Experimental Animals
Exp. Anim. 71(2), 150–160, 2022

Original

Comparison of survival rates in four inbred mousestrains under different housing conditions:effects of environmental enrichment
Kohei Kawakami1), Hiroyuki Matsuo 1), Naoyo Kajitani1),
Takaya Yamada1), and Ken-ichi Matsumoto 2)
1)Department of Experimental Animals, Interdisciplinary Center for Science Research, Head Office for Research andAcademic Information, Shimane University, 89-1 Enya-cho, Izumo, Shimane 693-8501, Japan2)Department of Biosignaling and Radioisotope Experiment, Interdisciplinary Center for Science Research, Head Office for

Research and Academic Information, Shimane University, 89-1 Enya-cho, Izumo, Shimane 693-8501, Japan

Abstract: Housing conditions can affect the well-being of laboratory animals and thereby affect the outcomes ofexperiments. The appropriate environment is essential for the expression of natural behavior in animals. Here, we
compared survival rates in four inbred mouse strains maintained under three different environmental conditions.
Three mouse strains (C57BL/6J, C3H/HeN, and DBA/2J) housed under environmental enrichment (EE) conditions
showed improved survival; however, EE did not alter the survival rate of the fourth strain, BALB/c. None of thestrains showed significant differences in body weights or plasma corticosterone levels in the three environmental
conditions. For BALB/c mice, the rates of debility were higher in the EE group. Interestingly, for C57BL/6J andC3H/HeN mice, the incidence of animals with alopecia was significantly lower in the EE groups than in the control
group. It is possible that the enriched environment provided greater opportunities for sheltering in a secure locationin which to avoid interactions with other mice. The cloth mat flooring used for the EE group was bitten and chewed
by the mice. Our findings suggest that depending on the mouse strains different responses to EE are caused withregard to health and survival rates. The results of this study provide basic data for further studies on EE.
Key words: cloth mat flooring, environmental enrichment, inbred strain, lifespan, mice

Introduction
Human life expectancy is increasing globally due toimproved healthcare infrastructures and to the early diagnosis of disease. Age-related decreases in physiological functions can have a significant and adverse influence on daily life and quality of life in elderly populations[1]. The mechanisms of aging are currently the focus ofa wide range of investigations. Animal models are commonly used in aging research because they allow researchers to obtain data that are difficult or impossible
to obtain from humans. Laboratory mice are widely used
for aging and senile disease research because they are

genetically and physiologically similar to humans, easy
to breed and to produce genetically modified animals,
have a well-developed database, and are relatively shortlived with an average lifespan of about 3 years [2].
Advances in our understanding of the optimal housing
conditions for animals in biomedical research have led
to the increased use of “environmental enrichment (EE)”for rodents. EE can be defined as any modification in theenvironment of captive animals that seeks to enhancephysical and psychological well-being by providing
stimuli that meet the specific needs of the animals [3].
It is important to evaluate EE in terms of health benefit
and lifespan of the animals; assessment of the effects on(Received 5 July 2021 / Accepted 21 October 2021 / Published online in J-STAGE 16 November 2021)
Corresponding author: K. Kawakami. email: kkawaka@med.shimane-u.ac.jp
This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives(by-nc-nd) License <http://creativecommons.org/licenses/by-nc-nd/4.0/>.

©2022 Japanese Association for Laboratory Animal Science

ENRICHED HOUSING AND SURVIVAL RATE IN MICE

behavior and on physiological parameters can also be
used to investigate the potential benefits of EE.
In general, two types of experiment are performed toevaluate the potential improvement in rodent welfare by
EE: (1) experiments that examine the preference of animals for specific objects or environments (preference
testing); and (2) experiments that examine the effects ofenrichment on behavioral or physiological parameters[4]. Research on the effects of EE on mice has demonstrated significant differences in behavior: mice kept
under enriched conditions show increased locomotory
and exploratory activities [5], object exploration [6],learning ability [7], and decreased anxiety [8]. These
differences in behavior reflect differences in brain anatomy and chemistry, including brain size, brain weight,
density of spines, levels of dendritic branching, numberof synapses per neuron, and activity of neurotransmitters[9]. The influence of EE on cognition and memory has
been well documented [10–13].
In preference testing experiments, mice are provided
with a choice of different conditions, and the environment most frequently chosen is assumed to be that mostsuitable [14, 15]. In a previous study, we showed thatmice remained longer in cages with cloth bedding materials [16]; cloth bedding materials offer both a significant reduction in waste and an improvement in the
habitat of the mice [17]. Mice prefer an environment thatmatches their fur color as it provides greater protection[18]; mice also show a clear preference for cages containing a wooden quadrangle [19].
Particularly relevant to investigations of aging arestudies examining enrichment-induced alterations in
middle-aged and aged rodents. The majority of studies
on aged mice generally provide an enriched environment
for only a short time period [20–23] although one study
on rats examined enriched versus standard housing conditions over a 12 month period [24]. To date, however,no study has evaluated the effects of EE on lifespans andsurvival rates. The present study was initiated to examine the effect of three different environmental conditions
on the survival rates of four inbred mouse strains.
Materials and Methods
Animals and housing conditions
Six-week-old female mice from four inbred strains(BALB/c, C57BL/6J, C3H/HeN, DBA/2J) were used
(CLEA Japan, Inc., Tokyo, Japan). To eliminate theinfluence of the rearing environment, mice were housedin TPX plastic cages (220 × 320 × 135 mm, Natsune
Seisakusho, Co., Ltd., Tokyo, Japan) with wire meshfloors for a 2-week acclimatization period. All animals

were kept in an animal room at a constant temperature(23 ± 2°C), humidity (55 ± 10%), and 12 h light/12 hdark cycle (lights on at 07:00 and off at 19:00). The
mice were fed a plain commercial diet (NMF, Oriental
Yeast Co., Ltd., Tokyo, Japan) and had tap water ad
libitum. All experiments were performed according to
the Guide for the Care and Use of Laboratory Animals
at Shimane University and approved by the Institutional Review Board of the University (Approved number: IZ21-127).
Experimental procedure (housing conditions of test
animals)
Experiments were designed to compare the effects ofthree housing conditions on survival rates in the four
strains of mice (Fig. 1). For each strain, 75 female mice
were used: 5 × 5 animals were maintained in the threehousing conditions. Groups of mice were composed on
the basis of body weight of the animals at Six weeks ofage, so that groups had comparable mean body weights.
Standard housing
Mice were housed in cages containing standard woodshavings (Clean-chipTM, Shimizu, Inc., Kyoto, Japan).
Approximately 65 g of flooring material was placed ineach cage and this was replaced once a week. These miceserved as the control group.
AGMAT housing
The second group of mice was housed in cages containing a cloth mat flooring (AGMAT group, AgrebematTM, 160 × 270 mm, about 50 g, CLEA Japan, Inc.).
The mice were rehoused once per week. AGMAT iscomposed of meth acrylic acid graft copolymerized cellulosic knit fiber; this material has been reported to havethe property of adsorbing ammonia [17]. In addition,
unlike conventional flooring materials, it can be reused
after washing [25]. The amount of dust in the cages issignificantly reduced using this material compared toconventional flooring materials.
Enriched housing
The third group of mice was housed in cages withcloth mat flooring (Agrebe-matTM, 160 × 270 mm, about
50 g, CLEA Japan, Inc.), and containing a quadrangularwooden block with four small holes (110 × 110 × 75 mm,
one small hole (φ40), CLEA Japan, Inc.) [19] and a pieceof cloth (AgrebeTM, 450 × 650 mm, CLEA Japan, Inc.).
Housing materials of similar color to the mouse furwere used: the housing materials for BALB/c andDBA/2J mice were white, while those for C57BL/6J andC3H/HeN mice were black [18]. Cloths were dyed blackExp. Anim. 2022; 71(2): 150–160 | 151

K. KAWAKAMI, ET AL.

Fig. 1.	The three housing conditions evaluated here: (a) standard wood shaving cages (Control group); (b) cloth mat
flooring cages (AGMAT group); (c) cloth mat flooring cages containing a quadrangular wooden block withfour small holes; the cloth was color matched to the mouse fur color (Environmental enrichment (EE) group).

using India ink (calligraphy liquid, Juntendo Co., Ltd.,
Masuda, Japan) and sterilized in an autoclave.
Observation scores
We monitored the status of the mice daily. Body
weight, food consumption, and water consumption were
measured monthly using an electronic balance (PE-1600,
Mettler Ltd., Tokyo, Japan). Food consumption per animal was determined by placing a known weight andweighing the amount of food left after 7 days. Water
consumption was measured in the same way. Survival
rates were assessed over the entire lifespans of the animals. Deaths due to natural causes were recorded. Animals were euthanized under anesthesia with isoflurane(Escain, Mylan Seiyaku, Tokyo, Japan) using an anesthetic instrument (KN-1071-I, Natsune Seisakusho, Co.,
Ltd.) if they were too debilitated to reach food and water
and/or had a palpable mass of ≥2.0 cm in diameter [26].
Autopsies were performed on all mice, other than thatrotten carcase or partly eaten and all organs were subjected to macroscopic examination. Debility in the mice
was assigned to various categories: abnormal behavior;skin lesions; increased lordosis and kyphosis; alopecia,
which could develop in a diffuse manner or affectedcircular areas on the dorsal surface; tumors, which in152 | doi: 10.1538/expanim.21-0118

cluded various skin tumor types and cancers; ascites;
hepatic cirrhosis, which included hepatic failure andhepatic fibrosis; lesions of the spleen, including inflammatory and necrotic fibrosis; spleen atrophy; cataracts;
anal prolapse, including symptoms of intestinal inflammation such as perianal inflammation and bleeding.
Corticosterone (CORT) assay
After 12 weeks, five mice from each group of all four
stains were moved from the animal room to the laboratory and euthanized in their cages using CO2 supplied
through a safety glass lid. All animals lost consciousness
within 30 s and stopped breathing within 2–3 min. To
minimize the effect of sampling order on the levels of
CORT, the five mice were euthanized together in theirhome cage. The treatment groups were sampled as one
cage per group in the order: control, AGMAT, and EE.
This sampling order was maintained until all of the animals had been euthanized. Blood samples were collected by heart puncture directly after euthanasia. Bloodplasma was separated by centrifugation (1,530 × g for
10 min) and deep frozen (−80°C) until assayed. CORT
levels were determined using an ELISA kit according to
the manufacturer’s instructions (AssayPro, FunakoshiCo., Ltd., Tokyo, Japan).

ENRICHED HOUSING AND SURVIVAL RATE IN MICE

Cloth mat flooring
The weights of the cloth mat flooring in the AGMAT
and EE groups were compared before and after use (for1 week) using an electronic balance (PE-1600, Mettler
Ltd., Tokyo, Japan). During cage cleaning, the conditionof each mat was also subjectively assessed. The matswere then washed, reprocessed, sterilized, and weighed,and reused over about 4 months.
Statistical analysis
Data are expressed as means ± SD. Differences were
tested by analysis of variance (ANOVA) and Dunnet’s
PLSD post hoc test. The data from the post-mortem
analyses of animals euthanized during the experimental
period were analyzed using the chi-square test. Survival
rates were analyzed using the Kaplan–Meier survival
test and the log-rank test. Cloth mat flooring weights
were analyzed using the student’s t-test of unpaired comparisons. Analyses were performed using StatView (SASInstitute Inc., Cary, NC, USA). A P value less than 0.05
was considered statistically significant.
Results
Survival
Figure 2 shows the survival curves for BALB/c,

C57BL/6J, C3H/HeN, and DBA/2J mice during thisexperiment. In BALB/c mice, the numbers of mice dying
increased slightly earlier in the AGMAT and EE groups
compared with the control group, but the difference was
not significant. All of the BALB/c mice died before 900
days. BALB/c mice showed a mean lifespan of 660.3 ±22.2 days in the control, 615.1 ± 40.8 days in the AGMAT group (P=0.32), and 615.6 ± 29.2 days in the EE
group (P=0.32). C57BL/6J mice in the EE group showedsignificantly longer survival than the control group: mean
lifespan was 723.5 ± 39.2 days in the control group,
802.4 ± 30.2 days in the AGMAT group (P=0.08), and840.9 ± 24.1 days in the EE group (P<0.05). The 900-day
survival rate for the C57BL/6J mice was 15% in thecontrol group, 25% in the AGMAT group, and 25% in
the EE group. In C3H/HeN mice, survival in the EE
group was significantly longer than the control group:
mean lifespan was 814.4 ± 25.9 days in the control
group, 842.8 ± 11.9 days in the AGMAT group (P=0.30),
and 906.6 ± 16.6 days in the EE group (P<0.01). The
900-day survival rate for the C3H/HeN mice was 30%
in the control group, 15% in the AGMAT group, and 60%
in the EE group. In DBA/2J mice, survival was significantly longer in the EE group than the control group.
DBA/2J mice also tended to have a shorter lifespan thanthe other three strains: the mean lifespan of the DBA/2J

Fig. 2.	Survival curves for the four mouse strains in each housing condition. Survival rates were analyzed using
the Kaplan-Meier survival test and the log rank test. A P value less than 0.05 was considered statisticallysignificant differences from the control group.
Exp. Anim. 2022; 71(2): 150–160 | 153

K. KAWAKAMI, ET AL.

control group was 567.8 ± 37.9 days, that of the AGMAT
group was 644.4 ± 33.6 days (P=0.13), and that of theEE group was 677.0 ± 33.6 days (P<0.05). All of the
DBA/2J mice had died before 900 days.
Body weight
Figure 3 shows the changes in body weight in all fourmouse strains and in all three housing conditions during
the period of the experiment. Measurements of bodyweight and food consumption are useful methods forearly detection of the onset of disease or debilitation in
animals. BALB/c, C57BL/6J, and C3H/HeN mice
showed no significant differences in body weight at 26
months among the three groups. DBA/2J mice in the
AGTMAT and EE groups had slightly larger body weightincreases compared with that of the control group at
10–16 months, but these were not statistically significant.
Food and water consumption
As shown in Fig. 4, food consumption in BALB/cmice did not differ significantly among the three groups.
Food consumption of C57BL/6J mice decreased slightly in the AGMAT and EE groups during the experimental period but there were no significant differences infood consumption among the three groups. Food con-sumption of C3H/HeN and DBA/2J mice did not differ
significantly among the three groups. In all four mousestrains, the type of housing did not have a significant
effect on food consumption, although food consumption
varied from week to week.
As shown in Fig. 5, BALB/c mice showed no significant differences in water consumption among the threegroups, although there was a remarkable drop in the EE
group at 22 months. In C57BL/6J mice, daily water consumption varied widely in the control group throughout
the experiment but, overall, there were no significant
differences among the three groups. C3H/HeN and
DBA/2J mice showed no significant differences among
the three groups.
Plasma CORT
To measure the stress response, plasma CORT levels
were assessed. Figure 6 shows the plasma CORT levelsin the four strains after 12 weeks. Plasma CORT of
BALB/c mice represented the basal levels: control, 69.3
± 63.9 ng/ml; AGMAT group, 70.3 ± 87.1 ng/ml; EE
group, 72.9 ± 43.6 ng/ml. There were no significant differences among the groups. Housing conditions did not
influence CORT levels in C57BL/6J or DBA/2J mice;these strains showed similar levels of CORT as BALB/c

Fig. 3.	Average body weights for all mouse strains in each housing condition. Each value represents the mean ± SD.
154 | doi: 10.1538/expanim.21-0118

ENRICHED HOUSING AND SURVIVAL RATE IN MICE

Fig. 4. Food consumption in four mouse strains in each housing condition. Each value represents the mean ± SD.

Fig. 5.	Water consumption in four mouse strains in each housing condition. Each value represents the mean ± SD.
Exp. Anim. 2022; 71(2): 150–160 | 155

K. KAWAKAMI, ET AL.

mice. By contrast, plasma CORT levels were up to fourtimes as high in C3H/HeN mice as in the other strains.
Post-mortem observations
The results of the necroscopic analyses of mice fromthe four strains are shown in Table 1. In BALB/c, increases in neoplastic lesions, ascitic fluid, and splenomegaly were observed. The rates of debility in the EE
groups were significantly higher (P<0.05) compared withthe control group. In C57BL/6J mice, the rates of alopecia in the AGMAT and EE groups were significantly
lower (P<0.01 and P<0.05, respectively) compared withthe control group. The date of onset of alopecia was alsosignificantly later in the AGMAT group (n=5, 457.0 ±96.1 days) compared with the control group (n=15, 248.4± 37.6 days). Similarly, the onset of alopecia was significantly later in the EE group (n=8, 451.5 ± 45.1 days)
compared with the control group. In C3H/HeN mice therate of alopecia in the EE group was lower than in thecontrol group (P<0.05). However, there were no differences in timing of onset of alopecia among the threegroups: control, 473.1 ± 43.2 days; AGMAT, 533.6 ±
72.3; EE, 500.5 ± 70.6 days.
Condition of cloth mat flooring
Figure 7A shows representative images of cloth mat
flooring before and after use. In the AGMAT group andthe EE group, the mat displayed scratches and toothFig. 6. Corticosterone levels in four mouse strains in each housing condition. Data are presented
as the mean ± SD (n=5) and were analyzed using a variance (ANOVA) and Dunnet’s PLSD
post hoc test.
Table 1. Necropsy findings in four mouse strains in each housing condition
StrainGroup

Debility

Alopecia

Tumor

Ascites

Hepatic
cirrhosis

Spleenhypertrophy

Spleenatrophy

CataractAnalprolapse

BALB/cControl
AGMAT
EE
Control
AGMAT
EE
Control
AGMAT
EE
Control
AGMAT
EE

0/20
0/17
6/18*
2/19
4/18
7/20
7/20
10/20
7/19
1/18
1/20
2/18

7/20
2/17
0/18
15/19
5/18**
8/20*
12/20
5/20
4/19*
0/18
0/20
1/18

10/20
5/17
3/18
2/19
5/18
6/20
4/20
6/20
2/19
8/18
12/20
11/18

10/20
11/17
4/18
13/19
6/18
10/20
8/20
5/20
5/19
7/18
8/20
10/18

4/20
3/17
0/18
3/19
3/18
3/20
4/20
0/20
1/19
2/18
4/20
6/18

10/20
8/17
5/18
4/19
8/18
1/20
1/20
0/20
1/19
4/18
6/20
5/18

0/20
0/17
0/18
0/19
0/18
0/20
8/20
11/20
11/19
0/18
0/20
2/18

1/20
0/17
0/18
5/19
4/18
4/20
1/20
0/20
2/19
3/18
3/20
2/18

2/20
2/17
0/18
1/19
3/18
1/20
0/20
1/20
0/19
0/18
0/20
0/18

C57BL/6J
C3H/HeN
DBA/2J

The data from the post-mortem analyses of animals euthanized during the experimental period were analyzed using the chi-square test. **:P<0.01 vs Control, *: P<0.05 vs Control.
156 | doi: 10.1538/expanim.21-0118

ENRICHED HOUSING AND SURVIVAL RATE IN MICE

marks; these were observed for mats used in all strains.
The matting was replaced after about 4 months when it
became too tattered for use. Figure 7B shows the changes of average weights of the floor mats for the four strains
in AGMAT and EE groups. In both the AGMAT and EEgroup of BALB/c mice, a 45% rate of damage to the floor

mat was observed after 4 months; there was no significant difference between the two housing groups. In the
C57BL/6J mice, the floor mat weight in the AGMAT
group was significantly lower than in the EE group(P<0.05) after two and four months. In the C3H/HeN
mice, floor mat weight in the AGMAT group was sig-

Fig. 7. Comparison of the condition of the cage mat flooring for each mouse strain in AGMAT and EE housing
conditions. A: Representative images of the cloth mat before and after use. Each mat was used for about 4
months. B: Changes in weight of cage mat flooring before and after use. Four mats were assessed in the
AGMAT and EE groups of each strain. Each value represents the mean ± SD and were analyzed using thestudent’s t-test of unpaired comparisons. *: P<0.05.
Exp. Anim. 2022; 71(2): 150–160 | 157

K. KAWAKAMI, ET AL.

nificantly lower than in the EE group (P<0.05) at fourmonths. In the DBA/2J mice, floor mat weight in the
AGMAT group was significantly larger at two and fourmonths than in the EE group (P<0.05).
Discussion
In the present study, we examined the effects of enrichment of housing conditions on health and survival offour inbred mouse strains. The EE condition represents
specific measures to improve the psychological wellbeing of laboratory animals from an animal welfare
viewpoint [3, 15]. To date, the effects of EE on brain
structure and function [27], behavior [5], immune responses [28], pup mortality [29], physiological parameters [22], and aggression [30] have been investigated;
however, the effects of enrichment on lifespan and survival rates have not been evaluated. The results of ourstudy provide fundamental data for other studies seeking
to improve housing conditions as part of EE.
In general, when male mice are fostered in groups, thefrequency of external wounds is high due to fighting[15]; however, female mice rarely fight. To avoid aggressive behavior among cage mates, we examined theeffects of complex EE on female mice. The survival
times of three of the mouse strains examined (C57BL/6J,
C3H/HeN, and DBA/2J) were longer in the EE group;
however, no effect was observed in BALB/c mice. In theformer three strains, functional behavioral activity improved; the presence of places where they were securefrom attack may have been one cause [31, 32]. Tsai etal. [33] reported that EE stimulated physiological function and behavior in mice. For instance, although it is
not possible to escape from the cage, shelters or visualbarriers could allow mice to hide from a threatening
conspecific. However, in BALB/c mice, the survival rate
was not influenced by EE. BALB/c mice have been reported to be adversely affected by EE, female BALB/cmice kept in an enriched environment have fewer immature thymocytes [34]. BALB/c female mice tended tohave lower mature body weights when kept under enriched conditions, and a smaller number of litters were
produced by female BALB/c mice housed in an enrichedenvironment [4]. Even among studies using the same
mouse strain, both increases and decreases in locomotory activity have been reported for mice in EE conditions [35–38]. The reasons for these conflicting results
are unclear.
The use of an AGMAT cloth as a bedding material isa new concept in housing laboratory mice. The use of
the cloth can improve the habitat of the animals. The
AGMAT bedding was bitten and chewed by the mice.
158 | doi: 10.1538/expanim.21-0118

The mat floors in both the AGMAT and EE groups weredamaged in all the strains used here. The mice may havechewed the mat flooring to provide nesting material. The
goal of EE is to provide animals with opportunities toexpress their full range of species-typical behavioral
patterns [4]. Also, response-contingent enrichment objects (i.e., items the animals themselves can alter) areoften more effective at eliciting novel interactive responses. Therefore, AGMAT is considered to be a beneficial alternative to wood shavings.
CORT, the main glucocorticoid produced by rodent,
is a highly sensitive and reliable indicator of stress [39,40]. In our study, CORT levels in the EE groups weresimilar to those in control mice. CORT levels weresimilar to those previously reported that standard housing and enriched housing were no significant difference[33, 41, 42]. Presumably, the lack of any change here
among housing conditions indicates that the experiment
did not load extreme stress on the animals during theexperimental period. The C3H/HeN mice had slightly
higher CORT levels than the other strains. Marashi et al.
[43] reported CORT levels of 80–100 ng/ml in C3H/HeN
mice, suggesting that slightly higher CORT levels arecharacteristic of this strain. CORT levels have previously been examined in mice kept under EE conditions[44, 45]. EE may decrease emotional reactivity by lowering the levels of stress hormones such as ACTH and
CORT [46, 47]. While some studies found that EE decreased resting CORT levels [39] or following exposure
to stressors [11, 44], others have found that resting levels in EE animals did not differ from those of non-enriched controls [24, 42, 48] and that animals kept under
EE conditions had higher basal levels of CORT [41].
Possibly, the increases in CORT levels may be attributable to increased physical activity of animals in EE.
Future studies comparing the effects of various levels inenrichment settings on stress response are needed [49].
In our study, the incidence of C57BL/6J and C3H/HeN
mice with alopecia was significantly lower in the EE
group than in the control group. As C57BL/6J mice age,they can develop a mild punctuate ulcerative dermatitis[50]. Continuous grooming is an abnormal behavior thatcan cause alopecia in mice; it has been particularly
noted in C57BL/6J mice [51]. Skin lesions are common
in mice that pull or pluck hair fibers from the follicles[52]. This behavior is a symptom of brain dysfunction
and frequently occurs in a stressful environment [53].
Sundberg et al. [54] reported that the relative incidenceof alopecia areata in one production colony of C3H/HeJ
mice was 0.25% for female mice. The frequency in an
aging colony selectively bred reached 4.7% for miceover 18 months of age, suggesting that this might be a

ENRICHED HOUSING AND SURVIVAL RATE IN MICE

common aging change in C3H/HeJ mice. The EE usedin this experiment appears to have alleviated this hairloss symptom associated with aging phenomenon andexcessive grooming.
In conclusion, three mouse strains housed under EE
conditions showed improved survival; however, EE did
not alter the survival rate of the BALB/c mice. We suggest that there are inter-strain differences in response tochanges in environmental conditions. Variations in thepreferences of different species, strains, age groups, andsex require further evaluation. It is possible that the EE
provided greater opportunities for sheltering in a secure
location in which to avoid interactions with other mice.
Our findings here suggest that depending on the mousestrains different responses to EE are caused with regard
to health and survival rates. The results of this study
provide basic data for further studies on EE.
Conflict of Interests
The authors declare that have no conflict of interests.
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