Reithrodontomys microdon Merriam, 1901

Marines-Macías, Tania, Colunga-Salas, Pablo & León-Paniagua, Livia, 2022, Reithrodontomys microdon (Rodentia: Cricetidae), Mammalian Species 54 (1020), pp. 1-8 : 1-4

Reithrodontomys microdon Merriam, 1901

Small-toothed Harvest Mouse

Reithrodontomys microdon Merriam, 1901:548 . Type locality “Todos Santos, Guatemala (altitude 10,000 ft.).”

Reithrodontomys microdon microdon : Howell, 1914:80. Name combination.

CONTEXT AND CONTENT. Order Rodentia , suborder Myomorpha , superfamily Muroidea , family Cricetidae , subfamily Neotominae , tribe Reithrodontomyini , genus Reithrodontomys , subgenus Aporodon . Reithrodontomys microdon is a member of the R. tenuirostris species group. The following three subspecies are currently recognized ( Bradley 2017):

R. m. microdon Merriam, 1901:548 . See above.

R. m. wagneri Hooper, 1950:169. Type locality “ Michoacán, about 10 miles northwest of Ciudad Hidalgo, western flanks Cerro San Andrés, 9,400 feet elevation.”

DIAGNOSIS

Reithrodontomys microdon has a head–body length of 68–70 mm, which makes it smaller than R. gracilis (slender harvest mouse), R. paradoxus (Nicaraguan harvest mouse), R. bakeri (Baker’s harvest mouse), R. hirsutus (hairy harvest mouse), R. spectabilis (Cozumel harvest mouse), R. creper (Talmancan harvest mouse), R. rodriguezi (Rodriguez’s harvest mouse), and R. garichensis (Chiriquian harvest mouse), which have head–body lengths ranging from 71–72 mm ( R. paradoxus ) to 89–93 mm ( R. creper ). On the other hand, the head–body length of R. microdon is longer than those of R. brevirostris (shortnosed harvest mouse), R. darienensis (Darien harvest mouse), R. musseri (Musser’s harvest mouse), R. humulis (eastern harvest mouse), and R. montanus (plains harvest house), which have head–body lengths that range from 55–65 mm ( R. montanus ) to 62–69 mm ( R. brevirostris ). Although the measurements of R. microdon and R. brevirostris overlap, they can be distinguished by ear length and length of hind foot: R. microdon has an ear length of 16–19 mm and a length of hind foot of 19–21 mm, whereas these measurements for R. brevirostris are 14–16 and 16–19 mm, respectively. Species such as R. burti (Sonoran harvest mouse), R. zacatecae (Zacatecan harvest mouse), and R. raviventris (salt-marsh harvest mouse) have about the same head–body length as R. microdon , but their tail lengths range 56–95 mm compared to 101–117 mm for R. microdon ( Howell 1914; Hooper 1952; Gardner and Carleton 2009; Bradley 2017).

The only two species of Reithrodontomys with the same head–body length as R. microdon can be differentiated by coloration. Reithrodontomys microdon has a deep reddish-fulvous dorsum, bright fulvous sides of the body, and salmon fulvous or white underparts, lips, and sides of the nose. By contrast, R. mexicanus (Mexican harvest mouse) has a dorsum that varies from yellow-brown to orange-brown and a creamy white to light cinnamon or cinnamon orange belly, and R. cherrii (Costa Rican harvest mouse) has a brilliant orange dorsum and sides ( Howell 1914; Hooper 1952; Bradley 2017).

The geographic distributions of R. mexicanus and R. cherri do not overlap with that of R. microdon , which makes it easier to differentiate them. Nevertheless, the distribution of R. microdon does overlap with that of R. chrysopsis (volcano harvest mouse) in central Mexico, with R. fulvescens (fulvous harvest mouse) and R. sumichrasti (Sumichrast’s harvest mouse) in all of its geographic distribution, with R. megalotis (western harvest mouse) in some parts in central and southern Mexico, and with R. tenuirostris (narrow-nosed harvest mouse) in southern Mexico and adjacent Guatemala, but it can be distinguished from these species mainly by its coloration. These five species of Reithrodontomys have combinations of colors from orange, dark cinnamon, gray, and black, always resulting in an overall darker color. Reithrodontomys microdon also can be distinguished from the five sympatric species by its cranial characters, of which its long narrow rostrum, bulbous brain case, narrow zygomatic plate, weak zygomatic arches, and full mesolophs in M1–3 are noteworthy ( Howell 1914; Hooper 1952; Bradley 2017).

Reithrodontomys bakeri resembles R. microdon the most because it has a similar body size and coloration. The geographic distribution of R. m. wagneri does not overlap with that of R. bakeri , but they can be found in proximity, which may lead to confusion when recognizing one from the other. Reithrodontomys microdon can be morphologically distinguished by its shorter ear (14–19 mm), a shorter and narrower rostrum (8.0– 8.4 mm), a shorter palate (3.3–3.4), a shorter molar toothrow (3.2–3.3 mm), a narrower zygomatic plate (1.3 mm), and a smaller breadth of mesopterygoid fossa (1.6– 1.8 mm — Bradley et al. 2004; Ceballos et al. 2005; Müdespacher and Gaona 2005, 2014; Ceballos and Bradley 2014; Bradley 2017). There are helpful keys, which include both internal and external characters, to identify R. microdon and to differentiate it from other Reithrodontomys species ( Howell 1914; Hooper 1952; Álvarez-Castañeda et al. 2017).

GENERAL CHARACTERS

Reithrodontomys microdon is a small-bodied cricetid mouse with long, soft, and dense fur that is deep reddish-fulvous on the upperparts, color most intense on rump, and palest on head. The sides of the body are bright fulvous, and the underparts, lips, and sides of nose are salmon fulvous, sometimes white. Its forefeet and toes are white, its ankles and hind feet are dark brown, and it has a dark brown spot on its wrists. Its tail is monocolored, dark all around, but slightly paler beneath. Its ears are long, dark, and nearly naked and it has a dark ring around its eyes ( Fig. 1 ). Reithrodontomys m. albilabris has paler fur and its lips, sides of nose, and underparts are pure white instead of salmon. Reithrodontomys m. wagneri is also paler, with an ochraceoustawny rather than reddish-tawny fur and whitish or pale pinkishbuff underparts ( Merriam 1901; Hooper 1952; Bradley 2017). Information about guard hair is only available for R. m. albilabri s, which has two coloration types: some individuals have bicolored guard hair, and other individuals have a single band of color. The medulla of the guard hair shows cortical intrusions and uneven curly shaped scales ( Baca and Sánchez-Cordero 2004).

Ranges of external measurements (mm) of R. microdon were: total length, 169–187; tail length, 101–117; length of hind foot, 19–21; ear length, 16–19 ( Merriam 1901; Hall 1981; Bradley 2017). In general, R. m. wagneri has a slightly smaller head–body length than the other subspecies and has larger ears. Cranial measurements of the type specimen, which belongs to R. m. microdon (female, adult), were: basal length 17.5 mm; zygomatic breadth posteriorly, 11 mm; length of palate, 10 mm; length of molar series, 3 mm. Reithrodontomys m. albilabris has a slightly shorter basal length and slightly wider zygomatic breadth than those of the type specimen ( Merriam 1901). Other cranial measurements of R. microdon (not from the type specimen) were: length of molar row, 34–37% of depth of cranium; width of zygomatic plate, 14–15% of depth of cranium; breadth of rostrum, 123–135% of length of molar row; length of incisive foramina, 48–51% of depth of cranium ( Hooper 1952). The reported average weight of R. microdon is 20 g ( Müdespacher and Gaona 2005, 2014). Nevertheless, the average weight from 11 additional individuals of R. m. wagneri from Guerrero, Mexico were as follows: 29.4 g for males (n = 5) and 24.8 g for females (n = 6— Marines-Macías 2014).

DISTRIBUTION

The first records of Reithrodontomys microdon were from the highlands of Guatemala for R. m. microdon and Oaxaca, Mexico for R. m. albilabris , both of records report an elevation of 3,000 m (10,000 feet — Merriam 1901; Howell 1914). After that, R. m. microdon was found in a total of four localities that included southern Mexico in Chiapas, with an elevational range from about 2,200 to 3,200 m. Reithrodontomys m. wagneri was discovered in Distrito Federal (formerly Mexico City) and Michoacán, Mexico within an elevational range of 2,800 –3,000 m, although it could probably occur elsewhere on the mountains rimming the southern end of the Mexican Plateau ( Hooper 1952; Hall 1981; Musser and Carleton 2005). At present, R. m. wagneri is also known from Estado de México, the states of Morelos and Guerrero ( Fig. 2 ; González-Cózatl and Arellano 2015).

Specific localities reported in publications where R. microdon was captured included: Alta Verapaz, Baja Verapaz, Quetzaltenango, Totonicapán, El Progreso, San Juan Ixcoy, Todos Santos Cuchumatán, San Lorenzo Mármol, Huehuetenango, Cerro Raxón, and Cerro Pinalón, all within Guatemala, at elevations from 2,700 to 3,400 m. The last three localities are part of the Sierra de los Cuchumatanes, which extends the known geographic range of R. m. microdon some 50 km east–southeast of previous collecting sites at that mountain range ( Woodman and Timm 1999; Matson and McCarthy 2005; Woodman 2010; Matson et al. 2012, 2014, 2016; Matson and Ordóñez-Garza 2017). In Mexico, R. m. microdon has been found in the Cerro Tzontehuitz in Tenejapa, El Retiro in the Montaña Cuilco, and in Rayón, all in the state of Chiapas (Horvath and Navarrete-Gutiérrez 1997; Matson and Ordóñez-Garza 2017), whereas R. m. albilabris was collected at Pluma Hidalgo in the Copalita river basin in the state of Oaxaca ( Palacios-Romo et al. 2012). Reithrodontomys . m. wagneri has been found in the state of Morelos, at Coajomulco, Tepoztlán; in the state of Guerrero at “Cerro del Huixteco” State Park in the Sierra de Taxco; in the State of Estado de México at Zacualpan, Villa del Carbón, San Rafael, and Llanos de Aculco; in the state of Michoacán at Cerro San Andrés; and in Mexico City, at Cañón Contreras ( González-Cózatl and Arellano 2015; González-Cózatl et al. 2016; Marines-Macías et al. 2018). No fossil records are known for R. microdon .

FORM AND FUNCTION

Reithrodontomys microdon has a long, deep, and broad skull, markedly narrowing anteriorly ( Fig. 3 ). Its braincase is either moderately ( Howell 1914) or highly inflated ( Hooper 1952) and large, as well as narrowed and depressed posteriorly. Braincase breadth is slightly less than that of the zygomatic plate, which is always less than 1.5 mm and this in turn less than the width of the mesopterygoid fossa, which is about equal to or greater than the length of the crown of M1. The zygoma is slender and decidedly contracted anteriorly, the rostrum is long and narrow, the interpterygoid fossa is very broad, the auditory bulla is large and inflated, and the outer wall of the anteorbital foramen is very narrow. The pterygoid fossae are narrow, the hamuli of pterygoids are well inflated and reflexed laterad, and the angular process of mandible is inflected mediad but slightly dorsad. The skull has a palatal foramen ending on the plane of the front molars, a long incisive foramen in relation to palatal length, and comparatively erect incisors. The length of the molar row of R. microdon ranges from 3.0 to 3.3 mm, nevertheless, compared with the size of the cranium, the molars represent 34–37% of depth of the cranium, which indicates that R. microdon has the smallest teeth of the genus ( Hooper 1952). Reithrodontomys microdon , as do all members of the genus, has upper incisors with a deep longitudinal groove near the middle of the tooth ( Howell 1914; Hooper 1952; Hall 1981; González-Ruiz et al. 2007). The dental formula is: i 1/1, c 0/0, p 0/0, m 3/3, total 16.

Other than skull features, the only thing known about the anatomy of R. microdon concerns the distribution of glandular and cornified epithelium of the digestive system, in which the bordering fold extends across the lesser curvature near the pyloric orifice. About one-half of the antrum contains cornified epithelium, whereas glandular epithelium lies along the greater curvature and narrows toward the pyloric opening. As in the other members of Aporodon , the fornix ventricularis appears to arch farther craniad than in members of the subgenus Reithrodontomys ( Carleton 1973) .

ONTOGENY AND REPRODUCTION

Litter size of Reithrodontomys microdon has not been reported, but it has a total of six mammae, one pair of pectoral and two pairs of inguinal ( Howell 1914), suggesting the upper limit of litter size. Other members of the genus produce 3–7 young at a birth, and the breeding season extends in northern latitudes from April to October and in tropical regions may cover the entire year, which may be the case for R. microdon ( Howell 1914) . Reported data about the breeding season of R. microdon were as follows: 3 of 21 male individuals captured in April 1998 in Sierra de las Minas, Guatemala ( Matson et al. 2016) and 1 of 8 captured males in January 2005 in Huehuetenango, Guatemala ( Matson et al. 2012), had scrotal testes, whereas all females (n = 11) collected in the same areas of Guatemala did not show active-breeding evidence. Four mice captured in 2007 (January) in Alta Verapaz, Guatemala had no evidence of reproductive activity ( Matson et al. 2014). Also, of five specimens collected in Morelos and Estado de México, Mexico, in July and August of 2009, only one presented evidence of reproductive activity and one was a juvenile ( González-Cózatl and Arellano 2015). Elsewhere, of 20 individuals captured in April and May 2013 (Guerrero, Mexico) and March and April 2015 (Estado de México, Mexico), nine presented signs of reproductive activity, including three pregnant females, and two were juveniles ( Marines-Macías 2014, 2017; Marines-Macías T., in litt.). The only information available about nonadults is that the young tend to be browner instead of ochraceous and less mixed with tawny than the adults, with white underparts ( Howell 1914; Marines-Macías T., in litt.).

Figures

Fig. 1.—An adult female Reithrodontomys microdon from “Cerro del

Fig. 2.—Dorsal, ventral, and lateral views of skull and lateral view of mandible of an adult male Reithrodontomys microdon (MZFC [Zoology Museum of the Faculty of Sciences “Alfonso L. Herrera,” Universidad Nacional Autónoma de México] #8375) from San JuanAtepec, Oaxaca, Mexico. Photographs used with permission of the photographer Sara C. Lucero-Verdugo. Greatest length of skull is 23.77 mm.

Fig. 3.—Geographic distribution of Reithrodontomys microdon. Subspecies are: 1, R. m. wagneri; 2, R. m. albilabris; 3, R. m. microdon. Map drawn from data obtained from International Union for Conservation of Nature and Natural Resources (2016).