MAMMALS OF MISSISSIPPI 7:1-6
Muskrat (Ondatra zibethicus)
RYAN T. LANGLEY
Department of Wildlife and Fisheries, Mississippi State University, Mississippi State, Mississippi, 39762 USA
Abstract--Ondatra zibethicus (Lowery 1974) is a medium-sized rodent commonly called the muskrat. It is
the only species in the genus Ondatra. This species is endemic to the Northwest Territories and as far south
as the Gulf of Mexico occupying every state except Florida. It is also absent from parts of Georgia, South
Carolina, Texas, Arizona, Nevada, California, and Oregon. It prefers marshes, but it also occurs in rivers,
ditches, ponds, lakes and swamps. The International Union for Conservation of Nature and Natural Resource
Conservation classification for the muskrat is Least Concern.
Published 5 December 2008 by the Department of Wildlife and Fisheries, Mississippi State University.
Muskrat
Ondatra zibethicus (Lowery 1974)
CONTEXT AND CONTENT.
Order Rodentia, Suborder Myomorpha, Family
Muridae, Subfamily Microtinae
GENERAL CHARACTERS
Fur color is generally dark brown, but can vary
from white and silver to tan to reddish brown
and black (Fig. 1). The tail is usually dark
brown, and the feet are usually dark brown or
black in color.
Fig. 1. Adult bobcat Lynx rufus. http://www.kiawahisland.
org/Wildlife/Bobcats.aspx
The total body length of the muskrat is from
440 to 526 mm, the tail measures from 180
to 256 mm, the hind foot from 63 to 87 mm,
and the ears from 15 to 25 mm (Gould and
Kreeger in Lowery 1974). The total weight of
adults ranges from 700 to over 1,800 g (Walker
et. al.1975). Average skull length (Fig. 3) is
66.3 mm for males and 64.8 mm for females.
The average length of diastema is 22.8 mm in
males and 22.2 mm in females. The zygomatic
breadth averages 41.4 mm in males and 40.4
mm in females (Gould and Kreeger in Lowery
1974). The sexes are similar in size and
appearance and can be easily confused. The
female’s clitoris can be mistaken for the male’s
penis. In females, the space between the
vagina and the anal opening is bare, whereas in
Fig. 2. Geographic distribution of Lynx rufus. Map
redrawn from North American Mammals. http://www.
mnh.si.edu
1958). They are weaned about 4 weeks after
birth (Lowery 1974). During the second month
after birth, the tail becomes laterally flattened
(Errington 1963). Muskrats usually become
sexually mature the spring following their birth
(Errington 1963).
Fig. 3.––Distribution of Mustela frenata in North America.
males the area between the penis and the anal
opening is well furred (Gould and Kreeger in
Lowery 1974).
DISTRIBUTION
Muskrats range from northern Alaska and
Canada to the southern United States (Fig.
2). Muskrats occur throughout the state of
Mississippi and Louisiana. They are absent
from Florida, and parts of Georgia, South
Carolina, Texas, Arizona, Nevada, California,
and Oregon (Lowery 1974).
FORM AND FUNCTION
Most females have two pairs of mammae
between the front and hind legs, one pectoral
pair and two pairs in the groin. Some have
been known to have up to 4 or 5 pairs if
mammae (Godin 1977). The dental formula is i
1/1, c 0/0, p 0/0, m 3/3 (Hall and Kelson 1959).
The muskrat has large incisors well suited for
gnawing and transporting objects through the
water. The lips can close behind the incisors
to keep water out of the mouth when gnawing
under water (Godin 1977).
ONTOGENY AND REPRODUCTION
Muskrat neonates are blind with little hair and a
pink to grey color with a rounded tail (Errington
1963). Incisors begin to erupt between 6 and
7 days, and the eyes open between 14 and
16 days post-parturition. Young muskrats are
covered with a soft fur and are able to swim
within 14 days of birth (Erikson 1963; Sather
Muskrats in Mississippi breed throughout
the year, with reproductive activity peaking
during winter (O’Neil 1949). Estrous cycles
last between 3 to 6 days (McLeod and Bondar
1952). Average litter size varies from 4 to 8
young, with muskrats living in poor habitat
having fewer young per litter and fewer litters
per year (Neal 1968). Spermatogenesis occurs
from spring, through late fall (Errington 1963;
Forbes 1942).
ECOLOGY
The major cause of muskrat mortality is
trapping for fur (Errington 1963). The typical
lifespan of a muskrat is 3 to 4 years (Godin
1977).
Muskrats dig burrows into banks and also
create conical houses with underwater tunnels
to the entrances (MacArther and Aleksiuk
1979). Muskrat burrows can damage river and
pond banks, and can also affect agricultural
land. The two types of houses constructed
by muskrats are main dwellings and feeding
houses (Dozier 1948a). Feeding houses are
usually smaller than the main dwellings and
are usually largest in the winter, with thicker
walls and floors (MacArther and Aleksiuk
1979). The average size of a muskrat house
ranges from 1 to 2 m in diameter at the base
and from 600 to 1200 mm in height. Muskrat
houses provide a dry nest area, and protection
from predators and weather (Lowery 1974).
House construction begins during ice-free
periods usually peaking between May and
early June, and again in October (Danell 1978).
Temperatures inside muskrat houses are higher
than ambient temperature (Earhart 1969).
Diet consists of aquatic vegetation, and
occasionally some animal matter. Muskrats
living in marshes typically feed on cattails
(Typha), alligator grass (Alternanthera
philoxeroides), Roseau cane (Phragmites
communis) shoots, delta duck potato (Sagittaria
platyphylla), and dog-tooth grass (Panicum
repens). In the brackish Subdelta, their diet
consists of wire grass (Spartina patens),
hogcane (Spartina cynosuroides), Roseau cane
(Phragmites communis) shoots, giant bulrush
(Scirpus californicus), coco grass (Scirpus
robustus), oystergrass (Spartina alternatiflora),
spike rush tubers (Eleocharis sp.), delta duck
potato, pickerelweed (Pontederia cordata),
and black rush (Juncus roemerianus). In fresh
Subdelta marshes, plants eaten are paille
fine (Panicum hemitomon), cattails, wapato
(Sagitarria latifolia), Roseau cane shoots,
giant bulrush, and softstem bulrush (Scirpus
validus). In prairie marshes, muskrats eat coco
grass, wiregrass, hogcane, cattails, bulrush,
lakegrass, salt grass (Distichlis spicata),
dwarf spike rush (Eleocharis parvula), black
rush, pickerelweed, Roseau cane shoots,
oystergrass, and alligator weed (Lowery 1974).
Some diseases associated with the muskrat
include: adiasspiromyosis (Otcenasek et al.
1974), epizootic chlamydiosis (Spalatin et
al. 1971), hemorrhagic disease (Errington
1963), leptospirosis (Al Saadi and Post 1976),
pseudotuberculosis (Langford 1972), ringworm
disease (Errington 1963), salmonellosis
(Armstrong 1942), tularemia (Errington 1963),
Tyzzer’s disease (Chalmers and MacNeill
1977), and yellow fat disease (Debbie
1968). The mite, Tetragonyssus spiniger,
and flesh-fly larvae are abundant in the fur
and nests of muskrats. Other parasites of
the muskrat include: nematodes, Trichuris
opaca; trematodes, Echinostoma revolutum,
Plagiorchis proximus, and Quinqueserialis
quinqueserialis, and cestodes Hymenolepis
spp., and Taenia taeniaeformis (McKenzie and
Welsh 1979).
Muskrats and nutria (Myocastor coypus)
have similar feeding habits consuming large
quantities of stems; therefore competition may
occur between the two (Wilner et al. 1979).
Muskrats have many predators, especially
immatures. Predators include: mink, raccoons,
owls, alligators, ants, hawks, snakes, bullfrogs,
gar, bowfins, snapping turtles, bass, hogs, cats,
and dogs (O’Neil 1949).
BEHAVIOR
Muskrats become aggressive before and
during the breeding season. Dominance
is established through fighting, and the low
ranking muskrats leave the area and are more
susceptible to predators and cannibalization
(Steiniger 1976). Competition for breeding sites
is intense, and the larger and older members
of the population occupy the best burrows.
Females are more aggressive than males when
defending breeding territory, sometimes killing
intruders (Errington 1963).
Mating occurs while partially submerged in the
water. Prior to mating, scent is released from
anal glands to attract a mate (Svihla and Svihla
1931). Females give birth in nest chambers of
houses or in the nests of some ducks. Females
care for the young, and on some rare occasions
if the female dies, the male will care for the
young (Errington 1963).
CONSERVATION
Muskrats cause extensive damage to treatment
wetlands by grazing the vegetation and digging
burrows in levees and berms (Kadlec et. al
2007). Muskrats can be managed by lethal
and non-lethal trapping. Foothold and conibear
traps are examples of lethal methods. Foothold
traps hold on to the foot of the muskrat and
they drown after captured. Conibear traps kill
instantly when they are triggered. Muskrats
can also be captured alive in a family trap,
relocated and released. Family traps consist
of a cage that muskrats enter and cannot exit
(Snead 1950).
ACKNOWLEDGEMENTS
I would like to thank Dr. Jerrold L. Belant for all
of his time and help formatting and revising this
account.
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Contributing editor of this account was
Clinton Smith.