Peromyscus labecula Elliot, 1903:143.

Peromyscus sonoriensis blandus Osgood, 1904:56.

Peromyscus sonoriensis fulvus Osgood, 1904:57.

Peromyscus maniculatus fulvus Osgood, 1909:86.

Peromyscus maniculatus labecula Osgood, 1909:87.

Holotype.— Field Museum of Natural History (catalog number 8693), skin, skull, adult, sex unknown.

Type locality.— Ocotlán, Jalisco, Mexico; collected June 1901 by F. E. Lutz.

Subspecies.— Based on data obtained herein, we tentatively assign the following three subspecies recognized in Hall (1981) to P. labecula: P. m. blandus, P. m. fulvus, and P. m. labecula. It is possible that specimens from western Chihuahua, Mexico that were depicted by Hall (1981) as P. m rufinus (herein referred to P. sonoriensis) could be assigned to P. labecula. Samples from western Chihuahua, Mexico are needed to resolve this issue.

Diagnosis.— Upperparts pale (gray in winter), whitish, vinaceous buff and lined with dusky, occasionally russet; underparts white; ears dusky; tail sharply bicolored, brown above and white below (Osgood 1909). Size small to medium for species group; measurements obtained from Osgood (1909), for several of the subspecies now assigned to P. labecula, indicated a total length averaging 166 mm; (range 145–183 mm) and tail length averaging 70.5 mm (range 59–82 mm).

Genetically (mitochondrial DNA sequences; Dragoo et al. 2006; Kalkvik et al. 2012), P. labecula (specifically, samples referable to P. l. blandus) have been shown to differ from other populations formerly assigned to P. maniculatus from the southwestern United States (now referable to P. sonoriensis) as well as other regional members of the P. maniculatus species group now assigned to P. melanotis and P. sejugis. In this study, Cyt b sequences indicated that P. labecula differed from P. gambelii, P. maniculatus (sensu stricto), P. melanotis, P. sejugis, and P. sonoriensis, by 3.80%, 3.79%, 5.90%, 4.22%, and 4.28%, respectively. Genetic differentiation (= 0.70%) based on DNA sequences obtained from 11 individuals of P. labecula was among the lowest intraspecific values obtained in this study; however, given the small sample size, the level of genetic divergence may have been under-estimated.

Distribution.— Lower Sonoran Zone of southern New Mexico and southwestern Texas, southward from east-central Chihuahua (perhaps western Chihuahua, see above) and western edge of Tamaulipas, and southward to Colima, Veracruz, and northern Oaxaca.

Comparisons.— A member of the P. maniculatus species group. Phenotypically resembles other species in terms of size, coloration, and bicolored tail. Averaging smaller than P. sonoriensis and tail shorter (<75 mm); color more vinaceous. P. labecula can be distinguished from P. melanotis, with which it overlaps in distribution (the two species are separated by elevation), by larger body and lighter pelage (Osgood 1909).

Remarks.— Thirteen samples were examined in this study that are assignable to P. labecula. Of these samples, the closest examined herein was approximately 60 km northeast of the type locality in Ocotlán, Jalisco, Mexico.

Chromosomal variation reported for the three subspecies of P. labecula ranges from FN = 82–86 (Bowers et al. 1973; Peppers et al. 1997) with P. l. fulvus and P. l. labecula being reported as monomorphic (FN = 84; Bowers et al. 1973; Peppers et al. 1997) and P. l. blandus being polymorphic (FN = 82, 84, and 86; Bowers et al. 1973). The FNs reported for P. labecula fall within the upper ranges for reported for P. gambelii, P. maniculatus, and P. sonoriensis (see Table 2); but differed substantially from those observed for P. melanotis (FN = 62, Hsu and Arrighi 1968; Bowers et al. 1973), P. polionotus (FN = 69–71, Te and Dawson 1971), and P. sejugis (FN = 76, Smith et al. 2000).

Recognition of P. labecula as a species is supported by the analyses depicted in Dragoo et al. (2006), Gering et al. (2009), Kalkvik et al. (2012), Natarajan et al. (2015), and Kingsley et al. (2017) which showed a distinct lineage corresponding to samples residing in southern New Mexico, Texas, and Mexico. DNA sequence data (Cytb) and conformity to the Genetic Species Concept (see Bradley and Baker 2001; Baker and Bradley 2006) support elevation to species status. Although support for a P. labecula / P. maniculatus clade was weak, divergence time estimates indicated that P. labecula separated from P. maniculatus (sensu stricto) approximately 1.28 mya. The divergence of the P. labecula / P. maniculatus (sensu stricto) from a common ancestor that gave rise to P. polionotus and P. sonoriensis occurred approximately 1.56 mya, suggesting a rapid appearance of these four species with all four species appearing between 1.28 and 1.40 mya.

As discussed earlier, P. labecula appears to be sympatric with samples of P. sonoriensis in southcentral New Mexico (6.2 mi NW of Timberon). Additional data are need from these areas to determine if these genetic species (see Bradley and Baker 2001; Baker and Bradley 2006) are behaving as biological species (Mayr 1942) as well as genetic species.