Little Brown Bat
Last Update/Review: June 17, 2002
The
little brown bat (Myotis lucifugus) is the most common bat in
Alberta (Figure 3). The provincial population is estimated at 1 to
1.5 million individuals. The adult females are relatively conspicuous
as they roost together in colonies in occupied or unoccupied buildings
throughout the province. The colonies are common in central Alberta
and may be found in houses, schools, churches, garages, barns, shopping
malls and office towers.
Summer cottages close to bodies of water, for example Wabamun Lake, Pigeon Lake, Gull Lake, Beauvais Lake, are often colonized by little brown bats. The largest colonies may be in buildings in good repair. Females are usually larger then males. On average, adult little brown bats weigh approximately 8.5 g (0.3 oz) and have a wing-span of up to 20 cm (8 in).
The life history of little brown bats in Alberta is presented in Figure 4. Adult females appear at summer roosts in early May. During the summer colonies consist almost exclusively of pregnant or lactating females and their young (thus the term "nursery" or maternal colonies). Colonies can consist of 3 to 1100 bats but typically have from 50 to 300 individuals. Up to 98 percent of the females are pregnant each year; after a gestation period of 50-60 days, each female gives birth to a single young in late June or early July. The young are born blind, hairless, and unable to fly. They are completely reliant upon the female for care and maintenance during the first three weeks of life. The females are capable of identifying their own young and this is no doubt aided by the continuous squeaking of the juveniles whenever the female is absent. By three weeks of age the young bats generally are able to fly and begin to feed themselves.
Nursery colonies are characterized by hot dark conditions. They are poorly ventilated but often have more than one entrance. The high temperature promotes the rapid growth of young bats. The females exhibit high site fidelity and return to the same nursery roost year after year. Females relocated to other roosts during the summer rapidly returned to the original roost.
Little is known about where or how male little brown bats spend their time. A few have been found roosting individually in concealed roosts throughout the summer and are, consequently, difficult to locate. Similarly, juvenile females in Alberta are seldom found during their second summer but apparently join the other adult females after this time.
In the summer roosts, bats are often torpid or inactive during the day, especially during cool weather. Activity begins shortly before nightfall as the bats prepare to emerge for the evening feeding period. Not all individuals leave the roost each time. The bats return to the roost during the late evening, but leave again for another feeding period near dawn. They all return to the roost before sunrise and remain inside during the day.
Throughout August and September, many individual bats are seen flying at night or resting on walls and in alcoves during the day. At this time it is easy to find bats on buildings along the main street of many communities such as Edmonton, Innisfail, Lac la Biche or Slave Lake. Red brick buildings seem to be preferred. The use of exposed roosts by bats may be more common in the northern part of their range and may be an adaptation to cooler temperatures. Direct sunlight may be important in providing heat energy during cool days.
In the fall, bats of all ages and both sexes rendezvous at caves in the mountains. This is the only period when large numbers of males have been found. This swarming activity is probably important in bringing large numbers of bats together for breeding. During this period, there is a rapid turnover of individuals at each cave which maximizes the exchange of diverse genetic material within the population. Sperm are stored by the hibernating females and fertilization occurs in spring.
Where most little brown bats hibernate in Alberta is not known. Hibernacula usually are cool, dark, humid places in which the air does not move and, thus, the environment is very stable. In the eastern United States, little brown bats hibernate in caves and abandoned mines. In Alberta, a few bats have been found in caves at Cadomin and Nordegg. No doubt other caves in the mountains or foothills also serve as hibernacula, but many bats may migrate to the northern United States or southern British Columbia. A few little brown bats have been found hibernating in caves in Wood Buffalo National Park. Males predominate in the winter roosts and the location of most females during the winter is unknown.
