Synaptomys cooperi Baird, 1857

Rose, Robert K. & Linzey, Alicia V., 2021, Synaptomys cooperi (Rodentia: Cricetidae), Mammalian Species 53 (1007), pp. 95-111 : 95-101

Synaptomys cooperi Baird, 1857

Southern Bog Lemming

S [ynaptomys]. cooperi Baird, 1857:558 . Type locality not given; fixed as “Jackson, Carroll County, New Hampshire,” by B. P. Bole, Jr. and P. N. Moulthrop (1942); correct original spelling of S. cooperii Baird, 1857 , by selection ( Coues, 1877:235). First use of the current name combination (see “Nomenclatural Notes”).

Myodes (Synaptomys) cooperii Baird, 1857 :xliv. Incorrect original spelling of S. cooperi Baird, 1857 . Name combination.

Arvicola (Synaptomys) gossii Coues, 1877:235 . Type locality “Neosho Falls,” Woodson County, Kansas.

Synaptomys stonei Rhoads, 1893:53 . Type locality “May’s Landing [Egg River, Atlantic County], N[ew]. J[ersey].”

Synaptomys fatuus Bangs, 1896:47 . Type locality “Lake Edward, Quebec,” Canada.

Synaptomys helaletes Merriam, 1896:59 . Type locality “Dismal Swamp [Norfolk County], Virginia.”

Synaptomys helaletes gossii: Merriam, 1896:60 . Name combination.

Synaptomys cooperi saturatus Bole and Moulthrop 1942:149 . Type locality “Bloomington, McLean County, Illinois.”

Synaptomys cooperi jesseni Long, 1987:324 . Type locality “Swenson Road, N. side of Washington Island, Door County, Wisconsin.”

CONTEXT AND CONTENT. Context as for genus. Seven subspecies are recognized ( Cook 2017, who no longer recognized formerly described subspecies fatuus , jesseni, and saturatus). Wilson and Choate (1997) speculated that S. c. paludis and S. c. relictus might be extinct because no specimens of these biogeographical relicts had been collected since 1942 and 1956, respectively.

S. c. cooperi Baird, 1857:558 . See above.

S. c. gossii ( Coues, 1877:235) . See above.

S. c. helaletes Merriam, 1896:59 . See above.

S. c. kentucki Barbour, 1956:414. Type locality “Sadieville, Scott County, Kentucky.”

S. c. paludis Hibbard and Rinker, 1942:26. Type locality “from the bog area surrounding brooder pond No.1, Meade County State Park, fourteen miles southwest of Meade, Meade County, Kansas.”

S. c. relictus Jones, 1958:387. Type locality “Rock Creek State Fish Hatchery, 5 mi. N, 2 mi. W Parks, Dundy County, Nebraska.”

S. c. stonei Rhoads, 1893:53 . See above.

NOMENCLATURAL NOTES. The year of Baird’s original description of Synaptomys cooperi has been reported as 1857 (e.g., Wilson and Reeder 2005; Integrated Taxonomic Information System 2021) and 1858 (e.g., Hall 1981; Cook 2017). The publication date of Volume 8 on Mammals in the “Reports of explorations and surveys to ascertain the practicable and economical route for a railroad from the Mississippi River to the Pacific Ocean” is clearly 1857 on the title page, and there is no compelling evidence to suggest it was published in 1858 ( Moore 1986; A. L. Gardner, in litt.). Therefore, we changed the date to 1857 from 1858 as given in the first Mammalian Species account for S. cooperi by Linzey (1983). Synaptomys is the combined form of the Greek word syn (juncture) plus haptein (to fasten), which means “to fasten together,” and the Greek word for mouse, mys. Baird (1857) believed that Synaptomys was a link between true lemmings ( Lemmus ) and other mice. Other common names include lemming mouse, Stone lemming mouse, Illinois lemming mouse, and Cooper’s mouse (Mumford and Whitaker 1982).

DIAGNOSIS

The two species of Synaptomys are similar in color and dimensions but S. cooperi has six mammae versus eight in S. borealis (northern bog lemming), and S. cooperi lacks the distinctly rust-colored hairs at the base of the ear, seen in S. borealis . Differences in the skull include closed triangles on the labial (outer) sides of mandibular molars in S. cooperi , but these molars lack closed triangles in S. borealis . The mandibular incisors are large in S. cooperi but relatively slender in S. borealis . Finally, the median tip of the palate is broad and blunt in S. cooperi but is pointed in S. borealis .

Other short-tailed arvicoline rodents with which S. cooperi might be confused include Microtus pennsylvanicus (meadow vole), which is dark brown with a grayish venter and Microtus ochrogaster (prairie vole), which is light brown with a yellowto-orangish venter; adults of all three species are similar in head– body length, 106–131 mm and body mass, 35–45 g ( Cook 2017). But the tails of the voles are twice the length of the hind foot (21–23 mm), compared to the similar size of hind foot and tail (18–20 mm) for S. cooperi . Microtus pinetorum (woodland vole) has an 18–19 mm tail too, but is shorter in head–body length and lighter in mass (25–30 g) than S. cooperi ; further, it has a short velvety chestnut-colored pelage in contrast to the grizzled graybrown coat of S. cooperi . Myodes gapperi is smaller but similar in proportions to meadow and prairie voles but is red-backed, its vole common name.

GENERAL CHARACTERS

Synaptomys cooperi is a comparatively small lemming with small eyes and ears and a short tail ( Fig. 1 ). The head appears large relative to overall body size, and the skull is massive ( Fig. 2 ). Facial whiskers can be erected to form a “facial disk” around the blunt snout. S. cooperi is the only arvicoline rodent in the continental United States with grooved upper incisors (except for the rarer S. borealis ). The short tail is about the same length as the hind foot. Dorsal color ranges from bright chestnut to dark grizzled gray, whereas the venter is silver to light gray. Juvenal pelage is dark gray-brown and is replaced by subadult pelage that resembles the adult pelage but is darker and duller ( Connor 1959). The fresh winter pelage of S. c. cooperi is longer, softer, paler (Bole and Moulthrop 1942), and denser ( Howell 1927) than the dark brown summer pelage. Many adult males have bilateral flank glands, and the hairs above the glands are paler and finer than surrounding hairs ( Quay 1968). An albino S. cooperi with pink eyes was reported by Manville (1955).

Among the subspecies, S. c. kentucki and S. c. cooperi are smallest in overall body size, and S. c. paludis and S. c. relictus are largest. A summary of published measurements (mm; weighted averages for total length, length of tail, length of hind foot, in ascending order of total length) were: 117.2, 18.9, 18.8 for S. c. kentucki (n = 76— Barbour 1956); 118.3, 18.5, 18.5 for S. c. cooperi (n = 177— Bangs 1896; Howell 1927; Wetzel 1955; Whitaker and Hamilton 1998); 120.2, 18.2, 18.6 for S. c. helaletes (n = 33—RKR); 127.0, 21.5, 19.8 for S. c. stonei (n = 54— Howell 1927; Wetzel 1955; Whitaker and Hamilton 1998); 128.8, 19.2, 19.5 for S. c. gossii (n = 98— Wetzel 1955); 143.0, 22.5, 20.0 for S. c. relictus (n = 2— Jones 1964); and 144.4, 21.6, 21.6 for S. c. paludis (n = 8— Wetzel 1955). Ranges of linear measurements (mm) from these same published sources were: total length 94–151, tail 13–25, and hind foot 16–21. Mass of adults was 20.0– 47.4 g. Although Wetzel (1955) noted that sexes did not differ in size, adult male S. c. helaletes in southeastern Virginia were significantly heavier than adult females in a capture–mark–release study (Rose and Ford 2012).

DISTRIBUTION

Synaptomys cooperi is present in the colder regions of eastern North America from northeastern Canada westward to southeastern Manitoba, south through Minnesota, central Nebraska and Kansas, through much of Arkansas, then northward through the Appalachian Mountain region, and eastward to northeastern North Carolina and adjacent Virginia ( Fig. 3 ). The distribution in counties in parts of five states within the southern Appalachian Mountains region, based on records of museum specimens or published accounts, suggests its presence throughout most of Kentucky ( Campbell et al. 2010), but few states or provinces have such detailed information. Within this general geographic distribution, populations often are localized, and presence can change when habitat changes.

Some subspecies have broad distributions: S. c. cooperi broadly in northern and eastern North America, S. c. gossii in the western Great Plains states, and S. c. stonei restricted to higher elevations in the southern Appalachian Mountains region and farther north into New Jersey. The other subspecies have smaller or tiny distributions: S. c. kentucki in Kentucky, S. c. helaletes in eastern Virginia and adjacent North Carolina, S. c. paludis in south-central Kansas, and S. c. relictus in southwestern Nebraska.

Presence of S. cooperi in central Nebraska ( Jones 1964) was confirmed and extended even farther westward by Benedict et al. (2000). Wilson and Choate (1997:453) speculated that S. c. relictus merely represents “the terminus of a geographic cline” within S. c. gossii . S. c. relictus, known only from the type locality in the southwestern corner of Nebraska, has not been recorded since 1956; later attempts to find this subspecies failed (e.g., Benedict et al. 2000). Nevertheless, in June 2014, one of us (RKR) detected green feces at multiple places downstream from the Rock Creek Fish Hatchery, the type locality in Dundy County, so a small population of S. c. relictus probably survives at and near this site.

Similarly, S. c. paludis is known only from the type locality in southern Kansas and has not been collected since 1942 (Wilson and Choate 1997); RKR found no green feces in 1 day of searching at the Meade County fish hatchery site in June 2014.

Synaptomys c. helaletes also has a disjunct distribution, its type locality being Dismal Swamp of southeastern Virginia and adjacent North Carolina ( Merriam 1896). After no captures for 85 years, Rose (1981) caught 13 S. c. helaletes in pitfall traps in the northwestern section of the Great Dismal Swamp National Wildlife Refuge in late winter 1980. Additional field studies indicated that this subspecies was more abundant and widespread than previously believed, occurring up to 170 km south of Dismal Swamp in North Carolina ( Lee et al. 1983; Webster et al. 1984, 1992; Clark et al. 1993) and 80 km west in Virginia ( Rose 2005, 2006, 2011).

FOSSIL RECORD

During the Pleistocene, much of the current distribution of Synaptomys cooperi was occupied by S. borealis , with fossil remains of the boreal species as far south as Kansas and Tennessee ( Hibbard 1954; Guilday et al. 1978). Remains of S. borealis outnumbered those of S. cooperi at sites in Pennsylvania and Virginia but were rare in Tennessee cave deposits ( Guilday et al. 1978). Both species were recovered from a Pennsylvania cave ( Guilday et al. 1977), where a stratigraphic shift in relative numbers indicated that S. cooperi replaced the northern species in late Pleistocene (about 11,000 years ago). Except for late Wisconsinan sites in Tennessee, S. cooperi fossils constituted a small percentage of arvicoline remains in Pennsylvania and Virginia, leading Guilday et al. (1978) to suggest that S. cooperi was uncommon even in precolonial times.

The late Pleistocene distribution of S. cooperi extended much farther south than the present distribution, reaching at least to San Josecito Cave (elevation 2,256 m) near the town of Aramberri, southern Nuevo León, Mexico ( Cushing 1945). A more extensive distribution to the west is suggested by its occurrences at the following sites: Lubbock Lake on the Southern High Plains of Texas ( Johnson 1987); Schulze Cave in Edwards County, Texas ( Hafner 1993); alluvial terraces along the Colorado River at Austin, Texas ( Lundelius 1992); Snowmass Village, Colorado ( Sertich et al. 2011); and Dark Spring Cave in southeastern New Mexico ( Tebedge 1988, as cited in Morgan and Lucas 2006). Many records point to a distributional retraction during the Pleistocene–Holocene transition (Dalquest and Schultz 1992); however, other records, such as those from Lubbock Lake that date to late Quaternary (about 9,000 years ago— Johnson 1987), indicate that its disappearance from some areas did not occur until beyond that transition. Distributional shifts of S. cooperi during Pleistocene were correlated with temperature changes and not with moisture gradients ( Lyons 2003).

Pleistocene Synaptomys australis , such as from Saber-tooth Cave, Citrus County, Florida ( Simpson 1928), also was reported from coastal Georgia (Hulbert and Pratt 1998), several caves in northern Florida ( Olsen 1958), southwestern Kansas ( Hibbard 1955), and several counties in Texas ( Dalquest 1962; Dalquest and Schultz 1992; Kaspar 1992). Olsen (1958), noting that S. australis differed from S. cooperi largely in size, questioned whether S. australis warranted specific designation. Fossil material from Pennsylvania, Virginia, and Tennessee examined by Guilday et al. (1978) indicated that S. cooperi increased in size with decreasing latitude. These authors suggested a size continuum between S. cooperi and the larger S. australis but indicated a need for specimens from Alabama and Georgia. The question of whether the size difference between S. australis and S. cooperi represents clinal variation within a single species has not been resolved.

There are several hypotheses that address the origins of S. cooperi and its relationship to S. borealis . In his revision of the genus, Howell (1927) designated the subgenera Synaptomys for the southern species ( cooperi ) and Mictomys for borealis . Mictomys was elevated to generic status by von Koenigswald and Martin (1984). Although Musser and Carleton (2005) recognized that elevating Mictomys to a genus might be appropriate, they retained Mictomys as a subgenus of Synaptomys , pending further study. Remains assigned to Synaptomys (Mictomys) vetus ( Wilson 1933) were found in late Pliocene deposits in southern Idaho (Ruez and Gensler 2008) and in the oldest interglacial fauna of Kansas ( Hibbard 1941). Recognition of a bog lemming intermediate between Synaptomys and Mictomys in an 840,000-year-old fauna from the Cheetah Room of Hamilton Cave, West Virginia, led Repenning and Grady (1988) to conclude that S. cooperi evolved from Mictomys . Although Martin et al. (2003) acknowledged that evolution from Mictomys was a viable possibility, they argued that S. cooperi might have originated from a newly described species, S. morgani , from the early Pleistocene of Florida.

It is difficult to determine the earliest appearance of S. cooperi in the fossil record due to uncertainty in identification and systematic relationships. The earliest positive identification of fossil S. cooperi , based on two lower molars, dates to 830,000 years ago from the Fryllan Cave and Kitchen Door sites on the Edwards Plateau of Texas (Winkler and Gose 2003). Because 76% of the Cheetah Room sample conforms to S. cooperi (Repenning and Grady 1988) , it could be argued that these remains dating to 840,000 years ago constitute the oldest S. cooperi fossils, but these authors did not make a species assignment. Synaptomys fossils from Hillsborough County, Florida dated to early Pleistocene (1.64 million years ago— Morgan and White 1995) were not identified to species but are similar in size to S. australis . Thus, if S. cooperi and S. australis are found to be conspecific, the oldest S. cooperi fossils date to 1.64 million years ago (Morgan and White 1995). If the two remain distinct species, the origin of S. cooperi is more recent (830,000–840,000 years ago).

FORM AND FUNCTION

Form.— The large skull of Synaptomys cooperi has thick zygomatic processes to accommodate the large masseter muscles needed for teeth to gnaw through woody and herbaceous tissues ( Fig. 2 ). The grooved upper incisors ( Howell 1927:11, figure 2) and the bright orange pigment of each incisor might strengthen the anterior teeth. Mandibular molars have welldeveloped outer reentrant angles and closed triangles on the labial side. Molars are hypsodont but not as high-crowned as in most arvicoline rodents. All 16 teeth are open-rooted and thus ever-growing. The dental formula is i 1/1, c 0/0, p 0/0, m 3/3, total 16, as in other arvicoline rodents. The palate has a poorly developed posterior spinous process.

Published cranial measurements for S. cooperi (mm; weighted averages) were: condylobasilar length, 24.6; lambdoidal width of cranium, 12.9; cranial depth including bullae,10.0; zygomatic breadth, 16.3; interorbital constriction, 3.3; length of maxillary toothrow, 7.2; width of upper incisors, 3.5; incisive foramina, 4.7; length of rostrum, 6.2; rostral width, 5.7; nasal length, 7.2; nasal width, 3.5 ( Howell 1927; Bole and Moulthrop 1942; Wetzel 1955; Jones 1964; Long 1987). The tympanic bullae are prominent but not excessively large.

Skulls of adult S. cooperi from throughout its distribution examined by Wetzel (1955) showed that five (of 13) characteristics were useful in distinguishing among the five subspecies he studied: (1) width of upper incisors; (2) width of nasal bone; (3) condylobasilar length; (4) zygomatic width; and (5) height of skull. A regular increase in dimensions of these features was observed, as follows (arranged from smallest to largest): cooperi , stonei , helaletes , gossii , and paludis. Skull measurements of the more recently described subspecies indicate that relictus ( Jones 1958) is closest in size to paludis, whereas kentucki ( Barbour 1956) is most similar to cooperi .

Before the advent of multivariate analyses of skull or skeletal features and molecular methods were used to form clades, the baculum was an important morphological feature to distinguish among related groups. The baculum of S. c. cooperi ( Dearden 1958:542, figure 1) was described by Hamilton (1946:378) as “the stalk has a broad, somewhat rounded base, a long narrow shaft, and an expanded, knob-like distal portion... digital processes are all small and may be closely appressed or widely separated,” leading him to speculate that Synaptomys is more closely related to Dicrostonyx than to Lemmus . Later, an analysis of several features of the masticatory musculature revealed that Synaptomys and Lemmus are more similar to each other than either is to Dicrostonyx ( Kesner 1980) . Based on features of the myology of the manus, all three genera, plus Lagurus , are more closely related to each other than to other arvicoline rodents ( Kesner 1986). All four of these arvicoline rodents have the reduced hand muscles characteristic of graminivores, but because these features are highly sensitive to selective pressures, parallelism or convergence could equally explain these similarities (M. Kesner, Indiana University of Pennsylvania, pers. comm., 31 October 2019). Genomic analyses, coupled with other methods, would reveal new information about the subspecies of S. cooperi and the relatedness of Synaptomys to other North American genera of arvicoline rodents.

Bones of the pelvic girdle of S. cooperi show sexual dimorphism, with the shape of the ischium of females being triangular compared with oval in males ( Guilday 1951). Ischium and pubis are also thicker in males than in females, and the pubis of females, at its narrowest point, is half the thickness of that of males. The pubic symphysis, connected by a cartilage in both sexes, separates shortly before parturition to expand the birth canal and remains open afterward. The slightly open or open pubic symphysis is a useful feature by which to evaluate the current reproductive status of female S. cooperi in the field, just as descended testes is for males.

Function. —The alimentary canal of Synaptomys cooperi differs from that of prairie and meadow voles, and perhaps other arvicoline rodents, because its large cecum is spiral-shaped rather than one large transparent sac (observed by RKR). The spiral shape of the cecum, like a staircase inside a cylindrical tower, provides much more surface area than the simple sac of prairie or meadow voles. The value of this shape is unclear because the tissue-thin transparent cecum wall has few blood vessels (RKR, pers. obs.) and thus uptake of nutrients is unlikely in this region. Because fermentation and breakdown of cellulose by microbes in the foregut produce simple sugars and amino acids, the longer the digesta remains in the cecum, the longer these degradative processes can occur. Perhaps the value of the spiral shape lies in slowing the passage of digesta even more than the flow through the simple cecum.

Fecal pellets of S. cooperi are bright green and rounded at both ends ( Fig. 4 ), unlike those of Microtus , which are brown or black and tapered at one end ( Burt 1928; Cockrum 1952; Connor 1959; Linzey 1981), suggesting inherent differences in digestive physiology. Interestingly, the difference in the color of feces of Synaptomys (green) and Microtus (brown) is seen even in the digesta in the tissue-thin cecum ( Connor 1959). Reasons for green feces are unclear, but this feature is useful in detecting the presence of S. cooperi , as shown by Rose (2011) who in two nights of livetrapping caught S. cooperi at three of five locations with green feces, verifying the usefulness of green feces in identifying its presence. Similarly, the microdistribution of S. cooperi at study sites in southwestern Virginia was studied by noting the presence of green droppings on dropping boards (Linzey 1984). In southeastern Virginia, the 3‒4 cm discarded sections of American cane ( Arundinaria gigantea ) also foretell the presence of S. cooperi because other local rodents avoid eating this woody grass. Sometimes neat stacks of long sedge or grass leaves also indicate nearby activity by S. cooperi (RKR, pers. obs.).

Synaptomys , like some other arvicoline rodents, has flank glands, first noted by Howell (1927) as pale regions just anterior to the hind limb in old males; almost all adult males but less than 10% of adult females have such glands. The glandular region has parturition, by which time the ligament connecting the pelvic bones has lengthened, expanding the birth canal. After parturition, the ligament shortens, but the pelvic bones remain slightly apart. When not reproductive, the vaginal orifice is again covered with squamous cell epithelium (Whitaker and Hamilton 1998).

enlarged skin gland units, each composed of globular sebaceous acini averaging 150 by 70 microns ( Quay 1968). Histochemical studies ( Quay 1952) indicate that flank glands secrete a lipid material that might contribute to differences in color (paler), texture (finer), and length (longer) of hair compared with hair in nearby regions. Furthermore, the cycle of hair growth (molt) in the glandular region often is out of phase with that of nearby hairs, producing variable patterns among individuals within a season ( Quay 1968).

Little is known about the metabolic rate, digestive efficiency, or daily energy budget of S. cooperi . The one measured caloric efficiency of 4.85% was extremely low, as was the daily energy budget in winter, 12.67 Kcal−1 animal−1 day ( Knopf 1978), seemingly too low for a species that eats low caloric foods, such as mosses, lichens, fungi, bark, and woody tissues, while remaining active year-round in temperate to subarctic environments. Somehow, S. cooperi must be able to extract a large proportion of energy from its foods to survive. Its small size means it has a high metabolic rate ( Kleiber 1961) and also that long insulative fur and thick body fat cannot be part of a strategy to conserve heat. Its small ears, short tail, and dense fur do reduce heat loss, and during winter, it often lives below the snow, thus escaping the worst cold of winter in northern locations. Regardless, efficient digestive physiology must be crucial to winter survival at high latitudes. In southeastern Virginia (37°N), a necropsied S. cooperi has little fat during any season (RKR), so daily energy costs must be paid by daily consumption. In most years, paying the higher energy costs of winter is not difficult in southeastern Virginia because grasses and sedges continue to grow slowly, so green vegetation is available year-round.

Female S. cooperi have six mammae: four pectoral and two inguinal (Whitaker and Hamilton 1998). The vaginal orifice is covered with a layer of squamous cell epithelium during most of pregnancy, becoming perforate only a day or two before

ONTOGENY AND REPRODUCTION

Information on growth and development of Synaptomys cooperi comes from one female that produced six litters in the lab over a 26-week period ( Connor 1959) and from the young of two wild-conceived litters born in the laboratory ( Stegeman 1930). Average interval between litters was 23–26 days ( Connor 1959), suggesting postpartum estrus, which is common in arvicoline rodents. Average measurements for 11 neonates ( Connor 1959) were tail (5.7 mm), length of hind foot (7.9 mm), and body mass (3.9 g). Neonates are pink except for light gray pigmentation on the dorsum. Besides mystacial vibrissae, hair is present on the head and dorsum at birth, eyes are closed, ear pinnae are folded, and claws are present on webbed toes. Ear pinnae unfold by the second day, lower incisors erupt and young are well furred at 6–8 days, and eyes open at 10–11 days. Young are nursed regularly until day 16, and adult size (~ 20 g) is reached by about 3 weeks when grassy nests are permanently vacated ( Connor 1959). Life span in the wild is at least 7–8 months ( Connor 1959; Rose and Ford 2012) but potentially is much longer ( Linzey 1983).

As might be expected for a species with a broad latitudinal distribution, patterns of reproduction differ among geographical regions; northern populations of S. cooperi usually suspend breeding in autumn and resume in spring (e.g., Connor 1953), whereas more southerly populations sometimes breed throughout the winter and can depress breeding activity in summer heat ( Gaines et al. 1977; Robinson 1981; Beasley and Getz 1986; Rose 2005; Rose and Ford 2012). In eastern Illinois, some females showed signs of lactation or pregnancy throughout most of the year, but these breeding indices dropped sharply in two of three summers, suggesting few litters then (Beasley and Getz 1986:390, figure 2). Year-round breeding was observed in eastern Kansas ( Gaines et al. 1977). Although breeding activity usually was higher in summer than in winter, the highest densities, achieved in winter, probably resulted from high levels of breeding and survival in autumn. Both sexes generally had higher survival rates in winter than summer, and indices of early juvenile survival were also higher in winter (e.g., Gaines et al. 1977). Year-round studies of reproduction are lacking from northern states and southern Canada.

In an 18-month capture–mark–release study encompassing two winters in southeastern Virginia, 52% of adult males had descended testes, and 84% of adult females had two of three positive reproductive indices across the study, with monthly values having similar percentages, indicating year-round breeding (Rose and Ford 2012). In a pitfall study conducted in an adjacent county from 6 January to 6 February, two of seven necropsied females were pregnant, and all 13 males were fertile (confirmed by presence of convoluted cauda epididymides), plus two juveniles were trapped, further evidence of winter breeding ( Rose 2005). Based on necropsies of adults taken in pitfall traps in southeastern Virginia, pregnant S. cooperi were recorded in November–June but none in July–October ( Rose 2006), suggesting that breeding is reduced or suspended, perhaps in response to excessive heat. Males were infertile in July–September, lacking epididymal convolutions, another indication of reduced breeding in summer. A lactating female on 22 January indicated that some winter breeding is possible in Indiana (Mumford and Whitaker 1982). In Kentucky, pregnant females were caught only in December–April ( Robinson 1981). Gray juveniles and a female with two embryos were recorded in August ( Connor 1953), indicating summer breeding in New Jersey (no winter trapping was conducted in this study).

Mean litter size of nine necropsied female S. c. helaletes collected in pitfall traps in southeastern Virginia was 2.56 ( Rose 2016), and that of 10 females from earlier studies was 2.3. These litter sizes are smaller than those reported in other subspecies, such as 3.25 (n = 16; range 2–6) in Indiana (S. c. cooperi — Mumford and Whitaker 1982), 3.58 (n = 19; 2–6) in Illinois (S. c. gossii — Hoffmeister 1989), and 3.2 (n = 5) in Kentucky (S. c. stonei — Robinson 1981).

Figures

Fig. 1.—Photograph of Synaptomys cooperi cooperi in a bog near Brighton, Livingston County, Michigan. Note the sphagnum and sundews in the habitat. Photo by Larry Master (www.masterimages.org) used with permission.

Fig. 2.—Lateral views of skull and mandible and ventral and dorsal views of skull of a male Synaptomys cooperi stonei from Spruce Knob, Pocahontas County, West Virginia. Illustration from Linzey (1983).

Fig. 3.—Distribution of Synaptomys cooperi. Subspecies as listed by Cook (2017) are: 1, S. c. cooperi; 2, S. c. gossii; 3, S. c. helaletes; 4, S. c. kentucki; 5, S. c. paludis; 6, S. c. relictus; 7, S. c. stonei. A subspecies determination has not been made for areas of Kentucky and Tennessee designated with a question mark. Map modified from Linzey (1983).

Fig. 4.—Latrine of Synaptomys cooperi showing five bright green fecal pellets in the light, a cluster of green pellets below those, and several molding pellets above. Note also the cuttings at the latrine site. Photo by RKR.