MAMMALS OF MISSISSIPPI 1:1-8
Virginia Opossum (Didelphis virginiana)
BRITTANY L. WILEMON
Department of Wildlife and Fisheries, Mississippi State University, Mississippi State, Mississippi, 39762, USA
Abstract.—Didelphis virginiana is a small marsupial more commonly known as the opossum. Found
primarily in the eastern United States, it is a very hardy mammal that is usually gray with a lighter
shade in the north and a darker shade in the south. Known for its opposable tail and its ability to
feign death, this primarily nocturnal mammal prefers wooded and moist areas. Didelphis virginiana is
a species of little concern, with populations expanding to the north and west.
Published 5 December 2008 by the Department of Wildlife and Fisheries, Mississippi State University
Virginia opossum
Didelphis virginiana (Kerr, 1792)
CONTEXT AND CONTENT
Order Didelphimorphia, Family Didelphidae,
Subfamily Didelphinae, Genus Didelphis. Four
subspecies are recognized.
• Subspecies virginiana
• Subspecies californica
• Subspecies pigra
• Subspecies yucatanensis
•
GENERAL CHARACTERS
The Virginia opossum ranges in color from
a light gray in the north to a dark gray in
the southern part of the range. Males are
relatively larger than females (McManus
1974). Body length ranges from 38 to 51 cm
with males occupying the larger end of the
Fig. 1. Virginia opossum. Photograph by Karen.
Courtesy of www.flickr.com
spectrum. Weight ranges from 1.9 to 2.8 kg
(McManus 1974). Average life expectancy is
approximately 1.5 years. The length of the
tail is relatively large compared to the body
length. The tail is usually around 90 percent of
the body length (McManus 1974). The tail is
hairless and scale like. The ears are hairless
and are dark gray or black in coloration. The
adult dental formula (Fig. 2) of the Virginia
opossum is i 5/4, c 1/1, p 3/3, m 4/4, 50 total
(McManus 1974).
DISTRIBUTION
The Virginia opossum has been noted as one
of the most successful mammal species in
history. This is due to the recent expansion
of the species northern and western range
(Weber and O’Conner 2000). The Virginia
opossum is found in the south and northeastern
Fig. 2. Dorsal and lateral view of adult Didelphis
virginiana. Average length of skull is 11 cm.
Photograph courtesy of Skulls Unlimited.
Fig. 3. Geographic distribution of Didelphis
virginiana. Photo courtesy of the Smithsonian Book
of North American Mammals.
regions of the United States. The species is
widely distributed through all of Mississippi.
This species is found as far south as Central
America and has recently expanded as far
north as southern Canada (Kanda 2005).
The only limitation is the climatic limitation of
temperature. The northern boundary of the
opossums region is limited by the low winter
temperatures (Kanda et al. 2005). It is very
common for species to experience frostbite or
mortality due to starvation at the northern edge
of their boundaries (Kanda 2005).
FORM AND FUNCTION
Form.—The Virginia opossum is a relatively
hardy and stout individual. Males, at 2.8 kg,
are slightly larger than females, which usually
reach a weight of about 1.9kg. Species in the
north tend to have a denser underfur, which
serves a thermoregulatory function. The
underfur tends to be white in the northern
regions and may have dark coloring on the
tips. The species also has guard hairs.
Opossum populations in the south tend to have
a thinner underfur and the guard hairs tend
to be darker. This gives the animal a darker
overall appearance (McManus 1974). Sweat
glands have proved to be nonfunctional and
skin glands are located on the ventral side of
males. Female teat numbers can range from
ten to seventeen, but thirteen is most common.
The Virginia opossum has a relatively small
brain case, and the corpus callosum is absent
in this species. Vertebral numbers remain
fixed throughout growth and maturation – 7
cervical, 13 thoracic, 6 lumbar, 2 sacral, and
26 to 29 caudal (McManus 1974). The species
possesses an opposable hallux on the hind
feet. This hallux does not bear a claw, unlike
the other digits. Each of the opossum’s feet is
covered by ridges used for friction in climbing
(McManus 1974). The reproductive system
of the male is composed of a scrotum and a
hemipenis. The female reproductive system is
composed of three vagina, two vagina, which
receive the sperm and are laterally placed, and
a centrally placed median vagina, which serves
the purpose of a birth canal (McManus 1974).
Function.—The Virginia opossum has a
much lower amount of expanded energy due
to thermoregulation than any other placental
mammal (Weber and O’Conner 2000). It
is a homeotherm that can maintain its body
temperature at ambient temperatures that are
lower than zero degrees Celsius (McManus
1969). Many thermoregulatory measures are
taken by the opossum in low temperatures;
this species has been known to use shivering,
vasoconstriction, piloerection, and even
avoidance of the low temperatures (McManus
1974). Signs of temperature regulation were
first noted in young at the age of 55 to 60
days old, at which the young are still living
in the pouch of the female (McManus 1969).
At 95 days of age young Virginia opossums
were found to be able to hold a deep body
temperature constant at ambient temperatures
as low as five degrees Celsius for a time period
of two hours. The average heart rate of a
normal Virginia opossum is approximately 200
beats per minute. After ten days of age, young
develop the means to fight infection (McManus
1974).
ONTOGENY AND REPRODUCTION
The mating season of the Virginia opossum
begins in January or February and lasts
through June or July in southern states that
have been studied such as Florida, Georgia,
and Texas (McManus 1974). The reproduction
rate of the Virginia opossum was found to be
approximately 5.5 (Llewellyn and Dale 1964).
Reproduction yields the highest percentages
during late January through late March and
May through early June. The estrous cycle
averages about 30 days (McManus 1974).
The female is receptive for no longer than
a 36-hour period. Receptiveness ends with
copulation (McManus 1967). Each of the
two ovaries releases on average 11 ova.
Fertilization actually occurs in the fallopian
tubes, and the fertilized ovary reaches the
uterus approximately 24 hours after fertilization.
Parturition or birth usually takes place 13 days
after copulation. Even though the female
may posses as many as 13 teats the average
litter size is approximately 7 to 9 due to death
of some of the neonates. A female usually
produces two litters per year with young usually
weighing approximately 0.15 grams at the
time of birth. The young are considered to be
altricial at the time of birth (McManus 1974).
In the southern distribution range two litter per
year are most common even though a very low
percentage of females may produce three (Lay
1942). The further north the opossum lives,
the less likely they are to produce two litters
per year, but on average the same amount
of young are produced in the southern and
northern regions (Kanda and Fuller 2004).
The young remain in the pouch, attached to
the nipple up to 65 days of age. Mortality
may occur in the females pouch. The young
are then weaned from the mother at 95 days.
Estrus can be resumed after the young or
weaned (McManus 1974).
ECOLOGY
Population characteristics.—Young begin
to disperse and forage alone around 90 days
of age, but they still occasionally return to the
mothers den during the day (Gillete 1980). The
dispersion rate for this species was noted in a
particular study to be approximately four miles
per year (Long and Copes 1968). The sex ratio
is slightly preferential to males at 52 percent
compared to females at 48 percent. Of the 106
adults examined, 56 were males and 50 were
females (Llewellyn and Dale 1964).
Space use.—The Virginia opossum uses a
wide range of habitats. These habitats can be
arid, dry environments, but the species usually
prefers wet areas, near a stream or other body
of water. It is very difficult to estimate home
range due to the excessive movement of
opossums. One study provided a home range
of approximately 11.5 acres but that is only an
estimation. The opossum’s home ranges are
more often a longer course rather than a circle
shaped course (McManus 1974). One study
showed that one in every four dens in the range
was occupied by the opossum. The fact that
more dens were always available than were in
use proves that den factors place no limitation
on the opossum. The preferred dens were
usually covered with woody cover or another
type of protection (Lay 1942).
Diet.—The Virginia opossum is omnivorous,
but the opossums’ diet is mainly composed
of insects when they are available. Insects
consumed ranged from different species of
crickets to caterpillars. Opossums will also
consume a wide range of larvae. The diet of
the opossum is also comprised of vegetation,
grasses, and fruit. Most of the fruit that is
consumed has fallen from the tree or bush and
is readily eaten off of the ground (Hamilton
1953). The species has also been known
to consume amphibians, snakes, and birds
on occasion. The stomach contents of an
opossum population were examined and the
contents proved that the species will consume
small mammals such as cottontail rabbits,
squirrels, and mice. It was also found that the
species will consume other members of the
species (Taube 1947). In one particular study,
the Virginia opossum was found to heavily
rely on earthworms during the winter season
(Dexter 1951).
Diseases and Parasites.—Opossums can
become infected with protozoa’s such as a
trypanosome. The Virginia opossum was
first found to be infected with this disease
in Texas. Virginia opossums can also be
infected with Leptospirosis, which is caused
by L. Pomona. The first opossums to ever be
noted as carriers of L. Pomona were found in
Virginia and Louisiana (Barr 1963). Opossums
are also susceptible to yellow fever and they
do not produce a good antibody response to
the disease. Opossums have been know to
be carriers of rabies. Rabid opossums have
a very widespread distribution and have been
noted to be present in many different areas
of the United States. Opossums have been
known to contract rabies mainly through
contact with other infected species. In one
particular study, 34 opossums were examined
and only 4 of them showed symptoms of the
virus. The symptoms recorded were signs
of an affected central nervous system. The
4 opossums that were infected were capable
of transmission (Barr 1963). The main
ectoparasite that affects opossums is ticks. A
study examined 56 individuals and only 5 of the
individuals were completely free of ticks. The
infected individuals were somewhat less fit,
but there was no major impact on the infected
individuals. Fleas have also been observed in
many opossums (Lay 1942).
Interspecific interactions.—The great horned
owl is the major predator for Virginia opossums.
Other predators known to the opossum are
carnivores such as dogs and coyotes (Fitch
and Shirer 1970). Many avian predators can
be detrimental to young populations of young
opossums (McManus 1974). Opossums
have often been found living in dens that are
characteristic of armadillos, and on certain
occasions a male opossum was found to be
sharing a den with an armadillo (Lay 1942).
It has been noted in some specific research
experiments that opossums and raccoons will
often compete for resources that are necessary
for survival when their habitats are being
reduced (Ginger et al. 2003).
BEHAVIOR
Grouping behavior.—The Virginia opossum
uses three main types of locomotion, which
includes arboreal, terrestrial, and aquatic. The
terrestrial locomotion of the Virginia opossum
is characterized as plantigrade-quadrupedal.
Running speeds of the species have been
recorded up to approximately 7.4 kilometers per
hour. Arboreal locomotion is facilitated mainly
by the prehensile tail and friction ridges that are
found on the plantar surfaces on the bottom of
the feet. The opossum also has an opposable
hallux that aids somewhat in arboreal
locomotion. Arboreal locomotion is typically
slow (McManus 1974). It has been noted
that the opossum will employ swimming but
mainly as an escape mechanism. The Virginia
opossum has been described as a strong
swimmer and has been noted to have no fear
of water. All accounts state that immediately
after entering the water the opossum would
engage in underwater swimming. The species
engages in two different types of swimming
(Doutt 1954). The swimming technique used
most often resembles terrestrial locomotion
used by the animal and the other has been
described as “similar to that of a pacing horse”
(Doutt 1954). The toes are usually spread
apart during swimming, and the tail has been
noted to move from side to side. The species is
known to defecate in the water while engaging
in swimming activities. The opossum floats
with relative ease. The animal controls the
closing of its nostrils with ease and has been
observed resting while completely submerged
under water. The eyes may also remain open
during under water swimming. Shivering has
been recorded in response to swimming in
low temperature water. It has been stated
that large bodies of water, especially cold
water, might serve as a distribution barrier
due to the opossum’s rapid exhaustion and
shivering response (Doutt 1954). It has also
been noted that the opossum will dive and
travel under water up to a distance of 15 feet
without surfacing to breath. The opossum
was examined upon exiting the water and
the pouch carrying the young was sealed so
tightly that it could not be opened. This may
prove previously stated knowledge to be true
that a mother opossum can close her pouch
so tightly that water may not enter (Moore
1955). Extensive grooming is carried out by
the hind foot. Many defensive techniques and
behaviors have been noted and studied in the
opossum. In response to threatening stimuli
the animal will hiss and growl, bare teeth, and
discharge a secretion from two anal glands
(McManus 1974). Feigning death is one of
the opossums most noted and described
behavioral traits. This trait allows the opossum
to greatly decrease mortality due to predation.
Feigning death has been described as the
animal becoming very still or “freezing” and
somewhat curling its body and falling onto
its side. The tail is tucked between the legs
and the mouth is drawn back and the tongue
extends out of the mouth. The eyes have
been noted to remain open but slightly less
than when not feigning death (Franco1969).
Reactions to tactile stimulation are reduced
during this display. This can last from a few
minutes to up to six hours (McManus 1974).
Virginia opossums have been observed to be
most active soon after dusk until dawn. The
species is almost exclusively nocturnal. Social
behavior is not well developed, and with the
exception of mating, the species is solitary.
Most encounters between adult opossums
are usually hostile and aggressive (McManus
1974).
Reproductive behavior.—The “metallic click”
is the only vocalization used by the opossum
during mating. Mating occurs when the male
straddles the female and holds the female’s
hind legs with his hind feet causing the pair to
fall over to the side. Insertion of the penis then
occurs and the couple can remain joined for
up to twenty minutes. After insemination has
occurred the female resumes her aggressive
behavior toward the male (McManus 1967).
The female opossum participates in very little
maternal behavior. The mother will groom
the young, known as joeys, as long as they
remain attached to the nipple. Once they are
detached, the young must groom themselves.
Females observed in captivity have never been
seen defending their young (McManus 1974).
The main use of the opossums’ tail has been
noted to be nest building. On many occasions
the opossum has been observed gathering
leaves and twigs with the mouth and passing
them under the arched body with the help of the
forefeet. The material was then placed in the
curled tail and immediately taken back to the
nest box (Layne 1951). Another observation
of use of the tail for nest building described the
opossum using its tail to hold its hindquarters
off the ground while placing the gathered
leaves in the curled tail. This was repeated
many times and the opossum took the gathered
materials into the hole being observed. The
opossum defended the hole to a degree before
running away (Smith 1941).
Communication.—Vocal noises are comprised
of hisses and growls, and they are usually
used during aggressive encounters. Another
sound, the “metallic click” is used in a variety
of situations but mainly during mating. All
communications between adult opossums are
aggressive except during mating (McManus
1974).
Behavioral responses.—Many behavioral
responses to increased temperatures have
been noted in the Virginia opossum. The
response of saliva spreading was the first to be
observed. The feet and the snout were usually
the first to be licked. The opossum has also
been noted to urinate and defecate in response
to high temperatures before deep breathing
began. Many of the subjects were observed
lying on their sides while breathing rapidly
(McManus 1969).
GENETICS
The Virginia opossum possesses 22
chromosomes and 4 pair of sex chromosomes
(McManus 1974).
CONSERVATION
Conservation of the opossum is not a major
concern due to the widespread distribution
of the species. Opossums are used as a fur
resource in many southern areas. The fur or
pelts are not worth much individually, so a large
number of opossum skins must be harvested
(Lay 1942).
LITERATURE CITED
Barr, T. R. B. 1963. Infectious diseases in
the opossum: a review. Journal of
Wildlife Management 27:53-71.
Dexter, R. W. 1951. Earthworms in the winter
diet of the opossum and raccoon.
Journal of Mammalogy 32:464.
Doutt, J. K. 1954. The swimming of the
opossum. Journal of Mammalogy
35:581-583
Fitch, H. S., and H. W. Shirer. 1970. A
radiotelemetric study of spatial
relationships in the opossum. American
Midland Naturalist 84:170-186.
Franco, E. N. 1969. Behavioral aspects of
feigned death in the opossum, Didelphis
marsupialis. American Midland \
Naturalist 81:556-568.
Gillete, L. N. 1980. Movement patterns of
radio-tagged opossums in Wisconsin.
American Midland Naturalist 104 (1) 1.
Ginger, S.M., E.C. Hellgren, M.A. Kasparian,
L.P. Levesque, D.M. Engle, and D.M.
Leslie, Jr. 2003. Niche shift by Virginia
opossum following reduction of a
putative competitor, the raccoon. Journal
of Mammalogy 84(4):1279-1291.
Hamilton, W.J., Jr. 1953. The food of the
opossum in New York State. Journal of
Wildlife Management 15:258-264.
Kanda, L. L., and T. K. Fuller. 2004.
Demographic responses of Virginia
opossums to limitation at their northern
boundary. Canadian Journal of Zoology
82: 1126-1134.
Kanda, L.L. 2005. Winter energetics of Virginia
opossums Didelphis virginiana
and implications for the species’ northern
distributional limit. Ecography 28:731744.
Kanda, L.L., T.K. Fuller, and K.D. Friedland.
2005. Temperature sensor evaluation of
opossum winter activity. Wildlife Society
Bulletin 33(4):1425-1431.
Lay, D.W. 1942. Ecology of the opossum in
eastern Texas. Journal of Mammalogy
23:147-159.
Layne, J.N. 1951. The use of the tail by an
opossum. Journal of Mammalogy
32:464-465.
Llewellyn, L.M. and F.H. Dale. 1964. Notes
on the ecology of the opossum in
Maryland. Journal of Mammalogy
45:113-122.
Long, C. A., and F. A. Copes. 1968. Notes
on the rate of dispersion of the opossum
in Wisconsin. American Midland
Naturalist 80:283-284.
McManus, J.J. 1967. Observations on sexual
behavior in the opossum, Didelphis
marsupialis. Journal of Mammalogy
48:486-487.
McManus, J.J. 1969. Temperature regulation in
the opossum, Didelphis marsupialis
virginiana. Journal of Mammalogy
50:550-558.
McManus, J.J. 1974. Didelphis virginiana.
Mammalian Species 40:1-6.
Moore, J. C., 1955. Opossum taking refuge
under water. Journal of Mammalogy
36:559-561.
Smith, L. 1941. An observation on the nestbuilding behavior of the opossum.
Journal of Mammalogy 22:201-202
Taube, C.M. 1947. Food habits of Michigan
opossums. Journal of Wildlife
Management 11:97-103.
Weber, J.M. and T. O’Connor. 2000.
Energy metabolism of the Virginia
opossum during fasting and exercise.
The Journal of Experimental Biology
203:1365-1371.
Contributing editor of this account was
Clinton Smith.