Bird Animal of the Month

by Lewis Young

Little Brown Myotis bat
Little Brown Myotis – Photos Credit: Lewis Young

The Little Brown Myotis (Myotis lucifugus), also known as the Little Brown Bat, is the most common bat in Montana and one of the most common in the United States until recent years. It is well named since it is small and normally some shade of brown. Colors include cinnamon-buff to dark brown above, buffy to pale gray below, and with dark brown ears and wing membranes. Their wingspan is about 10 inches and they weigh about 6 grams (0.01 pound or 0.2 ounce). Life span is probably 15-20 years but may exceed 30. One was recaptured in Wisconsin that was 39 years old.

Little Brown Myotis, like most species of bats, have amazing adaptations for being active in the dark and surviving winter and are often under-appreciated for their importance to ecosystems perhaps due to lack of understanding and factual information about them. They are the bat species mostly likely to come into contact with people due to their summer use of houses, barns, and other human structures. They also roost in tree cavities especially dead trees, caves, mines, and cracks in rock outcrops and cliff faces. Hundreds of bats may be found together in good summer roosts. Males and females are mostly segregated during summer and in Montana, known maternity roosts are in buildings. If it is desired to exclude bats from buildings, non-lethal techniques are available through Montana Department of Fish Wildlife, and Parks; online through bat conservation organizations such as Bat Conservation International; and at knowledgeable private wildlife control services.

A high percentage of their diet is aquatic insects, as they like to forage over and near water. Typically they forage for the first few hours of the night then go to a night roost where they rest and digest before foraging again before sunrise. They are especially adept at foraging through swarms of insects, and a single Little Brown Myotis may eat 1,200 mosquito-sized insects in one hour. Enormous numbers of insects are consumed nightly by these bats. Normally they feed between 3 and 15 feet off the water or ground and fly in an erratic pattern as they detect and chase prey. Prey is often caught with the tip of the wing and rapidly transferred to the mouth unlike most bats that catch their prey in the web of the tail before transferring to the mouth.

Echolocation is used for both navigation and detection of prey items. Calls at frequencies of 40 to 80 kilohertz, well above human hearing capabilities, are sent out and the reflection is used to determine the position of objects. These high frequency calls are very loud (may exceed 110 decibels) and are emitted thousands of times per second allowing them to home in on insects or avoid collisions with inanimate objects while flying rapidly. Little Brown Myotis also make social calls to communicate with other individuals and many of these calls are within human hearing range so it is possible to hear squeaks and other sounds from these bats.

Little Brown Myotis breed in the fall and the female stores the sperm until coming out of hibernation in the spring when ovulation and conception occurs. Gestation is 50-90 days and females have one young per year that are born in late June or July. Young bats are called pups and grow rapidly as the mothers nurse them. Adult body weight is achieved in about 16 days and wing bone growth by about 20 days. The pups begin flying in 3-4 weeks and are weaned in about 5-6 weeks.

Little Brown Myotis roosting – Photo Credit: NPS

Hibernation takes place from around late September or early October until late March or early April at sites called hibernacula. Although considered a yearlong resident, very few hibernacula are known in Montana and some migration may occur because winter aggregations are much smaller than summer populations. Hibernacula offer protection from freezing temperatures and have high relative humidity that helps minimize water loss during hibernation. During this time the heart rate drops to 10-20 beats/minute compared to 200 beats/minute during rest and 1300 or more in flight. Periodically they must arouse out of the hibernation state to excrete body wastes like urea, but it may be up to 90 days continuously in full hibernation. Since they depend on stored body fat during hibernation they may lose up to 25% of their body weight during hibernation. Human disturbance in hibernacula may cause death by starvation due to the increased energy demands required to arouse from hibernation more frequently.

Little Brown Myotis range throughout most of North America from the Alaska-Canada boreal forest south through most of the continental U.S., except being generally absent from the southern Great Plains. They are common here in northwest Montana and throughout the rest of the state.

Little Brown Myotis are susceptible to White-Nose Syndrome, an introduced, cold-loving fungus that has killed approximately 6 million bats of several species in 26 states and 5 Canadian Provinces in the eastern part of North America since 2006. Little Brown Myotis populations in eastern U.S. and Canada have crashed with an estimated 95% mortality. So far, White-Nose Syndrome has not been detected in Montana but was found in Washington last year and has been predicted to eventually reach Montana. Montana bat researchers and cavers have implemented strict decontamination protocols when working with bats or being in caves or other bat roosting sites in an attempt to keep White-Nose Syndrome out of Montana.

Although not usually conspicuous, Little Brown Myotis deserve appreciation for their amazing adaptations and the role they play in ecosystems.