Cryptic species and taxonomic revision of kangaroo mice, the rodent genus <i>Microdipodops</i>

This study presents the first systematic revision of kangaroo mice, the heteromyid genus <i>Microdipodops</i>, in over 8 decades. The study relies on a suite of genetic (karyology and nuclear and mitochondrial DNA sequencing) and phenetic (cranial morphometrics and pelage colorimetry) characters to examine differentiation within the genus. We also present the first molecular phylogeny of the subfamily Dipodomyinae (includes kangaroo mice and kangaroo rats, <i>Dipodomys</i>) that incorporates all currently recognized species. Building on previous studies (including unpublished materials) and incorporating additional laboratory analyses and fieldwork, we find genetic evidence to recognize 6 species of <i>Microdipodops</i> organized into 2 subgenera as follows: <i>M</i>. <i>Microdipodops</i>, including <i>M</i>. (<i>M</i>.) <i>albiventer</i>, <i>M</i>. (<i>M</i>.) <i>megacephalus</i>, <i>M</i>. (<i>M</i>.) <i>oregonus</i>, and <i>M</i>. (<i>M</i>.) <i>polionotus</i>; and <i>M</i>. <i>Psammophilomys</i>, new subgenus, which includes <i>M</i>. (<i>P</i>.) <i>pallidus</i> and <i>M</i>. (<i>P</i>.) <i>ruficollaris</i>. All 6 species are endemic to the Great Basin Desert of western North America. We are unable to discriminate among these species using discriminate function analyses of cranial and pelage morphometric characters; indeed, the species are morphologically cryptic but genetically distinctive. We find strong molecular phylogenetic support for both a monophyletic <i>Microdipodops</i> and <i>Dipodomys</i>. Molecular divergence-time analyses show that the 2 subgenera of <i>Microdipodops</i> diverged from each other about 4.8 million years ago (Ma) and that the 3 sister-species pairs diverged approximately 1.2, 1.4 and 1.9 Ma, which compares to sister species of <i>Dipodomys</i> that diverged between 0.8 and 3.8 Ma in the same analysis. Expanded fieldwork, specimen collecting, and genetic sampling show range extensions and a reorganized distributional patterning of species of <i>Microdipodops</i> across the Great Basin Desert. Most noteworthy among the distributional changes is our discovery that kangaroo mice of the Owyhee Desert (ecoregion includes southwestern Idaho, southeastern Oregon and north-central Nevada), previously deemed a relictual isolate, all belong to the same species and that this species should bear the name <i>M</i>. (<i>M</i>.) <i>megacephalus</i> given that the type specimen of the genus (from Halleck, in northern Nevada) is genetically most closely related to other kangaroo mice of the Owyhee Desert. The ecological preferences of <i>M</i>. (<i>M</i>.) <i>megacephalus</i> appear to be strikingly dissimilar to those of its congeners. With the descriptions of 4 new species, and revised taxonomic concepts for 2 others, this study has important implications for the conservation and management of kangaroo mice across their distribution.