Point Arena mountain beaver
(PAMB)
Aplodontia
rufa nigra
Taxonomy
PAMB is
one of seven recognized subspecies of Aplodontia.
Aplodontia Range
The seven subspecies range from
southern British Columbia to Point Reyes, California and the Cascade and
Sierra Nevada Ranges.
Reasons for Subspecies Status
Genetic isolation
Unique black
coloring
Certain anatomical
features
PAMB Distribution
PAMB is known
only from 62 square kilometers (15,320 acres) in coastal
Mendocino County.
The USFWS considers the
potential range to be located south of a point
two miles north of Bridgeport landing, north
of a point five miles south of the town
of Point Arena, and to a distance
of five miles inland from the Pacific Ocean.
In 1988, 26 separate known populations had been
documented, with an estimated 200-500 animals total. To date more PAMB
locations have been discovered.
Description
Mountain
beaver are semi-fossorial rodents and well adapted for their burrowing
lifestyle. They
are stout, compact, and shaped like a football.
The front legs are muscular and well adapted for digging with long
curved claws.
An average
adult Aplodontia weights between 2-4 lbs and measures approximately 12
inches.
PAMB are
the smallest of the California subspecies and are distinguished by their
unique black coloring.
Life History
Reproduction:
Aplodontia
have an unusually low reproductive rate for rodents; females usually do not
bear young until their second year, have a single litter, with an average of
2-3 young (Scheffer 1929).
Pfeiffer (1958) performed
reproductive studies on several subspecies of mountain beaver and found that
there is a sharply defined and extremely limited breeding season of
approximately 5-7 weeks.
All the females of a given
population ovulate spontaneously together at about the same time each year.
Breeding Season:
The USFWS PAMB
restriction period:
Breeding Season-
Dec. 15 through June 30
Dispersal
Season - April 15 through Sept. 30
the
temperate northern California coast may have an
earlier breeding season than those in the Pacific Northwest.
Aplodontia in the Pacific
Northwest breed in February / March
with parturition in late March and early April.
Pfeiffer estimated a
PAMB parturition date of February 20,
this indicates a breeding
season from December to early
January.
Life Span
5-6 years
Predators of
Aplodontia
Bobcat
Great horned owl
Mountain lion
Coyote
Golden eagle
Mink
Fisher
Domestic dogs
Short-tailed
weasel
Red-tailed hawk
Water and Dietary
Requirements
Aplodontia have
primitive simple kidney structure
Unable to
concentrate urine effectively
Require large
quantities of water daily- a diet
heavy with succulent plants (Nungesser and Pfeiffer 1965).
Ecology
Distribution
Pattern:
Aplodontia exhibit a contagious/clumpy
distribution pattern
A population
consists of groups of individuals, each
with their own burrow system.
Aplodontia will readily move
into vacated systems
Burrow System:
A typical burrow system includes
chambers for nests, latrines, food storage and feeding, and perhaps earth
ball storage areas. The
complexity and numbers of chambers and burrows depends upon the age of the
system. Old systems are
recognized by well worn, hard packed burrow floors and smooth burrow walls.
A sub-adult system may contain only two or three chambers, for a
nest, fecal deposition, and food storage/feeding.
Home Range Size:
Aplodontia are
considered solitary animals
and defend their individuals nest sites
and adjacent burrows, but tolerate sharing
the outer burrows where home ranges
overlap.
They will avoid
contact except for the short breeding
period, even though their home ranges
overlap significantly.
Martin (1971) reported the
average home range size varied
from 0.08 to 0.5ac with an average of 0.3ac.
Lovejoy (1979) reported the
average home range size of
0.79ac for males and 0.42ac for females.
New research at the Olympia Field
Station in Washington reported a substantially
larger average home range size of
10.27ac and 2.89ac on a
second site of different habitat.
Plant Community
Types Associated With
PAMB
PAMB are found in
many plant community types
and areas of transition.
1. Northern coastal
scrub
2. Coastal bluff
scrub
3. Northern riparian
scrub
4. North coast
riparian
5. Coastal prairie
6. Northern dune
scrub
7. Fresh water seep
8. Alder/herbaceous
ground cover
9. Conifers/sword fern
10. Roadside habitat
Areas not considered
as potential habitat are those
containing only annual grasses, or those
comprised of conifers or hardwoods with
little or no ground cover.
