Anim. Behav ., 1970, 18, 747-752

DIFFERENTIAL REACTIVITY OF WILD AND SEMI-DOMESTICDEERMICE (PEROMYSCUS MANICULATUS)

BY EDWARD PRICEDepartment of Forest Zoology, State University College of Forestry, Syracuse, New York 13210

The process of breeding `wild' animals incaptivity is an active evolutionary phenomenoninvolving changes in the gene pool of the popu-lation concerned (Hale 1962 ; Lorenz 1965 ; Fox1967; Price & King 1968 ; Berry 1969). Sinceselection `pressures' differ in nature and cap-tivity, `natural' selection will occur during thedomestication process for traits facilitatingadaptation to the captive environment .

It has been suggested (King 1939 ; Richter1954; Barnett 1958 ; Robinson 1965 ; Price1967) that fitness in captivity is enhanced bylow reactivity . This study was designed tomeasure differences in reactivity between astrain of deermice (Peromyscus maniculatusbairdii) maintained in captivity for 17 years(twenty to twenty-five generations) and thelaboratory born offspring of a field-caughtpopulation of the same subspecies . The hypo-thesis tested was that the propensity to approachand investigate a novel environment (open-field)would be greater for the semi-domestic strainthan for their wild counterparts . This hypothesiswas based on the premise that the lesser re-activity of the semi-domestic strain has resultedfrom changes in the gene pool (in the absence ofartificial selection) rather than changes associ-ated with the post-natal or post-weaning en-vironments.In mammals the maternal environment is

often critical in the behavioural development ofthe young (Rheingold 1963) . If the field-trappedmothers of the wild genotype strain were stressedby the transition from the field into the labor-atory, the resulting change in physiology and/orbehaviour could have had a permanent influenceon their offspring, mediated through the pre-and post-natal environments. Consequently, acontrol for the post-natal maternal environmentwas established by fostering offspring betweenand within strains .

The nature of the post-weaning experiencemay be critical for behavioural development(Newton & Levine 1968). Early experience inlaboratory cages or in the field represent twowidely divergent experiences for the developingsmall mammal. Consequently, the potential

747

importance of the post-weaning experience wasassessed by rearing individuals of both strainsin a semi-natural field enclosure in addition torearing in standard laboratory cages .

MethodsSubjects

Wild genotype strain. The eighty wild geno-type subjects were the laboratory born off-spring of fifty pairs of field-caught deermicetrapped in the vicinity of East Lansing, Michigan .during November 1964 and April 1965 .Semi-domestic genotype strain. The wild

ancestors of the eighty semi-domestic subjectswere trapped in the vicinity of Ann Arbor,Michigan (approximately 95 km from EastLangsing) in 1948 by Harris (1952) . At the timeof the study this stock had been in captivityfor 17 years and was estimated to be approx-imately twenty to twenty-five generations re-moved from the wild. During this period aconscious effort to avoid inbreeding was madeand artificial selection for fast and slow eye-opening (in its fifth generation at the time of thestudy) was the only conscious selection applied .Treatments

Post-natal maternal environment . A total ofsixty deermice of each genotype was born andreared in the laboratory by : (1) their naturalmothers (N=ten males and ten females),(2) foster mothers of the opposite genotype(N=ten males and ten females, and (3) fostermothers of the same genotype to control for theeffect of fostering per se (N=ten males and tenfemales) .Post-weaning rearing environment . Twenty

deermice (ten males and ten females) of eachgenotype (born in the laboratory) were reared inan outdoor enclosure from 21 to 55 days of age .These subjects were subsequently comparedwith animals reared in standard laboratorycages .All subjects were 58 to 62 days of age on

test day 1 .Care and Handling

All mice (regardless of treatment) were bornin the laboratory in clear plastic cages (12 .5 x

748

30 .0 x 15 .2-cm deep) with removable wirelids. Wood shavings were used for bedding andcotton was provided for nesting material. Food(Purina Mouse Breeder Chow) and water wereprovided ad libitum . Litters were reduced tofour young (two males and two females,when possible) shortly after birth . Litters con-taining less than three young were rejected fortesting purposes . All fostering was completedon or before 3 days of age .

Litters were weaned at 21 days of age, and thesubjects were numbered by toe clipping andear notching. The animals were placed either bysibling pairs (like sex) into standard laboratorycages or, by unisexual groups of four into thesixteen areas of the outdoor enclosure .The laboratory reared subjects were left

undisturbed except for periodic cage cleaning.After 5 weeks, the enclosure-reared mice weretrapped and brought into the laboratory wherethey were maintained in standard cages in uni-sexual pairs until test day 1, 6 days later.

Handling prior to testing (except where other-wise stated) was accomplished by grasping thetail with 30 . 5 cm metal forceps (tips covered withrubber hose) .

Outdoor EnclosureThe outdoor enclosure was located in a grass-

land similar to the natural habitat of the species .The outside dimensions were 30 .5 x 7 .6 mwith internal partitions dividing the enclosureinto sixteen equal areas, 3 .8 m2. Hardwarecloth fastened along its length to 35 .5 cmaluminium flashing provided the escape-proofwalls. The `free' side of the hardware cloth wasfolded 10 cm along its length to form a 90° bend .This side was buried approximately 10 cm in thesoil with the bend projecting toward the insideto prevent the mice from digging under . Thewalls were anchored around the periphery bywooden posts .

Each area in the enclosure was equipped with anest chamber made from a 30-cm piece of 10-cmdiameter drain tile buried vertically in theground and covered with a piece of 2-54-cmpine. Purina Mouse Breeder Chow plus ear cornwere provided to supplement the natural foodspresent in each area. Water was provided adlibitum during the dry months .

ApparatusOpen-field. Six open-field boxes (25 x 76 x

56 cm) were constructed from plywood (naturalfinish). The hardware cloth floor was marked off

ANIMAL BEHAVIOUR, 18, 4

into five equal sections of 15 cm each . A 4-cmdiameter hole was located midway along itslength, serving as the point of entry for thesubjects. A 71-W bulb placed over each open-field provided light from 06.00 to 18.00 hourseach day.

Three open fields were placed in each of thetwo sound-deadened test rooms . Stop cocksand counters located in a third room wereactivated by silent manually operated mercuryswitch keyboards in each test room . A blindconcealed the experimenter during the tests .

Six 10 .0 CM3 nest boxes were used for trans-porting and handling mice during the testperiods and as living quarters between testperiods. A plywood plunger served as a falseback to the nest boxes and facilitated the remov-al of mice from the latter with a minimum ofdisturbance. A nest box was positioned adjacentto the entrance hole of each open field betweentest periods during which time food and waterwere available ad libitum .

Start box . Two 6 .0 x 6 .0 x 13 .0-cm plywoodand masonite start boxes were used for intro-ducing the subjects to the open field duringtesting. A vertically sliding sheet metal doorformed the front side of the start box .

Weasel cage . During certain tests a least weasel(Mustela nivalis) was introduced in each open-field in a 10 .0 x 20 .0 x 15 .0-cm wooden cagewith hardware cloth front and sides. Fourseparate weasels and cages were used duringthe habituation period .

ProcedureEach subject was tested four times in the

following sequence :

On the day prior to test 1 a nest box wasplaced in the home cage of each of six pairs ofsubjects to facilitate subsequent handling . Ontest day 1 each pair of mice was removed fromits home cage in the nest box provided and in-

Testno .

Hours of exposureto open field

1 0 Initial reaction to open-field

2 48 Habituated reaction to open-field

3 72 Initial reaction to open-fieldcontaining caged least weasel

4 120 Habituated reaction to open-fieldand weasel

PRICE : DIFFERENTIAL REACTIVITY OF WILD AND SEMI-DOMESTIC DEERMICE

749

Table I. Median Latency to Enter Scores (Seconds) for all Laboratory-Reared Subjects

NM=reared by own mother, WF=reared by foster mother of same strain, CF=reared byfoster mother of opposite strain .

dividually transferred to a start box . The startbox was positioned at the entrance hole of oneof the six open-field apparatuses (each pair wasassigned to a different apparatus) and after 8min, the start box door was raised and securedin an open position . During each 2-min test trialthe observer recorded the following data : (1)latency to enter the open-field, (2) activityin the open-field (number of floor units crossedper unit time in the open-field), and (3) totaltime in the open-field . After each test trial thesubject was removed from the open-field (inthe start box) and returned to its respective nestbox. When all subjects had been tested on agiven day each pair was returned to its re-spective open-field where it lived until thesubsequent test period . All subjects were re-moved from the open-field apparatuses im-mediately prior to tests 2, 3 and 4 .

Although four caged weasels were used duringthe habituation period, a single weasel wasused for all tests on test days 3 and 4. Fortesting, the caged weasel was placed directlyopposite the entrance hole of the open-field .

Before beginning tests 2, 3 and 4, all micewere taken from their respective open-fields(while in their nest boxes) to an adjacent room .Each mouse was tested in its own open-field tokeep room cues and odours as nearly constantas possible. The tests were conducted between13.00 and 15 .00 hours each day.Two sound-deadened rooms were employed

in test administration . Thus, one subject couldbe in starting position while another was beingtested in the adjacent room .

ResultsSeparate statistical analyses were made on thepost-natal maternal and post-weaning experi-ence treatment effects . Only the `latency to

enter' and `entry versus non-entry' data wereanalysed since the experimenter could notseparate activity associated with escape andexploration and because the `time in the openfield' data were generally the reverse of the`latency to enter' scores .The propensity to enter the novel environ-

ment (open-field) was significantly greater forthe semi-domestic strain . Table I presents themedian latency to enter values (seconds) foreach of the 4 test days . Sexes were combinedand the scores were transformed to log valuesprior to conducting the analysis of variance.Test day 4 scores were not included in theanalysis since an insufficient number of weaselswere available for all fostered groups .

Significant strains (F=68 .6, P<0 .005) andday (F=5 .54, P<0 .005) effects were obtained .The post-natal maternal environment effectwas not significant (F=1 . 36, P<0 .10). The wildgenotype subjects exhibited significantly longerlatencies than the semi-domestic mice in enteringthe open-field. A `New Multiple Range' test(Li 1964) conducted on the `day factor' (maternaltreatments combined) indicated that the semi-domestic subjects entered more readily (P<0 .01)on test day 2 (habituated response to open-field) than on test days 1 and 3 . The latencyscores for the wild genotype subjects did notdiffer over test days.

Strain differences in the number of subjectsentering the open-field during the 2-min testtrial are given in Table II (maternal treatmentscombined). Chi-square contingency tests in-dicated that a significantly greater number ofsemi-domestic subjects entered irrespective oftest day.

Whereas the number of subjects entering theopen-field did not change appreciably for eitherstrain following 48-hr exposure to the open-field,

Test day Wild Semi-domestic

NM WF CF NM WF CF

1 104 .0 47 .4 75 . 6 28 . 6 21 . 8 41 .0

2 92 . 8 51 . 8 89 . 6 15 . 0 13 .6 14 . 3

3 120 .0 120 .0 120 . 0 17 . 0 22 . 6 20 . 7

4 120 .0 120 . 0 32 .6 - 64 . 6

750

ANIMAL BEHAVIOUR, 18, 4

Test day

Table II. Number of Laboratory-Reared Subjects which Entered (E) and Failed toEnter (F) the Open-Field (Sexes and Maternal Treatments Combined)

*P<0 . 005 .

Table III . Number of Laboratory (Natural Mothered) and Enclosure-Reared Subjects Entering the Open-Field (Total N perSubclass = 10)

Wild

Laboratory

Enclosure

a substantial decrease in the number of entrieswas noted with the caged weasel present (testdays 3 and 4) . Chi-square contingency tests(test days 2 and 3) indicated that this decreasewas significant for both wild genotype (x2=8 .83,P<0 .005) and semi-domestic (x2=4 .78, P<0 .05) strains. As Table II indicates, this decreasewas much more marked in the wild than in thesemi-domestic strain .

Table III presents the number of enclosure-reared subjects entering the open-field duringthe four test periods. Sexes are given separatelydue to an obvious difference between male andfemale enclosure-reared semi-domestic subjects .

Strain differences among the enclosure-rearedsubjects were tested by the Fisher Exact Proba-bility Test (Siegel 1956). The results indicatedthat the male wild and semi-domestic subjectsdid not differ on any test day, whereas thefemale semi-domestic subjects entered the open-field more readily than the wild females on testdays 1 (P<0 .01), 2 (P<0 .05), 3 (P<0 .05) and4 (P<0 .05) .

The Fisher Exact Probability Test indicatedthat frequency of entry was significantly reduced

Semi-domestic

Laboratory

Enclosure

in the male semi-domestic subjects due to post-weaning experience in the outdoor enclosure .This relationship was consistent over days oftesting (P<0 .05, P<0 .005, P<0 .005 and P<0 .01, respectively) . Female semi-domestic sub-jects were unaffected by this treatment .

Fisher Exact Probability analyses showed thatsex was an important variable in the enclosure-reared semi-domestic subjects only . The per-centage of male subjects entering the open-field was significantly lower on all 4 test days(P<0 .05 in each case).

No significant change in the frequency ofentering was obtained over test days for anygroup .

DiscussionThe preceding results suggest that successivegenerations of breeding Peromyscus in captivitytends to reduce the caution with which theseanimals approach or investigate unfamiliarstimuli. The implication is that this reductionin reactivity has been brought about by changesin the population gene pool resulting from

Male Female Male Female Male Female Male Female

1 6 5 3 1 7 9 2 7

2 5 6 3 1 8 8 1 6

3 3 2 1 0 7 7 0 4

4 4 2 1 0 6 6 0 4

Wild Semi-domesticTest day x2 value

E F E F

1 36 24 51 9 8 . 19*

2 33 27 52 8 15 . 50*

3 16 44 41 19 20 .88*

4 9 21 28 14 9 .42*

PRICE: DIFFERENTIAL REACTIVITY OF WILD AND SEMI-DOMESTIC DEERMICE

751

differential selection pressure in the field andlaboratory environments .

An alternative explanation is that the wildcaught Lansing stock was genetically differentfrom the wild caught ancestors of the semi-domestic stock from Ann Arbor . (These an-cestors are no longer available for testing .)However if the observed strain differences inreactivity are the result of differential selection,it is more probable that the selection differentialinvolved was generated in contrasting laboratoryand field environments than between two fieldpopulations separated spatially by 95 km andtemporally by 17 years .

If reduced reactivity enhances fitness in thelaboratory environment, it is highly possiblethat the genetic factors influencing approach-avoidance behaviour could have changed duringtwenty to twenty-five generations of captivebreeding even in the absence of artificial select-ion. This assumes, of course, that sufficientgenetic variability (for this trait) exists in thewild-caught population to permit natural select-ion to occur in captivity .

Although a control was not conducted for thepre-natal maternal environment, fostering off-spring both within and between strains revealedno permanent post-natal maternal effect on thebehaviour studied. This finding was in agreementwith Broadhurst (1961) who failed to demon-strate a post-natal maternal effect from fosteringoffspring between low and high reactive strainsof domestic rats .The behaviour of the wild genotype and

female semi-domestic subjects was not changedby 5 weeks of early experience in an outdoorenclosure. On the other hand, the propensityof the male semi-domestic subjects to approacha novel environment was significantly reducedby this treatment . The sex differential treatmenteffect in the semi-domestic strain was repli-cated at a later date with six male and sevenfemale subjects . As before, failure to enterthe open-field was greater for the enclosure-reared males than for the females (ratio entryversus non-entry : males 2 : 4 ; females 6 : 1) .

The hypothesis does not account for the sexdifferential response of the semi-domestic miceto post-weaning treatment. Several hypothesesmay be postulated however, based on the resultsof similar studies conducted largely on dom-esticated animals (Hafez 1962 ; Newton &Levine 1968). First, the sexes may have beendifferentially responsive to the increased sensoryand locomotor experience afforded by the en-

closure environment . Secondly the transitionfrom the outdoor enclosure to the laboratoryjust 6 days prior to testing may have beendifferentially stressful to the male and femalesemi-domestic mice .

Most likely the explanation for the differentialsex response concerns the phenomenon ofhabituation. Little is known about the rateand extent to which field-reared animals habit-uate to the laboratory environment (Melzack1968). If males habituate more slowly thanfemales, 6 days in the laboratory may haveeffectively segregated sexes . In a subsequentstudy allowing 35 to 55 days of habituation tothe laboratory environment, enclosure-rearedmale and female semi-domestic mice did notdiffer in either running wheel activity (Price1969) or novelty-induced food deprivation(Price, in preparation) . If this explanation iscorrect it implies that the effect of early en-closure experience on the semi-domestic mice istransient.Only the semi-domestic subjects exhibited

decreased reactivity toward the open-field in thesecond test period . Apparently 48 hr was notsufficient time for habituation to occur in thewild genotype strain .

Latency scores increased for both strainswith the caged weasel present . The use of anatural predator of Peromyscus was deliberatealthough the intent was to present a strongfear-provoking novel stimulus rather than tostudy innate responses of prairie deermice tothe least weasel . Since responses to the weaselwere not compared with responses to otherpotential novel stimuli (animate or inanimate),it is not known if the subjects were respondingto specific predator-associated cues (selectedavoidance response) or simply to an unfamiliaranimal per se (generalized avoidance) .

On initial exposure to the caged weasel bothstrains display ambivalent behaviour charact-erized by an alternation of cautious approachand rapid retreat . The retreat was often in-itiated when the weasel moved. Since the weaselmade no audible sounds during the tests it isassumed that the important stimulus cues werevisual and olfactory. The least weasel is notknown to emit ultrasonic vocoli7..ations (Huff& Price 1968) . It may be significant that neitherstrain exhibited evidence of habituation to thecaged weasels after 48 hr of exposure to them .

The failure of many wild genotype subjectsto enter the open-field in the 2-min test periodreduced the significance of some of the data

752

ANIMAL BEHAVIOUR, 18, 4

collected . The entry versus non-entry categorywas perhaps the most valid measurement .However, had the test trial been lengthenedsufficiently to permit an entry by all subjects, itis doubtful that the conclusions reached wouldhave differed .

A check on the reliability of the above datawas made by retesting eight laboratory-rearedsubjects of each strain 11 to 12 months later .The semi-domestic strain continued to ap-proach the open-field more readily than thewild strain .

The above study warrants the tentative con-clusion that the transition from field to labor-atory environments is accompanied by a shiftin selection pressures affecting those behavioursconcerned with the propensity to approach andinvestigate novel stimuli .

SummaryA strain of deermice (Peromyscus maniculatusbairdii) maintained in captivity for 17 years(twenty to twenty-five generations) was com-pared with the laboratory-reared offspring of afield-caught population in regard to the pro-pensity to enter an unfamiliar open-field .Controls were established for the post-natalmaternal environment and post-weaning rearingexperience (laboratory cages versus outdoorenclosure) .

The semi-domestic subjects exhibited sig-nificantly shorter latencies in entering the open-field and significantly fewer subjects failed toenter during the 2-min test trial. Fosteringoffspring both between and within strains did notaffect these results.Whereas early experience in a semi-natural

field enclosure did not change the open-fieldbehaviour of the wild genotype subjects, pro-pensity to enter was significantly decreased inthe male semi-domestic animals .Only the laboratory reared semi-domestic

subjects showed significantly decreased avoid-ance after 48 hr habituation to the open-field .The propensity of both strains to enter the open-field was significantly decreased in the presenceof a caged least weasel .

The reduced reactivity of the semi-domesticsubjects was explained by differential selectionpressure in field and laboratory environments .

AcknowledgmentsThe author would like to thank Dr John A .King for his advice and guidance during thisstudy and his critical review of the manuscript .Dr Rollin Baker, Dr Walter Blinn and Dr

William Cooper also provided invaluable assist-ance. This study was conducted at MichiganState University and was supported by GrantM-5643 to John King from the National In-stitute of Mental Health, United States PublicHealth Service. It represents a portion of theauthor's doctoral dissertation .

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(Received 2 January 1970 ; revised 17 June 1970 ;MS. number : A925)