The body of hoary bats is about the size of a fat mouse. Hoary bats weigh 20 to 35 g. The length from the tip of the nose to the end of the tail is 13 to 15 cm. The wingspan is 43 cm. These bats have blunt, rounded noses and small, beady eyes. The ears are short, thick, broad, and rounded. When laid forward they do not reach the nostrils. The hindfoot has thick fur on the upper side. The thumbs are long. These bats have four mammary glands.
Thick, long, soft hair covers the back, extending to the elbow, the border of the undersides of the wings, the underside of the arms, and the lower surface of the tail membrane. The coloring of the back (including the tail membrane) is a mixed brown-gray with a heavy white tinge, giving these bats a frosty appearance. In fact, these bats' scientific name means "frosty or ash colored hairy tail." The individual silky hairs are dark at the base, yellowish in the middle, and black at the end with white tips. The belly of these bats is not heavily frosted. The throat has a distinct yellow patch. The hair on the elbow, at the base of the clawed thumb, and the upper arm is yellowish as well. The ears are yellow with black edges. Brownish fur extends out on the underside of the wing nearly to the wrist.
Juveniles appear nearly grayish, but still have a frosty appearance.
These bats have large, strong teeth.
Hoary bats are the most widespread of all bats in the United States. Though not yet recorded in Alaska, these bats are thought to occur in all 50 states. They range from the tree limit in Canada down to at least Guatemala in Central America, and throughout South America. They are the only bats found in Hawaii. There are records of migrant hoary bats on Southampton Island off of Northern Canada, and from Iceland, Bermuda, and the Orkney Islands off Scotland. They are rare in most of the eastern United States and northern Rockies and common in the Pacific Northwest and prairie states. They are abundant in California, Arizona, and New Mexico, where they winter. They spend the winter in southern California, southeastern United States, Mexico, and Guatemala, but have also been found in Michigan, New York and Connecticut during December and in Indiana during January. This suggests that some may winter farther north than was previously expected.
Sexes are generally only found together in parts of Nebraska, Montana, and the Badlands of South Dakota. Males and females are usually separated during the warmer months in North America, except during the mating season. Females appear to be more concentrated in the western part of North America. There is evidence for a separation of sexes by altitude in California, with females concentrated in the lowlands and coastal valleys and males higher up in the foothills and mountains.
There is disagreement about whether hoary bats prefer trees with needles or trees with broad leaves. Hoary bats are thought to prefer trees at the edge of clearings, but have been found in trees in heavy forests, open wooded glades, and shade trees along urban streets and in city parks.
In North America, the breeding range of hoary bats extends across Canada and northcentral and northeastern United States down to at least Kansas and Kentucky, and perhaps to Arkansas, Louisiana, and Georgia. Hoary bats are thought to mate around the time of autumn migration. Researchers are uncertain about whether mating occurs before, during, or after the southward migration. Courtship is believed to happen while the bats are flying during the day. Mating may also occur at southern wintering grounds.
Mating is followed by delayed fertilization, a process in which the sperm is stored in the female reproductive tract all winter and doesn't fertilize the egg until the next spring. Little is known about the length of these bats' pregnancy. Births appear to occur from the middle of May into early July. Litter size is usually two, but can range from one to four.
Hoary bats give birth to their young while hanging upside down in the leafy shelter of their daytime retreat. The newborns' skin is brown, darker on the body than on the wings, and lighter beneath. The throat and head are much paler and their feet are nearly black. Fine, silver-gray hair covers their back. The hoary bats' ears and eyes are closed at birth and open on days three and twelve, respectively. The young bats are able to fly by the thirty third day. The young cling to the mother in the day, while she sleeps, and hang on a twig or leaf while she hunts at night.
Hoary bats are solitary. They roost 3 to 5 m above ground during the day, usually in the foliage of trees. They prefer thick leaf coverage above and an open area below. They also prefer trees on the edges of clearings. They have been seen roosting in a woodpecker hole in British Columbia, in the nest of a gray squirrel, and under a driftwood plank. Occasionally they are found clinging to the overhangs of buildings and in caves in the late summer. They often have trouble finding their way out of the caves and die there.
Hoary bats reach their peak activity at about five hours after sunset, although they may occasionally be seen flying on warm winter afternoons. Their flight is stong and direct, reaching speeds of thirteen miles/hr. While hunting, they soar and glide. They search for food about the tree tops, along streams and lake shores, and in urban areas where there are lots of trees. These bats stop to rest between meals at night. Feeding is the only time that hoary bats appear to associate with other bat species. Hoary bats often form groups when hunting for insects.
These bats wrap their hairy tail membrane around their curled up bodies for insulation while resting during harsh weather conditions. Their body temperature drops when they are inactive during the day, as well as between feeding flights at night.
Hoary bats can be seen flying in large groups in spring and autumn, during the time of breeding and migration. They are believed to migrate through Florida from late October to late November and from February through early May. Autumn migration occurs in waves, whereas spring migration appears to be less organized. Some hoary bats are believed to remain in the north and hibernate for the winter, rather than moving south of the United States like most do.
Like all Michigan bats, hoary bats use echolocation while flying. They make a shrill, hissing sound when disturbed. They are one of the only bats in their family to make an audible chatter during flight.
Moths make up the bulk of the diet of hoary bats. These bats are also known to feed on flies, beetles, small wasps and their relatives, grasshoppers, termites, and dragonflies. The bat approaches the insect from behind, taking the rear portion in its mouth and biting off and swallowing this area of the insect, while dropping the wings and head. In comparison to other bats, hoary bats feed on relatively few kinds of insects. On rare occasions, these bats have been observed to feed on leaves, grass, shed snake skin, and eastern pipistrelles.
The main enemies of hoary bats are hawks and owls. American kestrels and rat snakes have on rare occasions been reported to feed on hoary bats. These bats are also known to become entangled in barbed wire fences. Another important source of mortality is females falling out of their roost with attached young, thus becoming easy prey for predators on the ground.
Hoary bats have an important ecosystem role as insect consumers. These bats are often infested with several different types of parasites, including mites, worms, and microorganisms.
Hoary bats occasionally hang out under overhangs of houses and garages, but this is only menacing and they rarely cause any true disturbance to homeowners.
These bats often carry rabies. In some years, 25% of sick bats collected were found to be rabid.
Hoary bats prey on many insect species that are considered to be pests.
Hoary bats are widespread and secure over much of their range. One subspecies, the Hawaiian hoary bat (Lasiurus cinereus semotus), is listed as endangered by the U.S. Fish and Wildlife Service.
Temperate North American bats are now threatened by a fungal disease called “white-nose syndrome.” This disease has devastated eastern North American bat populations at hibernation sites since 2007. The fungus, Geomyces destructans, grows best in cold, humid conditions that are typical of many bat hibernacula. The fungus grows on, and in some cases invades, the bodies of hibernating bats and seems to result in disturbance from hibernation, causing a debilitating loss of important metabolic resources and mass deaths. Mortality rates at some hibernation sites have been as high as 90%. While there are currently no reports of Lasiurus cinereus mortalities as a result of white-nose syndrome, the disease continues to expand its range in North America. (Cryan, 2010; National Park Service, Wildlife Health Center, 2010)
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