Nelsonia goldmani Merriam, 1903

León-Tapia, M. Ángel, 2023, Nelsonia goldmani (Rodentia: Cricetidae), Mammalian Species 55 (1025), pp. 1-8 : 1-5

Nelsonia goldmani Merriam, 1903

Goldman’s Diminutive Woodrat

Nelsonia goldmani Merriam, 1903:80 . Type locality “ Mt. Tancitaro , Michoacan, Mexico.”

Nelsonia neotomodon goldmani : Hooper, 1954:7. Name combination.

Nelsonia neotomodon cliftoni Genoways and Jones, 1968:97 . Type locality “ 2.5 mi. ENE Jazmin , 6,800 ft, Jalisco,” Mexico.

Nelsonia goldmani goldmani : Engstrom, Sánchez-Herrera, and Urbano-Vidales, 1992:880. Name combination.

the collector P. L. Clifton ( Genoways and Jones 1968). Another common name is rata enana michoacana (Spanish— Álvarez-Castañeda and González-Ruiz 2018).

Nelsonia goldmani cliftoni : Engstrom, Sánchez-Herrera, and Urbano-Vidales, 1992:880. Name combination.

CONTEXT AND CONTENT. Context as for genus. Currently two subspecies are recognized ( Musser and Carleton 2005; Burgin et al. 2020; León-Tapia and Cervantes 2021):

N. g. cliftoni ( Genoways and Jones, 1968:97). See above.

N. g. goldmani Merriam, 1903:80. See above.

NOMENCLATURAL NOTES. Nelsonia is considered a member of the Neotomini tribe based on morphological similarities with Neotoma, Xenomys, and Hodomys ( Musser and Carleton 2005). However, recently genetic analysis and morphological characteristics of Nelsonia, including fossil information of its likely closest relatives, revealed that Nelsonia is the unique, current living member of the Galushamyina subtribe ( Martin and Zakrzewski 2019; León-Tapia and Cervantes 2021; Kelly and Martin 2022). The first members of this genus were collected by E. W. Nelson and E. A. Goldman. The generic name Nelsonia honors one collector ( Merriam 1897), whereas the species name, goldmani, honors the other ( Merriam 1903). The subspecies cliftoni honors

DIAGNOSIS

Nelsonia goldmani resembles N. neotomodon (western diminutive woodrat), but can be distinguished by a set of morphological characters. The dorsal pelage of N. goldmani is grayish with a fulvous lateral line ( Fig. 1 ) versus the lighter cinnamon-buff line present in N. neotomodon . The dorsal surface of the hind feet in N. goldmani are blackish instead of white as in N. neotomodon . The tail is indistinctly bicolored in N. goldmani with a tip that is slightly darker than dorsal color of the tail ( Fig. 2 ), whereas in N. neotomodon the tail is bicolored and has a white tip ( Engstrom et al. 1992; León-Tapia and Cervantes 2019a). The juvenile dorsal pelage in N. goldmani is dark slate, whereas in N. neotomodon it is buffy grayish ( Merriam 1903; Hooper 1954). Cranially, the zygomatic plate is wider in N. goldmani (3.0– 3.8 mm) than in N. neotomodon (2.6–3.1 mm), when like age groups are compared. The two species are distinguished by the presence of a distinct anterior notch in the zygomatic plate of N. goldmani and its absence (a conservative character in neotomine–peromyscine rodents) in N. neotomodon , suggesting a shift in the position of the anterior slip of the masseter lateralis muscle and changes in associated patterns of mastication between the species ( Engstrom et al. 1992).

Finally, the basic shape of the phallus is distinct; in N. goldmani the shape resembles a bottleneck, compared to the finger-like shape in N. neotomodon . The total length and diameter of the phallus in N. goldmani are slightly greater than in N. neotomodon , in a proportion of 6 to 5. The baculum of both species is different in shape and location; in N. goldmani the proximal section is rounded and extends to 1.76 mm before the distal section, whereas in N. neotomodon the proximal section of the baculum lies below the base and extends to the distal section of the organ ( León-Tapia and Cervantes 2019a).

GENERAL CHARACTERS

Nelsonia goldmani is a medium-sized woodrat with vibrissae extending to the lumbar region ( Hooper 1954). There is no sexual dimorphism in the specimens examined to date ( Engstrom et al. 1992; León-Tapia and Cervantes 2019b). The dorsal pelage is dark slate-gray with a fulvous lateral line. The tail length is almost one-half the total length with a blunt end, covered with short black hairs becoming gradually paler ventrally, and with a black terminal pencil. The ventral pelage is white with a plumbeous underfur ( Merriam 1903). The juvenile pelage is dark slate ( Merriam 1903). The hind foot is dusky-colored above and the length is 25–30 mm ( Hooper 1954; Engstrom et al. 1992). Mean external measurements (mm ± SD) of five adult specimens (two males; three females) recently collected in Jalisco and Estado de México states were: total length 249 ± 8.63, tail length 123.2 ± 12.5, length of hind foot 25.4 ± 1.52, and ear length 23.6 ± 2.19. The mean body mass of these specimens was 50.52 g ( León-Tapia and Cervantes 2019b).

The skull of N. goldmani is flattened between the orbits, deeply concave laterally with the zygomatic arches slightly extended anteriorly, and with a well-developed anterior zygomatic notch ( Fig. 3 ). The occlusal surface of the upper molars is flat-crowned with deep reentrant angles ( Merriam 1903; León-Tapia and Cervantes 2019b). Mean skull measurements (mm ± SD) for the same five specimens mentioned previously were: greatest length of skull 32.77 ± 0.33; zygomatic breadth 16.76 ± 0.31; interorbital constriction 4.63 ± 0.1; length of rostrum 13.26 ± 0.19; breadth of rostrum 5.22 ± 0.2; mastoid breadth 13.79 ± 0.15; length of interparietal bone 4.5 ± 0.38; width of interparietal bone 10.69 ± 0.37; breadth of zygomatic plate 3.88 ± 0.17; length of palate 6.33 ± 0.24; length of maxillary toothrow 6.44 ± 0.29; breadth across molars 6.26 ± 0.25; length of auditory bulla 5.4 ± 0.2; depth of skull 10.32 ± 0.19, and length of mandibular toothrow 6.97 ± 0.24 ( León-Tapia and Cervantes 2019b). Five age classes were described based on the degree of eruption and wear of the upper molars ( Engstrom et al. 1992).

The two subspecies of N. goldmani differ in coloration ( Fig. 4 ): N. g. goldmani has dark gray dorsal pelage and is paler laterally, whereas N. g. cliftoni is distinguished by paler and more fulvous pelage both dorsally and laterally (Engtrom et al. 1992). The only skull difference is a greater length of the rostrum in N. g. cliftoni (12.6–13.7 mm) than N. g. goldmani (11.6–13.2 mm).

DISTRIBUTION

Nelsonia goldmani is endemic to the highlands of central Mexico ( Fig. 5 ), where its geographic distribution is fragmented and limited to elevations of suitable habitat in an area of 19,533 to 20,225 km 2 ( León-Tapia and Cervantes 2019b). It is known from only 15 localities in the highlands of the central-west of the Trans-Mexican Volcanic Belt biogeographic region in central Mexico ( González-Cózatl et al. 2016; León-Tapia and Cervantes 2019b; León-Tapia 2021). The localities where specimens of N. goldmani were collected range in elevation from 2,000 to 3,100 m. Nelsonia g. goldmani is distributed in Michoacán, Estado de México, and Morelos states, and N. g. cliftoni found in only a few localities in Jalisco state, so as presently understood, their geographic distributions do not overlap ( León-Tapia and Cervantes 2019b; León-Tapia 2021). No fossils are known.

FORM AND FUNCTION

The pelage of Nelsonia goldmani contains guard hairs that are characterized by an abrupt enlargement in the distal part of the stem; the medulla has cortical intrusions. Based on optical microscopy analyses of the pelage, the mean total length (µm ± SD) of the hair (middle part) is 13.86 ± 1.11, diameter 0.056 ± 0.003, and medulla diameter 0.05 ± 0.003 ( Monroy-Vilchis and Rubio-Rodríguez 2003). The dental formula is i 1/1, c 0/0, p 0/0, m 3/3, total 16. Molars are prismatic, the occlusal surface of the upper molars is flat-crowned, presenting deep reentrant angles of enamel occupying one-half of the tooth width, and the third molar is without the inner fold ( Merriam 1903; Hooper 1954; León-Tapia et al. 2019b).

The skull of N. goldmani is flattened between the orbits with a wide zygomatic plate, is deeply concave laterally with the zygomatic arches slightly extended anteriorly and has a well-developed anterior zygomatic notch. There is a stapedial and sphenofrontal foramen on the anterior part of the braincase and laterally there is a well-developed groove on the squamosal bone. The lacrimal bone is scarcely evident dorsally ( Merriam 1903; León-Tapia et al. 2019b). The palatine is narrow and rough, and does not lie immediately below the palatine foramina, which is situated posteriorly. The incisive foramina are large and open, and broader anteriorly. The angular process of the dentary bone is directed posteriorly in the same plane as the ascending ramus ( Merriam 1897, 1903; Hooper 1954; Carleton 1980; Engstrom et al. 1992). The postcranial skeleton contains 13 thoracic, 6 lumbar, and 32–36 caudal vertebrae. The second thoracic vertebra has a well-developed neural spine ( Carleton 1980). The ilium and ischium of the pelvis are slender, and the neck of scapula is broad and short ( Hooper 1954).

The stomach of N. goldmani is bilocular–discoglandular and the incisura angularis is deep and projects beyond the esophageal opening. The fornix ventricularis arches are located closer to the esophageal opening than the incisura angularis and are slightly curved toward the esophagus. The corpus is elongated horizontally. The antrum walls of the inner stomach are muscular, more visible near the pyloric opening. A bordering fold surrounds the glandular region, and cornified squamosal epithelium coats the remaining areas of the antrum and corpus ( León-Tapia et al. 2018).

The reproductive structures were described from one adult male collected in Estado de México and showed that externally the phallus was simple, compared to the forms and ornamentations presented in other species of the subfamily Neotominae , with a length of 9.5 mm and shaped like an elongated bottleneck, being broad and cylindrical at the base and narrowing in diameter toward the tip ( León-Tapia et al. 2019a). The external surface of the phallus is rigid with one-quarter of the surface bare, whereas the remainder of the surface is dotted with conical tubercles or spines surrounded in a circular depression. The spines are larger at the base of the organ (0.31 mm) and double in size compared those on the distal area (0.15 mm — León-Tapia et al. 2019a). The baculum of N. goldmani has 7.74 mm of length and consists of two sections: an osseous structure and a cartilaginous part in the distal section. The osseous structure has a length of 6.59 mm and is simple in form; its proximal portion is concave, becoming narrower toward the distal end. The cartilaginous part is approximately one-sixth the total length of the baculum resembling a cone. The inner position of the baculum is the same as the phallus and the proximal section is the same as the base of the phallus ( León-Tapia et al. 2019a). Sperm are large (mean length 182.58 µm ± 1.615 SD), have asymmetric heads, and are rounded at the base with a slight depression; the acrosome extends as a strong and large recurved hook. The middle piece is separated clearly from the head by a narrower section; its length is four times that of the head. The tail of the sperm is long, comprising 85% of total length ( León-Tapia et al. 2019a).

ONTOGENY AND REPRODUCTION

One adult male was collected in October 2010 with scrotal testes, 10.94 mm in length and 7.47 mm in breadth, with a paired weight of 0.79 g; the testicular volume was 319.64 mm 3 and 1.34% of gonadal somatic index ( León-Tapia and Cervantes 2019a). Histological analysis showed active spermatogenesis ( León-Tapia and Cervantes 2019a). One adult female collected in December 2012 showed prominent nipples and signs of active nursing ( León-Tapia and Cervantes 2019b).

Figures

Fig. 1.—An adult female Nelsonia goldmani cliftoni from El Nevado de Colima, 1.5 km SE El Izote, San Gabriel, Jalisco, México, 12 December 2012 (CNMA [Colección Nacional de Mamíferos] 46614). Photograph by MAL-T.

Fig. 2.—An adult female Nelsonia goldmani goldmani from Las Peñas 2.6 km SE Dexcaní Alto, Jilotepec, Estado de México, México, 17 October 2010 (CNMA [Colección Nacional de Mamíferos] 46292). Photograph by MAL-T.

Fig. 3.—Dorsal, ventral, and lateral views of skull and lateral view of mandible of an adult female Nelsonia goldmani cliftoni (CNMA [Colección Nacional de Mamíferos] 46614) from El Nevado de Colima, 1.5 km SE El Izote, San Gabriel, Jalisco, México. Occipitonasal length is 32.66 mm.

Fig. 4.—Color differences in dorsal pelage between female adults of Nelsonia goldmani.Nelsonia g. cliftoni (left) from El Nevado de Colima, 1.5 km SE El Izote, San Gabriel, Jalisco, México, 14 December 2011 (CNMA [Colección Nacional de Mamíferos] 46290) and N. g. goldmani (right) from Las Peñas 2.6 km SE Dexcaní Alto, Jilotepec, Estado de México, México, 17 October 2010 (CNMA 46292). Photographs by MAL-T.

Fig. 5.—Known geographic distribution of Nelsonia goldmani. The localities where N. goldmani has been collected are indicated on the map. The subspecies are N. g. cliftoni (squares) and N. g. goldmani (circles). Map redrawn from León-Tapia and Cervantes (2021) and León-Tapia (2021).