The northern bat (Eptesicus nilssonii ) is the most abundant species of bat in northern Eurasia. It is found from England to Hokkaidō and south to northern India. It is closely related to the serotine bat (Eptesicus serotinus ).
The northern bat is a medium-sized animal with short and rounded ears. Specimens range from 8 to 16 g in weight depending on the season, 54 to 64 mm in body length and 240 to 280 mm in wing span, which is a moderate size for a bat. The nose, ears, tail and wings are black or blackish brown. Most of the body is coated with dark brown or black with some gold touched at the tip of the hairs in the head and back region. The coat on the ventral side is yellowish brown. Like other dental structures of Eptesicus genus, there is no presence of peculiarities, but the dental structure of the northern bat is large compared to the size of the skull.
It is widespread throughout Eurasia, and is the most common bat in the northern part of the continent. It is found from northern Scandinavia (beyond the Arctic Circle) to northern Italy, and eastern England to northern Japan. The bat mostly favours forest uplands about 200–2000 m above sea level.
It was thought that the northern bat is a sedentary species, but research shows colonies have moved as far as 450 km. It does not migrate seasonally but over a period of years.
Breeding season is in late autumn, and the females stores the male sperm over the winter. Hibernation begins in early winter (November and December), and lasts until March or April; only then do the females become pregnant, pregnancy lasting for 50–60 days. In summer, males dwell alone, but females form a colony of 10-80 adults. A colony is formed in early summer and disbanded in August, when young bats are able to fly. Winter colonies are often found in houses, and natural or artificial underground habitats.
