Assessment of the use of two commercially available environmental enrichments by laboratory mice by...

The provision of environmental enrichmentfor laboratory animals is becomingincreasingly less controversial. ManyEuropean institutes involved withlaboratory animals have by now adoptedenvironmental enrichment for their animalsas a standard operating procedure. The

Council of Europe has included a section onenvironmental enrichment in its Resolutionon Accommodation and Care of LaboratoryAnimals (Council of Europe 1997) andin the revision of the guidelines onAccommodation of Laboratory AnimalsETS 123 (Kornerup-Hansen 1999). Withinthe scientific community however, manystill object to the use of environmentalenrichment for its alleged threat to validexperimental results (Dean 1999).

Assessment of the use of two commercially

available environmental enrichments by

laboratory mice by preference testing

Pascalle L. P. Van Loo1, Harry J. M. Blom1, Margot K. Meijer1 & Vera Baumans1,2

1Department of Laboratory Animal Science, Utrecht University, The Netherlands and 2Karolinska

Institute, 17177 Stockholm, Sweden

Summary

In the field of biomedical research, the demand for standardization of environmentalenrichment for laboratory animals is growing. For laboratory mice, a wide variety ofenvironmental enrichment items are commercially available. Most of these comply withthe demands for standardization, hygiene and ergonomics. Whether these items also complywith their actual purpose, to enhance the well-being of the mice, is often not assessedscientifically. In this study, we tested the preference of mice for two commercially availablenest boxes differing in shape and material: the Shepherd Shack/DesRes (SS/DR) and theTecniplast Mouse House (TMH), in a simple preference test. To indicate strength ofpreference, both nest boxes were also tested against a highly preferred nesting material.Preference for the most preferred nest box was investigated further. Our results indicateda strong preference by mice for the SS/DR, but not for the TMH. Furthermore, nestingmaterial was almost always combined with the SS/DR, but not with the TMH. Moreelaborate testing of the SS/DR in an automated preference test system confirmed that micespent significantly more time in a cage in which an SS/DR is provided. Differences betweenboth nest boxes are discussed with regard to their attractiveness to mice. It is also arguedthat enrichment should primarily be developed in concordance with the animals’ needsprior to the marketing of enrichment tools.

Keywords Mice; preference; welfare; environmental enrichment; nest box; nestingmaterial

Accepted 5 April 2004 © Laboratory Animals Ltd. Laboratory Animals (2005) 39, 58–67

Correspondence to: Dr P. L. P. Van Loo, Department ofLaboratory Animal Science, Utrecht University, PO Box80.166, 3508 TD Utrecht, The NetherlandsE-mail: [email protected]

Commercially available enrichment for mice 59

Laboratory Animals (2005) 39

Environmental enrichment should in thefirst place be designed to cater for the healthand well-being of laboratory animals. Ofequal importance is that it satisfies experi-mental demands. The by far most importantexperimental demand is for standardization.Standardization in experimentation has beenintroduced in the mid-20th century in orderto reduce inter- and intra-experimental vari-ability and to increase the reproducibility ofresults within and between laboratories.However, any housing or managementprocedure can be standardized, irrespectiveof economic and ergonomic demands.Although the variation in individual param-eters may differ when measured in differenthousing conditions, an increase ofcomplexity in housing conditions does notas a matter of course increase the variationin results (Dean 1999, Eskola et al. 1999,Tsai & Hackbarth 1999, Van de Weerd et al.2002). In view of the above, the introductionof standardized environmental enrichmentfor laboratory animals emphasizing species-specific behavioural needs would be alogical step fulfilling both the demandfor improved animal welfare and thedemand for standardization in the animalexperiment.

Different kinds of environmental enrich-ment items for laboratory mice are commer-cially available, ranging from nest boxes andnesting materials to running-wheels andgnawing blocks (Mortell 2001). Some ofthese commercially available enrichmentitems can comply with the demand forstandardization. Furthermore, they usuallycomply with hygienic and ergonomicdemands. Manufacturers recommend ‘their’enrichment item as being user-friendly, nottime consuming, and advantageous to themice’s well-being. The latter claim,however, usually lacks any scientific back-ground and is sometimes assessed from ananthropomorphic perspective. Since the firstand foremost aim of environmental enrich-ment is to enhance animal well-being, thisis at least questionable. A better approachwould be to choose the mouse’s perspectivewhen assessing benefits to animal well-being. Tests investigating the preferences forand use of enrichment items by mice are

numerous, especially with regard to nestingmaterial and nest boxes or shelters (see Vande Weerd & Baumans 1995 and Olsson &Dahlborn 2002 for a review). In general,mice show a strong preference for nestingmaterial, irrespective of strain or gender(Van de Weerd et al. 1997, 1998b). Theymake extensive use of different types ofnesting material to build complicated nests.Nest boxes and shelters that have beentested in the past were much less preferredby mice, and often used as explorationobjects or toilets, rather than as refuges andsleeping areas (Ward et al. 1991, Van deWeerd et al. 1998a,b). Wild mice, however,used nest boxes as feeding posts, for storageof food, or to build nests for bearing andraising offspring (Truszkowski 1974).Commercially available nest boxes arerelatively easily applicable and standardizedtypes of environmental enrichment formice. We therefore thought it worthwhile totest the preference of mice for two commer-cially available nest boxes differing in shapeand material: the Shepherd Shack or DesRes(SS/DR), and the Tecniplast Mouse House(TMH). Manufacturers of both nest boxesstated that their product is readily used asnesting area by mice, but failed to producescientific evidence to back up their claims.In a pilot experiment, we conducted asimple preference test to investigatewhether mice chose to sleep in either of thetwo nest boxes. In a more elaborate experi-ment, we further investigated the preferencefor one of the two nest boxes.

Experiment 1

The protocols of the experiments wereapproved by the Ethics Committee of theVeterinary Faculty of Utrecht University(IACUC), and peer-reviewed by thescientific and ethical committee of theDepartment of Laboratory Animal Science.

Methods

Nest boxes The Tecniplast Mouse House(Tecniplast, Milan, Italy) is a red transparentperspex triangular-shaped nest box(11.7 �11.7 �6.5 cm) with a small tunnel

opening on the long side (5 �3 cm) and anextra triangular opening in the top(4.5 �4.5 �6 cm). It weighs approximately95 g. The Shepherd Shack (ShepherdSpecialty Papers, Kalamazoo, Michigan US),also known as DesRes (Lillico, Surrey, UK),is a paper-based triangular-shaped nest box(14.6 �8.9 �6.4 cm) with one small hole(Ø4 cm) in the side. It weighs approximately20 g (see Fig 1). Both items are comparablein shape and volume; they differ in materialand weight.

Animals and husbandry Forty-nine femalemice of three strains (BALB/cANCrlBr(n �13), C3H/HeNHsd (n �10) andC57BL/6JIco (n �26)) were used in this partof the study. The mice were aged between6 and 18 months and had all been usedpreviously for experimental or educationalpurposes with no more than minor discom-fort. All mice were familiar with differentkinds of environmental enrichment, butwere naive to both the Shepherd Shack/DesRes (SS/DR) and the Techniplast MouseHouse (TMH). The animal room had a con-trolled photo and background noise period(lights and radio on between 07:00 h and19:00 h), temperature (21–23°C), and relativehumidity (50 �5%). Three days prior to

preference testing, all the home cages wereprovided with a non-transparent PVC tube,similar to those used to connect cages in thepreference test system, in order to familiar-ize the mice with the test system.

The preference test system consisted oftwo Makrolon type II cages (375 cm2)connected with a PVC tube (inner dimen-sions 2.6 �2.6 cm, 25 cm long). All the testcages were provided with 50 g of sawdust(Lignocel 3/4, Rettenmaier and Söhne,Ellwangen-Holzmühle, Germany), andad libitum food pellets (CRM, SDS,Witham, Essex, UK) and tap water. One ofthe test cages was additionally furnishedwith either a TMH (n �7) or a SS/DR (n �8).A total of 15 of these housing systems wereused to allow simultaneous testing of allgroups of mice. At the start of the experi-ment, 15 groups of three or four familiarmice were placed randomly in one of thetwo test cages and were allowed to choosea cage for a period of 2 days. After 2 days,the empty test cage was furnished with theother nest box. All groups could now choosebetween the SS/DR and the TMH foranother period of 2 days. Finally, in theoverall least preferred test cage, two Kleenextissues (Kimberly-Clark Corporation®, EC)were added for an extra 2 days.

60 Van Loo et al.


Fig 1 The two enrichment items tested: the Shepherd Shack/DesRes (SS/DR, left) and the Techniplast

Mouse House (TMH, right)

Data collection and statistics During theexperiment, the nest site of the mice wasdetermined by checking twice daily at10:00 h and 15:00 h in which test cage themice slept. Sleeping site within the cage(inside or outside nest box and/or tissues)was noted as well. Strain differences inpreference as analysed using a �2 test werenot present. Data of different strains weretherefore pooled in subsequent tests. Dataon test cage preference were analysed usinga two-tailed binomial test with group as thestatistical unit (SPSS 9.0). P values wereBonferroni corrected when more than onecomparison was made (indicated by P

B).

Descriptive statistics were used to outlinethe sleeping site within the test cages.

Results: Experiment 1

During the first 48 h of the test period,most groups of mice significantly choseto sleep in the cage containing a nest box(Fig 2, P �0.007). This was primarily due to astrong preference for the SS/DR (P

B�0.016).

All the groups slept inside the SS/DR.Preference for the TMH was not significant(P

B�0.906). From the five groups that chose

the cage containing the TMH, two groupsslept inside the TMH, and three groups sleptoutside the TMH. When mice were givena choice between the SS/DR and the TMHduring the second test period of 48 h, they

significantly chose to sleep in the SS/DR(P �0.001). The single group that chose tosleep in the cage with the TMH did so inthe sawdust outside the TMH. When tissueswere provided in the cage containing theTMH, preference for the SS/DR was notsignificant anymore (P �0.118). However, 11out of 15 groups dragged the tissues to theSS/DR through the passage tube and builta tissue nest inside the SS/DR. The fourgroups that did not take the effort todrag the tissues built a nest of the tissuesnext to the TMH. None of the groups combined tissues with the TMH.

Experiment 2

To further investigate the preference for theSS/DR in comparison to nesting material,we compared the SS/DR in a more elaboratepreference test with two types of paper-derived nesting material: Kleenex tissuesand Enviro-dri. In a previous experimentthese were found to be most and leastpreferred, respectively (Van de Weerd et al.1997).

Methods

Animals Twelve male and 12 female miceof the BALB/cAnNCRLBr strain were used,different individuals from those used in

Fig 2 Number of test groups that made their nest in the control cage, next to the Tecniplast Mouse

House, in the Tecniplast Mouse House or in the Shepherd Shack/DesRes



62 Van Loo et al.


Experiment 1. At the time of testing, themice were about 6 months old, and werepreviously used for experimental purposeswith no more than minor discomfort. Halfof the male and half of the female micewere familiar with the SS/DR, and half ofthem were naive. The mice were housed insame-sex groups of three in elongatedMakrolon II cages (530 cm2), provided withsawdust, food pellets and tap water in condi-tions similar to Experiment 1. Three daysprior to preference testing, all the homecages were provided with a non-transparentPVC tube, similar to those used to connectcages in the preference test system inExperiments 1 and 2, in order to familiarizethe mice with the test system.

Preference test system The preference testsystem used in this study has been validatedand described in detail by Blom et al. (1992).In short, a housing system was used consist-ing of four test cages connected to a clearperspex central cage (15 �15 �18 cm). Thetest cages were Makrolon type II cages(375 cm2) provided with 50 g of sawdust, andad libitum food pellets and tap water in abottle as described in Experiment 1. The

central cage had no food, water or bedding(Fig 3). A total of six of these housing sys-tems were used to allow the simultaneoustesting of six mice. To minimize any exter-nal influences on choice behaviour, eachsystem rotated slowly during testing. Themovements of the mice between the testcages were detected automatically by meansof photoelectric sensors in the passagetubes. The signals were sent to a computerthat calculated dwelling times per cage(Gate-Watch, Metris BV, Hoofddorp, TheNetherlands).

Procedure and data collection Mice wereintroduced into the test system between15:00 and 18:00 h and dwelling times ineach of the test cages monitored over threeperiods of 24 h. During the first 24 h, themice were given a choice between a controlcage without enrichment, a cage providedwith an SS/DR, a cage provided with 5 g of Enviro-dri (ED; folded paper strips,11 �0.3 cm, Shepherd Specialty Papers,Kalamazoo, Michigan, USA), and a cageprovided with both SS/DR and 5 g of ED(combi). At the end of Period 1, the mostpreferred cage was closed by a trap door, and

Fig 3 The preference test system, top view

the shift in preference was measured foranother 24 h. At the end of Period 2, thecontrol cage was provided with two Kleenextissues (tissues), the trap door to theinitially most preferred cage was re-opened,and preference was measured for another24 h. Dwelling times in each of the cageswere calculated automatically. Furthermore,the sleeping site was scored daily and amark was made if nesting material wasdragged to other cages.

Statistical analysis Data on dwellingtimes on each of the 24 h test days wereanalysed separately. First, a one-sampleKolmogorov–Smirnov test was used toestablish whether data were normallydistributed. This was the case for alldatasets used. Then, a Student’s t-test wasused to establish whether male and femalemice differed significantly in their choicebehaviour, or whether prior familiarity withthe SS/DR influenced choice behaviour. Formost datasets, this was not the case;however, some datasets revealed smallgender differences. Gender was thereforeincluded as a factor in the final preferenceanalysis. The preference for a cage wastested with an analysis of variance forrepeated measures with cage as the withinfactor, and gender as the between factor. Ifan overall cage effect was present, differ-ences between dwelling times in two cageswere tested with a paired t-test includingcage (all test periods) and gender (Period 1)as factor. In this way, six comparisons weremade for the first 24 h period and final 24 hperiod, and three comparisons were madefor the second 24 h period (since during thisperiod, one cage was closed). A Bonferronicorrection (P value times 6 or times 3,respectively) was applied to correct for type

II statistical errors. All statistical tests werecarried out using SPSS for MS WindowsRelease 9.0. Due to technical failure,datasets of some animals at certain timeperiods were not reliable. These weretherefore omitted from statistical analysis.The number of valid datasets (out of a totalof 24) in each of the three time periods was19, 21 and 21, respectively.

Results: Experiment 2

During all three time periods, dwellingtimes in the test cages differed significantly(see Table 1). Furthermore, an overall gendereffect was present during the first testperiod. This was due to the fact that femalemice spent more time in the central cagethan did male mice. This gender differencehad therefore no effect on the final compar-isons between cages.

The three test periods In the first testperiod, mice could choose between a controlcage (control), a cage with SS/DR, a cagewith Enviro-dri (ED) and a cage with bothSS/DR and ED (combi). Paired t-tests of thefirst 24 h period reveal that both male andfemale mice dwelt significantly longer inthe combi cage (Fig 4, all tests: P �0.001).Furthermore, dwelling times in the SS/DRcage were significantly higher than inED cages (P �0.042), and higher thancontrol cages, although this was only atrend (P �0.060). Dwelling times in controland ED did not differ significantly.

From this first test period, the combi cagewas clearly the most preferred cage. Forthe second test period this cage was subse-quently closed by a trap door, forcing themice to shift their preference to a previouslyless preferred cage, the choice being

Table 1 Overall statistical differences for dwelling times and gender (Anova for

repeated measures)

Test period (h) Cage effect Gender effect Subsequent paired t-tests

0–24 �0.001 0.021 For both cage and gender

24–48 �0.001 NS Only for cage

48–72 �0.001 NS Only for cage

If P �0.05, paired t-tests were performed to establish the most preferred cage. NS �notsignificant



64 Van Loo et al.


between control, SS/DR or ED. Paired t-testsreveal that the SS/DR cage was now highlypreferred by both male and female mice(both tests: P �0.001) while dwelling timesin control and ED cages again did not differsignificantly (Fig 4).

At the start of the final test period,Kleenex tissues were provided in the controlcage, and the trap door to the combi cagewas re-opened. The choice for the mice wastherefore: tissues, SS/DR, ED or combi.Paired t-tests reveal that, again, the combi

cage was the most preferred cage (all tests:P �0.001). Dwelling times in other cages didnot differ significantly (Fig 4).

Dragging of nesting material and sleepingarea In many cases, the mice dragged bothEnviro-dri and Kleenex tissues completelyor partially to the SS/DR or combi cage(Table 2). Furthermore, if a cage containingan SS/DR was the preferred cage (as inalmost all cases), the mice always used theSS/DR to sleep in.

Fig 4 Dwelling time in each of the four test cages for males (top) and females (bottom), separated for the

first, second and third 24 h period

General discussion

The first experiment clearly showed thatmice had a strong preference for theShepherd/Shack compared to the TMH.Almost all the mice used the SS/DR forsleeping and, when available, draggednesting material into it to build elaboratenests. Several studies have indicated astrong preference of mice for tissues. Ifavailable, mice will almost always use it tobuild a nest (Van de Weerd et al. 1997,1998b, Baumans et al. 2002, Olsson &Dahlborn 2002). Combining features of theleast preferred cage (TMH) with tissuescould therefore provide information on thestrength of preference for the most preferredcage (SS/DR). The preference for the SS/DRwas not significant when tissues were pro-vided in the cage containing the TMH. Still,one would not expect any of the mice todrag tissues to a nest box in another cage ifa suitable nest box is already present in thecage containing tissues. After all, draggingtissues through a small tube is energy andtime consuming. Dragging of nesting mate-rial through a small tube has been observedbefore in a preference test by Van de Weerdet al. (1997). In this test, approximately 50%of the mice combined two types of nestingmaterial by dragging one to the other. In asubsequent test in which tissues werepresented on a grid floor, none of the micedragged these tissues to the cage containingsawdust or to a metal nest box that was pre-viously preferred while comparing a series ofnest boxes (Van de Weerd et al. 1998a). Itseems, therefore, that dragging of nestingmaterial only occurs if the other cagecontains features that are attractive to themice or when they want to avoid a certainfeature.

The first experiment was solely conductedwith female mice. It is possible that malemice would demonstrate a different prefer-ence; however, results from the secondexperiment and from previous preferencetests (Van de Weerd et al. 1997, 1998a)indicate that enrichment preferences ofmale mice do not differ drastically fromthose of female mice.

The second experiment confirms thatmice show a strong preference for theSS/DR. In this experiment too, 62% (ED) to88% (tissues) of the mice dragged nestingmaterial to a cage containing an SS/DR. Inview of the results presented here, and theexperiments performed by Van de Weerdet al. (1997, 1998b), one might presume thatthe SS/DR has features which provide addedattractive value to mice with regard to theirchoice of nesting area. The TMH seems tobe lacking these features (this paper) as doother perspex or metal nest boxes (Van deWeerd et al. 1998a). The SS/DR and theTMH differed with regard to constructionmaterial (rough surface paper vs smoothsurface Perspex, respectively), and weight(light vs heavy, respectively). These featuresmake it possible to manipulate the SS/DR,while the TMH is more rigid. The TMHalways stayed in position once placed in thecage, while mice usually moved around andmanipulated the SS/DR. We observedseveral mice lift the SS/DR, so that itrandomly provided an exit anywhere in thenest box. Other mice nibbled an extra holein the side, or shredded part of the box,using the shreds to strengthen their nest.We also observed that some groups hadtossed the SS/DR over, thus creating a‘rocking-chair’ in which a nest was built.For future research, systematic gathering ofmore detailed information on the use of

Table 2 Dragging of nesting material (in number of times observed)

Not dragged To combi cage To SS/DR cage Other

Tissues 2 15 6 1 to central cage

Enviro-dri* 8 5 10 1 to tissue cage

*Enviro-dri was often partially dragged. The number of times mentioned are total amount of times (e.g.if two mice each dragged half of the Enviro-dri, this was counted as one time dragging).SS/DR �Shepherd Shack/DesRes



66 Van Loo et al.


enrichment items is suggested. Still, thesecasual observations provide evidence thatthe SS/DR increases environmental controlfor mice. Environmental control is animportant stress-reducing propensity (Weiss1972, Wiepkema & Koolhaas 1993,Sambrook & Buchanan-Smith 1997). Severalauthors have argued that active behaviourmay have a reward value in itself, i.e. thatan animal’s needs may lie as much in the‘doing’ as in the ‘achieving’ (Hughes et al.1989, Jensen & Toates 1993, Wemelsfelder1997, Poole 1998). Items providing for thisneed, such as the SS/DR and nesting material,would thus be more suitable enrichmentitems than rigid structures such as the TMH.This is in concordance with results from aprevious experiment in which we compareda rigid shelter with tissues as environmentalenrichment (Van Loo et al. 2002).

Concluding remarks

The most important aim of environmentalenrichment is to meet essential behaviouralneeds of animals in order to improve theirwell-being in captivity. This paper providesevidence that some commercially availableenrichment items are more suitable in thisrespect than others. It seems, however, thatthe design of environmental enrichmentitems for laboratory animals is primarilybased on human demands (standardization,economy and ergonomics), withoutscientific evidence that these items meetthe animals’ needs and preferences. Westrongly question whether this is the waytowards an improvement of animal welfare.To comply with the animals’ needs whilekeeping in mind human requirements, wepropose that the ultimate standardizedenrichment item should be designed andtried on the basis of knowledge that hasbeen gained from many enrichment studiesalready performed.

Acknowledgments We gratefully acknowledge thefinancial support and provision of the ShepherdShack/DesRes items for this study by ShepherdSpecialty Papers (Kalamazo, Michigan, USA) andLillico (Surrey, UK), and the provision of theTecniplast Mouse Houses by Tecniplast (Milan,Italy).

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