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Behavior, Dispersal, and Survival of
Captive-Raised Idaho Pygmy Rabbits (Brachylagus idahoensis)
Released onto the INEEL in Idaho
Background
The
pygmy rabbit (Brachylagus idahoensis) is the smallest
rabbit in North America, a sagebrush foraging specialist, and
one of only two North American rabbits to dig its own burrow.
The long-isolated and genetically unique population of Columbia
Basin pygmy rabbits located in eastern Washington State has
declined precipitously to dangerously low levels and the U.S.
Fish and Wildlife Service (USFWS) recently listed the Washington
pygmy rabbits as an endangered population segment under the
Endangered Species Act. Because little is known about successful
captive-rearing and methods for restoring pygmy rabbits back
into vacant natural habitats, reintroduction techniques in
southeastern Idaho are being tested to develop protocols for the
eventual restoration of endangered pygmy rabbits in Washington
State. Idaho pygmy rabbits are propagated in captivity at
Washington State University (WSU) and elsewhere and released
into the wild in southeastern Idaho. The Idaho Fish and Game
Department supervises these releases to determine whether
selected captive rearing and release methods influence the
behavior, dispersal, and survival of pygmy rabbits reintroduced
into suitable sagebrush habitat.
Objectives
- Develop techniques to enhance
the survival of captive-bred Idaho pygmy rabbits released into
natural habitats for the p
 urpose
of establishing new local populations of pygmy rabbits.
- Test the effects of
captive-rearing and release methods on the resulting behavior,
dispersal, and survival of reintroduced pygmy rabbits.
- Develop recommended protocols
for restoring pygmy rabbits in areas of vacant, suitable
sagebrush habitat, and model the numbers of captive-bred
animals and survival rates needed to establish new local
breeding populations.
Accomplishments through 2004
A total of 42 pygmy rabbits were released from 2002-2004 at
the INEEL to study behavior and survival of reintroduced
animals. Rabbits originating from a source population in Idaho
were raised in captivity at WSU, fitted with radio collars
weighing < 2 percent of body weight, and released into
temporary, weld-wire containment pens on the INEEL. The
temporary pens surrounded the two openings of 3.0 to 4.5 m (10
to 15 ft) long plastic drainage tube burrows dug into the soil
about 0.75 to 1.0 m (2.5 to 3.5 ft) deep in the center. The
plastic-tubing burrows were used to partially replicate a
natural pygmy rabbit burrow system and provide both thermal
buffering and some protection against digging predators. Another
goal of the artificial burrow system was to reduce premature
dispersal of rabbits away from the release site selected in good
sagebrush habitat. Released rabbits were monitored almost daily
to record behavior, dispersal and habitat use.
Results
All released rabbits readily adapted to the small, temporary
holding cages surrounding their burrow openings and continued
normal feeding on provided foods (i.e., sagebrush tips, spinach,
lettuce, pellet food). All containment pens were removed from
the burrows by the fourth day, allowing free movement and
dispersal of the animals.
Rabbits moved an average of 54.1
m (177.5 ft) from their initial release burrow during their
first week after soft release. Most rabbits remained fairly
localized on the release site. Mean movement distances did not
vary significantly among the first, second, or third week after
soft release. Most captive-bred, dispersing animals selected an
appropriate habitat consisting of relatively tall, dense big
sagebrush with relatively good grass and forb availability.
Released animals appeared to adapt to natural local forage
quickly and appeared to use a high proportion of grass and forbs
until colder weather in fall and winter, which prompted greater
use of sagebrush.
Predation was the main source of
mortality for released pygmy rabbits. Of the 42 released
animals, approximately 26 percent were censured from the study
(primarily because radio signals were lost and because of one
collar malfunction), 42 percent were lost to predators, 19
percent were lost to unknown mortality factors, and 12 percent
were alive at the end of the project. Eighteen of the 27
documented mortalities were caused by predators. Four
mortalities were caused by raptors; northern harriers (Circus
cyaneus) were directly observed in two predation events.
Twelve animals were killed by long-tailed weasels (Mustela
frenata) and two were confirmed coyote (Canis latrans)
kills.
Survival -
Total survivorship for the release population was 0.138
(Standard Error ± 0.085). This survivorship translates to an
annual survival rate of 32 percent. Age and sex did not
significantly influence survival, although the ability to detect
such differences was limited. Males and females had similar
survivorship; however, females experienced a higher mean
survival time (175.7 days) than males (83.6 days). Annual
survival rate was 18 percent for males and 30 percent for
females.
Survival varied significantly among seasons (i.e., release
groups). The annual survival rate was 0 percent for July, 24
percent for August, 32 percent for September, and 18 percent for
February. However, the February release group had 50 percent of
the rabbits released from the soft-release cages survive until
the breeding season.
Survival quantiles for the released rabbits show a 76 percent
survivorship for the first six days post soft-release, declining
to 28 percent by day 95. Survivorship did not drop below 25
percent until day 260.
Reproduction of Reintroduced Pygmy Rabbits - At
least two of the surviving released females appear to have given
birth on the INEEL release site. One of the females was observed
in 2003 and one in 2004. Consequently, it appears that surviving
females will produce litters in the first spring after their
release.
Plans for
Continuation
This study on the INEEL has been a major research component
of the recovery program for the endangered Columbia Basin pygmy
rabbit in Washington, but will also provide valuable information
in the event that local reintroductions are ever needed for
populations of Idaho pygmy rabbits. The study was terminated on
the INEEL and two graduate theses were completed at WSU in
summer 2004, to finish the research project. Technical research
publications currently are being prepared from the theses for
publishing in scientific journals. Interested parties may
contact the investigators for more information.
Investigators and Affiliations
Rodney D. Sayler, Associate Professor, Department of Natural
Resource Sciences, Washington State University, Pullman, WA
Lisa A. Shipley, Associate Professor, Department of Natural Resource
Sciences, Washington State University, Pullman, WA
Robert Westra, Graduate Student, Department of Natural Resource
Sciences, Washington State University, Pullman, WA
Funding Sources
Washington Department of Fish and Wildlife
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