Little Brown Myotis
The little brown myotis is the most common, widespread bat of temperate North America.
About This Species
This is a relatively nondescript, medium-sized, small-eared bat best recognized by eliminating the several more distinctive species that might be confused with it. The Yuma myotis is of similar size but is paler in color and the hairs are black metallic grading to burnished tips. The California myotis may be as dark but is considerably smaller; the long-legged myotis is a heavier bat with a keeled calcar (a spur of bone that projects inward from the ankle) and a wing that is furred beneath from body to elbow. The fringed myotis has longer ears (as has the long-eared myotis) and a distinctive fringe of stiff hairs on the trailing edge of the uropatagium.
This is perhaps the most common, widespread bat of temperate North America, and it is one of the best studied. It deserves greater attention in Colorado, however, particularly because former colonies have disappeared or have been reduced in size.
Living with Bats
There are 18 species of bat that call Colorado home. These amazing mammals are not only fantastic insect control, but are also valuable pollinators. Learn more about how humans and bats can live in harmony in our guide.
Bat Conservation
Bats remain one of the most mysterious land mammals, especially in Colorado, where 18 species spend at least part of the year. For example, the little brown bats in our state seem to hibernate differently than those on the east coast, and we are constantly learning about their whereabouts, habitats and behaviors. Our current research suggests that bat populations in Colorado are doing quite well.
One little brown bat can eat up to 18,000 small insects (like mosquitoes) each night. Their economic impact is estimated to be billions of dollars, and much of the agricultural industry relies on their insect control.
Beyond their economic impact, bats flaunt some impressive adaptations. They are the only mammal that truly flies, which requires a very unique anatomy. Their upside-down lifestyle requires adaptations in their “feet” to consistently grip. If people hanged upside down for even an hour, we’d lose consciousness; bats have developed amazing blood pressure accommodations to fit the lifestyle.
We are currently involved in a monitoring partnership called the North American Bat Monitoring Program (NA Bat). The program utilizes acoustic surveys to collect information on bat populations without interfering with their behaviors. We also are using PIT tags, like the chips put in household pets, to monitor a number of maternity sites. While data collection will take years, we’ve already learned new information about Colorado’s bat population, the most heartening of which is that the population appears larger and more widespread than we’d thought. This is an excellent start to learning more about the species, but we’ve got lots of problem-solving and technological innovation left to do.
Our priorities also include learning more about bats so that we are prepared for the potential spread of White-nose Syndrome. Finding and monitoring bats is crucial to this effort, but also comes with its own challenges. Because bats are small, flying mammals, much of the equipment used to track other species is too heavy for them. And during the winter, Colorado bats are very elusive, choosing to hibernate in small groups often in locations that are inaccessible to people.
More Information:
Physical Characteristics
Average length is 97.9 mm; average length of forearm is 39.3 mm. Weights average about seven g, and the wingspan is about 220-270 mm.
Range
The little brown bat ranges across North America, from Alaska across Canada to Newfoundland and south, mostly in forested areas, to central Mexico. In Colorado the species may occur statewide in suitable habitat, ranging as high as 11,000 feet in Lake County. However, in the eastern two-fifths of the state, records are generally limited to towns and cities.
Habitat
This is a species of wooded areas -- including riparian woodland in the mountains and lower valleys -- urban areas, woodlots and shelterbelts. The little brown bat is one of the most tolerant of bats in terms of roost selection. Night roosts are located in tree hollows, beneath tree bark, in or under buildings, bridges, crevices in rock, behind shutters or beneath eaves. They may share roosts with other species of bats. Day roosts in attics or mines may be used by large concentrations of bats. Hibernation sites include rock crevices, caves, mines and to a lesser extent buildings. Winter habits are poorly known in Colorado and elsewhere in the West.
In Ontario, hibernation lasts from September to May. In some parts of the range, the animals may move several hundred kilometers from nursery colonies to hibernacula, but such long-distance movements are not documented in the West. The animals can move 50 miles a night at speeds up to 19 miles per hour. Maximum longevity in these bats may be remarkably long; the current record is some 31 years.
The average lifespan, however, is much shorter, as over half the young die in their first year.
Diet
Little brown bats emerge at dusk to feed, often following the same foraging route repeatedly through the night and on successive nights. They forage over water or among trees, 3 to 6 meters above ground. Foraging flight is erratic for a bat of this size. The diet consists largely of aquatic insects, including caddis flies and midges, but also includes moths, mosquitoes and other flies. The prey is knocked from the air with a wingtip, captured in the membrane between the legs and lifted to the mouth. Foraging behavior has to be learned, and adults are much more efficient feeders than are young; adults fill their stomach in as little as 15 minutes.
Reproduction
A great deal is known about the breeding habits of the little brown bat in the eastern part of its range, but the species has not been studied intensively in the West. Breeding takes place in autumn or early winter. There are two phases, an active phase in which males and females are alert and a passive phase in which males mate with torpid females. Breeding of both sexes is promiscuous. Sperm are stored by the female in the uterus until spring, when fertilization, implantation and gestation take place. Gestation lasts 50 to 60 days, depending on temperatures. The young are born almost always singly in nursery colonies from late May to early June. At birth, little brown bats are blind, but their eyes open in two days. The mother can fly with her offspring attached to a nipple, but the young usually is left in the roost during the mother's foraging flights. Young can fly on their own by 3 weeks and reach adult weight about a month after their first flight. About half the females breed their first autumn. Males breed first as yearlings. Nursery colonies of several hundred females are known. Non-breeding females and males roost away from nursery colonies.
Threats to Species
Predators include raccoons, mink, snakes and owls.