E-Fauna BC: Electronic Atlas of the Wildlife of British Columbia

Eptesicus fuscus (Palisot de Beauvois)
Big Brown Bat
Family: Vespertilionidae
Species account authors: David Nagorsen and Mark Brigham.
Extracted from the Bats of British Columbia
Photo of species

© Diane Williamson  Email the photographer   (Photo ID #7101)

E-Fauna BC Static Map
Distribution of Eptesicus fuscus in British Columbia
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The information provided below is extracted from the Bats of British Columbia, and may be dated. Check the status section below for current status information.

Species Information

Click on the image(s) below to view an expanded illustration for this taxon.

Illustration Source: Bats of British Columbia by David Nagorsen and Mark Brigham © Royal BC Museum

One of our larger bats, the Big Brown Bat has a large, broad head, broad nose and long, lax fur. Its fur colour varies from pale to dark brown; the fur tends to be oily in texture. Its flight membranes and ears are black. The ears just reach the nose when pushed forward; the tragus is short and blunt. The calcar has a prominent keel. The skull is robust, with thick, heavy jaws, a flattened brain-case and large teeth.


Although the Big Brown Bat resembles several of our Myotis species in colour, it can be readily distinguished by its large size (forearm greater than 42 mm), large head and long fur. The combination of 2/3 incisors and 1/2 premolars is unique among British Columbian bats.

Dental Formula

incisors: 2/3
canines: 1/1
premolars: 1/2
molars: 3/3


total length: 116 (98-131) n = 126
tail vertebrae: 46 (37-55) n = 123
hind foot: 12 (8-15) n = 115
ear: 15 (11-21) n = 53
tragus: 8 (5-11) n = 40
forearm: 47.5 (43.0-52.0) n = 109
wingspan: 328 (205-393) n = 62
weight: 15.2 (8.8-21.9) n = 36



Mating is thought to take place in autumn and winter. Fertilization occurs in spring after departure from the hibernaculum. Males may attain sexual maturity in their first year; females may breed in their first year but many do not breed until their second. Although twins are typical in eastern North America, single young are usually produced in the west. The scanty data available for the province suggest that the young are born in late June. Nevertheless, birth dates and development of the young can be extremely variable, differing among roosts and even among individuals within a roost. Pregnant females, females with newborn young and females with well-developed young can be found together in the same roost. In the Okanagan, considerable year-to-year variation has been observed, with dates for the first appearance of the young varying as much as a month. This variation probably results from year to year differences in climate. Newborn young weigh about three grams. They are naked and blind although the eyes open within a few hours after birth. Juveniles are capable of flying at 18 to 35 days of age. There are longevity records showing that this species can live at least 19 years in the wild.

This species emerges around dusk to feed. It is regarded as a generalist that hunts in a variety of situations: over water, over forest canopies, along roads, in clearings, and in urban areas, often around street lights that attract insects. The intensity of its echolocation calls is very high. In open areas such as forest clearings and over water the Big Brown Bat may first detect prey as far as 5 metres away. In most locations beetles are an important part of the diet and the heavy teeth and strong jaws of this bat appear to be an adaptation for chewing hard-shelled insects. Nevertheless, remains of moths, termites, carpenter ants, lacewings and various flies also have been identified in faecal pellets.

The only data on feeding biology for the province comes from research undertaken at Okanagan Falls. There the Big Brown Bat emerges 30 to 40 minutes after sunset and hunts 5 to 10 metres above the water along a 300-metre stretch of river. It is a relatively fast flyer, able to attain a speed of 4 metres per second when pursuing prey. The Big Brown Bat makes several feeding flights during the night, each 30 to 60 minutes in duration. Between flights it usually roosts near the feeding area. During the day it roosts as far as 4 kilometres from the feeding area, even though there are sites near the Okanagan River suitable for day roosts.

Large caddisflies are the major prey at Okanagan Falls. Midges are the dominant insect group in the area, but the Big Brown Bat does not eat them. The echolocation calls of this bat should enable it to detect midges at a distance of about a metre. Either it is unable to react quickly enough to capture them or it simply prefers larger prey.
Natural History

Throughout its range this bat demonstrates a strong affinity for buildings and it is often referred to as a house bat or barn bat. In British Columbia, maternity colonies have been found in the attics of houses, cabins and barns. But in the Okanagan Valley this species rarely roosts in man-made structures; there, maternity colonies have been found in cavities in dead Ponderosa Pines and rock crevices.

The Big Brown Bat is less tolerant of high temperatures than other species that use attics such as the Little Brown Myotis and Yuma Myotis. Temperatures above 35°C will often force the Big Brown Bat to move to a cooler site within the roost or change roosts altogether. Maternity colonies as large as 700 individuals have been reported, but those associated with buildings in British Columbia are small, comprising no more than 50. The tree colonies studied in the Okanagan contain any number from a few to 200 bats, with an average of about 100. Loyalty to roosting sites seems to depend on the type of roost. Big Brown Bats roosting in buildings or rock crevices show a strong fidelity to those sites; however, those roosting in trees seem to move regularly between several established roosts. Some recent genetic research has revealed that most individuals in maternity colonies are closely related.

This species' hibernation period varies with local climate and geography. In the interior of the province, it lasts from November to April. In British Columbia's southern regions, the Big Brown Bat may be active for short periods in winter. Migrations of almost 300 kilometres have been documented, but most individuals travel no more than 80 kilometres between summer and winter roosts. Populations exploiting buildings may even use the same site in winter and summer. (Despite its rather sedentary nature, experiments have demonstrated that this species possesses a remarkable homing ability: individuals released 400 kilometres from their roosts managed to find their way back.)

The Big Brown Bat hibernates in buildings, caves and mines; in western Canada most populations appear to use buildings. Hibernating colonies are typically small, although large colonies have been found in a few caves and mines in eastern North America. As with most of our bats, information on winter biology in British Columbia is sketchy and derived largely from incidental information on museum specimens. There are coastal winter records from Victoria and interior records from as far north as Williams Lake and Prince George. Most of these winter records are from buildings; this mammal is rarely found in caves or mines in the province.

In eastern Canada, swarming behaviour often takes place at mines and caves before this species enters hibernation. The Big Brown Bat hibernates alone or in clusters of as many as 100. This is a hardy bat capable of withstanding low temperatures. It selects a dry microenvironment with temperatures of -10 to -5°C, although temperatures below -4°C will usually stimulate arousal. When the Big Brown Bat enters hibernation in late autumn it weighs about 25 grams; when it completes hibernation in spring it weighs about 19 grams, having lost about 25% of its body weight. In caves or mines it hibernates in exposed areas near the entrance where it hangs from the ceiling and walls, or it wedges itself into small crevices or under rocks. Big Brown Bats often arouse from hibernation to change roosting locations; there are several observations of this bat outside on the walls of buildings when temperatures were well below freezing. There is some evidence that winter activity is more common in buildings where conditions for hibernation may not be ideal.


The Big Brown Bat inhabits a variety of habitats in the province, including arid grassland and interior and coastal forests. Its elevational range is from sea level to 1070 metres.


The Big Brown Bat has a vast range that extends from northern South America to southern Canada. In British Columbia, it is found on Vancouver Island, the coastal mainland north to the Bella Coola River Valley and the interior where its northern limits are unknown. Northernmost localities in the province are from the Prince George and the Peace River region, but because there is a record from the interior of Alaska the range may extend into extreme northern British Columbia.


All British Columbian populations belong to the race E. fuscus bernardinus, a subspecies that ranges from California to British Columbia.


This species may have some claim to being the "provincial bat" because it often turns up in the provincial legislative buildings in Victoria during autumn and winter.

Status Information

Origin StatusProvincial StatusBC List
(Red Blue List)
NativeS5YellowNot Listed
BC Ministry of Environment: BC Species and Ecosystems Explorer--the authoritative source for conservation information in British Columbia.

Additional Photo Sources

General References

Recommended citation: Author, Date. Page title. In Klinkenberg, Brian. (Editor) 2021. E-Fauna BC: Electronic Atlas of the Fauna of British Columbia [efauna.bc.ca]. Lab for Advanced Spatial Analysis, Department of Geography, University of British Columbia, Vancouver. [Accessed: 2024-07-18 4:10:30 PM]
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