Microtus quasiater
Microtus quasiater (Coues, 1874) Arvicola ( Pitymys ) pinetorum var. quasiater Coues, 1874: 191. Type locality: “Xalapa (=Jalapa), Veracruz, Mexico.” Arvicola ( Pitymys ) quasiater : Coues, 1877:226. Name combination. Arvicola quasiater : True, 1885:596. Name combination. M [ icrotus ( Pitymys )]. q...
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2018
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| Summary: | Microtus quasiater (Coues, 1874) Arvicola ( Pitymys ) pinetorum var. quasiater Coues, 1874: 191. Type locality: “Xalapa (=Jalapa), Veracruz, Mexico.” Arvicola ( Pitymys ) quasiater : Coues, 1877:226. Name combination. Arvicola quasiater : True, 1885:596. Name combination. M [ icrotus ( Pitymys )]. quasiater : Miller, 1896. First use of current name combination. Microtus quasiater : Allen and Chapman, 1897:207. Name combination. Pitymys quasiater : Miller, 1912:229. Name combination. Microtus ( Pitymys ) quasiatur Baker and Villa-R., 1953:149. Incorrect subsequent spelling of Arvicola ( Pitymys ) pinetorum var. quasiater Coues, 1874. M [ icrotus ]. ( Pitymys ) quasiator Anderson, 1960:190. Incorrect subsequent spelling of Arvicola ( Pitymys ) pinetorum var. quasiater Coues, 1874. CONTEXT AND CONTENT. Order Rodentia, suborder Myomorpha, superfamily Muroidea, family Cricetidae, subfamilyArvicolinae, genus Microtus , subgenus Pitymys (Musser and Carleton 2005). Microtus quasiater is a monotypic species (Ramírez-Pulido et al. 2005). NOMENCLATURAL NOTES. Originally, Microtus quasiater was recognized as a subspecies of M. pinetorum (Coues 1874), but soon its specific status was established (Bailey 1900; Hall and Cockrum 1953; Repenning 1983). Although its phylogenetic relationships were initially uncertain, it was considered to be closely related to M. pinetorum (Hall and Cockrum 1953; Anderson 1960; van der Meulen 1978). Analyses of allozymes and dental characters suggested that it had a closer relationship with M. ochrogaster (Repenning 1983; Moore and Janecek 1990). More recent phylogenetic analysis using mitochondrial genetic markers has confirmed its closer relationship with M. pinetorum (Conroy et al. 2001; Jaarolaa et al. 2004). The status of Pitymys is unresolved. Some authors prefer the name Pitymys quasiater (Ellerman 1941; Corbet and Hill 1980; Honacki et al. 1982; Repenning 1983), but others treat Pitymys as a subgenus of Microtus , arguing that it does not warrant full generic status (Hooper and Hart 1962; van der Meulen 1978; Hall 1981). In an analysis of the subfamily Arvicolinae, Pitymys was treated as a subgenus of Microtus , but it was recommended that the existing classification not be modified until every species of Microtus has been reviewed and phylogenetic relationships among them evaluated (Carleton and Musser 1984). As a result, endemic microtines of Mexico, previously included in the genus Pitymys , have been reclassified under Microtus in accord with the Carleton and Musser (1984) classification and the more recent review by Musser and Carleton (2005). The name Microtus comes from the Greek words mikro , meaning small, and otus , ears (Álvarez-Castañeda and Álvarez 1996). The specific name comes from the Latin words quasi , as if, or almost, and ater , black or gloomy, which refer to the darker color of its pelage. Its Spanish vernacular name is “metorito de Jalapa” (Villa-R. and Cervantes 2003). DIAGNOSIS The tail is faintly paler below and the ears are small yet evident (Hall and Cockrum 1953; Hall 1981; Fig. 1). Specimens from Oaxaca have moderately longer, softer, and thicker pelage, which is Prout’s Brown in dorsal coloration (Goodwin 1969). Bailey (1900) described the ears as large for a Pitymys and the tail about as long as the hind foot. The skull (Fig. 2) as described by Goodwin (1969:213) based on Oaxacan specimens, “The skull is large and broad; zygomatic arches widely spaced; rostrum short and depressed anteriorly; braincase large and broad; bullae moderately large; anterior palatine foramena short.” Bailey (1900:67) described the skull as Microtus quasiater (Fig. 1) is easily distinguished from the Mexican vole M. mexicanus , the only other microtine recorded in sympatry, by a combination of morphological characters. M. quasiater is usually smaller in body size (total length 110– 143 mm), has a much shorter tail (13–30 mm), and slightly smaller length of hind foot (15–20 mm) compared with M. mexicanus (total length 125–159 mm, tail length 28–44 mm, and length of hind foot 17–22 mm). The pelage of M. quasiater tends to be longer, softer, and uniform dark umber or seal brown above. The 3rd upper molar has only 2 closed triangles instead of 3 on the lingual side as occurs in M. mexicanus (Goodwin 1969; Hall 1981; Ramírez-Pulido et al. 1991; Álvarez-Castañeda 1996; Sánchez et al. 1996). Also, M. quasiater has a broader and shorter baculum (greatest length of stalk 2.6–3.2 mm) in contrast to the baculum of M. mexicanus (3.1–3.4 mm), described by Anderson (1960) as having an attenuated stalk and a slenderer shaft distally. These 2 species differ ecologically and chromosomally. M. quasiater is restricted to the montane cloud forest zone and its diploid number (2n) is 62 chromosomes with a fundamental number (FN) of 66, while M. mexicanus inhabits a greater variety of habitats varying from dry grasslands to meadows in montane coniferous forest, and has a 2n = 48, FN = 58 karyotype (Hall and Dalquest 1963; Getz 1985; Hoffmann and Koeppl 1985; Ramírez-Pulido et al. 1991; Cervantes et al. 1994; López- Wilchis and Torres-Flores 2005). GENERAL CHARACTERS The dorsum and flanks of Microtus quasiater are uniform dark umber or seal brown above, and the venter is slightly paler. “similar to that of M. pinetorum , but with larger, more quadrate braincase, more prominent postorbital ridges, narrower interorbital space, and larger audital bullae; dentition slightly heavier; molar pattern the same.” Microtus quasiater is the smallest microtine in Mexico and does not exhibit obvious sexual dimorphism in size, nor does it show marked geographic variation (Ramírez-Pulido et al. 1991). Ramírez-Pulido et al. (1991) reported mean values (mm; sexes combined) for standard external measurements (ranges and sample sizes in parentheses): total length, 126.9 (110–140, n = 106); tail length, 20.6 (13–30, n = 108); length of hind foot, 17.6 (15– 20, n = 108); and height of ear from notch, 13.4 (9–18, n = 93). Average body mass (g) of 180 adults, sexes combined, from Apulco, Puebla, was 26.1 ± 2.39 SD , range 21.6–35.1 (López- Wilchis and Torres-Flores 2005). Measurements of 6 individuals from Teziutlán, Puebla, given by Heaney and Birney (1977) were: total length 137.3 mm (133–143) and body mass 32.1 g (27.4–35.4). Ramírez-Pulido et al. (1991) reported means, ranges, and sample sizes used for cranial measurements (mm) separately for adult males and females. Because they did not find sexual dimorphism in size, we have combined their measurements as follows: length of skull, 25.5 (24.0–27.1, n = 89); basilar length, 23.5 (21.4–25.4, n = 86); palatal length, 13.8 (12.7–15.1, n = 110); length of incisive foramina, 4.1 (3.4–4.8, n = 110); palatal bridge length, 5.7 (4.9–6.8, n = 112); length of maxillary toothrow, 5.9 (5.2–6.7, n = 111); breadth across maxillary molars, 5.0 (4.3– 5.4, n = 112); breadth of bulla, 5.25 (4.8–6.2, n = 104); length of bulla, 6.6 (5.7–7.9, n = 102); zygomatic breadth, 15.1 (13.7–16.8, n = 98); mastoid breadth, 12.3 (11.1–13.7, n = 91); interorbital breadth, 3.6 (3.0–4.1, n = 111); rostral breadth, 5.0 (3.9–5.8, n = 110); nasal length, 6.8 (5.7–8.1, n = 99); height of skull, 9.6 (7.5–11.0, n = 93); greatest length of dentary, 14.8 (13.3–16.6, n = 112); length of mandibular toothrow, 5.9 (5.2–7.1, n = 110); greatest breadth of dentary across coronoid processes, 12.3 (10.9–14.0, n = 85); breadth across molars, 4.9 (4.0–6.2, n = 90); breadth across angular processes, 12.1 (11.1–13.8, n = 71); and posterior height of mandible, 8.9 (8.1–9.9, n = 71). DISTRIBUTION Microtus quasiater is endemic to Mexico where it is distributed at elevations from about 700 to 2,150 m (Ramírez-Pulido et al. 1991) along the Sierra Madre Oriental from Xilitla in southern San Luis Potosí, south through the states of Querétaro, Puebla, Hidalgo, and Veracruz, to Huautla de Jiménez in northern Oaxaca (Fig. 3). Some databases (e.g., Comisión Nacional para el Conocimiento y Uso de la Biodiversidad 2013a; Unidad de Informática para la Biodiversidad 2013; Global Biodiversity Information Facility 2015) contain unpublished records outside the known distributional range; some of which clearly represent erroneous identifications, and all need to be confirmed. FOSSIL RECORD According to Repenning (1983), all American microtines originated from Asian forms of Microtus that dispersed to America through Beringia. The probable ancestor of M. quasiater immigrated to America in the early Pleistocene along with M. meadensis , a closely related but now extinct species. M. quasiater could either represent a peripheral isolate of the woodland vole Microtus pinetorum or is a relic from the 2nd arvicoline dispersal event in the New World at middle Pleistocene (Hoffmann and Koeppl 1985). Other authors consider M. quasiater , like M. oaxacensis (Tarabundi vole), M. pinetorum , and M. ochrogaster (prairie vole), probable descendants of the “ Allophaiomys ” pliocaenicus complex, a microtine radiation in the New World originating from the 2nd wave of Palearctic immigrants that crossed the Bering land bridge during the middle Pleistocene (Harris 1988; Martin and Tesakov 1998; Golenishchev and Malikov 2006). However, further morphological analysis of fossil molars indicate that many taxa traditionally considered as potential ancestors (including “ Allophaiomys ” pliocaenicus ) to some of the North American species of Microtus would be excluded from consideration (Bell and Bever 2006). Molecular analysis supports the monophyly of North American species of Microtus as well as 2 invasions, the 1st resulting in species restricted to North America and the 2nd resulting in the tundra vole M. oeconomus , but the estimated phylogeny should be tested by including other suspected early colonizers such as M. quasiater , M. oaxacensis , the Zempoaltépec vole M. umbrosus , and the Guatemalan vole M. guatemalensis (Conroy and Cook 2000). Fossils of molars of M. quasiater have been found in the late Pleistocene-Holocene deposits at La Cinta, Michoacán, Mexico (Pérez and Godínez 2007). A late Pleistocene specimen found in El Tajo de Tequixquiac, Estado de Mexico, was originally identified as M. meadensis (Repenning 1983), but referred to M. quasiater by Martin (1987). Arroyo-Cabrales et al. (2002) treated M. quasiater as part of the late Quaternary mammalian fauna of Mexico. FORM AND FUNCTION Microtus quasiater shows no significant intraspecific, geographic, or sexual variation. As in M. ochrogaster and the California vole M. californicus , young males are slightly smaller than same-age females in size and body mass; yet this difference disappears when they reach adulthood (Ramírez- Pulido et al. 1991). This species shows age-related variation in skull morphology and pelage color. Juveniles have a narrower, more downcurved rostrum, a globose braincase lacking ornamentation or crests; the zygomatic arches are not broadly expanded outward; the dorsal profile of the skull is convex due to an anterior nasal curvature. The pelage is dark and dull with a plumbeous hue instead of dark brown (Fig. 1). The skull in subadults is longer, with conspicuous postorbital processes and low lambdoid crests; globose braincases are common, as well as a more developed rostrum; the zygomatic arches are more expanded outward; and the pelage has the characteristic dark brown color of the species. Adults have a more robust cranium and larger rostrum; fully expanded zygomatic arches; the skull appears rounded and all of its crests and processes are fully developed; interorbital ridges may be present. The pelage of adults is dark brown, but darker than the grayer pelage of juveniles (Ramírez-Pulido et al. 1991). The dental formula is: i 1/1, c 0/0, p 0/0, m 3/3, = 16; possessing only 2 closed triangles in the last upper molars (M3) is diagnostic. The last 2 lower molars (m2 and m3) have 3 transverse loops and no closed triangles (Hall 1981). Microtus quasiater has the longest baculum, and the largest base and median process of the 3 American species of the subgenus Pitymys . The baculum was illustrated in Anderson (1960:190) and the following description is based on his description (pp. 208–209). The baculum stalk (main shaft) is broad, its greatest length (2.6–3.2 mm, n = 3) is 1.3–1.6 times the greatest breadth, and 3.3–3.6 times its greatest depth. The medial distal process is 0.25–0.33 the length of the stalk and depressed ventrally. The process is ossified and pressed closely to the tip of the shaft and is wider than deep proximally; terminus is relatively broad. The lateral cartilaginous processes are small and attenuate. In dorsal view, the stalk is broadly rounded, bilobate, or trilobate (Anderson 1960:209). The median lobe is formed by a posterior projection of the dorsal shelf between enlarged lateral tuberosities that form outer lobes. In end view, the dorsal surface is slightly concave, the ventral concavity is broad and deep; the shaft is flattened except the tip that is more cylindrical and bowed dorsally, relatively slender terminally and narrower than median process. The gross stomach morphology of M. quasiater was described briefly by Carleton (1981). The incisura angularis is deep; the edge of the bordering fold is emarginate with short processes projecting over the glandular epithelium and concentrated on the left rim of the fold. The glandular zone is intermediate grade III, possesses a small narrow patch of glandular epithelium on the lesser curvature near the pyloric orifice, and this is in addition to the discoglandular zone on the greater curvature. These morphological characteristics of the stomach are similar across species of Microtus . ONTOGENY AND REPRODUCTION Microtus quasiater is polyestrous and breeds year-round. Pregnant and lactating females can be found in all seasons, and males with scrotal testes are present throughout the year, but most frequently encountered in August (Ramírez-Pulido et al. 1991; López-Wilchis and Torres-Flores 2005). Greatest reproductive activity occurs in the rainy season (late summer and early autumn), but a less intense reproductive peak may occur in late autumn. Reproductive activity is reduced in the dry season (winter), but this pattern may vary from year to year (López- Wilchis and Torres-Flores 2005). Pregnant and lactating females were reported in mid-October from Veracruz (Hall and Dalquest 1963) and pregnant females were found at the end of April in Puebla (Heaney and Birney 1977). Adult males with scrotal testes ranging from 7 to 10 mm in spring and summer ( n = 3) and 6 to 10 mm in autumn and winter ( n = 8) have been reported (Ramírez-Pulido et al. 1991). Testes of subadults measured 5–8 mm in autumn and winter ( n = 4), and 5–10 mm in spring and summer ( n = 7). Heaney and Birney (1977) reported 3 males in April that had mean testis length and width of 8 and 6 mm, respectively. Embryonic litter size varies between 1 and 4 at crownrump lengths between 6 and 35 mm and postpartum litter sizes vary from 1 to 4 with a mean of 1.83 ( n = 18—Ramírez-Pulido et al. 1991). Other litter sizes reported are 1.87 ( n = 8—López- Wilchis and Torres-Flores 2005) and 2.1 ( n = 12—Hall and Dalquest 1963). One of us (RL-W) has observed that young develop rapidly and become indistinguishable from adults at about 1 month of age. ECOLOGY Population characteristics .— The only published account on the population dynamics of Microtus quasiater is the study by López-Wilchis and Torres-Flores (2005) conducted in Apulco, Puebla, between June 1990 and September 1994. During those years, the population was mainly composed of adults (91.9%). Two population peaks were observed within each year, the primary peak in mid-summer and a 2nd in late winter. Lowest population numbers were found during the autumn. The maximum density was 152 individuals/ha and the lowest was 16 individuals/ha. Unlike other microtines, M. quasiater appears to have relatively stable populations, and does not display marked cyclic differences in abundance. Neither seasonal reproductive pattern nor food habits can explain these demographic features. Long-term studies and surveys of other populations are needed to test the generality of these patterns. The sex ratio was 1:1 throughout the year. The residency time in a particular area varied greatly among individuals (range 0–41 months, SD = 8.47), with a longer average residency in males ( X = 5.7 months) than in females ( X = 4.4 months). Most individuals remained less than 6 months, with only a very few individuals recovered in the same area over 9 months, probably because of a high mortality rate or high levels of dispersal. There is a trend to form stable populations in which loss and recruitment of males and females appears to occur in equal proportions. The annual recruitment varied from 17.5% to 23.4% of the total population. Losses of individuals occur with nearly the same frequency in both dry and rainy seasons. The annual losses varied from 10.1% to 18.3%. Although M. quasiater breeds throughout the year, the combination of relatively small litter sizes, the longevity of some individuals, and the low rate of recruitment make the population comparatively stable and lessens the probability of reaching irruptive densities. Maximum longevity documented for this vole was 41 months, the longest recorded in the wild for any species of Microtus (López-Wilchis and Torres-Flores 2005). Space use. — Microtus quasiater is a habitat generalist occurring in mountain cloud forest dominated by sweetgum, pine, and oak, with an understory of perennial herbs, shrubs, and small trees (Hall and Dalquest 1963; Ramírez-Pulido et al. 1991). Within this forest zone, M. quasiater has been found in marshy meadows, grassy swales, grasslands in forest openings, and edges of fruit orchards. It has also been documented in cornfields, cattle pastures, sugarcane plantations, herbaceous secondary growth vegetation resulting from agricultural practices, and along stream an : Published as part of Torres-Flores, José Williams & López-Wilchis, Ricardo, 2018, Microtus quasiater (Rodentia: Cricetidae), pp. 59-66 in Mammalian Species 50 (961) on pages 59-65, DOI: 10.1093/mspecies/sey009, http://zenodo.org/record/4573517 : {"references": ["COUES, E. 1874. Synopsis of the Muridae of North America. Proceedings of Academy National Sciences of Philadelphia 3: 173 - 196.", "COUES, E. 1877. Family Muridae. Pp. 1 - 264 in Monographs of North American Rodentia (E. Coues and J. A. Allen, eds.). Report of United States Geological Survey of the Territories, Vol. XI: 1 - 1091. Government Printing Office, Washington, D. C.", "TRUE, F. W. 1885. A provisional list of the mammals of North and Central America and the West Indian Islands. 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