PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON 112(4):813-856. 1999. Identification and distribution of cotton rats, genus Sigmodon (Muridae: Sigmodontinae), of Nayarit, Mexico Michael D. Carleton, Robert D. Fisher, and Alfred L. Gardner (MDC) Division of Mammals, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, U.S.A.; (Ba)F, ALG) Patuxent WildUfe Research Center, Biological Resources Division, U.S. Geological Survey, National Museum of Natural History, Washington, D.C. 20560, U.S.A. Abstract. — Morphological, chromosomal, distributional, and ecological data are presented for three species of Sigmodon (S. alleni, S. arizonae, and S. mascotensis) from the state of Nayarit, Mexico. The species were collected in all possible pairwise combinations of sympatry, including the first record of such documented for S. arizonae and S. mascotensis. Emphasis is devoted to the discrimination of S. arizonae and S. mascotensis, from each other and from typical S. hispidus, using qualitative features of the skin and skull and mor-phometric analyses of craniodental measurements. Based on these results and examination of type specimens, additional synonyms of S. mascotensis are identified, with reassignment of two forms, tonalensis Bailey (1902) and ob-velatus Russell (1952), currently mistaken as subspecies of S. hispidus. Sig-modon mascotensis emerges as a species distributed from southern Nayarit and Zacatecas to extreme western Chiapas, where it inhabits deciduous or semi-deciduous tropical vegetation having a pronounced dry season. These reallo-cations and other reidentifications remove any documentation for S. hispidus along the entire Pacific versant of Mexico. A useful form of research communica-xico, and the taxon of interest is the genus tion that sees less application today is the Sigmodon, the ecologically abundant cotton brief expeditionary account or short faunal rats that inhabit open landscapes from the note. Aside from the practical enhance-southern United States, through Mexico and ments in knowledge of a taxon's distribu-Middle America, to northern South Amer-tion and habitat, such reports offer the ad-ica (Hall 1981, Voss 1992). Situated along vantage of bringing regionally focussed no-coastal westcentral Mexico, the state of menclatural clarity to complex taxonomic Nayarit encloses a varied topography and problems that seem incomprehensible over diverse natural environments, a biogeo-a broader geographic scale. One recalls that graphical setting that has proven pivotal for the prolific literature appearing over the illuminating the systematics of other small past two decades on the Peromyscus boy Hi mammals (for example, Fisher & Bogan group emanated from Hooper's (1955) im-1977, Gardner 1977, Bogan 1978, Diersing memorable commentary in "Notes on & Wilson 1980, Carleton et al. 1982, Wil-Mammals of Western Mexico," in which he son 1991). recorded the sympatric occurrence of vari-The excellent series of Nayarit cotton rats ous "morphological types" of boylii at sev-collected by personnel of the U.S. Fish and eral collecting localities in Jalisco, Nayarit, Wildlife Service in the middle 1970s war-and Sinaloa (see systematic reviews by rant report in view of Zimmerman's (1970) Carleton 1989, and Bradley et al. 1996). seminal report on Sigmodon taxonomy. His The regional focus here is Nayarit, Me-study, and the subsequent contributions of
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