330
Laboratory Animals (1993) 27, 330-341
The effects of group housing on the research use of the
laboratory rabbit
MARK WHARY, RANDALL
FREDERICK
FERGUSON
PEPER,
GARY BORKOWSKI,
WENDY
LAWRENCE
&
The Pennsylvania State University, Office of the Sr. Vice President for Research and Dean of the Graduate
School, Intercollege Research Programs, Laboratory Animal Resources Program and the Veterinary Science
Department of the College of Agricultural Sciences, University Park, Pa, 16802, USA
Summary
This project evaluated the influence of group
housing on common aspects of research use of
female laboratory rabbits (Oryctolagus cuniculus).
Eight rabbits housed individually in conventional
cages were compared to a second group of 8
housed as a social group in a proportionately
larger enclosure. The group housing method
provided increased opportunities
for exercise,
social contact, and a more novel environment.
As a function of housing style, the 2 experimental groups were compared on humoral and
delayed hypersensitivity
response, feed intake,
growth rate, and selected physiological parameters that are considered to reflect stress in
most species. Single and group housed rabbits
did not significantly differ in physiological and
immunological measurements, indicating that the
practical research performance (immune response,
stress level, growth rates etc.) of these rabbits was
not significantly
affected by group housing
compared
with the more traditional
single
housing. Analysis of group social behaviour
indicated that the rabbits preferred small social
groups, had preferences for microenvironments
within the enclosure, and exhibited behaviours
that are not possible when housed singly.
Group housing appeared to be a successful
method for enriching the environment of female
rabbits and aspects of it should be considered
Correspondence to: Mark T. Whary, 101 Centralized
Biological Laboratory, The Pennsylvania State University,
University Park, Pa., 16802 USA
Received 5 October 1992; accepted 15 March 1993
in the approach
research.
to housing
rabbits
used in
Keywords: Laboratory Animals; Rabbits;
Behaviour; Housing; Environmental Enrichment
Current recommendations
for enriching the
physical and mental health of dogs and nonhuman primates specify increased exercise or
group housing of compatible animals in larger
enclosures when possible (Code of Federal
Regulations,
1985). Similar to the traditional
housing of these higher species, laboratory
rabbits (Oryctolagus cuniculus) are conventionally
housed individually in cages that are psychologically unstimulating, socially isolating, limiting
for exercise and unlikely to be enriching to their
quality of life. Identification and use of the most
suitable housing method for rabbits is important
for humane reasons and for understanding what
impact the housing environment may have on
research performance.
One alternative to the individual housing of
rabbits is group housing, which has received
attention recently (Bell, 1984; Bell & Bray, 1984;
Heath & Stott, 1990; Love & Hammond,
1991;
Huls, Brooks & Bean-Knudsen, 1991; Podberscek,
Blacksawand & Beattie, 1991). Group housing
has greater potential than single housing for
positive enrichment through social interaction,
increased exercise, and mental stimulation from
the exploration of more complex environments.
It has greater potential for adverse effects also.
Wild rabbits are territorial and live dispersed
Rabbit housing and research
within expansive warrens (Cowan & Bell, 1986;
Roberts, 1987), therefore, it is possible that
grouping domestic rabbits in a small space could
impose stress. Additionally, grouping as a single
sex is also unlike the natural state. Cage
population density in rodents has significant
effects on aggression and other indicators of
stress (Peng et at., 1989) and may have similar
effects in rabbits as well. Abrupt social regrouping of rodents increases plasma corticosterone levels (Barrett & Stockham, 1963;
Benton, Brain & Goldsmith, 1979), alters antibody response (Edwards et at., 1980)and natural
killer function (Hoffman-Goetz, Simpson &
Arumugam, 1991), and may induce aggression
(Christian, 1955). These physiological and behavioural alterations may have deleterious effects
on practical aspects of research performance.
Importantly, the type and magnitude of housing
stress rabbits can tolerate before adverse effects
on research performance develop has not been
established.
Despite the artificial environmental conditions
used in rodent stress studies, the results suggest
that the most important stress-inducing factor is
a sudden change in housing method rather than
the method itself, however presumably adverse
(Vessey, 1964; Benton, Brain & Goldsmith,
1979). For practical application of environmental
enrichment techniques, studies of acclimated
animals under more typical housing and research
conditions are needed to evaluate the practical
influence of housing method on performance
parameters that can influence the outcome of
experiments. For example, there are no known
correlations between housing method and the
most common use of the rabbit, antibody
production (Cohen & Tissot, 1974).
Traditional single caging of rabbits has been
shown to alter some aspects of physiology
and prevent behaviours common to both wild and
domesticated rabbits. Podberscek, Blackshawand
and Beattie (1991) found that the space restrictions of standard cages (2600 cm2) frustrate
normal hopping, locomotion and that isolated
rabbits, compared with group housed rabbits,
spent more time on self-directed grooming, were
331
more restless, and exhibited stereotypic behaviour such as repetitive chewing on cage
equipment. Group housed rabbits in their study
displayed a higher incidence of self-comforting
behaviours, such as yawning and stretching.
Love and Hammond (1991) also observed that
space restriction and the isolation of single
housing prevented mutual grooming, play, and
the erect 'prairie dog' posture that rabbits assume
when investigating disturbances. Additionally,
from a physical welfare standpoint, Lehman
(1984 cited in Lehman, 1987) concluded that
rabbits confined to standard size cages have an increased risk of developing atrophic osteoporosis.
While these observations suggest that single
housing in standard cages may have important
consequences, others have found no significant
difference in feed intake, growth rate, plasma
hormone levels, or organ weights between rabbits
housed individually or in groups (Bell, 1984; Bell
& Bray, 1984). As a result, group housing may
only have intangible, psychological benefits for
rabbits. The purpose of our study was to
compare effects of single and group housing on
practical performance parameters important to
biomedical research in order to estimate the
tangible value of an enriched housing environment. Important determinations included whether
increased opportunities for exercise, social contact, and a more novel environment significantly
benefit rabbits and their use in research or
alternatively, whether group housing of rabbits
should be avoided due to potential effects of
social stress.
Specifically, this study compared single and
group housed rabbits on immune responses, feed
intake, growth rate, and stress level as a function
of housing method. Immune response capabilities
were estimated by development of antibody titre
and delayed hypersensitivity response using
common research protocols. Selectedphysiological
parameters that are considered to reflect stress
in most species, specifically adrenal gland size,
circulating corticosterone levels and lymphocyte
counts, were used to determine if either single
or group housed rabbit were experiencing significant stress. Behavioural monitoring evaluated
Whary et al.
332
the group housed rabbits for social group size,
social interaction,
reaction to an enriched
environment and space utilization patterns of the
larger enclosure.
Materials
and methods
Animals
Sixteen female, young adult (2' 3-2' 7 kg), SPF*
New Zealand White rabbits were purchased from
a commercial vendor (Hazleton Labs, Denver,
Pa., USA) and randomly assigned to single or
group housing. Each group housed rabbit was
marked with non-toxic paint on the dorsal pinnae
for identification
at a distance. Both experimental groups were monitored for a total of 12
weeks.
Housing
Fig. 1. Group housing enclosure modified from a swine
farrowing pen. Space available per rabbit was similar to single
cage floor area.
their holding room. A resting shelf, O' 3 m W x
1 . 5 m L, was mounted along one end wall of the
pen at a height of O' 25 m off the pen floor. A
litterbox, 60 cm W x 20 cm H x 60 cm L and containing 5 cm of absorbent pellets, was provided
at the opposite end wall. Any theoretical loss of
usable space in the group housing design due to
the area occupied by the litterbox was compensated by the extra space provided by the
resting shelf surface.
The single caged rabbits were fed from stainless steel J hoppers and drank from sipper tube
bottles. The group housed rabbits were fed from
a 16cm Wx90cm
Lx 12cm H galvanized
trough mounted on the front pen wall. This
feeder was large enough to avoid competition
between rabbits for access to feed and held more
food than could be consumed by the entire group
in 24 h. Feed intake by both experimental
groups was measured daily by weight. Water for
the group housed rabbits was available from 4
one-litre sipper bottles mounted at equidistant
locations around the pen perimeter. All rabbits
were handled once weekly for cage cleaning,
body weight measurement and other test procedures. The litterbox and resting shelf in the
group housing pen were cleaned daily.
Husbandry
practices
for room temperature
(21 ± 2°C), humidity (40-70070), fresh air changes
(10-15 h), 12: 12 h light/dark
cycle (0600 to
1800) and cage sanitation (weekly), followed the
guidelines from the NIH Guide for the Care and
Use of Laboratory Animals (Committee on Care
and Use of Laboratory Animals of ILAR, NRC,
and PHS, US Dept of Health and Human
Services, NIH 1985). The 2 experimental groups
were in adjacent rooms (16 m2) and exposed to
identical levels of human traffic and work-related
noise. Eight rabbits were housed individually in
stainless steel bank-style cages measuring 60 cm W
X 60 cm L X 40 cm H with expanded metal rod
flooring (I'44cm2
openings) (Hoeltge Inc.,
Cincinnati, Ohio, USA). The remaining 8 were
group housed in a modified swine farrowing pen
measuring 1· 5 cm W X 2 m Lxi m H (Fig. 1).
The floor space allowance per rabbit was comparable for each experimental
group (singles
0'36m2/group
0'375m2)
but the group enclosure provided a full metre of vertical space.
The pen floor was rubber coated, expanded
metal mesh (2 cm2 openings), suspended O· 5 m
off the room floor. The pen sides and roof were
constructed of large mesh steel-rod panels that
permitted the rabbits an unobstructed
view of
Diet
*Free of Pasteurella multocida and Eimeria stiedae.
All rabbits were limit fed, in accordance with
supplier recommendations,
for the first 2 weeks
Rabbit housing and research
to allow for adaptation to the diet and minimize
the chances of digestive disturbances. Commercial
rabbit pellets, (Rabbit Lab Chow HFTM, Purina
Mills, St Louis, Mo., USA), were thereafter
provided ad libitum to both experimental groups.
Data collection on daily food intake began as ad
libitum feeding was instituted (10 weeks total).
Immune responses
Humoral response.
Pre-immune serum was
collected during week 4. Both experimental
groups were then injected subcutaneously with
3' 0 mg human serum albumin (HSA) that was conjugated to dinitrophenol (DNP) and emulsified
in complete Freund's adjuvant* (Sigma, St
Louis, Mo., USA). The same quantity of HSADNP mixed in incomplete Freund's was administered 3 weeks later. Pre-immune, primary and
secondary humoral responses (weeks 4, 7, and 12
respectively) were measured by ELISA antibody
capture as previously described (Ausubel et al.,
1989). The optical density (00) of the antigenantibody reactions was measured by an automated
ELISA reader (Bio-Tek EL311 Autoreader, BioTek Instruments, Winooska, VT, USA) and
compared with positive and negative controls on
the same microtitre plate. The humoral response
of each rabbit was determined by subtracting the
00 of the pre-immune (week 4) serum ELISA
reaction from the primary (week 7) and secondary
week (week 12) humoral responses respectively.
The humoral response was considered specific
and significant if the primary and secondary 00
values exceeded a value equal to the pre-immune
mean OD plus 3 standard deviations for all
rabbits within the experimental group.
Delayed-type hypersensitivity response.
Delayed hypersensitivity response (DTH) was
measured 3 weeks after initial priming with HSADNP. An intradermal injection of o· 6 mg HSADNP mixed in 0,05 ml of incomplete Freund's
adjuvant was compared with an adjacent control
*HSA-DNP emulsified in an equal volume of complete
Freund's adjuvant. A total of O' 25 ml was injected
subcutaneously over 5 separate sites.
333
saline injection. The area of induration was
measured by 2 observers at 24, 48, 72, 96, and
120 h post-inoculation.
Stress profile
Lymphocyte counts.
Blood was collected in
EDT A at weeks 4, 7, and 12 by venipuncture of
the central ear artery with the rabbit held in a
stainless steel restrainer. Total leukocyte numbers
were counted by an automated cell counter
(Coulter Electronics Ltd, Hialeah, FL., USA).
Differentials of 100 leukocytes were counted
manually on blood smears stained with JennerGiemsa. The absolute number of lymphocytes
was derived by multiplying the total leukocyte
number by the subset percentage.
Circulating corticosterone.
Circulating corticosterone was measured from blood samples
obtained during the mid-afternoon of the fifth
day of each week 4, 7, and 12. The first portion
of each blood sample was used for the assay to
standardize the amount of time between placement of the rabbit in the restrainer and sampling.
The serum was stored in plastic vials at -70°C
until analysed by radioimmune assay (Jones,
1992).
Behavioural observations
The daily behaviour of the group housed rabbits
was monitored during the first 4 days of each
week from weeks 2 through 6. To avoid potential
effects of handling for cage cleaning and other
test procedures on social behaviour, the fifth day
of each week was reserved for these procedures
and 2 more days were allowed to pass before
behaviour was again monitored. Social group size
and area use were recorded by hourly visual observations through the animal room door window
during the hours of 0800 to 1700. Still photography with a remote shutter release and videotaping in the absence of an observer were recorded
during the same time period to provide data on
social interaction and interest level in novel
stimuli that was unaffected by human presence.
Painted lines on the pen floor visually subdivided the available pen space into 8 equal-sized
334
Whary et at.
Rabbit housing and research
squares (Fig. 2). A social group was defined as
the number of rabbits either occupying the same
visually-defined square or if observed occupying
adjacent squares, were judged to be interacting
by close proximity and similar body alignment.
Cage design symmetry resulted in 4 different
microenvironments (Fig. 3) and preference of
rabbits for occupying these unique areas was
measured from the same observations used for
determining social group size. Novel objects used
for environmental enrichment included the litterbox, resting shelf and a section of plastic polyvinyl chloride (PVC) tubing (30 cm diameter x
75 cm L), that was anchored to the floor in Area 2.
Post-mortem
evaluation
A complete gross necropsy examination of all
rabbits was performed at the end of the study
period. Representative samples of all thoracic
and abdominal organ systems were placed in
10070neutral buffered formalin and processed for
histological examination.
At necropsy, the adrenal glands were trimmed
of fat, weighed and then placed in 10070neutral
buffered formalin. The total weight of both
adrenal glands, the ratio of adrenal weight to
100g body weight, and the cross-sectional area *
of the adrenal cortex were measured for each
individual rabbit.
335
independence from random probability and by
ANOV A for differences in area usage on the
basis of time of day (a.m. versus p.m.), total
observations for each week and total observations for the period of weeks 2 through to 6.
Differences were considered significant if the P
value was less than 0'05 (Ott, 1988).
Results
Clinical health
Clinical signs or post-mortem lesions due to
infectious disease did not develop in any
rabbits. One group housed rabbit developed a
mild limp during week 7 which did not require
its withdrawal from the study. A diagnosis of
ruptured anterior cruciate ligament was confirmed at necropsy at the conclusion of the
study.
Housing
management
There were no problems associated with housing
and handling the single rabbits. The group
housed rabbits typically fled under the resting
shelf when first approached. This actually
facilitated their restraint because it was then easy
to remove them individually from under the shelf
without further escape attempts. The mesh
flooring kept the rabbits clean and dry but the
solid floors of the litter box and resting shelf
required daily maintenance.
Statistics
Mean values for each measurement are reported
along with the standard error. Physiological
measurements of the single and group housed
experimental groups were compared at each time
point for significant difference by the Student's
t-test. The number of rabbits comprising different
social group sizes was compared by analysis of
variance (ANOVA) for significant differences on
the basis of time of day (a.m. versus p.m.), total
observations for each week and total observations
for the period of weeks 2 through to 6.
Occupancy rate of microenvironments within the
pen were compared by the Chi-squared test for
"Measured by computer morphometry (R & M Biometrics
Inc., Nashville, Tennessee, USA).
Peed intake and growth
rate
The mean daily feed intake for the single housed
rabbits was 192± 2 g which was less than the
group housed daily intake of 210± 3 g (P<O·Ol).
The net growth rate over the 12-week study
period for both experimental groups met normal
expectations for the age and sex of the rabbits
(Templeton, 1968) and was not significantly
affected by housing method. The single housed
rabbits gained a mean of 1·34 ± O' 13 kg compared with a gain of 1.26 ± O' 18kg for the group
housed rabbits. The final body weight of each
single housed rabbit was a mean 54 ± 5070greater
than the starting weight and was similar to the
52 ± 7070growth over starting weight exhibited by
the group housed rabbits.
Whary et at.
336
Immune responses
Humoral immunity.
HSA-DNP, with a large
molecular weight and conjugated hapten (DNP),
has been shown to be immunogenic in rabbits
(Menard & Demers, 1977, Snippe et al_, 1982)
and all of the rabbits in this study developed a
significant humoral response (P<O-OI). The
primary response by the single housed rabbits
with a corrected mean OD of 0-185 ± 0-018 was
not significantly different from the group housed
corrected
mean OD of 0-211 ± 0-018. The
secondary response was also similar between
experimental groups, with a corrected mean OD
of 0·203 ± 0·020 for the single housed rabbits
and a corrected mean OD of 0·227 ± 0·023 for
the group housed rabbits_
Delayed-type hypersensitivity response.
The
mean DTH response to intradermal HSA-DNP
was not statistically different between the single
and group housed rabbits. Induration increased
at each time point of 24, 48, 72 h and decreased
through time points 96 and 120 h post-injection
for both experimental groups. The area of induration in response to HSA-DNP was maximal at 72 h
for both experimental groups and was a mean
93 ± 14 mm2 for the single housed rabbits and a
mean SO± I I mm2 for the group housed rabbits.
Stress profile
housed rabbits were 42 ± 8, 49 ± 9, and 29 ±
4 f-tg/dl at weeks 4, 7, and 12, respectively_These
values were similar to the values of 45 ± 9, 32 ± 13,
and 28 ± 15 f-tg/dl recorded for the group housed
rabbits at the same time points.
Lymphocyte counts.
The only difference in
lymphocyte counts between experimental groups
occurred at week 7 (2nd of 3 time points).
The group housed rabbits had a lower count
(P<O·OI) of 3459±289Iymphocytes/f-t
P compared with a mean of 6778 ± 379 Iymphocytes/
f-tPin the single housed rabbits. The mean total
leukocyte count of 7756 ± 374 cells/ f-tP in the
group housed rabbits was also lower than the
value of 10 568 ± 566 cells/f-tP observed in the
single housed rabbits at week 7 (P< 0·01).
Behavioural observations
The behaviour patterns of the single housed
rabbits were not evaluated because of the space
restriction and the absence of physical or social
contact that are inherent in the single caging
system. The static 'single housing environment
was considered as a control for the more dynamic
test environment of group housing which allowed
data collection on social interaction, use of space
and interest in novel stimuli.
Adrenal gland measurements.
There were no
significant
differences
between experimental
groups in mean total adrenal gland weight (single
housed 0·55±0-06g,
group housed 0-54±
0·05 g), mean ratio of adrenal weight to 100 g
body weight (single housed 0·014 ± 0- 001, group
housed 0·015 ± 0·002), or in the mean crosssectional area of the adrenal cortex (single housed
8-03± I·Omm2, group housed 8·51 ±O-SI mm2).
The adrenal gland weights and adrenal gland
weight to body weight ratios for both experimental groups were similar to measurements
previously reported (Kozma et al., 1974).
Social group size in group housed rabbits.
The
group housed rabbits were observed, during
weeks 2 through to 6, to socialize in small group
sizes (one to 3 rabbits) compared to infrequent
larger group sizes (4 to 8) (P<O-OI) (Fig. 4).
Social group size did not change significantly
with time of day, or day of the week (data not
shown). Group sizes larger than 3 rabbits
accounted for less than 120/0 of all observations
and were not statistically different in frequency.
Group sizes of 6 or more rabbits were infrequent
and usually formed only when the feed trough
was refilled or during a startle response when the
rabbits intermingled under the resting shelf.
Circulating
circulating
Social interaction among group housed rabbits
There was no evidence of overt aggression or
corticosterone
corticosterone
levels.
The mean
levels In the single·
Rabbit
housing
and
337
research
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Number of Rabbits In Social Group
•'ig. 4. Mean ± standard
weeks 2-6. Rabbits had
of I to 3. There were no
day, between days of the
error of social group sizes during
significant preferences for groups
differences associated with time of
week or between weeks 2 through 6.
competition for space, feed or water access
during the entire study period. Patterns of
behaviour recorded by visual observations,
videotape and still photography indicated that
mutual grooming, spontaneous bursts of running
activity that involved one or more rabbits, and
investigation of auditory stimuli by assuming an
erect 'prairie dog' position are behaviours
repeated several times an hour during the daytime observation period. Between bursts of
activity, small social group(s) of one to 3 rabbits
rested evenly dispersed within the available
space. Rabbits rested in both sternal and lateral
recumbency.
Space utilization.
Chi-square analysis of
area use shows that preferences for microenvironments within the pen were non-random
(P<O'Ol) (Figs 1 & 5). As in social group size
observations, there was not a significant difference in space utilization between weeks 2
through to 6 or associated with day of the week
or time of day (data not shown). There were
strong preferences for occupying the corners of
Area 1 near the litterbox and the open central
Area 2 that contained the PVC tube, food
and water (42070 and 45070 of observations,
o
--r
Arlla 1
Arlla 2
Area 3
Arlla 4
Fig. 5. Mean ± standard error for area use during weeks 2-6 .
Rabbits preferred the open areas. There were no differences
associated with time of day, between days of the week or
between weeks 2 through 6.
respectively). Area 3 under the resting shelf
provided cover during startle responses but was
used only 3070 of the time for undisturbed resting.
Use of Area 4, the resting shelf surface itself, was
moderate at 10070 of observations. Rabbits used
the shelf both for resting and exploratory
behaviour.
Novel stimuli.
The litter box was not
preferentially used for urination and defecation
but was used frequently as a resting area and for
play activity when rabbits would hop in and out
of the litterbox in rapid succession. The resting
shelf stimulated intense exploratory interest with
many brief mountings during the first few days
and thereafter was used occasionally for resting or
exploration. The PVC tube was very popular as a
resting area and as an object for investigation.
Individuals often rested in the tube for several
minutes before being apparently amiably displaced by another rabbit entering the tube. The
tube was rarely empty during the observation
periods.
Discussion
The Animal Welfare Act (USA Code of Federal
Regulations 1985) and the Guide for the Care
338
and Use of Laboratory Animals (Committee on
Care and Use of Laboratory Animals of ILAR,
NRC, and PHS, US Dept of Health and Human
Services, NIH 1985), effectively charge the
laboratory animal care profession with developing
housing methods that not only satisfy the
physical needs of research animals, but importantly, their potential need for environmental
enrichment as well. The results of this study indicate that the group housing method employed
here provided environmental enrichment without
significantly affecting immune or physiological
variables when compared with single housed
rabbits. Additionally, the opportunity to exhibit
natural behaviours that could increase the research
rabbit's quality of life was amenable to manipulation in the research setting through group
housing.
From a clinical and management perspective,
group housing of SPF rabbits in this study did
not increase the incidence of infectious disease.
A recent study on group housing of non-SPF
rabbits in floor pens with contact bedding also
concluded that there was not an associated risk
of infectious disease (Love & Hammond, 1991).
Both the rubber-coated mesh flooring used in the
group housing and the steel rod flooring used for
single caging in this study effectively separated
the rabbits from most faecal and urinary waste.
These materials and designs also appeared to be
comfortable for resting and locomotion and
consequently did not appear to affect activity
patterns. The cause of the ruptured cruciate
ligament in one group housed rabbit was undetermined. Because the injury occurred during
week 7, it appears unrelated to prior handling
and suggests that larger space available for
activity, the use of cage furniture such as the
resting shelf, or the use of mesh flooring could
all potentially be risk factors for injury.
Feed intake was significantly higher in the
group housed rabbits although growth rates
between experimental groups were not different,
indicating better feed conversion to body weight
by the single housed rabbits. The difference in
feed intake may be related to potentially higher
caloric requirements of the group based rabbits
Whary et al.
due to more opportunity for exercise. Although
this study did not find differences in growth rate,
a significant effect of opportunity for exercise
in a larger space may be expected in older rabbits
that are more prone to obesity than the growing
rabbits used in this study. The floor space
available to the group housed rabbits allowed for
many bursts of play activity exemplified by
running, jumping, and kicking in the air. Others
have also noted that increased opportunities for
social interaction and exploration of more
complex environments helps prevent obesity and
that higher activity may reduce the time spent
eating, which may be a displacement behaviour
in rabbits lacking environmental stimuli (Metz,
1987).
Aggression and dominance hierarchies were
not evident in these young, non-breeding female
rabbits which is probably related to their
immaturity. Although there may have been
subtle social ranking among the group housed
rabbits for access to the feeder, this was not
apparent. The similar weight gains belie any
significance to potential dominance hierarchy
influencing feed access on a group basis. There
were no injuries traceable to aggression and the
use of common areas, such as the feed trough,
litterbox, PVC tube and resting shelf was
amiable. Aggression would be more of a concern
in group housing mature rabbits, especially
mature males, which are more likely to inflict
serious injuries compared to mature females.
The group housed rabbits appeared to experience a greater sympathetic 'flight' response
when approached due to the larger space
available for attempted escape, but this response
did not significantly affect any measurements.
There was a downward trend in the corticosterone values for both experimental groups that
did not reach statistical significance but may
reflect a physiological adaptation to repeated
handling. The flight response of the group
housed rabbits when their pen was approached
by humans may have been decreased if the group
rabbits had been handled more often, but this
study was designed to mimic the actual handling
that rabbits used in biomedical research for
Rabbit
housing
and research
antibody production would normally experience.
Because our data did not indicate any effects of
stress, the relative lack of handling of either
experimental group was not a confounding
variable on the more chronic effects of housing
method.
The only time point involving a statistically
significant difference in lymphocyte counts was
week 7 when the group housed rabbits were lower
in lymphocyte counts than the single housed
rabbits. The absence of a relative leukocytosis
in the group housed rabbits and the additional
lack of any significant difference in corresponding
corticosterone levels indicate that the lower
lymphocyte count measured in the group housed
rabbits was not typical of a stress leukogram
(Jain, 1975)and although statistically significant,
lacks any apparent biological significance.
It also needs to be considered that we did not
use positive controls involving a known stressor.
Little is known about the definitive level of
circulating corticosterone, changes in adrenal
gland size or the responses of the lymphocyte
population as specific indicators of stress for the
laboratory rabbit (Redgate, Fox & Taylor, 1981).
Nonetheless, we have taken the liberty of
estimating stress by these measurements because
the extrapolation from data in other species
339
Since social groups of one to 3 rabbits were
most common, it may be optimal to group house
in small groups and conserve floor space by using
stackable group housing units. This would allow
rabbit density per animal room to be comparable
with commonly used single cage systems. Smaller
group sizes than used here would agree with
recently suggested methods of pairing primates
and canines for psychological enrichment (USA
Code of Federal Regulations, 1985)and with the
results of previous work that showed that rabbits
preferred pair-housing over single housing when
given the choice (Huls, Brooks & Bean-Knudsen,
1991). Despite the rabbit's preference for small
social group sizes, the results of our study show
that larger grouping can be incorporated into an
enrichment programme without inducing significant behavioural or physiological stress. The
cooperative cohabitation observed here agrees
with previous observations that wild rabbits
will evenly disperse over maximum ranges if
optimum soil conditions exist for burrowing
but will congregate in smaller areas and accept
the presence of others if soils are hard-packed
and burrowing is difficult (Cowan & Bell,
1986).
Resting with bursts of either play or exploratory
activity were the predominant daytime behaviours
appears reasonable (Christian, 1955; Peng et al.,
observed. Even greater use of cage furniture and
1989).
The rabbits showed strong preferences for
using nearly all of the available pen space, which
may be related to their preference for dispersion
or small social groups. Resting in the central
open area and infrequent use of the resting shelf
as cover suggests that domesticated rabbits adapt
well to open caging and do not feel threatened.
This contrasts with the observations on guineapigs by White et al. (1989) who reported that
guinea-pigs prefer grouping at the pen periphery
and consequently do not efficiently take advantage
of a larger enclosure. Our observation that the
group housed rabbits spent a significant amount
of time resting in the open agrees with a previous
report that wild Oryctolagus spend significantly
more time above ground than other more timid
rabbit species (Stodart & Meyers, 1964).
open cage space would be expected during
nocturnal activity. Frequently, rabbits would lie
in full stretched-out lateral recumbency which,
for a large rabbit, may exceed the space available
in standard single caging. In addition, the increased vertical height of the group housing
enclosure allowed the rabbits to investigate sights
and sounds by assuming an erect stance, as has
been noted by others (Love & Hammond, 1991).
Many social behaviours of wild rabbits such as
mutual grooming and play activity were observed
in their domesticated counterparts during our study
and single caging prevents these natural behaviours. We were not able to demonstrate any
practical physiological difference attributable to
social opportunities but the reality is that such
psychological benefits probably cannot be easily
quantified.
340
Efforts to enrich the environment of rabbits
should appeal to their denning and exploratory
instincts by providing PVC tubes, a resting shelf
and other cage furniture such as a litterbox. The
mesh floor used in our design allowed faeces and
urine to fall out of contact and may have
decreased use of the litterbox for waste elimination. The results of this study would indicate
that efforts spent on enriching the rabbit's
environment probably will have intangible results
and the decision to group house will then depend
primarily on experimental design, available space
and equipment resources. The option of changing
from conventional single caging to group
housing is a viable alternative supported by
evidence that it will not negatively affect research
Whary et al.
productivity. Continued study and use of group
housing for research rabbits should be encouraged to address animal welfare concerns and
proactively to avoid regulation that could require
psychological enrichment of this species if the
public perceives that their quality of life is in
doubt.
Acknowledgments
The investigators appreciate the expertise and
guidance of Dr Byron Jones of The Pennsylvania
State University on determination of corticosterone levels.
Animal use in this project was approved by the
Institutional Animal Care and Use Committee
of The Pennsylvania State University.
References
Ausubel FM, Brent R, Kingston RE, et al. (1989) Current
protocols in molecular biology. New York: John Wiley and
Sons, Appendix II, pp. 313-319
Barrett AM & Stockham MA (1963) The effect of housing
conditions and simple experimental procedures upon the
corticosterone level in the plasma of rats. Journal of
Endocrinology 26, 97-105
Bell OJ (1984) The behavior of rabbits: implications for their
laboratory management. Standards In Laboratory Animal
Management. Proceedings of a symposium sponsored by
UFAW, London, /51-162
Bell OJ & Bray GC (1984) Effects of single- and mix-sexed
caging on post-weaning development in the rabbit.
Laboratory Animals 18, 267-270
Benton 0, Brain PF & Goldsmith JF (1979) Effects of prior
housing on endocrine responses to differential caging in
male TO-strain mice. Physiological Psychology 7(1), 89-92
Christian JJ (1955) Effect of population size on the adrenal
glands and reproductive organs of male mice in populations
of fixed size. American Journal of Physiology 182, 292
Code of Federal Regulations (1985) In Federal Register (ed.
APHIS, USDA). Washington, DC: Office of the Federal
Register
Cohen C & Tissot J (1974) The biology of the laboratory
rabbit (eds SH Weisbroth, RE Flatt & AL Kraus), p. 167.
New York: Academic Press
Committee on Care and Use of Laboratory Animals of
ILAR, NRC, and PHS, U.S. Dept. of Health and Human
Services, NIH (1985). Guide for the care and use of
laboratory animals. NIH Publication 86-23
Cowan DP & OJ Bell (1986) Leporid social behavior and
social organization. Mammal Review 16(3/4), 169-179
Dechambre RP & Gosse C (1973) Individual versus group
caging of mice with grafted tumors. Cancer Research 33,
140-144
Edwards EA, Rahe RH, Stephens PM, et al. (1980) Antibody
response to bovine serum albumin in mice: the effects of
psychosocial environmental change. Proceedings of the
Societyfor Experimental Biology and Medicine 164, 478-481
Heath M & Stott E (1990) Housing rabbits the unconventional
way. Animal Technology 41(1), 13-25
Hoffman-Goetz L, Simpson JR & Arumugam J (1991)
Impact of changes in housing condition on mouse natural
killer cell activity. Physiology & Behavior 49, 657-660
Huls WL, Brooks DL, Bean-Knudsen 0 (1991) Response of
adult New Zealand White rabbits to enrichment objects
and paired housing. Laboratory Animal Science 41(6),
609-612
Jain NC (1975) Schalm's veterinary hematology, 3rd edition,
pp. 128. Philadelphia, Pa: Lea & Febiger
Jones B (1992) Personal communication
Kozma C, Macklin W, Cummins L, Mauer R (1974) The
biology of the laboratory rabbit (eds SH Weisbroth, RE
Flatt & AL Klaus), p. 52. New York: Academic Press
Lehmann M (1984) PhD Thesis, University of Berlin. Cited
in Lehmann 1987
Lehmann M (1987) In Rabbit production systems including
welfare (ed. T Auxilla), pp. 257-268. Luxembourg: Official
Publications of the European Communities
Love JA & Hammond K (1991) Group-housing rabbits. Lab
Animal 20(8), 37-43
Menard HA & Demers JC (1977) Use of a hapten-carrier
system in experimental immune arthritis in the rabbit.
Arthritis and Rheumatism 20(7), 1402-1408
Metz JHM (1987) In Rabbit production systems including
welfare (ed. T Auxilla), pp. 221-230. Luxembourg: Official
Publications of the European Communities
Ott L (1988) An introduction to statistical knowledge and
data analysis, 3rd edition. Boston, Mass: PWS-Kent
Publishing
Rabbit
housing
and research
Peng X, Lang CM, Drozdowicz CK, et al. (1989) Effect of
cage population density on plasma corticosterone and
peripheral lymphocyte populations of laboratory mice.
Laboratory Animal Science 23(4), 302-306
Podberscek AL, Blackshawand JK & Beattie AW (1991) The
behavior of group penned and individually caged laboratory
animals. Applied Animal Behaviour Science 28, 353-363
Redgate ES, Fox RR & Taylor FH (1981) Strain and age
effects on immobilization stress in JAX rabbits (41088).
Proceedings of the Society For Experimental Biology and
Medicine 166, 442-448
Roberts SC (1987) Group-living and consortships in two
populations of the European rabbits (Oryctolagus cuniculus).
Journal of Mammalogy 68(1), 28-38
341
Snippe H, de Reuver MJ, Beunder JW, et al. (1982) Delayedtype hypersensitivity in rabbits. International Archives of
Allergy and Applied Immunology 67, 139-144
Stodart E & Myers K (1964) A comparison of behaviour,
reproduction and mortality of wild and domestic rabbits
in confined populations. C.S.I.R.O. Wildlife Research 9,
144-159
Templeton GS (1968) Domestic rabbit production, p.68.
Danville, Illinois: Interstate Printers and Publishers Inc.
Vessey SH (1964) Effects of grouping on levels of circulating
antibodies in mice. Proceedings of the Society for
Experimental Biology and Medicine 115, 252-255
White WJ, Balk MW & Lang MC (1989) Use of cage space
by guinea pigs. Laboratory Animals 23, 208-214