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Bats Northwest

"helping bats in Washington State"

Big Brown Bat (Eptesicus fuscus)

by Susan Montana

Big? It's all relative. Common names can be misleading and all too generic, especially when it comes to creatures like brown bats, of which there are many. Eptesicus fuscus, compared to her cousin, the Little Brown Bat, Myotis lucifugus (who is in a different genus altogether), is big – about twice as big – with an average weight of 15.2 grams (~1/2 oz.), total length of 116 mm (~4 ½ in.) and wingspan of 328 mm (~13 in.). Brown? Yes, but not boring. Her long hair coat ranges from very light to dark brown, with a dark brown muzzle and black wing membranes and short, black ears with a blunt tragus. The calcar is keeled and two tail vertebrae extend just beyond the uropatagium1. She boasts a robust skull with formidable jaws and teeth for munching on the larger insects she prefers, such as large beetles. This is a fast flying bat, clocked at up to 4 meters (~13 feet) a second in pursuit of prey! Eptesicus fuscus is one of our most common bats in the Northwest as well as in the entire continental United States and as far north as Alaska. This is the bat we are most likely to find behind a shutter or in an undisturbed cranny of our home or garage at chilly times of year. This is a very hardy bat and one of the last to go into hibernation, having a high tolerance for cold temperatures. Given stretches of moderate weather, even warmer than -4 Celsius (24.8 F), she will arouse from hibernation for short periods of activity. Some banding studies indicate that this bat may hibernate locally, within an 80 km (~50 mi.) range, rather than migrating long distances as do many other species. Big Browns tend to hibernate in attics, caves and mines, singly or in small clusters.

During the summer, large maternity colonies of up to 700, but more typically 100-200, grace many attics and barns (their choice real estate) throughout urban and rural settings. Tree hollows and rock crevices are also selected sites. However, frequent association with man-made structures make this bat more familiar to humans than most. In the Puget Sound area we see the Big Brown in early evening skies, flying and feeding over and around tree tops and bodies of water. Green Lake, for example, is an excellent location for observing her foraging behaviors during the summer months. Set your bat detectors between 27-48 kHz to pick up their searching mode echolocation calls.

It's believed that Big Browns mate in autumn and winter, delaying fertilization until leaving the hibernaculum in spring. Females can be found pregnant or with their single pup (two are common in the eastern U.S.) in all stages of post-natal development within the same roost throughout early summer. The young are flying by 35 days of age and the first survivors (~59%) of a new generation may live to be 19 years or more. Our conservation efforts help to insure that this valuable insectivore will remain a common sight in our evening skies.

Note 1: uropatagium: Skin membrane between the hind legs.