Geographic variation of the western chipmunks Tamias senex and T. Siskiyou, with two new subspecies from California

Abstract

Three species of western chipmunks, Tamias senex, T. siskiyou, and T. ochrogenys, were removed from T. townsendii because discrete character variation, especially of genitalic morphology, suggested lack of intergradation and species-level distinction. Two of these species, T. senex and T. siskiyou, show substantial geographic variation in cranial morphology and pelage characteristics across their ranges, from humid coastal forests to the relatively arid Sierra Nevada and Cascade ranges in northern California. In both species, specimens from inland localities are paler and smaller than those collected near the Pacific coast. This also is true for another species in Oregon and Washington, where coastal T. t. townsendii is replaced by inland T. t. cooperi. Interspecific convergence in pelage and external variation is so strong that inland T. senex and T. siskiyou resemble each other more strongly than either resembles conspecifics from coastal forests, and vice versa. In spite of this convergence, genital bones confirm the specific identity of inland and coastal specimens. Because both T. senex and T. siskiyou were originally described from inland localities, the novel patterns of color and size described here provide the basis for naming the coastal forms as new subspecies.