Integrative species delimitation reveals an Idaho-endemic ground squirrel, <i>Urocitellus idahoensis</i> (Merriam 1913)

The "small-eared" species group of <i>Urocitellus</i> ground squirrels (Sciuridae: Xerinae: Marmotini) is endemic to the Great Basin, United States, and surrounding cold desert ecosystems. Most specific and subspecific lineages in this group occupy narrow geographic ranges, and some are of significant conservation concern; despite this, current taxonomy remains largely based on karyotypic or subtle pelage and morphological characteristics. Here, we leverage 2 multilocus DNA sequence data sets and apply formal species delimitation tests alongside morphometric comparisons to demonstrate that the most widespread small-eared species (<i>U. mollis</i> Kennicott, 1863 sensu lato; Piute Ground Squirrel) is comprised of 2 nonsister and deeply divergent lineages. The 2 lineages are geographically separated by the east-west flowing Snake River in southern Idaho, with no sites of sympatry currently known. Based on robust support across the nuclear genome, we elevate populations previously attributed to <i>U. mollis</i> from north of the Snake River to species status under the name <i>Urocitellus idahoensis</i> (Merriam 1913) and propose the common name "Snake River Plains Ground Squirrel" for this taxon. We delimit 2 subspecies within <i>U. idahoensis</i>; <i>U. i. idahoensis</i> (Merriam 1913) in western Idaho and <i>U. i. artemesiae</i> (Merriam 1913) in eastern Idaho. <i>Urocitellus idahoensis</i> is endemic to Idaho and has a maximal range area of roughly 29,700 km² spanning 22 counties but occurs discontinuously across this area. Our work substantially expands knowledge of ground squirrel diversity in the northern Great Basin and Columbia Plateau and highlights the difficulty in delimiting aridland mammals whose morphological attributes are highly conserved.