Reithrodontomys mexicanus ( Saussure, 1860 )

Martínez-Borrego, Daily, Arellano, Elizabeth, González-Cózatl, Francisco X & Rogers, Duke S, 2020, Reithrodontomys mexicanus (Rodentia: Cricetidae), Mammalian Species 52 (996), pp. 114-124 : 114-119

Reithrodontomys mexicanus ( Saussure, 1860)

Mexican Harvest Mouse

Mus tazamaca Gray, 1843:79 . Type locality “Coban, Guatemala.” Nomen nudum (see Alston 1876:756).

R [eithrodon]. mexicanus de Saussure, 1860:109 . Type locality “Habite les montagnes de la province de Véra-Cruz [ Mexico],” restricted to “Mirador, Veracruz ”, Mexico by Hooper (1952:140).

Reithrodon mexicana: Tomes, 1861:284 . Incorrect subsequent spelling of Reithrodon mexicanus de Saussure, 1860 .

Ochetodon mexicanus: Coues, 1874:186 . Name combination (see “Nomenclatural Notes”).

Hesperomys (Vesperimus) cherrii Allen, 1891:211 . Type locality “ San José,” Costa Rica.

Sitomys cherriei Allen, 1893:238 . Name combination.Unjustified emendation of the specific name cherrii Allen, 1891 .

Reithrodontomys mexicanus: Allen, 1895:135 . First use of current name combination. Not Reithrodontomys mexicanus fulvescens Allen 1894:319 .

Reithrodontomys costaricensis Allen, 1895:139 . Type locality “La Carpintera (alt. 6000 ft.), Costa Rica.”

Reithrodontomys Söderströmi Thomas, 1898:451 . Type locality “Quito,” Province Pichincha, Ecuador.

Reithrodontomys costaricensis jalapae Merriam, 1901:552 . Type locality “[J]alapa, Vera Cruz, Mexico (altitude 4,000 ft.).”

Reithrodontomys goldmani Merriam, 1901:552 . Type locality “Metlaltoyuca, Puebla [ Mexico] (altitude 800 ft.).”

Reithrodontomys cherriei Osgood, 1907:50 . Type locality “ San José,” Costa Rica. Incorrect subsequent spelling of the specific name cherrii Allen, 1891 .

Reithrodontomys soderstromi Trouessart, 1910:22 . Incorrect subsequent spelling of R. söderströmi Thomas, 1898 .

Reithrodontomys milleri Allen, 1912:77 . Type locality “Munchique (alt. 8325 ft.), Cauca, Colombia.”

Reithrodontomys cherriei cherriei: Miller, 1912:129 . Incorrect subsequent spelling of the specific name cherrii Allen, 1891 .

Reithrodontomys cherriei jalapae Miller, 1912:130 . Type locality “Jalapa, State of Vera Cruz, Mexico. Altitude, 4,000 feet.” Incorrect subsequent spelling of the specific name cherrii Allen, 1891 .

Reithrodontomys mexicanus goldmani: Howell, 1914:72 . Name combination.

Reithrodontomys mexicanus minusculus Howell, 1932:125 . Type locality “Comayabuela (just south of Tegucigalpa), Honduras;” corrected to Honduras, Dept. Francisco Morazán, Comayaguela by Hooper (1952:150).

Reithrodontomys mexicanus soederstroemi Arellano, 2015: 63 . Justified emendation of spelling of the specific name Söderströmi Thomas, 1898 [article 33.2.2 and 32.5.2.1 of the International Code of Zoological Nomenclature (ICZN 1999)].

CONTEXT AND CONTENT. Order Rodentia , suborder Myomorpha , family Cricetidae , subfamily Neotominae , tribe Reithrodontomyini , genus Reithrodontomys , subgenus Aporodon (Musser and Carleton 2005) . There are 24 species of Reithrodontomys ( Bradley 2017) , grouped into two subgenera ( Howell 1914; Hooper 1952). Hooper (1952) also proposed four species groups, two for the subgenus Reithrodontomys ( fulvescens and megalotis ), and two for the subgenus Aporodon ( mexicanus and tenuirostris ). R. mexicanus was formerly considered a polytypic species with 13 subspecies by Hooper (1952, 1955), but in recent years its taxonomic status has been reviewed and changes have been proposed. R. cherrii was proposed as a distinct species by Arellano et al. (2005), and later recognized as such by Gardner and Carleton (2009). Gardner and Carleton (2009) also regarded R. m. garichensis as a species, based mainly on craniodental characteristics. In addition, they considered populations of R. m. potrerograndei as a synonym of R. brevirostris . Here we follow Bradley (2017), who recognizes 10 subspecies:

R. m. eremicus Hershkovitz, 1941:4. Type locality “Near Pimanpiro, Imbabura Province [Valle de la Chota], Ecuador.”

R. m. howelli Goodwin, 1932:560. Type locality “Chichicastenango (Santo Tomas) District of El Quiché, Guatemala.”

R. m. lucifrons Howell, 1932:125. Type locality “Cerro Cantoral, Honduras.” See above; minusculus Howell is a synonym.

R. m. mexicanus (de Saussure, 1860:109) . See above; jalapae (Merriam), goldmani (Merriam) are synonyms.

R. m. milleri Allen, 1912:77 . See above.

R. m. ocotepequensis Goodwin, 1937:1. Type locality “Monte Verde, Department of Ocotepeque, Honduras, 30 miles northeast of the city Ocotepeque.”

R. m. orinus Hooper, 1949:169. Type locality “ El Salvador, Dept. Sonsonate, about 12 miles southeast of Sonsonate, near summit of Balsam Range, Hacienda Chilata.”

R. m. riparius Hooper, 1955:12. Type locality “ México, Michoacán, 2 ½ mi. SW Coalcomán.”

R. m. scansor Hooper, 1950:418. Type locality “ México, Chiapas, Villa Flores.”

R. m. soederstroemi Thomas, 1898:451 . See above.

NOMENCLATURAL NOTES. Reithrodontomys mexicanus was originally classified in the genus Reithrodon , which grouped rodents from South America and North America. Coues (1874) recognized the North American species as a different group and included them in the genus Ochetodon . However, this name was abandoned because Giglioli (1873, not seen, cited in Merriam 1892) proposed the genus Reithrodontomys , including all species of Ochetodon . The type specimen of R. mexicanus is deposited in the Natural History Museum of Geneva, number 510/100.

The generic name Reithrodontomys derives from the Greek rheithron (stream or channel), odous (tooth), and mys (mouse). The specific epithet mexicanus comes from Latin, meaning “from or belonging to Mexico,” referring to the country where the type specimen was collected. One of the Spanish common names is ratón cosechador mexicano ( Tirira 2004).

DIAGNOSIS

Reithrodontomys mexicanus and all other species in the subgenus Aporodon (for list of species, see Wilson and Reeder 2005) typically are distinguished from members of the subgenus Reithrodontomys by their relatively brighter dorsal fur coloration that can vary in reddish tones. The ventral coloration is generally white, gray, or cinnamon; the dark tail is longer than the head– body length, monochromatic, and with short hairs (members of the subgenus Reithrodontomys usually have a bicolored tail with long hairs— Hooper 1952). Molars have a more complex pattern (well-developed enamel loops) than species in the subgenus Reithrodontomys ( Howell 1914) . M3 and m3 represent threequarters of the size of M2 and m2, respectively, constituting compact replicas of these molars versus M3 and m3 equal onehalf or less the size of M2 and m 2 in members of the subgenus Reithrodontomys ( Hooper 1952) .

Reithrodontomys gracilis (slender harvest mouse) is most closely related to R. mexicanus (both are arranged in the mexicanus species group— Hooper 1952). The two species differ in overall size (measurements in mm and R. mexicanus presented first in each pair): body length (171 versus 181), length of hind foot (19–20 versus 17–20), and skull length (21–24 versus 20–23). The tail of R. mexicanus represents 130–160% of the head and body together, whereas in R. gracilis it constitutes 97–155%. Although both species are distributed from the southeast portion of Mexico to Nicaragua ( Hall 1981), R. mexicanus mostly inhabits highlands, mainly pine-oak and subtropical forests, whereas R. gracilis is found in tropical arid lowlands ( Hooper 1952). R. mexicanus is sympatric with other species in the subgenus Aporodon such as R. brevirostris (short-nosed harvest mouse—Jones and Genoways 1970), R. tenuirostris (narrow-nosed harvest mouse), and R. microdon (small-tooth harvest mouse— Hall 1981). R. brevirostris from Nicaragua resembles R. mexicanus but the former is cranially smaller (mean skull length 22.0 mm in R. b. nicaraguae versus 23.4 mm in R. m. lucifrons) with a much narrower rostrum (Jones and Genoways 1970). R. tenuirostris is larger in size of body and skull (mean total length 211 mm versus 181 mm in R. m. howelli; mean skull length 24.9 mm versus 22.4 mm in R. m. howelli — Hooper 1952). R. microdon is slightly smaller than R. mexicanus (mean total length 180 mm versus 181 mm), but R. microdon has greater depth of the skull (8.9 mm versus 8.4 mm) and longer rostrum (8.3 mm versus 7.7 mm) than does R. mexicanus ( Hooper 1952) .

GENERAL CHARACTERS

Reithrodontomys mexicanus ( Fig. 1 ) is a moderate- to largesized harvest mouse. Dorsal pelage is brown to cinnamon orange, although it varies geographically and there are more intensely pigmented subspecies such as R. m. mexicanus and R. m. soederstroemi ( Hooper 1952) . The underparts can vary from whitish to pale cinnamon. The tail is noticeably longer than head–body length and maintains a uniform color, usually dark brown or blackish, rarely paler below or with a white tip (Hopper 1952; Hall 1981). The ears are fuscous in color and sparsely covered by black hairs. The upper surface of both the forefeet and hindfeet are covered with dark brown hairs and the toes are whitish ( Howell 1914). The pelage changes as juveniles transition to adults, and the molting process is very similar throughout the genus (description follows Hooper 1952). Juveniles have a dense coat with long guard hairs and shorter cover hairs, always with a dark and dull coloration. Subadults have thicker and brighter hairs, very similar to adult pelage, which is the most intense and bright with the distinctive marks expressed in their entirety. Hypopigmentation in the fur coat (leucism) has been reported in an individual of the subspecies R. m. soederstroemi ( Ramírez-Jaramillo et al. 2019) .

The skull of R. mexicanus ( Fig. 2 ) shares the general characteristics found in members of the subgenus Aporodon . The braincase is wide and shallow, the anterior part of the zygomatic arch is weak ( Hooper 1952). The rostrum is wide, and the palate is usually as long as the molar toothrow, but shorter than the incisive foramina ( Hall 1981). In comparison to members of the subgenus Reithrodontomys , the dentition of R. mexicanus has a relatively complex pattern on the occlusal surface of the molars. This pattern is characterized by the presence of mesolophs (ids) and mesostyles (ids) on all teeth, and a third molar that is essentially a smaller replica of the second ( Hooper 1952). The incisors are moderately recurved and, as in all species of Reithrodontomys , they have a longitudinal groove located medially on the upper incisors ( Le Conte 1853).

An assessment of nongeographic variation in body mass and four external, 13 cranial, and three postcranial measurements showed no sexual dimorphism in a population of R. mexicanus from La Esperanza, Oaxaca, Mexico ( Arellano et al. 2012). There were significant differences due to age in all but the postcranial measurements, comparing five age classes (I–V) assigned according to the molar wear (I and V being the youngest and oldest age classes, respectively). The most distinct classes were II and III, whereas IV and V were similar in size ( Arellano et al. 2012).

Mean external measurements (mm; range in parentheses) for 51 individuals of age class IV (M3 with more than 50% worn occlusal surface) from La Esperanza, Oaxaca were: total length, 198.0 (173–229); tail length, 117.4 (90–144); length of right hind foot, 20.9 (18–24); and length of right ear, 16.1 (13–19— Arellano et al. 2012). Mean body mass (g; range in parentheses) for age class IV was 14.0 (8–18), and mean cranial measurements (mm; range and n in parentheses) were: greatest length of skull, 23.8 (22.5–24.9, 51); skull width, 11.5 (10.7–12.1, 54); skull depth, 8.7 (7.9–9.4, 54); zygomatic breadth, 12.4 (11.6–12.7, 10); nasal length, 8.5 (6.4– 9.1, 54); length of molars, 3.5 (3.2–4.4, 54), length of incisive foramen, 4.6 (3.8–5.1, 54); rostral length, 8.6 (7.9–9.4, 54); rostral width, 4.3 (3.9–4.6, 54); least interorbital constriction, 3.8 (3.6–4.3, 54); and width of zygomatic plate, 1.8 (1.5–2.1, 54— Arellano et al. 2012).

DISTRIBUTION

Reithrodontomys mexicanus has the largest geographical and elevational range among members of the subgenus Aporodon ( Arellano 2015) . It is distributed from the mountainous systems of northern Mexico south to Nicaragua, and in western Colombia and northwest Ecuador in South America ( Fig. 3 ). R. mexicanus can generally be found in an elevational range from near 1,000 to 3,800 m, but this varies depending on the geographic region. In North and Central America, the lowest reported record of occurrence is 91 m, near Gómez Farías in Tamaulipas, Mexico (Jones and Anderson 1958), and the highest is 2,438 m in Los Esesmiles, El Salvador ( Hooper 1952). In Colombia, its elevational range varies from 500 to 3,000 m (Marín-C. 2014), and in Ecuador, it occurs from 1,800 to 3,800 m ( Tirira 2007).

The distribution of Mexican subspecies includes the Sierra Madre Oriental for R. m. mexicanus and the Sierra Madre Occidental for R. m. riparius ( Hooper 1952, 1955; Hall 1981). R. m. scansor is found in Oaxaca and Chiapas and R. m. howelli inhabits areas from the center and east of Chiapas to the west and center of Guatemala. Central American subspecies are distributed almost continuously in the highland areas of this region, excluding Costa Rica and Panama ( Bradley 2017). In South America, R. m. milleri is found in northern Colombia and both R. m. eremicus and R. m. soederstroemi occur in northwestern Ecuador.

FOSSIL RECORD

Information related to fossil remains of Reithrodontomys is generally scarce ( Ruez 2000). Reports of the subgenus Aporodon in northern Ecuador are dated from the late Pleistocene and Holocene ( Moreno et al. 2018). A revaluation of the fossil species Copemyodon ecuadorensis by Fejfar et al. (1996) resulted in synonymizing this taxon with Reithrodontomys . Despite the great similarity of the molars between R. ecuadorensis and R. mexicanus soederstroemi , it was considered a species-level taxon by Moreno et al. (2018). The remains of two upper molars in a Pleistocene deposit from La Palmera, Costa Rica, were identified by Laurito (2003) as belonging to R. mexicanus .

FORM AND FUNCTION

Like other species in the genus, female Reithrodontomys mexicanus have six mammae: one pectoral pair and two inguinal pairs ( Hooper 1952). The dental formula is i 1/1, c 0/0, p 0/0, m 3/3, total 16 (Emmons and Feer 1999). R. mexicanus has a moderately complex molar pattern ( Carleton 1980). The groove of the upper incisors is deep, and the molars are bunodont. The lingual root of M1 and M2 is single and large, and M3 has three roots. Also, the labial root of M1 is absent ( Carleton 1980). Four main cusps are present in M3, the protoconus is the largest followed by the paraconus, hypoconus, and metaconus. The major fold in M3 is short and deep while the minor fold is usually indistinct. First and second primary and secondary folds are well-developed ( Hooper 1952). Both lingual and labial roots are absent in m1, whereas m2 and m3 have two roots each. In m3, the entoconid and hypoconid cusps are greatly reduced and wear to a “C-shape” ( Carleton 1980). The major and minor folds in m3 are relatively long and deep, and the second primary fold is well-developed. Mesolophs (ids) and mesostyles (ids) present in all molar teeth, except possibly in m3. The molar toothrow varies in length from about 3.2 mm, in smaller subspecies, to 3.7 mm in larger subspecies ( Hooper 1952).

The anterior edge of the zygomatic arch is parallel to the anterior edge of the zygomatic plate in R. mexicanus , allowing the former to curve evenly and away from the rostrum (Rinker and Hooper 1950). Stapedial and sphenofrontal foramina are present with a groove existing on squamosal. The foramen ovale is present as is the postglenoid foramen and subsquamosal foramina, which are large. The sphenopalatine vacuities are elongate and extending over one-half the length of the presphenoid bone. When present, the posterolateral palatal pits have one or two small foramina. A pair of palatine foramina are found at the junction of the maxillary and palatine bones. Sometimes, paired foramina are large and have one or two small openings ( Carleton 1980). In R. mexicanus , there is little difference in size between pterygoid and mesopterygoid fossa (Rinker and Hooper 1950), and the lateral pterygoid fossa is flat. The supraorbital shape is smooth on the sides and the temporal ridges are absent. The zygomatic notch is absent, and the postorbital process and the angular processes of the dentary are not deflected. In the hyoid apparatus, the entoglossal process forms a small knob, the shape of the basihyal bone is arched, and the thyrohyal bone is long, greater than or equal to the length of the basihyal. The malleus is parallel with the manubrium forming a right angle with the frontal plane of the skull. Additionally, an orbicular apophysis is present. The accessory tympanum is large, the small mastoid bullae are unmodified, and the tympanic bullae are also small. The tentorium cerebellum is present as a low relief crest ( Carleton 1980).

Reithrodontomys mexicanus possesses 13 thoracic, six lumbar, and more than 36 caudal vertebrae. Additional features of the postcranial skeleton include the presence of an entepicondylar foramen and the second thoracic vertebra with a pronounced neural spine. The first rib articulates with the transverse processes of both the first thoracic and seventh cervical vertebrae. The trochlear process of the calcaneum is broad and it is found at the level with posterior articular facet. The tibia and fibula are extensively fused ( Carleton 1980).

Differences in cranial musculature associated with the zygomatic plate and the mesopterygoid fossa were examined between R. mexicanus and R. megalotis (western harvest mouse) by Rinker and Hooper (1950). R. mexicanus has small pterygoideus internus muscles with a minor area for their origin and insertion. The temporalis muscles are large, and their ventral boundaries extend below the dorsal lip of the auditory meatus. The anterior fibers of the masseter lateralis muscle originate slightly medial to the keel of the zygomatic plate, wrapping its anterior margin; and its insertion is posterior to the mental foramen. These myological characteristics have been associated with the movements of the jaw, probably the chewing process. Thereby, muscles responsible for moving the jaw are well-developed while those related to occlusion are relatively weak (Rinker and Hooper 1950).

The anterior ridge of the soft palate is complete with low relief and has two complete and five incomplete palatal ridges ( Carleton 1980) in R. mexicanus . It has a discoglandular stomach, with a small area of glands located in the ascending part of the greater curvature of the stomach ( Carleton 1973). Like all species of Reithrodontomys , the incisura angularis is intermediate in development compared to most cricetids ( Carleton 1973, 1980); the sulcus on the greater curvature of the stomach is not present and the saclike gallbladder is generally located between the cystic lobes of the liver. The first segment of the large intestine consists of either zero, one, or two coils and the cecum is simple internally, with a moderate length ( Carleton 1980).

The glans penis of R. mexicanus is similar in shape and structure to other species of Reithrodontomys such as R. fulvescens (fulvous harvest mouse), R. megalotis , and R. sumichrasti sumichrasti (Sumichrast’s harvest mouse) although larger (mean length = 7 mm, mean diameter = 1.7 mm, n = 2 versus mean length = 5.9 mm, mean diameter = 1.4 mm, n = 5— Hooper 1959). The glans is elongated, and rod-shaped, and consists of two divisions. One is the conical tip, somewhat protractile with soft, flexible, and nonspiny outer tissues, and the other has a main body composed of dense tissue ( Hooper 1959). The spines on the glans cover one-half to three-quarters of the spiny body and there are no spines on the inner wall of the crater ( Carleton 1980). Compared to R. megalotis , the spines are longer and thicker in R. mexicanus ( Hooper 1959) . The urinary meatus is located at the base of the protractile tip ( Hooper 1959), in a subterminal position ( Carleton 1980). Both ventral and dorsal lappet projections, and the urethral process, dorsal papilla, the lateral bacular mounds, and the crater hood, are absent in R. mexicanus ( Carleton 1980) . The baculum is simple, enlarged toward the base, and blunt distally. It does not consist of a terminal capsule or a cartilage spine, and the bone is much longer than the glans. The mean length of the baculum, based on two specimens, was 8.4 mm ( Hooper 1959). Male accessory reproductive glands lack preputials in R. mexicanus . The medial and lateral ventral prostates are present as are the dorsal and anterior prostates. In addition, the bulbourethral glands, ampullaries, and vesiculars (J-shaped inverted) are all present, except for the ampullae of ductus deferens ( Carleton 1980).

The hind foot of R. mexicanus has a bare or slightly furred plantar surface on the heel. The first interdigital plantar pad is positioned back toward the heel, not opposite the fourth, while from the second to the fourth, they are closer to each other. The thenar and hypothenar pads are subsequently displaced and strongly alternated ( Carleton 1980).

ONTOGENY AND REPRODUCTION

In Veracruz, Mexico, a female Reithrodontomys mexicanus with four embryos was reported in February (Hall and Dalquest 1963), and in Nicaragua a female with the same number of embryos was trapped in July (Jones and Genoways 1970). Females typically have three to four offspring per litter ( Reid 1997). Nestling juveniles have been reported in December from El Salvador, and juveniles or subadults were trapped in March and April in Mexico ( Hooper 1952). R. mexicanus taken in Nicaragua in April had juvenile pelage, and the upper third molar had not fully erupted (Jones and Genoways 1970).

Reithrodontomys mexicanus apparently reproduces during several months of the year, although its breeding season is not well-defined ( Hooper 1952). Lactating females were noted in February, March, April, and November in El Salvador ( Hooper 1952), and in April ( Hooper 1952), February, and September ( González 2012) in Mexico. Females from Nicaragua collected in July and April did not show evidence of reproductive activity (Jones and Genoways 1970). Scrotal males were collected in Guatemala during January ( Matson et al. 2014; OrdóñezGarza et al. 2014), and in early February both females and males from Cerro Celaque, Honduras were reproductively active (lactating females, females with embryos, and males with testes scrotal or enlarged— Matson et al. 2016). Males with well-developed testes have been captured in Ecuador during November (Vallejo and Boada 2017). Adult males captured in July from Nicaragua had testes with a mean length of 11.3 mm (9–17 mm), and one taken in April had testes measuring 8 mm (Jones and Genoways 1970). Of 11 male R. mexicanus trapped in Puebla, Mexico, in May, September, and November, five specimens had testicular measurements, ranging from 5 to 7 mm ( González 2012).

Figures

Fig. 1.—An adult of Reithrodontomys mexicanus from Palenque, Chiapas, Mexico. Used with permission of the photographer Oscar Miguel Pérez Macías (CONABIO).

Fig. 2.—Dorsal, ventral, and lateral views of skull, and lateral view of mandible of an adult male Reithrodontomys mexicanus (BYU [Monte L. Bean Life Science Museum, BrighamYoung University] 15439) from 18 km NW Teocelo, Municipio Ixhuacán, Veracruz, México. Greatest length of skull is 22.4 mm.

Fig. 3.—Geographic distribution of Reithrodontomys mexicanus. Subspecies are: 1, R. m. eremicus; 2, R. m. howelli; 3, R. m. lucifrons; 4, R. m. mexicanus; 5, R. m. milleri; 6, R. m. ocotepequensis; 7, R. m. orinus; 8, R. m. riparius; 9, R. m. scansor; 10, R. m. soederstroemi. Map redrawn from Hooper (1952) and Hall (1981) with modifications.