Identification and distribution of cotton rats, genus Sigmodon (Muridae : Sigmodontinae), of Nayarit, Mexico

Morphological, chromosomal, distributional, and ecological data are present ed for three species of Sigmodon (S. alleni, S. arizonae, and S. mascotensi s) from the state of Nayarit, Mexico. The species were collected in all pos sible 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 m orphometric analyses of craniodental measurements. Based on these results a nd examination of type specimens, additional synonyms of S. mascotensis are identified, with reassignment of two forms, tonalensis Bailey (1902) and o bvelatus Russell (1952), currently mistaken as subspecies of S. hispidus. S igmodon mascotensis emerges as a species distributed from southern Nayarit and Zacatecas to extreme western Chiapas, where it inhabits deciduous or se mideciduous tropical vegetation having a pronounced dry season. These reall ocations and other reidentifications remove any documentation for S. hispid us along the entire Pacific versant of Mexico.