MAMMALS OF MISSISSIPPI 5:1–8
Big brown bat (Eptesicus fuscus)
ALISHA A. WORKMAN
Department of Wildlife and Fisheries, Mississippi State University, Mississippi State, Mississippi, 39762, USA
Abstract—Eptesicus fuscus (Beauvois, 1796) is a vespertilionine commonly called the big brown bat.
It is sexually dimorphic with the female being slightly larger than the male. Dorsal color ranges from
pinkish tan to rich chocolate and the ventral color ranges from pink to olive buff. It represents one
out of twenty four in the genus Eptesicus. Eptesicus fuscus is distributed through all of the United
States except Hawaii, most of Central America except the Yucatan Peninsula, and its southern limit is
northwestern South America. Eptesicus fuscus generally prefers to live in caves, trees, bridges, and
buildings. Eptesicus fuscus is currently not listed as a species of special concern.
Published 5 December 2008 by the Department of Wildlife and Fisheries, Mississippi State University
Big Brown Bat
Eptesicus fuscus (Beauvois, 1796)
CONTEXT AND CONTENT.
Order Chiroptera, Suborder Microchiroptera,
Family Vespertilionidae, Subfamily
Vespertilioninae, Tribe Vespertilionini.
Figure 1. Big brown bat (Eptesicus fuscus) in Ann Arbor,
Michigan. Photo used with permission of the photographer
Phil Myers under a Creative Commons Attribution,(http://
animaldiversity.ummz.umich.edu/site/resources/phil_
myers/classic/eptesicus_best.jpg/view.html)
GENERAL CHARACTERS
The big brown bat, Eptesicus fuscus (Fig. 1), is
one of the most common bats in North America
(Barbour and Davis 1969). The pelage color
varies by geographic location, but ranges from
pinkish tan to rich chocolate on the dorsal
side and pink to olive buff on the ventral side.
The bare-skin areas of the face, wings, and
ears are black. The fur, while extremely soft,
has a somewhat oily texture (Miller 1907).
Figure 2. Geographic distribution of Eptesicus fuscus.
Map used with permission of NatureServe, (http://www.
natureserve.org/explorer/servlet/NatureServe?searchNa
me=Eptesicus+fuscus).
DISTRIBUTION
Eptesicus fuscus occurs throughout a large
portion of North and Central America and as far
south as northwestern South America (Fig. 2).
It also resides in Cuba, Jamaica, Puerto Rico
(Hall 1981), and some of the Bahama Islands
(Buden 1985). The big brown bat occurs
throughout the United States except Hawaii
(Hall 1981) and in Mexico except the Yucatan
Peninsula (Buden 1985). Although there are 11
subspecies of E. fuscus (Neubaum et. al 2007),
only E. fuscus fuscus occurs in Mississippi (Hall
1981).
Figure 3. Dorsal, ventral and lateral views of the skull
and a lateral view of the mandible of an adult female
Eptesicus fuscus in Lexington, Massachusetts. Greatest
length of the skull is 18.2 mm. Drawings by J. Love.
Measurements (in mm) of E. fuscus are: total
length, 87–138; length of tail vertebrae, 34–57;
length of hind foot, 8–14; length of ear from
notch, 10–20; length of tragus, 6–10; length
of forearm, 39-54; length of third metacarpal,
43-50; length of tibia, 17-21; greatest length
of skull, 15.1–23.0 (Fig.3); zygomatic breadth,
11.1–14.2; breadth of braincase, 7.5–9.6;
length of maxillary toothrow, 7.0–9.8. Adults
typically weigh 11-23g. This species is sexually
dimorphic, with females slightly larger than
males (Burnett 1983a). The big brown bat and
the hoary bat (Lasiurus cinereus) are the only
vespertilionids that produce an audible sound
during flight. Wing and skull size is positively
correlated with the amount of environmental
moisture (Burnett 1983b).
FORM AND FUNCTION
Form.—Each June, Eptesicus fuscus molts by
shedding its winter coat (Phillips 1966). The
dental formula is i 2/3, c 1/1, p 1/2, m 3/3, total
32. The crowns on M1 and M2 are narrower
than those of M3. The two pectoral mammae
contain milk that contains 2.5% lactose, 6.2%
protein, and 16.4% fat (Kunz et. al 1983).
This species displays pararhinal glands on
the sides of the nose that consist of apocrine
tubules (for producing sweat), which set atop
sebaceous units (Dapson et. al 1977). The red
blood cell count is 11.96 x 106/mL (Dunaway
and Lewis 1965). The bones that make up
the right and left appendages weigh the same,
which indicates that E. fuscus is ambidextrous
(Dawson 1975). Polydactyly, deformed
vertebrae, and underdeveloped radii are a
few of the skeletal anomalies that can occur
in this species (Kunz and Chase 1983). Over
time, this species has lost the ceratohyal of the
hyoid (Griffiths 1983). The baculum is about
0.8 mm long (Hamilton 1949). E. fuscus will
defecate within 90–130 minutes of eating. It
will completely pass a meal within 24 hours of
eating (Luckens et. al 1971).
Function.—The big brown bat has a large
heart that can represent 0.9% of its fat-free
body mass. The resting heart rate is about 450
beats/min. During flight, the heart rate more
than doubles to about 1,022 beats/min (Studier
and Howell 1969). During torpor, the heart
rate dramatically drops between 4–62 beats/
min. After arousal from hibernation, the heart
rate increases from 12–800 beats/min (Rauch
1973).
Rhodopsin and an unidentified molecule are
the two photopigments in the retina (Kurta
and Baker 1990). A few ocular abnormalities
are known to occur, such as an unpigmented
choroid, undifferentiated retina, and
underdeveloped lenses (Kunz and Chase
1983).
When the ambient temperature is <30ºC, the
bat will go into a torpid state. During torpor,
the body temperature is between 32 and 36
C (Herreid and Schmidt-Nielsen 1966). If
overheating occurs, E. fuscus is able to lower
its core body temperature by dilating blood
vessels that lead to the wings (Kluger and
Heath 1970). Some bats in this species have
been observed panting and spreading saliva
on their bodies when the ambient temperature
exceeds 40ºC (Kurta and Baker 1990).
Eptesicus fuscus uses echolocation by sending
out sonar signals to detect prey and avoid
obstacles. Echolocation is split into three
phases: search, approach, and terminal. When
the bat begins hunting it sends out narrow
signals at a low rate of 2-5 Hz. Once it detects
something of interest, it begins the approach
phase where signals increase progressively
from 5–10 to 30 Hz as the bat moves closer to
the target. During the terminal phase, the bat is
almost to its target and emits signals between
100–150 Hz. The sounds in this phase can
be heard on a bat detector and are commonly
called the “terminal buzz.” The bat decodes
this acoustic information to determine how
close the prey is (Sanderson and Simmons
2005).
ONTOGENY AND REPRODUCTION
Eptesicus fuscus copulates between September
and March (Phillips 1966). However, females
delay ovulation and fertilization until arousal
from hibernation occurs (Kurta and Baker
1990). Males are sexually mature by the end
of their first autumn (Kurta and Baker 1990).
During early stages of pregnancy, females
accumulate extra fat. During pregnancy,
females require about 48.9 kJ of assimilated
energy/day to sustain her and her developing
young. During the 32–40 lactation period,
females need 105.1 kJ/day (Kurta and Baker
1990). Female big brown bats usually give
birth to one pup in the western United States
and two pups in the eastern United States and
in Cuba (Barbour and Davis 1969). One theory
on why this occurs is that several biogeographic
events resulted in regional separation of
ancestral bats. This separation led to several
different mitochondrial DNA (mtDNA) lineages,
which likely resulted in the observed regional
difference in litter sizes (Neubaum et al. 2007).
Females can release up to five eggs/ovary
(Birney and Baird 1985). Gestation lasts for
two months. Young are born from May to July,
with earlier births occurring in lower latitudes
(Barbour and Davis 1969). The newborns are
altricial at birth and fledge at 18–35 days (Gould
1971). The fledgling’s body mass is about 75%
of adult body mass (Burnett and Kunz 1982).
ECOLOGY
Population characteristics.—Eptesicus
fuscus abundance decreases from the
deciduous forest biome to the coniferous forest
biome (Kurta and Baker 1990). Big brown bats
often live >10 years; the oldest recorded age
is 19 years (Paradiso and Greenhall 1967).
Males live longer than females. Males occur at
higher elevations in mountainous regions than
females (Kurta and Baker 1990). Postnatal
mortality before weaning is 7–10%. In adults,
common mortality factors are predation, failure
to store enough fat for hibernation, accidents,
and inclement weather.
Space use.—Bat-habitat relationships can
be very complicated due to their high mobility.
Therefore, studies have failed to quantify
habitat requirements of Eptesicus fuscus.
However, this species occurs in both urban
and rural areas. Night roosts are frequently in
locations that allow it to forage and engage in
social interactions. Big brown bats frequently
use buildings as roosts, often near a light where
insect densities are likely higher. They will also
roost in caves, tunnels, mines, tree cavities,
rock crevices, and under bridges (Agosta,
2002). They seldom move farther than 80 km
between their summer and winter roosts (Kurta
and Baker 1990).
Diet.—Eptesicus fuscus is insectivorous
(Agosta 2002). Big brown bats are also
generalists and do not have a preference for
over-water versus over-land foraging sites.
They begin foraging within an hour after
sunset and spend an average of 100 min/night
foraging. Prey varies by geographic location
but generally consists of beetles, moths,
mosquitoes and dragonflies (Kurta and Baker
1990).
Diseases and parasites.—There are many
ectoparasites known to affect Eptesicus fuscus
including Cimex (bedbugs), Basilia (bat flies),
Ornithodoros (soft ticks), Leptotrombidium
(chigger mites), and Myodopsylla (bat fleas)
(Kurta and Baker 1990). One species of
rosensteiniid mite,Nycteriglyphus fuscus, lives
in the guano of E. fuscus (Dood and Rockett
1985). To avoid ectoparasites, female big
brown bats may frequently change roost sites
(Agosta 2002).
Many endoparasites are also known to affect
E. fuscus, including nematodes from the
genera Allintoshius, Cyrnea, Physocephalus,
and Capillaria. Maseria vespertilionis is a
nematode that only infects the subcutaneous
tissue in the plantar surface of the bat’s feet.
Females that roost in colonies are frequently
infected whereas males are not. Big brown
bats are also infected with cestodes such
as Hymenolepis (tapeworms). Parasitic
trematodes include Dicrocoelium (liver flukes),
Limatulum, and Glyptoporus (Kurta and Baker
1990).
The big brown bat is a vector for St. Louis
encephalitis. A mosquito will bite an infected
bat and then bite a human, which is how the
virus is transmitted (Herbold et. al 1983).
Histoplasma capsulatum, a fungus, is found
in the big brown bat’s tissues and guano.
This fungus causes histoplasmosis (Darling’s
disease) in humans, cats, and dogs, which
affects the lungs (Bartlett et. al 1982).
Eptesicus fuscus is known to carry rabies
throughout the United States (Trimarchi and
Debbie 1977). However, local epizootics are
rare (Kurta 1979; Pybus 1986). In this species,
rabies infects the brain, brown fat, and salivary
glands (Kurta and Baker 1990) but is not
transmitted across the placenta (Constantine
1986). Incubation of the disease has been
observed to last up to 209 days (Moore and
Raymond 1970).
Interspecific interactions.—Common
predators include common grackles, longtailed weasels, various owl species, house
cats, and bullfrogs. Interspecific competition
in foraging areas is known to occur between
Eptesicus fuscus and Lasionycteris noctivagans
(silver-haired bat), as well as Chordeiles minor
(common night hawk). However, the extent to
which these interactions occur in Mississippi
is not known (Reith 1980). Intraspecific
competition occurs at highly aggressive levels
but the significance of these events is unknown
(Kurta and Baker 1990).
BEHAVIOR
Infant big brown bats will sound an “isolation
call” that can be heard from about 10 m away
when they are separated from their mothers
or fall from the nest. Females respond with
an ultrasonic chirping noise (Gould 1971).
Eptesicus fuscus has demonstrates a 24-hr
behavior rhythm that is persistent but inexact
(Twente and Twente 1987). Big brown bats
can use olfactory cues to distinguish among
individual colony members as well as between
young in a maternity colony, which may reduce
agonistic interactions (Boss et. al 2002).
Decreasing ambient temperature appears to
trigger hibernation in big brown bats. While
both the males and females will deposit fat in
anticipation of hibernation, females deposit fat
about one month earlier than males (Pistole
1988). Although females begin fat deposition
earlier, males enter hibernation first (Phillips
1966). Big brown bats typically hibernate at
ambient temperatures below freezing (Barbour
and Davis 1969), using rock crevices, caves,
old buildings, and mines as hibernacula (Mills
et. al 1975). Most big brown bats do not
hibernate in colonies, but small groups are
common (Mumford 1958).
Female big brown bats will usually form
large maternity colonies in trees, caves, and
buildings to give birth and raise their young.
By doing so, thermoregulation costs go down
which is important as stable temperatures are
vital for proper development of young (Boss
et. al 2002). They also take advantage of
cooperative foraging to reduce energy spent
searching for prey. Also, individual predation
risks may be lower in the colonies (Willis and
Brigham 2004).
CONSERVATION
The IUCN lists the big brown bat as a species
of least concern which means it is not in
danger of becoming threatened or endangered
(IUCN 2007). They are, however, ‘critically
imperiled’ in Louisiana. Eptesicus fuscus are
important members of the ecosystems in which
they live. They are primary insect consumers,
which fills an important ecological niche. Also,
pesticides and other man-made chemicals
increasingly pose threats to E. fuscus (Agosta
2002).
ACKNOWLEDGMENTS
I wish to thank my professor Dr. J. Belant for
his help with this project, as well as my fellow
peers who served as editors. I also wish
to thank the staff of the Mitchell Memorial
Library and the College of Forest Resources
at Mississippi State University for assistance
in gaining access to valuable information and
materials.
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Contributing editor of this account was
Clinton Smith.