Myotis grisescens

A bat with unicolored dorsal fur (gray after the mid-summer molt, at other times sometimes chestnut brown or russet); paler below, with hairs darker basally; wing membrane (gray) connects to the foot at the ankle; calcar is unkeeled; total length 80-105 mm; forearm length 40-46 mm; ear length 14-16 mm; tail length 33-45 mm; hind foot 9-12 mm; mass 7-16 g (usually 8-10 g). wingspread 275-300. Distinct sagittal crest on skull. Teeth 38; dentition I 2/3, C 1/1, P 3/3, M 3/3 (Barbour and Davis 1969, Sealander 1979).

Most likely to be confused with M. lucifugus, M. sodalis, M. austroriparius, and M. septentrionalis. Distinguished from these by uniform-colored dorsal fur from base to tip (all others have contrasting shades, bi- or tri-colored dorsal fur) and by attachment of wing membrane at ankle, not at base of toe (Barbour and Davis 1969, Tuttle 1978).

Roost sites are nearly exclusively restricted to caves throughout the year (Hall and Wilson 1966, Barbour and Davis 1969, Tuttle 1976), though only a few percent of available caves are suitable (Tuttle 1979). Winter roosts are in deep vertical caves with domed halls. Large summer colonies utilize caves that trap warm air and provide restricted rooms or domed ceilings; maternity caves often have a stream flowing through them and are separate from the caves used in summer by males.

Occasionally non-cave roost sites are used. Hays and Bingman (1964) reported a colony in a storm sewer in Pittsburg, Kansas and, in 1988, a maternity colony was discovered using a storm sewer in Kansas (Decher and Choate 1988). Harvey and McDaniel (1988) located a maternity colony in a storm sewer in downtown Newark, Independence County, Arkansas. There are occasional reports of mines (Sealander 1979, Thom 1981, Brack et al. 1984, Harvey 1988) and buildings (Gunier and Elder 1971) being used as roost sites.

Winter caves are deep and vertical and provide a large volume of air below the lowest entrance that acts as a cold air trap (Tuttle 1976). Cold air flows in and is trapped during successive winters, providing mean annual temperatures 6 degrees C or more below the above-ground mean annual temperature (Tuttle 1978). Hibernation sites often have multiple entrances, good air flow (Martin 2007), and temperatures of approximately 5-9 C, though 1-4 C may be preferred (Tuttle and Kennedy 2005).

In the summer, maternity colonies prefer caves that act as warm air traps or that provide restricted rooms or domed ceilings that are capable of trapping the combined body heat from thousands of clustered individuals (Tuttle 1975, Tuttle and Stevenson 1977). Cave temperatures range from 14 to 24 C. Undisturbed summer colonies may contain up to 250,000 bats, and average 10,000 to 25,000 (Tuttle 1979). Summer caves are nearly always located within 1 km of a river or reservoir over which the bats forage (Tuttle 1979).

Tuttle (1979) showed that forested areas along the banks of streams and lakes provide important protection for adults and young. Young often feed and take shelter in forest areas near the entrance to cave roosts (Tuttle 1979). Do not feed in areas along rivers or reservoirs where the forest has been cleared (LaVal et al. 1977; Tuttle and Stevenson, in prep.).

Yearlings and adult males segregate into nomadic summer colonies that tend to roost in caves within a few kilometers of ones selected by adult females (Layne 1978).

Forage in loose groups, but become territorial when insect numbers decrease; territories seem to be controlled by reproductively-active females (Tuttle et al.).

Elder and Gunier (1981) determined that the mean annual survival rate is about 70% in males and 73% in females. Stevenson and Tuttle (1981) found that the after-first-year survival rate is about 55 to 85% in relatively undisturbed colonies, and 57 to 66% in disturbed colonies. Mortality is especially high in spring migration when fat reserves and food supply are low (Tuttle and Stevenson 1977).

Mating occurs in September-October. Adult females store sperm through the winter and become pregnant soon after emergence from hibernation (Gutherie and Jeffers 1938, Harvey 1994, Tuttle and Kennedy 2005)). One young is born late in May or in early June (reported as mid-June for Oklahoma; flying as early as late June or early July). In Florida, young are weaned in mid-July (Layne 1978). Larger colonies are more successful in raising young. Most young are able to fly in 20-35 days, depending on colony size. Individual females typically do not produce young until their second year. Recorded maximum longevity approximately 14-17 years but may be longer (Harvey 1992, Tuttle and Kennedy 2005). Maternity colonies include from a few hundred to many thousands of individuals.

Tuttle (1975) showed that growth rates of non-volant young are positively correlated with colony size, probably because increasing numbers of bats clustering together reduce the thermoregulatory cost per individual (Herreid 1963, 1967). In larger colonies, most young begin to fly from 20 to 25 days after birth, while in smaller colonies, or where colonies have been reduced due to disturbance, this time is increased to 30 to 35 days (Tuttle 1976). In severely reduced colonies, the young sometimes die before achieving flight (Tuttle in Brady et al. 1982). For newly volant young, growth rates and survival are inversely proportional to the distance from their roost to the nearest over-water foraging habitat (Tuttle 1976). Although mothers continue to nurse young for a period after the young are flying, juveniles are apparently left to learn how to hunt on their own (Tuttle and Stevenson).