationship. Relationships among geomyine pocket gopher genera remain only partially resolved despite a number of earlier attempts to reconstruct their phylogenetic history and despite the newly generated sequence data analyzed in this study. This lack of resolution does not appear to result from insufficient or inappropriate DNA data, nor is it caused by inadequate sampling of taxa. Rather, molecular data and fossil data together lead to the conclusion that diversification within the Geomyini likely occurred during a geologically brief period in the Blancan. Rapid climate change during the Blancan, the origin of patchily distributed grasslands, and the evolution of hypsodonty may have triggered the rapid diversification that eventually produced the five extant genera of the Geomyini.