BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Ondatra zibethicus | Muskrat 



TIMING OF MAJOR LIFE HISTORY EVENTS : 
Breed - March through October, peaking from March through June 
Age of Maturity - from 6 to 8 weeks or 1 year
Gestation - 28 to 30 days
Litter - 4 to 7 kits north of 37 degrees north latitude, with 3 or fewer
         litters per year; 3 to 4 kits south of 37 degrees north
         latitude, with 3 or more litters per year; young are altricial
Weaning - 4 weeks
Life Span - up to 4 years in the wild [14]


PREFERRED HABITAT : 
Muskrats prefer sloughs, marshes, oxbow lakes, streams, levees, dikes,
and small lakes and ponds [1].  Along the Gulf Coast, they prefer
brackish marshes over freshwater marshes.  Muskrats build lodges in or
near water (within 3.3 feet [1 m]), using marsh vegetation.
Alternatively, they construct elaborate bank burrows that may be up to
45 feet (15 m) long [1,14].  Entrances to both lodges and burrows are
usually under water, and both are multichambered.  During periods of low
water, muskrats dig canals from lodges and burrows to deeper water
areas.  They also build feeding platforms to get out of the water to
eat, or feeding huts for protection from the elements and predators
[14].  Muskrats will usually stay within 45 feet (15 m) of their lodges
while foraging, although they can range out to 550 feet (183 m).  Habitat
suitability index models have been developed for muskrats inhabiting
inland freshwater and Atlantic or Gulf coast estuarine areas [1].


COVER REQUIREMENTS : 
Muskrat populations tend to be higher in areas with dense aquatic,
emergent vegetation that is surrounded by terrestrial herbaceous
vegetation.  Forested riverbanks usually do not support muskrat
populations.  High quality habitat is characterized by 50 percent or
more of an area having dense emergent species, although if habitats
become "choked" with vegetation, muskrat numbers will be low.  Ideal
ratios for vegetation to water are 75:25 to 80:20 [1].

Water levels and velocities affect muskrat habitat [14].  Typically, if
levels are too low, food availability will also be low.  This is most
pronounced in winter when low water levels allow freezing of the
substrate, killing food and cover species [1,14].  Stream gradients and
velocities were studied in Massachusetts to determine habitat selection
by muskrats.  Streams with gradients more than 47.5 feet per mile (9.0
m/km) and flows less than 4 cubic feet per second (cfs) (0.1 m3/sec) did
not support muskrats.  However, muskrats were found in streams with
gradients less than 32.2 feet per mile (6.1 m/km) and flows greater than
4 cfs.  River habitats with flows greater than 1,000 cfs (28 m3/sec)
typically experience scouring and water level fluctuations too great to
support muskrat populations [1].

In some cases more than a 2-foot (0.6 m) rise in lake levels forces
muskrats out of burrows and lodges, although some fluctuation is
necessary for regeneration of emergent vegetation [1].  Lake and pond
depths of between 18 inches and 4 feet (0.46-1.2 m) may be ideal.
Islands and coves can usually provide additional shoreline and more
vegetation for food and cover than bodies of water without these
features.  Recommended water depths in Gulf coastal marshes are 0.8 to
11.8 inches (2-30 cm) year-round.  Levels should not fall more than 3.1
inches (8 cm) below the substrate so that Olney bulrush (Scirpus
olneyi), a highly preferred food for mushrats, can regenerate [1].

Muskrats need emergent vegetation and a firm substrate for building
lodges [1].  They rarely use submergent vegetation.  Optimum sites for
bank burrows are on slopes of 30 degrees or more, with a minimum height
of 1.6 feet (0.5 m).  Maximum breeding density for muskrats is 5 pairs
per hectare [14].


FOOD HABITS : 
Muskrats eat the basal parts, rhizomes, and leaves of aquatic emergent
vegetation.  Although they consume mostly plant material, they eat some
fish, crustaceans, dead birds, and frogs [14].  In Gulf coastal marshes,
Olney bulrush is an important food [16].  Plant food species vary with
muskrat distribution, but some of the major foods are cattail, bulrush,
sedge, arrowhead (Sagittaria spp.), waterlily (Nymphaea spp.), wild rice
(Zizania aquatica), sweetflag (Acorus calamus), pondweed (Potamogeton
spp.), pickerelweed (Pontederia cordata), spikerush (Eleocharis spp.),
smartweed (Polygonum spp.), clover (Trifolium spp.), bluestem
(Andropogon spp.), rice (Oryza spp.), panicgrass (Panicum spp.),
paspalum (Paspalum spp.), burreed (Sparganium spp.), millet (Echinochloa
spp.), willow (Salix spp.), poplar (Populus spp.), and some crops.  They
also consume acorns and maple (Acer spp.) samaras [1,14,16].


PREDATORS : 
Muskrat predators include humans, mink (Mustella vison), raccoon
(Procyon lotor), bobcat (Felix rufus), house cat (F. domesticus),
domestic dog (Canis familiaris), coyote (C. latrans), red fox (Vulpes
vulpes), barn owl (Tyto alba), barred owl (Strix varia), great horned
owl (Bubo virginianus), northern harrier (Circus cyaneus), bald eagle
(Haliaeetus leucocephalus), eastern cottonmouth (Agkistrodon
piscivorus), alligator (Alligator mississippiensis), snapping turtle
(Chelydra serpentina), bullfrog (Rana catesbeiana), garfish (Lepisosteus
spp.), bowfin (Amia calva), and largemouth bass (Micropterus salmoides)
[14].  Muskrats will kill the young of other muskrats when populations
are too dense.


MANAGEMENT CONSIDERATIONS : 
A variety of animals use muskrat lodges including snakes, turtles,
toads, Canada geese (Branta canadensis), and black terns (Childonias
niger) [14].  

Muskrats can reduce cattail enough to allow purple loosestrife (Lythrum
salicaria), an undesirable weed, to replace cattail and degrade marsh
quality [15].  

Management of water levels can have a strong impact on muskrat habitat
because of fluctuation influences on certain food species [14].
Drawdowns can have a negative impact on muskrat populations [17].  Low
water levels may encourage undesirable species to take over.  Prolonged
flooding can destroy food plants [14].  Water levels in Louisiana
coastal marshes strongly influence the distribution of Olney bulrush
[13].  Recommended water levels for these areas are between 0.5 to 2.0
inches (1.3-5.0 cm), and never less than 2.0 to 3.1 inches (5-8 cm)
below the substrate [13,14].  Recommended water levels for Maine are
between 6 and 20 inches (15-51 cm) [14].  For detailed information on
how water levels affect cattail refer to Weller [19].  For more detailed
information on general affect of water level refer to Perry [14].

Muskrat populations tend to follow a roughly six- to 14-year cycle,
where low numbers leading to good food supplies are followed by a
population boom and a subsequent decline in muskrat numbers [4,14].  In
Gulf coastal marshes these booms cause "eat-outs", which are areas of
extensive overharvest of Olney bulrush by muskrats.  This usually occurs
in pure Olney bulrush stands, and can have a detrimental impact on
stands.  Bulrush must establish within 5 months after an "eat-out"
occurs or it will die out [16].  Regeneration can occur through
sprouting from rhizome fragments in the substrate.  Prolonged flooding
can delay regeneration in "eat-out" areas.  For more in-depth discussion
on management of Olney bulrush refer to Sipple [16].

In some areas mosquito control projects can destroy marshes, as can
dredging, diking, and urban sprawl [15].


REFERENCES : 
NO-ENTRY

Related categories for Wildlife Species: Ondatra zibethicus | Muskrat