Physical Features:  As suggested by the bat’s name, its fur is uniformly dark brown and glossy on the back and upper parts with slightly paler, greyish fur underneath.  Wing membranes are dark brown on a typical wingspan of 22–27 cm (8.7–10.6 in). Ears are small and black with a short, rounded tragus.  Adult bats are typically 6–10 cm (2.4–3.9 in) long and weigh 5–14 grams (0.2–0.5 oz). Females tend to be larger than males. The fore and hind limbs have five metapodials. The skull of the brown bat lacks a sagittal crest. Its rostrum is shortened and has upslope profile of the forehead. Its braincase flattened and sub-circular when observed dorsally. The bat has 38 teeth all of which including molars are relatively sharp, as is typical for an insectivore, andcanines are prominent to enable grasping hard-bodied insects in flight.  Brown bats live approximately 6 to 7 years and often live well beyond 10 years.
Habitat and roosting:  The brown bat lives in three different roosting sites: day roosts, night roost and hibernation roosts. Bats use day and night roosts during spring, summer and fall while hibernacula are used in winter. Day roosts are usually found in buildings or trees, under rocks or wood piles and sometimes in caves. Nursery roosts are found in both natural hollows and in buildings (or at least close to them). Nursery roosts have also been found under the sheet metal roofs of trappers’ caches and attics of buildings.  Night roosts tend to be in the same buildings as day roosts, but these roosts tend to be in different spots that are more constrained and the bats pack together for warmth. Bats rest in night roosts after feeding in the evening which may serve to keep their feces away from the day roosts and thus less noticeable to predators.  Brown bats typically hibernate in caves and perhaps unused mines. Northern populations of bats enter hibernation in early September and end in mid-May while southern populations enter in November and ends mid-March.   Brown bats are not true hibernators. As observed in the Mid-Atlantic States during periods of warming during the winter, typically over 50 degrees (F),  brown bats emerge from their winter torpor and hunt insects that have emerged as well in response to the warmer conditions.
Diet:  Brown bats are insectivores, eating moths, wasps, beetles, gnats, mosquitoes, midges and mayflies, among others. Since many of their preferred meals are insects with an aquatic life stage, such as mosquitoes, they prefer to roost near water.  Brown bats forage near bodies of water and move in and out of adjacent vegetation.  Evening forages are done in groups and above the water. They echolocate to find their prey. They are particularly good at hunting insects when they are at close range and packed together. When hunting,  brown bats capture prey both by gleaning and by catching them in the air.  When in flight, bats scoop up the prey with their wings, while prey above water is directly grabbed with the mouth. Brown bat do not claim feeding areas like a territory, however individuals frequently return to the same feeding sites where they have previously made successful catches.  When hunting swarms, brown bats usually select no more than two species. They feed on more species when they are scattered. If they do not catch any food, they will enter a torporsimilar to hibernation that day, awakening at night to hunt again.  The bats’ diet makes this species beneficial to agriculture as it eats many species of agricultural pests.
Mating and reproduction:  Applying a wing band to help biologists and researchers identify individual bats from year to year.
At least in Ontario, brown bat mating occurs in two phases, active and passive. In the active phase, both partners are awake and alert. In the passive state, active males try to mate with torpid bats regardless of their sex.  Active phase matings are more common as there are peaks in testosterone. There is some conflicting reports on whether active females store sperm.   Active mating is at its highest in August but passive mating lasts until winter.   During mating, the male mounts the female from the rear and may use a copulation call to calm her so she won’t struggle.   Bats mate promiscuously.  Neither sex is selective of their mates and males can’t monopolize females when torpid.  When they arise in the spring, the females go to nursery colonies which may often be the same place where they were born. These nursery colonies consist mainly of adult females and their young and can be located in the attics of warm buildings where there is high humidity. These colonies sometimes reach numbers of bats as great as 1,000 per cave/forest. Gestation lasts 50–60 days.   They usually have one baby per female each year, sometimes twins, born sometime from late May to early July. The young are born in an altricial state with their eyes closed and will hang in the nursery while their mothers forage at night. Their eyes open on their second day. They cling to a nipple constantly until they are two weeks old. At three weeks, they learn to fly. By four weeks, they are adult sized.   Females may be sexually mature in the fall after their birth, but males may take a year longer. About half of females and most males breed during their first autumn.