Sigmodontini. Hershkovitz (1955a) arranged Sigmodon with Holochilus, Neotomys, and Reithrodon as the sigmodont group, but other evidence has eroded the tribal-level affinity of each to Sigmodon (see those generic accounts) and leaves the genus as the sole living tribal member. Cladistic isolation from other sigmodontine genera, typically as a basal lineage, affirmed in broad taxonomic surveys of phallic morphology (Hooper and Musser, 1964a) and mitochondrial genes (D’Elía et al., 2003; Engel et al., 1998; Smith and Patton, 1999). Ambiguous evidence for geographic origin, North versus South America, discussed by Voss (1992) and Peppers et al. (2002).
Sigmomys, type species Sigmodon alstoni, has been variously treated as a distinct genus (Ellerman, 1941; Gyldenstolpe, 1932; Handley, 1976), a subgenus of Sigmodon (Husson, 1978), or a full synonym (Cabrera, 1961; Hershkovitz, 1955a; Voss, 1992). Peppers et al. (2002) discussed the possible recognition of alstoni within a monotypic genus, subgenus, or simply species group; in view of its morphological distinctiveness (Voss, 1992) and cladistic dichotomy relative to all other Sigmodon species (Peppers et al., 2002), the formal rank of subgenus seems appropriate.
North American forms revised by Bailey (1902), with subsequent partial revisions by Baker (1969), Zimmerman (1970), Voss (1992), Carleton et al. (1999), and Peppers and Bradley (2000); further alpha-taxonomic study dearly needed. Standard karyology summarized by Zimmerman (1970) and Carleton et al. (1999); banded karyotypes by Elder (1980) and Elder and Lee (1985); albumin differentiation by Fuller et al. (1984); and mitochondrial DNA variation by Peppers and Bradley (2000) and Peppers et al. (2002). Skin-and-skull morphology (Baker, 1969; Bailey, 1902), karyology (Elder and Lee, 1985; Zimmerman, 1970), and molecular data (Peppers et al., 2002) have yielded conflicting pictures of species-group associations. Species groups observed here follow the last study, which broadly treats most species under the revised understanding of S. hispidus.