Big Brown Bat

Eptesicus fuscus (Vespertilionidae) is the bat most often found living inside North American buildings. Adults are 10–13 cm long with a 32–40 cm wingspan and weigh 15–26 g, with glossy copper to chocolate-brown fur, a broad black muzzle, short rounded ears and black membranes. Females bear one pup in the east and commonly twins in the west, born late May to early July after 60 days of gestation; pups fly at about four weeks. The species is unusually cold-hardy, hibernating at 2–10°C and tolerating brief sub-freezing exposure, which lets it overwinter inside attics, wall voids and unheated buildings rather than migrating to caves.

Emergence is at dusk, with a strong, straight, relatively slow flight and echolocation calls sweeping from roughly 50 to 25 kHz. Diet is dominated by hard-shelled beetles, which its powerful jaws crush easily, along with moths, true bugs and flying ants. Maternity colonies typically hold 20–300 females and are highly site-faithful, returning to the same structure for decades. It is the species most frequently submitted for rabies testing in the United States simply because it lives with people, though the actual infection rate in wild populations remains under 1%. Accumulated guano in a long-occupied attic supports Histoplasma capsulatum growth and produces staining, odour and ceiling damage.