Myotis septentrionalis

Methods of aging individuals by morphological features are limited. Juveniles can be distinguished from adults by the incomplete ossification of the phalangeal epiphyses until late August of the year of their birth (Kunz 1971, Caire et al. 1979). After that time, all are typically classified as adults. Degree of wear of the teeth has been used to determine relative age of adults (Guthrie 1933), though Hall et al. (1957) found this to be unreliable. Examination of canine and molars of individuals known to be at least 18-19 years of age revealed very little wear.

In Missouri, prehibernation fat deposition period occurs from August to October and results in an increase in average weight of 41-45% (Caire et al. 1979). By spring, the same study found that both sexes weighed approximately what they had before the prehibernation fat deposition period.

This bat generally is associated with old-growth forests composed of trees 100 years old or older. It relies on intact interior forest habitat, with low edge-to-interior ratios. Relevant late-successional forest features include a high percentage of old trees, uneven forest structure (resulting in multilayered vertical structure), single and multiple tree-fall gaps, standing snags, and woody debris. These late successional forest characteristics may be favored for several reasons, including the large number of partially dead or decaying trees that the species uses for breeding, summer day roosting, and foraging. [Source: USFWS 2011, which see for citations of further literature]

Small, highly fragmented, or young forests that provide limited areas of subcanopy foraging habitat may not be suitable. Young forests may also lack appropriate nursery sites. However, recent studies indicate that these bats can exploit relatively isolated and small forest fragments (Caceres and Barclay 2000, Henderson et al. 2008, Johnson et al. 2008).

Foraging occurs within forests, along forest edges, over forest clearings, and occasionally over ponds (Ammerman et al. 2012). Eleven individuals (10 males, 1 female) tagged with chemical lights observed during the summer in Missouri (LaVal et al. 1977), foraged almost exclusively among the trees of hillside and ridge forests, rather than utilizing floodplain and riparian forests; frequently foraging occurred within 1 to 3 m of the ground. Foraging bats doubled back frequently and only slowly moved out of the observation area. In Iowa, Kunz (1973, 1971) found primarily females foraging in mature deciduous uplands with adjacent deep ravines and in a disturbed riparian area with an adjacent floodplain and agricultural lands.

Hibernation occurs primarily in caves, mines, and tunnels, typically those with large passages and entrances, relatively constant and cool temperatures, high humidity, and no air currents (Griffin 1940, Jackson 1961, Mumford and Cope 1964, Kurta 1982, Raesly and Gates 1987, Caceres and Pybus 1997, USFWS 2013). Hibernators frequently roost in crevices, drill holes, and similar sites (Griffin 1940, Layne 1958, Pearson 1962, Caire et al. 1979, Whitaker and Mumford 2009) where they may be overlooked during surveys, but roosting in the open is not uncommon (Barbour and Davis 1969, Whitaker and Mumford 2009). A lack of suitable hibernacula may prevent occupancy of areas that otherwise have adequate habitat (Kurta 1982).

Use of different types of hibernacula can vary considerably among areas, depending upon quality and availability of sites. In a study of 71 potential hibernation sites, including large and small caves, overhangs, and mines, on the Shawnee National Forest in southern Illinois (Whitaker and Winter 1977), mines were the only occupied habitat. Mines also are the principal hibernation sites in northern Michigan where there are no caves (Stones 1981). In the northeastern U.S., hibernation sites include mines and caves (Griffin 1940, Hall et al. 1957) as well as large, cavelike water conduit tunnels (G. Hammerson, pers. obs.; T. French, cited by USFWS 2013).

The principal requirements of a suitable hibernation site are winter-long, low temperatures above freezing, high humidity, and lack of disturbances, both natural (floods) and anthropogenic (visitation) (Barbour and Davis 1969, Hitchcock 1949). At least two studies have provided contradictory information on thermal habitat preferences, suggesting that warmer temperatures sometimes are selected or at least tolerated. In Illinois, Pearson (1962) found that the mean temperature at hibernation sites averaged 9.7 C. Stones (1981) studied the occurrence of bats in northern Michigan mines that were vertically thermally stratified. The mean ambient temperature was 5.9 C, with 43% of the population occurring in the range 7-8 C and 6.5% occurring in the range 9-11 C.

There appears to be a high degree of philopatry in hibernaculum use. In Missouri, over 90% of recaptured banded individuals, representing 5% of the original banded population of 945 (753 males and 192 females), were recaptured at their cave of origin (Caire et al. 1979). Mills (1971) recaptured 4.8% of 358 individuals at their cave of origin the year after banding. Griffin (1945) found that of over 13,000 banded bats of various species, of which about 8,500 were banded in their winter hibernacula, the ratio was 100:1 for bats that were observed to return to their cave of origin over subsequent winters vs. those that were recaptured elsewhere.

Night roosts used in summer between foraging bouts are in different habitats than day roosts. Caves, mines, and quarry tunnels are used as night roosts, typically by males, but also by nonreproductive females (Clark et al. 1987, Jones et al. 1967). They are joined later in the summer by juveniles and post-lactating females (Kunz, 1973). During the day, these same sites usually house no M. septentrionalis. Daytime observations typically are of individuals in crevices or hollows or under loose bark on trees (Foster and Kurta 1999) and in a variety of small spaces associated with buildings and other structures (Hoffmeister 1989, Caire et al. 1979, Hamilton and Whitaker 1979, Barbour and Davis 1969). At times M. septentrionalis has been found in or around caves on summer nights, but not actually roosting in them (Mills 1971). Early in the summer, these groups mostly comprise males, with females and young-of-the-year joining later in the season (Caire et al. 1979).

Nothing has been published on the fidelity of individuals or colonies to particular swarming sites, nor the relationship of swarming site selection to hibernaculum and summer roost selection. Given the low numbers found in most hibernacula and summer night roosts relative to the higher numbers found at swarming sites, it appears that certain caves serve as congregation points for fall mating activity. However, short-term banding returns at swarming sites are very low, indicating movement among swarming sites (Kurta, pers. comm.).

Most nursery colonies are in cavities or beneath loose bark in trees or snags in upland forests, with roost entrances generally below or within the tree canopy (Mumford and Cope 1964, Sasse and Perkins 1996, Lacki and Schwierjohann 2001, Menzel et al. 2002, Owen et al. 2002, Carter and Feldhamer 2005, Perry and Thill 2007, Lacki et al. 2009, Timpone et al. 2010, Silvis et al. 2012). Reproductive females use a wide range of tree species. For example, in summer in north-central Kentucky, Silvis et al. (2012) tracked 58 females to 105 roost trees of 21 species; sassafras was used as a day roost more than expected based on forest stand-level availability and accounted for 48.6 percent of all observed day roosts. Individuals frequently switch roosts (Menzel et al. 2002, Owen et al. 2002, Carter and Feldhamer 2005, Timpone et al. 2010).

Some summer roosts are in buildings or bat houses or under bridges (Brandon 1961, Barbour and Davis 1969, Cope and Humphrey 1972, Amelon and Burhans 2006, Whitaker and Mumford 2009). A large colony in a barn in Indiana (Cope and Humphrey 1972) on 22 June had 24 adult females, 12 immature females, and 18 immature males; 10 other adults escaped. Of the 24 females, 23 were lactating and 1 was pregnant. Roosts of males and nonreproductive females include tree hollows as well as cooler locations, including caves and mines (Barbour and Davis 1969, Amelon and Burhans 2006). In Arkansas, pine snags were important summer roosts for males (Perry and Thill 2007).

In West Virginia, these bats formed social groups whose roost areas and roost tree networks overlapped to some extent (Johnson et al. 2012). Most networks had a single central node roost tree. In control treatments, central node roost trees were in early stages of decay and surrounded by greater basal area than other trees within the networks. In prescribed fire treatments, central node roost trees were small in diameter, low in the forest canopy, and surrounded by low basal area compared to other trees in networks. The results indicated that forest disturbances, including prescribed fire, can affect availability and distribution of roosts within roost tree networks.

Syntopic species during hibernation include Myotis lucifugus, Pipistrellus subflavus, and Eptesicus fuscus. Myotis septentrionalis; generally comprises a small percentage (for example, <1% in Missouri, 6% in Quebec-Ontario, 8% in Michigan, 10% in New England, 15% in Illinois) of the bats found hibernating in any single site (Griffin 1940, Hitchcock 1949, Pearson 1962, Caire et al. 1979, Stones 1981). Summer surveys reveal similar figures. In a netting survey of Iowa bats utilizing stream corridors for foraging, Kunz (1973) captured 64 Myotis septentrionalis over three years, out of an eight-species sample totaling 540 individuals (12%); Myotis septentrionalis was the third most abundant species, ranking far behind Eptesicus fuscus (243) and Lasiurus borealis (124). At Renfrew mine, Fenton (1969) found 117 Myotis septentrionalis compared to 5,712 Myotis lucifugus.

Rarely are there more than 100 individuals per hibernation colony (Barbour and Davis 1969, Caire et al. 1979). However, Stones (1981), found over 100 individuals (mean = 226) in 5 of 21 mines in which M. septentrionalis occurred in northern Michigan. In that study, 73% of the entire population was found in 5 mines and 86% in 8 mines of the 21 mines containing the species. Individuals usually roost solitarily.

In summer, these bats generally are colonial, but reproductive females and juveniles often roost alone. As many as 60 adults have been found in a single tree (Foster and Kurta 1999).

No single population of significant size has been studied intensively or long enough to determine population structure. Habitat utilization biases are reflected in sex ratios of animals captured during the summer, when females are more frequently taken near streams and males are more frequently taken at caves. Sex ratio data from hibernacula are more consistent. Griffin (1940) reported on sex ratios from New England hibernacula, where he found males comprised 77.8% of a sample population of 877 individuals over an 8 year period. In southern Illinois, Pearson (1962) found 72% males among the groups hibernating in silica mines and Hitchcock (1949) found that 76.0% of 242 individuals hibernating in eastern Canada were males and that the sexes did not segregate during hibernation. In northern Michigan, males comprised 60% of winter populations and were more abundant than females in all but 3 of 21 mines searched (Stones 1981).

The disparity in the sex ratio appears to be quite consistent among studies, seasons, and sites. Griffin (1940) suggested that females may have a higher mortality rate than males and consequently, a shorter life span and lower representation in the population. Hitchcock's (1949) original data recently were statistically analyzed and shown to support this hypothesis (Hitchcock et al. 1984).

Although age structure is not known for any population, potential longevity is at least two decades. Hall et al. (1957) reported one banded M. septentrionalis that was found recently dead in the cave in which it had been banded almost 19 years earlier.

In West Virginia, foraging home ranges of seven females averaged 61.1 hectares (Menzel et al. 1999).

In Michigan, radio-tagged bats in spring-summer changed roosts every 2 days; distance between roosts was 6-2000 m (Foster and Kurta 1999).

In an experiment to determine the homing ability of blinded and deafened bats, a blinded individual returned 32 miles to its home cave in 2.5 hr. after being held in captivity for 3 days (Stones and Branick 1969). The return rate of this animal was at an average, straight-line speed of 12.8 miles per hour. Overall, blinded bats returned to their home cave at the same frequency as did the control animals over the 6-week period following their release. However, none of the bats with impaired hearing returned during that time.

No significant predators are known (Baker 1983). Reported parasites include chiggers, mites, and trematodes (Whitaker and Winter 1977, Whitaker and Mumford 1971).

Copulation occurs in the late summer and early fall, during the swarming period when large numbers of bats congregate in and near certain caves (Baker 1983, Kurta 1980). Females store sperm during hibernation, though some may copulate again at spring emergence (Guthrie 1933, Racey 1982). Guthrie (1933) found a portion of the males of some species to be reproductively active in late winter and early spring. However, males emerging from hibernation in Missouri were found to be reproductively inactive (undescended testes) until late July, with the largest percentage of males becoming reproductively active in August and September (Caire et al. 1979). Females ovulate at the time of emergence and parturition occurs 50-60 days later (Baker 1983). Later parturition dates at higher latitudes are due to later emergence and therefore later ovulation (Racey 1982).

Females bear a single young, with parturition occurring in late May or early June in Missouri and Oklahoma (Caire et al 1979, Easterla 1965, Caire et al. 1989), in early to late June in Indiana (Cope and Humphrey 1972), and in late June to early July in Iowa, Illinois, Michigan, and New York (Kunz 1971, Hoffmeister 1989, Kurta 1980, Hamilton and Whitaker 1979). Post-lactating females were observed by mid-June in Missouri (Caire et al. 1979) and by mid- July to late July in Michigan and Iowa (Kurta 1980, Kunz 1971), with volant young observed at about that time in all studies. Young-of-the-year may reproduce in their first fall, but the proportion of the cohort doing so is unknown (Kurta, pers. comm.). Nursery colonies are relatively small, most often including 2-30 adults (10-90 individuals, including young, according to Layne (1978)).