Bat of the Month
By Lewis Young
The Hoary Bat (Lasiurus cinerus) is one of the most striking and recognizable bats in North America. It is the largest in Montana and one of the largest in the U.S. with a wingspan of about 16 inches. The fur color on its back is a mixture of browns and grays with tinges of white giving it a frosted or hoary look that provides the common name and helps make it easy to identify. Ears are short and rounded with a distinct black edging and the tail membrane is heavily furred on the top. When roosting in cool weather the tail can be pulled up around the bat like a blanket. Relatively long, narrow wings give it a fast, direct flight pattern. Rarely, they can be seen at dusk where they are noticeably larger than most other bats and generally high flyers. Although a “large” bat, they only weigh about 0.7 ounce! Lasiurus is Greek for hairy or shaggy tail, and cinereus is Latin for ash colored—referring to its color. The species was first described in 1796 from a specimen collected in Philadelphia, Pennsylvania.
Hoary Bats roost on trees and tree foliage and are solitary except when females roost with their pups. Being solitary roosters and blending in well, they are seldom observed. They have been documented to return to the same roost tree for several years in some cases. Life span is typically 6-7 years but up to 12 years has been documented.
The Hoary Bat’s diet consists of a variety of insects but they favor larger sized prey such as moths and beetles and like other bat species eat huge numbers of insects every night. Pregnant and lactating females may eat their own weight each night! They prefer to forage in uncluttered environments such as the edge between a meadow and forest, and usually come out to forage later than many species—often an hour after sunset. Like other bat species, they catch their prey in the web of the tail membrane, like a catcher’s mitt, then transfer it to the mouth all in a split second.
Echolocation is used to navigate and find food—even in total darkness. Echolocation is like radar in that the bat sends out a rapid pulse of high frequency sound waves that bounce off objects. Then the return signals are used to judge the location. Their echolocation calls at 20-30 kilohertz are at a frequency above human hearing but Hoary Bats also have a variety of social calls used to communicate with other individuals and these are within human hearing range. Electronic devices are now available that can detect echolocation calls, and some even identify the species.
Hoary Bats usually have 2 pups but it can vary from 1-4. They have 4 nipples allowing more than 2 pupa at a time to be fed. Breeding occurs in the fall but implantation is delayed so that fertilization occurs in the spring. Gestation is about 90 days with pups being born in June. Pups at birth are mostly hairless except for a few patches of fine gray fur. They are capable of flying in approximately 30 days and reach sexual maturity the same year they are born. Although both sexes may inhabit the same general areas during summer males generally are scarce where females are raising their pups. Mothers fly up to 12 miles from the roost for foraging and when returning must find their well-concealed young. They recognize the pup’s call and communicate with them when reuniting. If pups fall from the roost they give distress calls and the mother will retrieve them when possible.
With the widest range of any North American bat, Hoary Bats are found all across Montana and widespread in North America, Central America, and much of South America. It is the only bat found in Hawaii. Although widespread, they are usually in low densities except at times during migration when they may be concentrated in certain migratory pathways. They occur over a broad elevation range from the lowest valley bottoms to over 9000 feet.
A migratory species, Hoary Bats are present in Montana primarily from May or June through September. There are no records of overwintering in Montana. It is not known where Hoary Bats from Montana go in winter. Possibly, they migrate to mild coastal areas, the southern tier of states, and Mexico where they are known to occur in winter months. Males and females typically don’t overlap ranges during summer but do migrate together and breeding occurs at that time.
Conservation concerns exist even though Hoary Bats are considered “Apparently Secure” on a global basis. In Montana they are listed as a Species of Concern. Hoary Bats are the species most commonly killed by turbines at wind energy facilities and make up about 40% of all bat fatalities at those sites in North America. Although the number killed is large (estimated in the hundreds of thousands annually), the overall size of the Hoary Bat population is unknown so the population impact of wind energy associated mortality is uncertain. Fortunately, White-Nose Syndrome (a fungus killing millions of bats in eastern North America) is unlikely to affect Hoary Bats due to their solitary roosting habits outside of caves and mines.
Hoary Bats contribute to the wonderful wildlife diversity in Montana. Although they are not easily observed during their nightly activities, they are widespread, have many fascinating features, and provide an amazing amount of insect control.