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Little Brown Bat
THE SPECIES: Myotis lucifugus
Under the weak light of a new moon, a little brown bat flies low over the shoreline of Kathleen Lake. The bat scans the air with a slow series of squeaks, listening for the echo to reveal a moth or flying beetle in its path. When a squeak finally bounces back, the bat emits a rapid series of sounds and quickly homes in on the insect. In one quick motion, the bat folds its wing-to-tail membranes forward to form a pouch, scoops the moth from the air, then dips its head and eats the insect.
DISTRIBUTION
Ranging from the east coast to the west, and from the Mexican border to Alaska, the little brown bat is the most widely distributed bat in North America.
A migratory species, it arrives in the southern Yukon in early June and commonly reaches as far north as Dawson City. It is believed that some lone males may venture north of the tree line where they are able to survive the summer. But when autumn arrives, having ventured too far from their winter hibernation sites, they expire.
Maternal colonies use the same attics, abandoned buildings, culverts or hollow logs year after year. These sites are usually located close to water where abundant supplies of insects are available. On the Liard drainage of northern British Columbia, about 50 km from the Yukon border, an unusual colony of little brown bats is known to use a hotspring-heated cave as a maternal colony site.
By late August, most little brown bats have left the Yukon. Although it has not been confirmed, biologists believe that Yukon bats probably spend the winter in snug hibernation sites on the Alaskan coast.
CHARACTERISTICS
In southern bat populations, females and males are the same size. But an Alberta study revealed that among northern populations, females are larger than males. This is explained by the fact that northern bats are born larger than southern bats, which gives them a head start in preparing for their first winter. To give birth to a larger offspring, a female must be larger herself.
Although the bat is not the most physically attractive Yukon mammal, its body is wonderfully adapted to flight. The digits, or finger bones, have become long and hollow to support the wings with maximum strength and minimum weight. The thin membrane that forms the wing is continuous between the forelimbs, the hind limbs and the tail, providing enough surface area to lift a body weight averaging nine grams.
Bats need a height advantage to become airborne, which is one reason why they hang upside down. From this position, all they have to do is release their grip, dive into a swoop and fly away into the night.
By feeding at dusk and throughout the night, a bat avoids competition with insect-eating birds and escapes predation from all birds of prey except owls. But, unlike the owl, the bat does not have a highly developed sense of sight. Instead, it relies on echo-location, or bat radar, to navigate and to locate insect food. In flight, the bat emits a continuous series of sounds that bounce off nearby objects and back to the bat's sensitive ears. Although unique among land mammals, this ability is shared with dolphins and a few other sea mammals.
An important characteristic of the bat is its low rate of metabolism. Most mammals of similar size, such as shrews and voles, have rapid heartbeats and short life spans. But the heartbeat of the little brown bat slows dramatically whenever it is inactive, which is most of the time. This bat hibernates for as long as eight months during the winter, sleeps all day in the summer, and takes frequent rest breaks throughout its nighttime feeding period. With this "take it easy" lifestyle, it is no surprise that little brown bats can live as long as 30 years.
A YEAR IN THE LIFE
Responding to the changing temperatures of spring, female bats are the first to emerge from the hibernation cave, mine shaft or building. The females ovulate, or release their eggs, as soon as they rouse from their winter sleep. If they mated the previous fall, their pregnancy finally begins. In a unique form of reproduction, the male's sperm survives in the female's uterus throughout the winter, waiting up to six months to fertilize an egg.
As soon as they emerge, females begin their migration to the summer maternal colony site. Following the broad river valleys, they fly several hundred kilometres from the Alaskan coast and arrive in the Yukon, at their familiar colony sites, in early June. Located in caves, buildings, culverts or hollow trees, the colonies are found near water bodies that support abundant insects.
A maternal colony contains anywhere from 30 to 1,000 bats, but no adult males. The males roost apart from the colony, singly or in small groups under tree bark, behind house shutters or in other hiding places.
About one hour after sunset, little brown bats waken and fly to their feeding areas that are usually within three or four kilometres of the colony site. Although these bats sometimes feed over land, in the insect canopy just above the tree tops, they more commonly feed on the aquatic insect blooms that are found over water. The bats eat a variety of insects -- moths, flying beetles, mosquitoes -- switching from one species to another as their abundance varies.
It has been calculated that a colony of 100 bats will consume 19 kg (42 lb.) of insects over the course of four months of summer activity. Most feeding occurs along the shorelines of lakes within two metres of the water surface. Insects are captured in the air or plucked from the water. Shorelines are favoured because insect blooms are often found in the quiet air along the lee shore.
Bats pause to roost two or three times during their night of foraging. They roost more frequently and longer when temperatures are below 10 degrees C. Below 5 degrees C, they may roost through the entire night.
It is thought that roosting allows the bats to digest their food before returning for another stomach-full. Males roost singly in trees, but pregnant females return to sheltered nooks and crannies where they can cluster together for warmth. A high body temperature speeds gestation, and the earlier the young are born, the better chance they have to survive their first winter.
Most births occur in late June or early July. When a birth is about to take place, the bat changes its normal head-down position and hangs from its thumbs. The newborn bat emerges tail first, drops into the same membrane-formed pouch that scoops insects, then climbs up its mother's body to nurse. For two or three days, the new mother flies on her nightly feeding forays with the young clinging to her underside. Later, the young remain together in their communal nursery while their mothers forage.
In late August, the bats prepare to migrate to their hibernation sites. Hundreds or even thousands of bats may swarm around the entrance to the maternal colony before the migration begins. The same hibernation sites are used every year. They are characterized by temperatures a degree or two above freezing and a constant humidity. Although bats may rouse to change position or to drink during the winter, they do not eat, but instead survive on stored body fat.
Before settling in for the winter, the bats mate in the hibernation cave. Mating successfully occurs between active males and active or dormant females.
BATS AND PEOPLE
The bat is one of the few mammals that has prospered as a result of human settlement. It is thought that bat populations are limited by the availability of suitable colony and hibernation sites. Today, buildings, mine shafts and other human workings are heavily used. If a building is still in use, the odours and stains of bat guano can be a problem. To get rid of the bats, wait until they leave for the night or for the season, then block up all openings to the site. A little brown bat can enter a hole as small as 1.6 x 2.2 cm.
VIEWING OPPORTUNITIES
Bats will be found near water bodies where insects are abundant. Like their insect prey, the bats will be most active above 10 degrees C and almost totally inactive below 5 degrees C.
If, at dusk, you spot bats making feeding passes over a certain lake or river, remember that the maternal colony is likely within four kilometres of your location. Good luck in finding it.