Myotis sodalis

Pelage very fine and fluffy, dull grayish chestnut above (hair tips slightly glossy; basal two-thirds blackish, followed by a grayish band and cinnamon tip), pinkish white underparts; membranes and ears blackish-brown; total length 75-102 mm; tail length 27-44 mm; wingspread 240-267 mm; length of head and body 41-49 mm; ear 10-15 mm, does not extend past end of nose when laid forward; forearm 36-41 mm; calcar obviously keeled (not always evident in dried study skins); hind foot small, 7-11 mm, hairs do not extend beyond toes; mass 5-11 g; greatest length of skull 14.2-15.0 mm, usually greater than 14.5 mm; length of maxillary toothrow 5.2-5.6 mm; complete sagittal crest usually present in adults; dentition I 2/3, C 1/1, P 3/3, M 3/3 (Hall 1981, Barbour and Davis 1969, Sealander 1979).

Differs from M. grisescens, M. lucifugus, M. velifer, and M. austroriparius in having an obviously keeled calcar. Has shorter ears than does M. septentrionalis (in which the ears extend more than 2 mm beyond the nose when laid forward). Lacks the distinct black face mask of M. leibii, which has a shorter skull (greatest length 13.1-14.7 mm, usually less than 14.5 mm). Skull is smaller, narrower, and lower than that of M. lucifugus. Myotis lucifugus has long hairs on toes, foot length greater than 10 mm, fur with a glossy sheen. Myotis grisescens has fur the same color from base to tip, and web attached to ankle. Myotis austroriparius has long hairs on toes and foot is greater than 10 mm. Pipistrellus subflavus is smaller, with a lightly colored forearm, partially furred tail membrane, and paler fur. See Barbour and Davis (1969).

Myotis sodalis hibernates primarily in caves (about 70 percent of population), also in mines and in one dam and one tunnel (USFWS 2009). Maternity sites generally are behind loose bark of dead or dying trees or in tree cavities (Menzel et al. 2001). Foraging habitats include riparian areas, upland forests, ponds, and fields (Menzel et al. 2001), but forested landscapes are the most important habitat in agricultural landscapes (Menzel et al. 2005).

In hibernation, limestone caves with pools are preferred. Hall (1962) noted that preferred caves are of medium size with large, shallow passageways. Roosts usually are in the coldest part of the cave. Preferred sites have a mean midwinter air temperature of 4-8 C (tolerates much broader range) (Hall 1962, Henshaw and Folk 1966), well below that of caves that are not chosen (Clawson et al. 1980). Roost sites within caves may shift such that bats remain in the coldest area (Clawson et al. 1980); individuals may move from a location deeper in the cave to a site nearer the entrance as the cold season progresses; they may move away from areas that go below freezing. Hibernation in the coldest parts of the cave ensures a sufficiently low metabolic rate so that the fat reserves last through the six-month hibernation period (Henshaw and Folk 1966, Humphrey 1978). Relative humidity in occupied caves ranges from 66 to 95% and averages 87% throughout the year (Barbour and Davis 1969, Clawson et al. 1980). Because of these requirements, M. sodalis is highly selective of hibernacula.

During the fall, when these bats swarm and mate at their hibernacula, males roost in trees nearby during the day and fly to the cave during the night. In Kentucky, Kiser and Elliott (1996) found males roosting primarily in dead trees on upper slopes and ridgetops within 2.4 km of their hibernaculum. During September in West Virginia, males roosted within 5.6 km in trees near ridgetops, and often switched roost trees from day to day (C. Stihler, West Virginia Division of Natural Resources, pers. obs., October 1996, cited in USFWS 1999). Fall roost trees tend to be in sunnier areas rather than being shaded (J. MacGregor, pers. obs., October 1996, cited in USFWS 1999).

In summer, habitat consists of wooded or semi-wooded areas, often but not always along streams. Solitary females or small maternity colonies bear their offspring in hollow trees or under loose bark of living or dead trees (Humphrey et al. 1977, Garner and Gardner 1992). Known roost tree species include elm, oak, beech, hickory, maple, ash, sassafras, birch, sycamore, locust, aspen, cottonwood, pine, and hemlock (Cope et al. 1974, Humphrey et al. 1977, Garner and Gardner 1992, Britzke et al. 2003, Britzke et al. 2006).

Humphrey et al. (1977) determined that dead trees are preferred roost sites and that trees standing in sunny openings are attractive because the air spaces and crevices under the bark are warmer. In Illinois, Garner and Gardner (1992) found that typical roosts were beneath the exfoliating bark of dead trees; other roost sites were beneath the bark of living trees and in cavities of dead trees. Kurta et al. (1993) found a large maternity colony in a dead, hollow, barkless, unshaded sycamore tree in a pasture in Illinois. In Michigan, a reproductively active colony occupied eight different roost trees (all green ash), all of which were exposed to direct sunlight throughout the day; bats roosted beneath loose bark of dead trees (Kurta et al. 1993). In western Virginia, a male used a mature, live, shagbark hickory tree as a diurnal roost; the bat foraged primarily among tree canopies of an 80-year-old oak-hickory forest (Hobson and Holland 1995). In Missouri, primary maternity roosts were in standing dead trees exposed to direct sunlight; there were 1-3 primary roosts per colony; alternate roosts were in living and dead trees that typically were within the shaded forest interior (Callahan et al. 1997). In eastern Tennessee and western North Carolina, several maternity colonies were in sun-exposed conifer snags (roost sites were above the surrounding canopy); some of these snags fell and were not used in subsequent years (Britzke et al. 2003). Rarely maternity colonies have been found in crevices in utility poles or in bat boxes (e.g., Ritzi et al. 2005). See Menzel et al. (2001) for a review of forest habitat relationships.

Though maternity sites have been reported as occurring mainly in riparian and floodplain forests (Humphrey et al. 1977, Garner and Gardner 1992), recent studies indicate that upland habitats are used by maternity colonies much more extensively than previously reported. Garner and Gardner (1992) reported that 38 of 51 roost trees in Illinois occurred in uplands and 13 trees were in floodplains. Of the 47 trees in forested habitat, 27 were in areas having a closed (80-100%) canopy, and 15 were in areas having an intermediate (30-80%) canopy. A single roost tree was found in the following types of habitat: a heavily grazed ridgetop pasture with a few scattered dead trees, a partially wooded swine feedlot, a palustrine wetland with emergent vegetation, a forested island in the Mississippi river, and a clearcut around a segment of an intermittent stream where dead trees were retained for wildlife. Roosts were not found in forests with open canopies (10-30%) or in old fields with less than or equal to 10% canopy cover.

In Illinois, Indiana bats used the same, evidently traditional, roost sites in successive summers. Recapture of the same individuals within traditional roost sites during subsequent summers suggests site fidelity (Garner and Gardner 1992, Gardner et al. 1996).

Relatively few individuals roost in caves at the mouths of which late summer swarming occurs (Cope and Humphrey 1977, Barbour and Davis 1969).

Hibernating individuals characteristically form large, compact clusters of as many as 5,000 individuals (averaging 500 to 1,000 bats per cluster; Hall 1962); the clusters may average 300 individuals per square foot (LaVal and LaVal 1980). Clusters form in the same area in a cave each year, with more than one cluster possible in a particular cave (Hall 1962, Engel et al. 1976). Clustering may perform certain functions, such as protecting the central individuals from temperature changes (Twente 1955), reducing the sensitivity of most bats to external disturbance (Hall 1962), or rapid arousal and escape from predators (Humphrey 1978).

Female survivorship in an Indiana population was 76% for ages 1 to 6 years, and 66% for ages 6 to 10 years (Humphrey and Cope 1977). Male survivorship was 70% for ages 1 to 6 years and 36% for ages 6 to 10 years. Maximum ages of banded individuals were 15 years for females and 14 years for males.

Mating occurs from late August to early October prior to hibernation, or in spring. Bats assemble at cave entrances at dusk and dawn in late August and September. Such staging is believed to facilitate breeding and reduce the chances of inbreeding in small summer colonies (Humphrey and Cope 1977). Males arrive first at the staging areas. Females begin to appear as early as late July, and the number of bats and the proportion of females rise to a maximum in early September. Ovulation takes place after the bats arouse in spring. Delayed fertilization (from sperm stored during the autumn matings) occurs in most reproductively active females (Guthrie 1933). Young are born in June-July. Litter size is 1. Young first fly at 25-37 days. Maximum longevity is about 15 years. A maternity roost in Illinois included up to 95 individuals (including juveniles) (Kurta et al. 1993). Main tree roost in Michigan was inhabited by up to 45 individuals (Kurta et al. 1993).

In Indiana, Humphrey et al. (1977) studied a maternity colony that contained between 25 and 28 reproductively active females. Females arrived at the roost sites starting on May 4, with the majority arriving in late May. Each female bore a single young between June 25 and July 4. The young required about 25 to 37 days before learning to fly; this time interval was dependent on weather, particularly the temperature. Mortality between birth and weaning was about 8%. The authors observed mothers moving non-volant young to warmer roost spots. Apparently, the first flights of the young were tandem flights with the mother.