Common Name: Little Brown Bat
(Information for this species pages was gathered in part by Crystal Greenlund for Biology 220W at Penn State New Kensington inSpring Semester 2011)
The little brown bat (Myotis lucifugus) is a very common and formerly quite abundant resident of almost all of North America. Its optimal range is across the northern United States and southern Canada, but it is frequently found both far to the north and far to the south of this general distribution.
The little brown bat, as it names describes, is a small bat that is between three and five inches long weighing between one sixteenth and one half an ounce. Its wing span is nine to eleven inches. Females tend to be larger than males. The little brown bat has glossy, brown fur that is darker on its back and upper body parts and lighter on its chest and belly. Its wing membranes are also dark brown.
Image credit: Kevin Matteson, Flickr.
Activity and Roosting
The little brown bat tends to be nocturnally active except in the spring when it may fly about catching insects even during the day. Normally, during the daylight hours the little brown bat will hide and rest in its “day roost” (often spaces under roofs or eaves of buildings, or in wood piles or caves). It will emerge at sunset to begin a one to five hour feeding period and will then rest in its “night roost” which is often close to its day roost (frequently it is another part of the building or wood pile or cave in which the day roost is located). This night roost, though, allows the bats to pack very tightly together to help them stay warm in the cool, nighttime temperatures, and also enables the bats to deposit their feces (which can attract predators) away from their day roosts. Roosting seems to be a gender specific activity. Females form large, colonial roosts for both their day and night and nursery roosts, while males tend to roost individually or in much smaller groups.
Little brown bats can fly up to twenty miles per hour and use self-generated, high frequency sounds and echolocation to both avoid collisions while flying and also locate their flying insect prey. These high frequency vocalizations are inaudible to humans.
Little brown bats eat large numbers of flying insects (including midges, mosquitoes, caddisflies, mayflies, lace wings, moths, and beetles) during their nocturnal feeding periods. Females, especially if they are lactating and feeding a pup, may eat up to one hundred and ten percent of their body weight in insects during a feeding period. A single bat will eat between three hundred to three thousand insects a night according to the Penn State Newswire (June 3, 2013). A million bats, according to the Wisconsin Bat Monitoring Program, eat six hundred and ninety-four tons of insects a year! That's a lot of mosquitoes and potential crop pests! The Penn State Newswire article cited estimates that a farmer in a bat-deprived world would have to spend between four and five thousand dollars a year on pesticides just to achieve the insect pest control that the bats had provided for free.
A little brown bat may directly capture a flying insect in its teeth, or it may use its wings and tail to scoop insects out of the air and then transfer them to its mouth. Bats seem to concentrate on a specific type of insect during each of their hunting events. Possibly the techniques used to capture specific kinds insects differ and the bat gains a greater level of efficiency by utilizing a single hunting/capturing strategy at a time. Many of the insects taken by the little brown bat have aquatic life stages, and, so, it is not surprising that little brown bats often roost and hunt near streams and ponds. Individual bats tend to have specific hunting areas and specific flight pathways between their roosts and these hunting territories. They are not, however, aggressively territorial with regard to these hunting zones.
Little brown bats must find refuges within which they withstand the stresses of winter. These “hibernation roosts” are typically caves, rock fissures, or abandoned mines. The bats respond to a variety of environmental cues (shortening day length, cooling temperatures, and decreasing abundances of insect prey) and prior to the extreme onset of cold weather, make their short migrations to their caves where they enter a torpid, low metabolic rate state. The caves need to maintain temperatures around forty degrees F and have a high relative humidity in order to allow the fat reserves of the bat to carry it through to the spring. Unfortunately, these cool, humid conditions are also optimal for the growth of many species of fungi including Geomycus destructans the fungus that causes the disease called “white nose syndrome.”
White Nose Syndrome
A little brown bat relies on its accumulated fat reserves not only to live through the winter but also to have the energy in the spring to mate upon emergence from their hibernation caves and then fly to their summer roosting and hunting ranges. The fungus that causes white nose syndrome irritates the bat during its winter torpor causing it to wake up and become active at inappropriate times. This wastes precious metabolic energy and can result in the death of the bat. This fungus not only affects little brown bats but also five other bat species! It is estimated that millions of bats have died because of this fungal infection since it was first detected in 2006. This fungus has been detected in twelve states and two Canadian provinces. The control of this fungus and the stabilization of the hibernation roosts for these bats is critical to the survival of this and several other bat species.
Mating and Reproduction
The life cycle of the little brown bat begins at emergence from hibernation. Emerging males and females mate repeatedly and with multiple partners prior to flying to their summer roosting areas. The pregnant females group together in a nursery roost that is notable for its warm temperatures (pregnant females are not able to thermoregulate very efficiently). After fifty to sixty days gestation each female gives birth to a single pup. The pup will cling to the mother and even go out on her feeding flights tightly attached to her fur. Soon, though, the pup gets too large for these free rides and must remain in the nursery roost where it is cared for and fed by the mother. Pups are weaned in three or four weeks and then they join the females on their nightly forays and in both their day and night roosts. Females become sexually mature around nine months and males become sexually mature at one year of age. A little brown bat, especially if it survives its first winter, may live twenty or even thirty years.
Little brown bats are preyed upon by many roost predators. Weasels, raccoons, rats, mice, many species of snakes, and domestic cats readily take roosting little brown bats for prey. During flight, hawks and owls also kill and eat little brown bats. Also, martens and fishers have been observed feeding on hibernating little brown bats.