Sigmodon mascotensis Allen, 1897

Martínez-Chapital, Sara T, Schnell, Gary D, Sánchez-Hernández, Cornelio & Romero-Almaraz, María de Lourdes, 2017, Sigmodon mascotensis (Rodentia: Cricetidae), Mammalian Species 49 (954), pp. 109-118 : 109-113

Sigmodon mascotensis Allen, 1897

West Mexican Cotton Rat

Sigmodon mascotensis Allen, 1897:54 . Type locality “Mineral San Sebastian, Mascota, State of Jalisco, Mexico,” 3,300 ft.

Sigmodon colimae Allen, 1897:55 . Type locality “Plains of Colima, State of Colima, Mexico.”

Sigmodon hispidus mascotensis: Bailey, 1902:108–109 . Name combination.

Sigmodon hispidus tonalensis Bailey, 1902:109 . Type locality “Tonala, Chiapas, Mexico.”

Sigmodon hispidus inexoratus Elliot, 1903:144 . Type locality “Ocotlán, Jalisco, Mexico.”

Sigmodon hispidus atratus Hall, 1949:149 . Type locality “6½ mi. W Zamora, 5,950 ft., Michoacán, México.”

Sigmodon obvelatus Russell, 1952:81 . Type locality “ 5 mi. S Alpuyeca, 3,700 ft., Morelos.”

Sigmodon ischyrus Goodwin 1956:8 . Type locality “El Arco, gorge of the Río Grande, Santo Domingo Chontecomatlán, District of Yautepec, Oaxaca, México,” 2,600 feet.

CONTEXT AND CONTENT. Order Rodentia , suborder Myomorpha , superfamily Muroidea , family Cricetidae ,

subfamily Sigmodontinae , genus Sigmodon (Musser and Carleton 2005) . Traditionally, S. mascotensis was considered a member of the semi-naked-tail group also known as the hispidus group (Bailey 1902), but more recent studies indicate that groupings based primarily on external morphological features are no longer valid. Various analyses have resulted in varied arrangements of Sigmodon species (e.g., based on DNA sequences from beta-fibrinogen gene, mitochondrial cytochrome- b gene, and the nuclear gene encoding interphotoreceptor retinoid-binding protein; chromosomal configuration; serum-albumin analysis) but most place S. mascotensis as the sister clade of S. arizonae , with S. hispidus as sister to this group ( Elder 1980; Fuller et al. 1984; Elder and Lee 1985; Peppers 1998; Peppers and Bradley 2000; Peppers et al. 2002; Carroll and Bradley 2005; Carroll et al. 2005; Henson and Bradley 2009). Most include S. mascotensis in a revised hispidus species group. No subspecies are recognized (Musser and Carleton 2005), with former subspecies now considered to be synonyms.

NOMENCLATURAL NOTES. The genus Sigmodon was first recognized by Say and Ord (1825) with the description of Sigmodon hispidus . Allen (1897) described S. mascotensis based on 2 specimens from Mascota, Jalisco, he considered to be adults (male and female). Based on morphological resemblance, Bailey (1902) included mascotensis as a subspecies of hispidus , with S. h. mascotensis being used until 1969. Based on patterns of blood-serum transferrin (Dalby and Lillevick 1969) and chromosome analysis ( Zimmerman 1970), S. mascotensis and S. hispidus were judged to be different species. Prior to 1969, there were studies in western Mexico referring to cotton rats found as S. hispidus ; however, due to the systematic overlap, it often is difficult to tell whether the species involved was S. mascotensis or S. hispidus .

Sigmodon mascotensis and S. colimae (from Colima) were described in the same publication by Allen (1897), where he argued that S. mascotensis had a much browner color and a longer tail. Bailey (1902) stated that S. colimae was a synonym of S. mascotensis (S. h. mascotensis ) and described S. h. tonalensis as a separate subspecies from Chiapas, the latter being larger and browner than S. h. mascotensis . S. inexoratus was designated as a new species from Jalisco by Elliot (1903). It was judged to be as big as S. hispidus but grayer and with cranial features similar to S. mascotensis ; however, it was larger and different in color (darker above with paler sides) than S. mascotensis . Hall (1949) named S. h. atratus from Michoacán, indicating it differed from S. h. mascotensis by having shorter hind feet, darker upper parts, hairier tail, and varied cranial features, including a shorter skull and higher degree of convexity of the skull dorsally. S. obvelatus from Morelos was named by Russell (1952), claiming it differed from S. h. mascotensis in having smaller body size, being slightly paler color above and whiter below, and having a hind foot that was smaller and whiter rather than buffy. Goodwin (1956) named S. h. ischyrus from Oaxaca, declaring it could be distinguished from S. h. mascotensis by its richer color, smaller size, and dusky marks on the hind feet.

Sigmodon hispidus atratus , S. h. colimae , S. h. inexoratus , S. h. ischyrus ( Zimmerman 1970) , S. h. tonalensis, and S. h. obvelatus ( Carleton et al. 1999) were included as subspecies of S. mascotensis based on similarity in morphological, cranial, and chromosomal features (large size, grayish brown dorsum with strongly hispid fur, relatively long tail, cranium with relatively small auditory bullae and expansive temporal fossa, diploid number of 28 chromosomes, and fundamental number of 26). The subspecies are now considered synonyms of S. mascotensis (Musser and Carleton 2005) .

DIAGNOSIS

Sigmodon mascotensis has a grayish brown dorsal pelage that is uniform throughout ( Fig. 1 ). S. arizonae , Arizona cotton rat, has brown pelage too, but has darker upper parts and is yellowish on the flanks. S. alleni , Allen’s cotton rat, has uniformly rich brown dorsal pelage, occasionally with rufous or cinnamonbrown tones ( Carleton et al. 1999). The pelage of S. mascotensis may not be distinguishable from S. hispidus , hispid cotton rat, but morphological and cranial features differentiate the 2 species.

Size of the foramen ovale in S. mascotensis averages ≤ three-quarters of the diameter of M3; it is larger in S. arizonae (about the diameter of M3— Zimmerman 1970). Greatest length of skull averages 36 mm for S. mascotensis and 40 mm for S. arizonae ( Zimmerman 1970) . Presence of an oval-shaped fenestra on the parapterygoid fossa is common in S. mascotensis (70%) but rare in S. hispidus and S. arizonae ( Carleton et al. 1999) . Distance between superior and inferior ridges is> 3.9 mm and the hind foot is 34–36 mm for S. mascotensis , whereas for S. hispidus the measurements are 3.0– 3.2 mm and 31.0– 32.5 mm, respectively ( Zimmerman 1970; Carleton et al. 1999). In S. hispidus and S. alleni , the temporal fossa is trapezoidal in shape over the posterior half of the parietal and squamosal bones, whereas in S. mascotensis the fossa is bigger and rectangular ( Carleton et al. 1999).

GENERAL CHARACTERS

Sigmodon mascotensis is a large-sized rat (total length 220– 313 mm; 51–270 g — Carleton et al. 1999; Miranda 2002b) with relatively big eyes (Allen 1897). The ears are small (13–22 mm), rounded, and internally covered with hair (Allen 1897; Miranda 2002b).

Dorsal pelage—brown with pale saturation, bright tone, and grayish hue—is formed by a combination of long, thin black hairs and short, thick brown hairs ( Carleton et al. 1999; Ramírez and Chávez Tovar 2005). Ventral pelage is light gray formed by hairs with plumbeous gray bases and white tips ( Carleton et al. 1999). The pelage in the nose area often is rich brown, contrasting with the grayish facial coloration ( Fig. 2 ).

The tail is shorter than the length of the head and body combined (81–166 mm), nearly naked, covered with scales> 0.75 mm in length, dark above, and pale below (Ceballos and Miranda 1986, 2000; Sánchez Hernández and Romero Almaraz 1995; Miranda 2002a). The hind foot is 34–36 mm and covered with yellowish to whitish hair, with the 3 central digits longer than the external ones ( Zimmerman 1970; Ceballos and Miranda 1986, 2000; Sánchez Hernández and Romero Almaraz 1995; Miranda 2002a; Ramírez and Chávez Tovar 2005).

The skull is long and thin ( Fig. 3 ). The upper incisors are well developed. Dental formula is i 1/1, c 0/0, p 0/0, m 3/3, total 16 (Ceballos and Miranda 1986, 2000). The generic name Sigmodon is from the Greek and refers to the sigmoid pattern of the enamel of the 2nd molar with the crown worn down (Say and Ord 1825; Palmer 1904).

Measurements (mm; mean ± SD with range and sample size in parentheses) of specimens for 2 samples from Nayarit ( Carleton et al. 1999) with sexes combined were as follows: total length, 278.1 ± 20.1 (242.0–313.0; n = 20), 257.0 ± 21.4 (220.0–305.0; n = 32); tail length, 132.3 ± 12.3 (110.0–156.0; n = 20), 112.3 ± 10.5 (96.0–141.0; n = 32); hind foot length, 34.1 ± 1.9 (31.0–38.0; n = 21), 35.0 ± 1.7 (31.0–38.0; n = 33); occipitonasal length, 32.8 ± 1.9 (29.8–37.0; n = 27), 31.6 ± 1.7 (27.5–35.3; n = 40); zygomatic breadth, 18.7 ± 1.0 (17.0–20.7; n = 27), 18.3 ± 1.0 (15.6–20.7; n = 45); interorbital breadth, 4.9 ± 0.3 (4.5–5.5; n = 27), 5.0 ± 0.2 (4.6–5.4; n = 45); breadth of braincase, 14.0 ± 0.4 (13.1–14.7; n = 27), 13.9 ± 0.4 (12.6– 14.6; n = 43); breadth of occipital condyles, 7.4 ± 0.3 (6.9– 8.0; n = 27), 7.5 ± 0.3 (6.9–8.2; n = 41); depth of braincase, 11.3 ± 0.3 (10.8–12.2; n = 27), 10.8 ± 0.5 (8.9–11.7; n = 41); distance between temporal ridges, 4.0 ± 0.4 (3.3–4.8; n = 27), 3.7 ± 0.4 (2.5–4.4; n = 45); length of rostrum, 10.6 ± 0.9 (9.0– 12.7; n = 27), 10.3 ± 0.7 (8.1–11.7; n = 43); breadth of rostrum, 6.4 ± 0.5 (5.5–7.4; n = 27), 6.3 ± 0.4 (5.3–7.2; n = 45); postpalatal length, 11.5 ± 0.8 (10.0–13.2; n = 27), 10.9 ± 0.7 (9.2–12.7; n = 41); length of bony palate, 6.1 ± 0.4 (5.2–6.9; n = 27), 6.1 ± 0.4 (5.1–6.8; n = 45); breadth of bony palate, 7.2 ± 0.4 (6.7–8.0; n = 27), 7.4 ± 0.3 (6.8–8.0; n = 45); length of incisive foramen, 6.9 ± 0.6 (5.9–8.2; n = 27), 6.7 ± 0.5 (5.5– 7.8; n = 45); length of diastema, 8.7 ± 0.7 (7.6–10.3; n = 27), 8.3 ± 0.7 (6.3–9.7; n = 45); breadth of zygomatic plate, 3.6 ± 0.4 (2.8–4.5; n = 27), 3.7 ± 0.3 (3.0–4.6; n = 45); length of zygomatic spine, 4.3 ± 0.4 (3.3–4.9; n = 27), 4.4 ± 0.5 (3.2–5.5; n = 46); length of auditory bulla, 5.3 ± 0.2 (4.9–5.8; n = 27), 5.3 ± 0.2 (4.7–5.8; n = 43); length of maxillary toothrow, 5.95 ± 0.26 (5.59–6.76; n = 27), 6.23 ± 7.23 (5.80–6.63; n = 46); width of M1, 2.01 ± 0.09 (1.84–2.26; n = 27), 2.09 ± 0.08 (1.94–2.34; n = 46); depth of upper incisor, 1.81 ± 0.08 (1.52–2.07; n = 27), 1.85 ± 0.15 (1.47–2.17; n = 46); and depth of mandible, 5.9 ± 0.4 (5.3–6.7; n = 27), 5.8 ± 0.4 (4.7–6.7; n = 46). Masses (g) for the same 2 samples were 99.6 ± 22.6 (63.0–148.0; n = 21) and 77.4 ± 17.8 (51.0–122.0; n = 33). Martin (1979) provided the following measurements (sex not indicated) for S. mascotensis from Jalisco: mandibular alveolar length, 7.12 (6.90–7.28; n = 3); length of m1, 2.22 (2.18–2.25; n = 3); length of m2, 1.69 (1.62–1.81; n = 4); length of m3, 2.57 (2.48–2.67; n = 4); width of m1, 1.65 (1.56–1.73; n = 4); width of m2, 1.81 (1.78–1.83; n = 4); and width of m3, 1.73 (1.66–1.78; n = 4).

The pelage differs between adults and juveniles. Adults have a grayish brown back and whitish belly, while juveniles are yellowish tawny brown on the back and have a soiled buffy gray belly ( Schnell et al. 2010). Juveniles and subadults have similar coloration but, in general, juveniles have a total length <160 mm and subadults <200 mm ( Schnell et al. 2010). Subadults can be distinguished from juveniles by the eruption of upper 3rd molars ( Carleton et al. 1999). Juveniles that are independent of parents can weigh 10–50 g and subadults 40–80 g ( Miranda 2002a). A juvenile whose 3rd molars had not erupted had a mass of 23 g (Ramírez-Pulido et al. 1977).

A contrast has been reported between pelages in the wet and dry season ( Hooper 1955); specimens from the dry season had a grayish brown pelage, and those captured during the wet season showed a more reddish pelage. Sexual dimorphism has been reported for body mass, with adult males (mean ± SD, 104.32 ± 27.71 g; n = 47) significantly heavier than adult females (92.08 ± 26.67 g; n = 34— Schnell et al. 2010).

DISTRIBUTION

The geographic distribution of Sigmodon mascotensis ( Fig. 4 ) extends in western Mexico from southern Nayarit and southwestern Zacatecas south to southwestern Chiapas, and as far east as western Hidalgo, western Puebla, and northwestern Oaxaca (Musser and Carleton 2005). It has been found at elevations of 0–2,550 m ( Juárez Gómez 1992; Carleton et al. 1999; Briones-Salas et al. 2015). The ranges of Sigmodon , including S. mascotensis , in arid regions of Mexico probably are not continuous and, in many cases, distributions are comprised of isolated, local populations ( Zimmerman 1970).

The work of Carleton et al. (1999) demonstrated that most of the cotton rats in southwestern Mexico are S. mascotensis and not S. hispidus . Their findings along with those of Bradley et al. (2008) indicated that in Mexico S. hispidus is restricted to the northeastern part of the country.

FOSSIL RECORD

Fossils of Sigmodon have been summarized by Martin (1979), with additional fossil material described subsequently by Czaplewski (1987), Martin and Prince (1989), and others. No specimen was assignable to S. mascotensis . However, Martin (1979) pointed out that no relevant dental or mandibular features were identified that separated S. mascotensis and S. arizonae from each other or from S. hispidus . S. bakeri of the late Irvingtonian through the early Rancholabrean (Middle and Late Pleistocene epochs) from what is now Florida is the only recognized extinct member of the hispidus species group (of which S. mascotensis is a member). As indicated earlier, there is evidence from multiple sources that S. mascotensis and S. arizonae are sister taxa, with the sister taxon to this group being S. hispidus . Zimmerman (1970) suggested that speciation of these taxa occurred during the Middle Pleistocene. Based on their molecular studies, Peppers et al. (2002) estimated that divergence times for these species ranged from 2.5 to 3.9 million years ago. Peláez-Campomanes and Martin (2005) considered this estimate to be too high, given their evaluation of the fossil record and rates of change extrapolated from that record. They suggest that ancestry of the hispidus species group is probably no earlier than 2.5 million years ago and may well be much later, possibly <1.25 million years ago.

FORM AND FUNCTION

Sigmodon mascotensis has 5 pairs of mammary glands: 3 pectoral and 2 inguinal (Sánchez Hernández and Romero Almaraz 1995). The female reproductive tract has been described anatomically and histologically ( Ricardez Barbera 1991). Ovaries are located in the sublumbar area under the kidneys and are covered by follicles in different developmental stages. They exhibit high follicular activity, with formation of follicles beginning in the internal part of the ovarian cortex and extending to the external portion. Fertilization takes place in the ampulla of the oviduct. The oviduct contacts the antimesometrial side of the uterus through a papilla. The uterus is Y-shaped and formed by the following 4 layers of cells: cylindrical columnar epithelium associated with glands; vascular flat epithelium (not keratinized); relaxed conjunctive tissue and fibroblasts; and smooth muscle. The cervix is much keratinized and has 2 thin channels that connect the lumen from the uterus with the vagina. The vagina has average length of 12.7 mm and diameter of 8.41 mm. The orifice is covered with thin hair and exhibits differences in the mucosa depending on the stage of the estrous cycle ( Ricardez Barbera 1991).

The resting metabolic rate of S. mascotensis was evaluated for 10 individuals that were descendants of wild-caught animals from Jalisco (Bower 1971). Mean mass of individuals evaluated was 116.2 g (range 71–135 g). Mean oxygen consumption under resting conditions was 1.53 cm 3 g−1 h−1 (range 1.23–1.57 cm 3 g−1 h−1).

ONTOGENY AND REPRODUCTION

Gestation lasts 27–30 days (Sánchez Hernández and Romero Almaraz 1995; Ceballos and Miranda 2000). Litter size has been reported for Chamela, Jalisco, as 3–9 individuals, with 5–6 being most common ( Miranda 2002b). Neonates have a mass of 5–7 g ( Miranda 2002b). The lactation period is short, and weaning occurs during the 2nd week after birth ( Miranda 2002b). Cotton rats, including Sigmodon mascotensis , are highly precocial, and weaned animals enter rapidly into the trappable population ( Carleton et al. 1999). Juveniles become sexually mature in the 1st to 3rd month (Sánchez Hernández and Romero Almaraz 1995).

Sigmodon mascotensis reproduces throughout the year (Ceballos and Miranda 1986, 2000). Males with scrotal testes have been found January–May, July, and December (Álvarez et al. 1987; Juárez Gómez 1992; Sánchez Hernández and Romero Almaraz 1995; Schnell et al. 2010). Receptive females are known from January and March ( Juárez Gómez 1992; Schnell et al. 2010), pregnant females from January, May, and July (Matson and Baker 1986; Álvarez et al. 1987; Schnell et al. 2010), lactating females from January and July (Matson and Baker 1986; Schnell et al. 2010), and juveniles from January, May, and July (Álvarez et al. 1987; Schnell et al. 2010). At the Biological Station near Chamela, Jalisco, peak reproductive activity occurred during the rainy season with reproduction beginning in the dry season ( Miranda 2002b).

Figures

Fig. 1.—Adult specimen of Sigmodon mascotensis from 3.2 km E La Venta del Astillero, municipality of Zapopan, Jalisco, Mexico. Photograph by Edgar G. Godinez used with permission.

Fig. 2.—Detail of facial features of Sigmodon mascotensis from 1.5 km S and 3.0 km E of Los Asmoles, municipality of Colima, Colima, Mexico. Photograph by María de Lourdes Romero-Almaraz.

Fig. 3.—Dorsal, ventral, and lateral views of skull, and lateral view of mandible of adult male Sigmodon mascotensis (2940 GDS) from rancho Majagua, municipality of Manzanillo, Colima, Mexico. Greatest length of skull 37.6 mm. Photograph by Cornelio Sánchez-Hernández.

Fig. 4.—Geographic distribution of Sigmodon mascotensis in Mexico. Modified from Carleton et al. (1999).