One of North America's most abundant bat species is the big brown bat (Eptesicus fuscus), a close relative of Sweden's remarkable northern bat. Like the northern bat, its range extends almost to the Arctic Circle in Alaska.
The big brown bat has many other similarities to the northern bat. It too is a typical house bat and is one of the species most often encountered by humans. Today, its most favored roosts are in buildings. An exceptionally hardy bat, it will accumulate up to a third of its body weight in additional fat in preparation for the winter, most often hibernating in houses even in the northern U.S. and Canada.
If the temperature of their chosen hibernation site drops much below freezing, big brown bats may arouse and leave to seek better shelter. It is not unusual to see one flying about in extremely cold weather, even during a snowstorm, or to find one dead on the ground in the middle of winter, a victim of the cold. So if you have large bats wintering in your house, chances are good that they are big brown bats, North America's closest relative to the northern bat.