Big Brown Bat

Eptesicus fuscus (Beauvois)




 Color photo by Barbara L. Clauson.
Copyright 1999.   All rights reserved.

Description:   Big brown bats resemble the smaller myotis bats in general color, but can be distinguished from other Kansas bats by: 1) noticeably larger size, 2) long, soft hair on its dorsal surface being uniform brown in color while the underparts are lighter, 3) individual hairs having a black base and a light tip, 4) thick, black wing membranes, 5) broad black ears with slightly recurved, rounded tips, 6) tragus being less than half as long as the ear length, and 7) conspicuously keeled calcar. Skin glands in the upper lip produce a secretion that is used to mark its roosting or hibernating places. The sexes are alike and show no seasonal variation.

Size:   Adults may attain the following dimensions: total length 110-130 mm; tail 39-54 mm; hind foot 9-13 mm; ear 14-20 mm; weight 16.5-33 grams.

Range and Habitat:   Big brown bats are the most common and widespread bat in Kansas. In winter they normally hibernate in the drier parts of caves, but will also use mines, buildings, storm-sewers, hollow trees, and crevices in rocks. In summer they live in a variety of situations such as chimneys and other parts of buildings, between walls, in cornices and roofs, and under tree bark. This is an abundant bat in Kansas cities and towns. Both maternal roosts and hibernacula may be found in older buildings. Two subspecies occur in Kansas, Eptesicus fuscus fuscus in the eastern two-thirds of the state, and Eptesicus fuscus pallidus in the western third. The species is a resident in Kansas the entire year.

Reproduction:   Breeding occurs in the autumn before hibernation, during winter, and in spring prior to ovulation in the first week of April. In early April, females leave hibernation sites and form nursery colonies. Between the end of May and mid-June one or two (usually two in the east and one in the west) naked and blind young are born. On the second day their eyes are open. In three or four weeks the young are capable of flight, and in four or five weeks the young are weaned. In two months they are approximately adult size. Both sexes are believed to be sexually mature during their first autumn.

Habits:   This is the bat most commonly found flying in houses here in Kansas. They gain entry by way of chimneys, open windows, and doors, or small cracks between ceilings and walls. In winter, as homes are heated, the bats become active and must search for a cooler location to continue hibernating. Under natural conditions these bats select dry places in caves, and it is for this reason that they are able to hibernate in dry buildings. Flying within a room presents no problem, and, although the bat can bite if captured, it is otherwise harmless and will not intentionally make contact with humans. Big Brown Bats commonly forage around city lights and over pools of water (including swimming pools and fish ponds). In flight they follow a slow, straight course in open places, but are capable of abrupt turns when encountering an insect. They may emit an audible chattering sound in flight. When Big Brown Bats enter hibernation in late September or early October, females always precede the males. At the beginning of winter the bats have already accumulated a store of energy, in the form of fat, which maintains their metabolism throughout the winter. Body temperature becomes approximately the temperature of the cave, which must be cool enough to induce a low level of metabolism. If temperatures abruptly drop too low the bats will freeze, and if temperatures increase the bats will awaken and become active, thus depleting their fat stores. Except for females in maternity colonies, Big Brown Bats are essentially solitary. Even in hibernation they prefer to be alone or in small, loosely aggregated groups. This is not a wide-ranging bat and generally spends the summer near its winter hibernation site.

Food:   Most kinds of nocturnal insects are captured and eaten on the wing or, if the insects are too large to manage in flight, the bat will consume them at a nearby night feeding roost. The kinds of insects eaten are generally those available at the time of feeding, but beetles seem to be preferred, and moths may be taken. Hunting activity peaks occur relatively early, several hours after sunset, and by midnight many Big Brown Bats are at night roost, though some individuals may forage intermittently throughout the night. Most individuals return to their daytime roosts for a few hours before sunrise.

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