General Introduction
Last Update/Review: June 17, 2002
Bats constitute nearly one-quarter of all the known mammalian species in the world. They are nocturnal furbearing animals which feed their young milk produced in mammary glands. Bats are the only mammals to have achieved true flight and, thus, have many unique features. The bones of the hand are elongated to support the thin double membrane of skin which forms the wings (Figure 1). (The scientific name for the group — Order Chiroptera, means hand-wing). In most cases, there is also a membrane between the hind legs and the tail. This arrangement provides for amazing maneuverability, so that most bats are more adept at flying than many birds. The flight membranes are also used to collect insects before they are eaten and to catch the newborn bats at birth. Some bats use their wings as raincoats while others use them as blankets. Although well adapted for flying, bats spend most of their lives hanging upside down asleep.
The navigational system of bats consists of a highly developed sonar or echo-location system. Ultrasonic sound waves produced in rapid pulses ranging from 20 to more than 500 pulses per second are bounced off objects around the bat. Interpretation of the returning sound waves (the echo) allows the bat to determine the distance, speed, direction, texture, and size of an object. This system of echo-location is extremely precise and individual bats have a remarkable ability to identify their own signals and avoid confusion even when surrounded by thousands of other echo-locating bats. Vision is also important in navigating at short range. The aphorism "blind as a bat" has no factual basis.
Nine species of bats have been identified in Alberta; however, only three species are encountered frequently by people. The remaining species reside as solitary individuals where people seldom see them, or else they are easily confused with the more common species.
All bats in Alberta are insectivorous and feed almost exclusively on flying insects. (They are the only major predator of night-flying insects.) As bats have a high metabolic rate, they must feed voraciously in order to meet the high energy demands of flight, thermoregulation, and reproduction. A typical colony of little brown bats (Myotis lucifugus) can consume up to 50 kg (110 lb) of insects during one summer. An individual little brown bat has been reported catching and consuming 900 insects in an hour! Some prey species, particularly moths, have developed methods of detecting bat sonar and respond with patterns of evasive behavior to try to avoid the predator. A few even produce their own sonar in an attempt to jam that of the bat or to warn of their distastefulness. However, bats remain very efficient feeders.
In Alberta, bats are near the northern limit of their distributional range. Since their food source is abundant in summer but absent in winter, bats exhibit a seasonal pattern in their life history. They are true hibernators and spend from 5 to 7 months in a state of torpor. Hibernating bats do not feed or drink, and their body temperature and metabolic rate decreases markedly. In this energy-efficient state, bats will sleep away the long northern winters.
Bats in Alberta are all migratory, although the extent of migration differs among species. Individuals seldom spend summer and winter in the same roost. During the summer, most bats roost in colonies in buildings. They generally prefer small dark spaces that are poorly ventilated and heat up during the day. The optimum temperature in these roosts is 39° to 42°C. Bats commonly live in eaves and attics, behind facing boards, and around chimneys (Figure 2). They may take up temporary roosts behind shutters and sliding doors, under shingles and sidings or even in the open on walls. Roosting sites usually are within a few hundred metres of a source of water. The water provides moisture for drinking which the bats obtain by swooping low over the surface. It also supplies an aquatic habitat to support high concentrations of insects.
There is little information about the winter habits of bats in Alberta. A few hibernating bats have been found in natural caves in the mountains and a few individuals of one species have been located in buildings. Some species spend the winter in a milder southern climate. However, it is not known specifically where the majority of our bats spend the winter.
Bats are extremely energy-efficient. During the summer they rely on the sun to heat the roost and, thus, promote the rapid growth of young bats. Although they are warm-blooded animals, they can withstand a wide range of body temperatures. This allows them to take advantage of the heat energy on warm days, while on cold days during the winter their body temperature drops as they enter a state of torpor to conserve energy.
Very few animals eat bats as a main part of their diet. The major predators in Alberta are great horned owls (Bubo virginianus) and humans. Many bats are killed by people, either intentionally by homeowners and vandals, or unintentionally by people destroying buildings and other roost sites. Although some bats may survive for up to 30 years, most die in their first or second year.
