Anim. Behav ., 1977,25,479-488

TESTOSTERONE AND PERSISTENCE IN MICE

BY JOHN ARCHER*Ethology and Neurophysiology Group, School of Biological Sciences, University of Sussex

Abstract . In view of previous studies showing that testosterone increases persistence of food searchingin chicks, a single factorially-designed experiment was carried out to investigate whether a similarphenomenon occurs in male mice . Using a runway test, it was found that testosterone, injected intocastrated mice, did increase persistence . It was also shown that intact males resembled more thetestosterone-injected than control-injected castrates, and that females resembled neither intact malesnor either group of castrates . A larger number of training trials was found to affect feeding latencies ina similar way to testosterone . Comparison of two strains differing in emotional reactivity (BALB/cand Porton) showed differences consistent with their reactivity levels .

Testosterone injected into male chicks duringthe first week after hatching induces precocioussexual and aggressive behaviour (e.g. Andrew1975). It also produces effects on behaviourwhich have been described as increased 'per-sistence' (Andrew & Rogers 1972 ; Andrew1972a; Rogers 1974) : these effects were originallyfound in chicks feeding from grain scatteredonto a floor, testosterone-treated chicks showinglonger `runs' of pecks from a particular area ofthe floor (on a plain floor) or longer runs on oneparticular type of food grain (when taking thisfood against a distracting background ofsimilarly-coloured pebbles) .

Results attributed to the same effect oftestosterone on persistence have also been foundin a test involving a feeding response in a run-way : Archer (1974) trained chicks to run to adish in a straight runway, then injected themwith testosterone or oil, and subsequently testedtheir running responses during trials on whichthe properties of the runway or food dish hadbeen changed . It was found that changing therunway walls increased the running times ofcontrols to a greater extent than those of thetestosterone-injected chicks, whereas changesat the food dish produced the opposite result .These findings were explained in terms ofAndrew's (1972b) suggestion that testosteroneincreases the degree to which search specifica-tions relevant to the feeding response aresustained in the face of factors tending toproduce changes in these specifications. Morerecently, Andrew (1976) has offered a furtherexplanation, in terms of rules for the selectionof stimuli which control attention, testosteronebeing regarded as opposing changes in theserules once they have come into use. Tests of the*Present address : Division of Psychology, PrestonPolytechnic, Corporation Street, Preston, Lancs .

479

validity of such theoretical explanations mustawait more detailed experiments on the be-haviour of testosterone-treated chicks in runwayand related tests (in progress at Sussex) .

In addition to the precise mechanism under-lying the testosterone-induced changes in chickbehaviour, another important question concernstheir generality. It is known that similar effectsoccur when castrated cockerels are injected withtestosterone, and that intact cockerels showsearch behaviour similar to adult castrates(Rogers 1974). Thus the effect of testosteroneon persistence has so far only been investigatedin chickens : it has, nevertheless, already beengeneralized to humans in some discussions of thepossible biological bases of sex differences (e .g .Andrew 1972a, b ; Hutt 1972 ; see also Archer1976c ; Rogers 1976). It seemed important,therefore, to investigate the possible occurrenceof similar effects of testosterone in a mammalianspecies : such an investigation, carried out onmice, is described in this paper .

Of the two types of test situation used forstudies of this subject in chicks, food searchand runway tests, the second one has been usedfairly widely in rodents (e .g. in brain lesionwork). This consideration, together with thedifficulty of devising for rodents a food-searchsituation comparable to grain pecking in chicks,led to the choice of a runway test for the presentexperiment. Mice were trained to run and feedat the end of the runway, and the effects (ontheir feeding latencies) of introducing changeseither at the food dish or half-way down therunway were assessed. Several hypotheses wereinvestigated, the first being that testosteroneinjected into castrated adult mice would produceeffects on runway performance similar to thosefound when testosterone was injected into youngmale chicks. Essentially, these effects consisted

480

ANIMAL BEHAVIOUR, 25, 2

of a greater increase in feeding latencies when achange was introduced at the food dish, but alesser increase when a change was introducedhalf-way along each runway wall (Archer1974) . The second hypothesis was that adultmale mice would show feeding latencies similarto testosterone-injected castrates but differentfrom control-injected castrates. Thirdly, femaleswere compared to intact males, to indicatewhether there was a sex difference in feedinglatencies .

These four groups (intact males and females,and castrates injected either with testosteroneor with oil) were part of a 4 x 2 x 2 x 2factorial design, which also investigated twostrains (BALB/c and 'Porton' mice), two differ-ent numbers of training trials, and the influenceof test order. The two strains were chosen asdiffering in aggressiveness and emotional be-haviour, the BALB/c's being more aggressiveand emotional than the Porton strain (Archer1977, and unpublished observations) . Thetwo different training trials were used to testthe prediction that a larger number of trainingtrials would exert a similar influence on runwaydistractability to that produced by testosteroneinjection . This was derived from the followingargument : that as a mouse is trained, it attendsmore to stimuli associated with the food (i .e .those associated with reinforcement) and less tostimuli associated with the runway walls (whichare associated with non-reinforcement) . There-fore, after a larger number of training trials,changing the features of the runway wallsshould have become less effective in distractingthe mouse from running to the food dish andfeeding; on the other hand, changing the fooddish should be more effective because the mousewill be attending to more features of the feedingsituation, and hence any change will producea greater mismatch from what is expected,resulting in a longer latency to feed .

The fourth factor, order of testing, was in-cluded as a control procedure in view of therepeated-measures design of the experiment .

The three testing procedures used in thisexperiment were based on those used in theearlier chick experiment (Archer 1974), but werenot identical to them . The first involved twowhite panels being introduced into the (black)walls of the runway, half way along ; the secondchange involved replacing the usual white fooddish by a black one ; and the third changeinvolved placing, half-way along the runway,two food dishes identical to the one at the end :

previously a similar test had been used forchicks but without retaining the dish at the endof the runway. No differences were found inrunning times in this test, because as manytestosterone-treated as control chicks fed atthe dishes ; there, were however, differences inthe degree of interruption during feeding. Inthe present study, a food dish was placed at theend of the runway as well as at each side, since itwas supposed that differences which resultedfrom maintaining attention to the dish at theend would be more readily detected when achoice of feeding from the usual place at the endor from the dishes at the sides was given .

MethodsSubjects

Sixty-four mice, 48 males and 16 females,aged 24 months were housed singly in a roomwith day-night reversed lighting . Half of themice were BALB/c strain and the other halfwere 'Porton' mice (from Sussex Universitystocks, initially a random bred albino mouse,obtained from Allington Farm, Porton Down,and originally derived from a colony of whiteSwiss mice). Thirty-two of the males werecastrated and left to recover for 3 weeks .ApparatusThe runway was made of black Perspex

and measured 65 x 11 cm. A food dish, 2 .5 cmsquare and containing approximately 0 .05 gof crushed rat pellets (in a central 1 . 3 cmdiameter depression) was situated at one end ofthe runway. Sliding doors were situated 13 cmfrom each end of the runway, to enclose thestart-box and goal-box areas .Two identical runways were used, and these

were slotted into a wooden base lined withwhite absorbent paper. Observations were madefrom only one of these runways at a particulartime, the second one usually containing a mousewhich was habituating to the runway (see below) .The apparatus was housed in a soundproofchamber (open at the front), and lighting wasprovided by two 5-W red lights, one at each endof the runways .Procedure

Training. Pretraining consisted of 30-minhabituation to the runway without a food dishon day 1, and 10 to 15 min exposure to therunway with a food dish (containing food) ateach end on day 2 . The mice were deprived offood for 20 hr before the day 2 session, and afterthis session given ad libitum feeding for 2 days .

ARCHER : TESTOSTERONE AND PERSISTENCE IN MICE

They were then food-deprived throughout thetraining period (5 days or 8 days : see below) .They were weighed on each day during thetraining and testing period, when they were keptat approximately 80 per cent bodyweight. Foodwas provided in the home cage after each set oftraining trials . The total number of trials waseither 30 (six a day for 5 consecutive days) or48 (six a day for 8 days) .

On each training day the mice were placed byhand into the runway (with both doors open)for 10 min to habituate, and then given sixtraining trials. The sliding doors were openedat the beginning of each trial and the time takento run and feed was recorded (a maximum of 60 sper trial was allowed, and if the mouse did notfeed within this time the sliding doors wereclosed, and a time of 60 s recorded) . The firstsliding door was closed after the mouse had leftthe start-box, and the second one was closedafter the mouse had entered the goal box andbegan feeding. The goal-box served as the start-box for the next trial, and the mouse fed forapproximately 20 s (until all the food was eaten) .Each trial was followed by a 30 s inter-trialinterval .

Hormone treatment . After the training period,mice taking longer than an average of 8 s onthe last six trials were excluded (throughout theexperiment, nine castrates, seven intact femalesand four intact males were exluded at this stage) .The others were either left untreated (intactmales and females, referred to is M and Frespectively), or were injected (castrated males) .Injections were given subcutaneously, 0 .03 ml,of oil (control-injection, referred to as Cs),or testosterone oenanthate in 0 .03 ml oil(testosterone injection, referred to as Ts) .The dose of hormone used was 7 . 5 mg, calcu-lated from the amount of testosterone propionatenecessary to produce aggressive behaviour inimmature or castrated males (e .g. Levy & King1953). Since testosterone oenanthate is a long-acting androgen released over a period of 2 to 3weeks, a proportionately larger single dose thanthe equivalent daily propionate injection wasused in this study .

Testing. Beginning on the third day after thelast training trial (i .e. also the time of injection),the mice were tested on three consecutive daysin the runway. On each day they were given firstthe 10-min habituation period used in training,followed by three `control' trials, the same asthose during training : any mice not reaching a

481

criterion of an average of 7 s per trial on thatparticular day of testing were excluded from theresults (since some mice had already been exclu-ded after the training period, only two furtheranimals were excluded on test day 1, these beinga C and an M). The control trials were followedby a single trial on which one of three changes(see below) was introduced into the runwaysituation . After this trial, two further `control'trials were given.

The changes introduced into the runway were(1) a black dish instead of a white dish ('blackdish'), (2) two white panels (7 . 5 x 13 cm) placedon the side walls, half way along the runway('white panels') and (3) two additional fooddishes, containing food, placed on each sidehalf way along the runway ('dishes at the side') .

On each trial, the time taken to commencefeeding from the dish at the end of the runwaywas recorded . If the mouse ceased feeding within5 s, this was not counted as commencing feeding,since the earlier chick experiment had shownthat on some occasions a change in the appear-ance of the food dish led not to delayed onset offeeding but to its interruption soon after it hadbegun : in practice this procedure producedalmost identical results to those calculated fromthe time taken to commence feeding, (in onlyone mouse was there any difference) . A totaltime of 60 s was allowed on each trial for themouse to commence feeding from the end of therunway. If it did not do so before this time,the trial was ended and a time of 60 s recorded .Timing was carried out using a stop-clock,

recording by pen and paper .Analysis of results and experimental design . For

each mouse, the mean time on the first three('control') trials was calculated, and this wassubtracted from the time on the next ('experi-mental') trial : the resulting value was referredto as the `distraction time' . The three separatedistraction times (for the 3 days of testing) andthe overall mean of these three times, were usedfor statistical analysis . The overall mean valuewas included because the effect of testosteroneon distraction times in chicks operated in adifferent direction in the different tests : thusan overall mean from the three tests wouldenable us to distinguish between this type ofeffect and a more general increase or decreasein distraction time (which might, for example,be attributable to differences in emotionalreactivity) .The overall design of the experiment was a

4 x 2 x 2 x 2 factorial one (see Introduction)

482

and it was carried out over eight replications,since it was practical to train only eight mice at atime : the `hormone and sex', and `training trial'variables were the only ones kept constant inevery replication . Strain and test order differedin different batches . Additional mice to com-pensate for those excluded during training(see above) were run at the end of the experiment .The second half of the replications was run`blind', with the identification of the mice beingcoded and the cages rearranged in a way un-known to the experimenter . This proceduretook place immediately after training andinjection.

Table I . Analysis of Variance of Distraction Times for the Three Tests, Showing Main Effects and ThoseInteractions Where P < 0 .05

ANIMAL BEHAVIOUR, 25, 2

Analysis of variance (on both the raw dataand on log transformations) was carried out onthe distraction times to investigate the effects ofthe four factors described above .Non - parametric comparisons using the

Wilcoxon matched-pairs test (Siegel 1956) werealso carried out between Ts and Cs, Msand Cs, Ts and Ms, and Ms and Fs, to testthe hypotheses described in the introduction .The matched-pairs test was considered appro-priate because each member of a group couldbe matched with a mouse from the other groupon control times, strain, test order and numbersof training trials .

Source SS . Df M.S . F P

(1) Black dish test

Hormone/sex (H) 3351 .0 3 1117 . 0 11 . 7 0 . 001

Training trials (T) 900 . 0 1 900 . 0 9 . 4 0 .01

Strain (S) 493 . 9 1 493 .9 52 0 . 05

Test day (D) 24. 3 1 24 . 3 0 . 25 NS

HSD 1572 . 7 3 524 .2 5 . 5 0 . 01

STD 900 . 0 1 900 .0 9.4 0 . 01

HTSD 3714 . 7 3 12382 13 .0 0 . 001

Error 3055 . 5 32 95 . 5

(2) White panel test

Hormone/sex (H) 448 . 5 3 149 . 5 0 . 7 NS

Training trials (T) 1320 . 4 1 1320 . 4 6 . 7 0 . 05

Strain (S) 3619 .5 1 3619 . 5 18 . 3 0 . 001

Test day (D) 7 . 6 1 7 . 6 0.04 NS

ST 1002 . 5 1 1002 . 5 5 . 08 0 .05

Error 6315 .9 32 197 . 4

(3) Dishes at side testHormone/sex (H) 2330 .4 3 776 . 8 2 . 5 Ns (0 . 1)

Training trials (T) 703 . 6 1 703 . 6 22 NS

Strain (S) 1931 . 6 1 1931 . 6 6 . 1 0 .05

Test day (D) 19 . 8 1 19 .8 0 .06 Ns

STD 1526 . 9 1 1526 .9 4 .9 0 . 05

Error 10058 .4 32 314 . 3

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483

ResultsThe means and significance levels of the maineffects investigated by variance analysis areshown in Figs 1, 2 and 3 . Table I shows detailsof the variance analysis .

The values given are for the raw data, sincethe analysis of the log transformations producedno substantial differences in significance levels .The first main effect was the hormone and sexconditions (Fig . 1) : the differences found herereflect the individual comparisons describedunder the next three sub-headings .Effect of Testosterone

Figure 1 (columns 1 and 3 from the left)illustrates the distraction times for Ts and Csin all three tests (expressed separately and as anoverall mean) . Ts showed significantly greaterdistraction times than Cs (P < 0 .01) when thedish was changed (the `black dish test') whereasthe reverse was the case for the `dishes at theside' test (P = 0 .05 to 0 .02). In this test, dis-traction usually occurred because mice fedfrom the additional (side) dishes, and were thus

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WHITE

DISHES AT

MEANP<0-001

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P=0 - 1-005

TIMEN .S .

Fig. 1 . Distraction times for testosterone-treated castrates (T), intact males(M), oil-injected castrates (C), and intact females (F) in runway tests in-volving changes at the dish (`black dish'), and at the side walls ('whitepanels' and `dishes at the side') . Mean distraction time refers to the averagetime over all three tests. P values refer to the results of analysis of variancecarried out for all four conditions (main effect for hormone and sex, `H'in Table I) .

K

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delayed in running to, and feeding from, thedish at the end of the runway . This resultindicates, therefore, that Ts were more likely tofeed from the end dish than Cs, and Cs weremore likely to feed from the side dishes thanwere Ts. Analysis of the time spent feedingat the side dishes confirms that this is the case :Ts spent a mean time of 8 .9 s feeding from theside dishes, whereas Cs spent a mean time of17 .3 s there (0 .05 to 0 .02, Wilcoxon matched-pairs test, two-tailed) . In the third test ('whitepanels'), the direction of the (non-significant)difference between Ts and Cs was the same asthat found with the dishes at the side (and thesame as that found in the previous study usingchicks : Archer 1974) . The mean distractiontimes for all three tests combined showed littledifference between the Ts and Cs, indicatingno increase or decrease in overall distractabilityas a result of testosterone injection .Comparison of Intact Males and Castrates

The distraction times for the intact males(Ms) were intermediate between those of the

484

ANIMAL BEHAVIOUR, 25, 2

Ts and Cs in all three tests (Fig. 1) . On none ofthe tests were there any significant differences(or differences approaching significance) bet-ween the Ts and the Ms . The difference betweenthe distraction times of Cs and Ms was sig-nificant for the black dish test (P < 0 .05, one-tailed) . The difference between the Cs and Ms forthe time spent feeding from the dishes at theside (Cs : 17 .3 s ; Ms 7 .3 s) was also significant(P = 0 .05, two-tailed) .Sex Differences

The comparison between Ms and Fs showedthat the distraction times of the Ms were sig-nificantly longer (mean : 10 .2 s) than were thoseof females (mean : 2 .8 s) for the black dish test(P = 0 .02, Wilcoxon matched-pairs test, two-tailed) . Thus, females showed distraction timessimilar to those of castrates in this test (Fig . 1) .However, this was not the case for the other twotests : here the mean values for Fs were similarto those of the Ts and Ms (Fig . 1) .Training Trials

Figure 2 illustrates the mean distraction timesand the results of analysis of variance for themain effect `training trials' . The direction of theseresults is similar to the difference between Tsand Cs described above and illustrated in Fig . 1,

30

C

V/1 46TRIALS30TRIALS

V

and is consistent with the prediction made in theintroduction . Mice receiving 48 trials weresignificantly more distracted by the black dishthan were those receiving 30 trials, whereas thereverse was found for the white panels . For the`dishes at the side', the differences were similarin direction to those found with the white panels,but were not significant . Thus, the direction ofthese differences is the same as that betweenTs and Cs but a significant difference was foundfor the white panel whereas for the Ts and Cs itoccurred with the dishes at the side instead . Itwas also found that mice receiving 30 trials wereslower to run and feed on the three trials preced-ing each test than were those receiving 48 trials(Table II). These differences do not directlyaffect the distraction times (which were calcu-lated by subtracting the mean control time fromthe test time for each mouse, and thus representrelative increase in running times) .

Strain DifferencesFigure 3 shows the mean distraction times

and results of analysis of variance for the thirdmain effect, strain. In all three tests, and for themean of the three tests combined, BALB/cmice showed longer distraction times than didthe Porton mice .

Fig . 2. Distraction times in runway tests for mice receiving 48 or 30 trainingtrials . P values refer to results of analysis of variance (main effect for train-ing trials, `T' in Table I).

BLACK DISH WHITE DISHES AT MEANPANELS SIDE DISTRACTIONP<0 , 01P<0 05 N . S . TIME

N . S .

Day of TestingNo significant differences were found between

mice tested on different days .

First-order InteractionsAnalysis of variance revealed only one first-

order interaction between any of the mainvariables (Table I), an interaction betweentraining trials and strain for the white card test .This indicated that contrary to the main trendthere was little difference between the 30- and48-trial conditions in the Porton strain ; thispossibly occurred because their distraction timeswere low even in the 30-trial condition (mean7 . 3 s) .Second-order Interactions

Second-order interactions were found in threeinstances, and involved interactions between dayof testing and strain with in one case trainingtrials, and in two others sex and hormonetreatment. Further analysis and illustration ofthe source of these interactions revealed effectsof little general interest, and the results of thisanalysis are not presented here .

DiscussionThe results indicated that testosterone, injectedinto castrated male mice, produced similar

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ARCHER : TESTOSTERONE AND PERSISTENCE IN MICE

485

changes in runway distraction times to thoseproduced by injecting young male chicks withtestosterone (Archer 1974) : when a change wasintroduced at the food dish (a black dish in-stead of a white dish), testosterone-injectedcastrates (Ts) showed a greater increase infeeding latencies than control-injected castrates(Cs) did ; on the other hand, with changes at theside of the runway (white panel or two fooddishes containing food), Ts showed smallerincreases than Cs .

In these tests, intact males (Ms) resembled Tsmore than they did Cs, although they showedless marked differences from the Cs than did theTs. This suggests that physiological levels oftestosterone may be affecting runway distractiontimes in the same way that injected testosteronedoes, but to a lesser degree . The latter mightresult from one of two possibilities : either thathigher than physiological levels of testosteronewere reached as a result of testosterone injection,or that there were comparatively low levels oftestosterone in Ms as a result of their previoussocial housing conditions in all-male groups .The second possibility is raised by findings thatthe androgen levels of laboratory rodents showconsiderable variation (Bartke et al . 1973) andare affected by social conditions : e.g. they are

Fig . 3 . Distraction times in runway tests for mice of the Porton and BALB/cstrains . P values refer to the results of analysis of variance (main effect forstrains, 'S' in Table I) .

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P < 0 001 P<0 05 TIMEP<O 05

486

decreased by group-housing (Brain & Nowell1971) and increased by the presence of females(Purvis & Haynes 1974). It is possible, therefore,that prior to the present experiment testosteronelevels may have been lowered as a result ofgroup-housing and the absence of females, butthat this influence would have been counteractedto some extent by subsequent isolation. Whetheror not these factors can account for the differ-ences in behaviour between Ts and Ms mustawait further studies involving hormone assays .

In a previous study with chickens, Rogers(1974) found that intact males and testosterone-injected males were similar in showing more`persistent' visual search in a feeding situation,than did control-injected castrates . Rogersalso found that visual search of adult femaleswas similar to that of the control-injectedcastrates . Hence it appeared that the effect oftestosterone on persistence in chickens formedthe basis of a sex difference in the adults . In thepresent experiment, female mice (Fs) were foundto be similar to Cs in only one test, the `blackdish'. In the other two tests, they showed valueswhich were more similar to those of Ms and Ts .Thus, females appeared to show a lesser tendencyto be distracted by any of the changes in stimuli

Table II. Control (Pre-test) Running Times (s) for Main Effects of H (Treatment and Sex), T (Training Trials)and S (Strain)

ANIMAL BEHAVIOUR, 25, 2

Main effect H

(Fig. 1), although this tendency did not resultin a significant main effect . Since females alsoshowed lower bodyweights than the male groups,it is possible that their response to deprivationwas different from that of the three malegroups, and this may have been responsible fordifferences in runway performance between thesexes .

The influence of the two different numbers oftraining trials on distraction times supported theprediction that more training trials wouldproduce an effect similar to that resulting fromtestosterone treatment . It is interesting tocontrast the hypothetical processes involved inthe two treatments. After a larger number oftraining trials, an animal will attend to morefeatures relevant to the food and to fewerfeatures of the (non-reinforcing) walls of therunway. After testosterone-injection an animalwill, according to Andrew (1972b ; 1976),have a greater ability to maintain search speci-fications or rules of selection . Thus in the firstcase the animal is presumed to attend to morefeatures of the food dish and in the second tomaintain greater stability of attention to fewerfeatures . Both processes result in greaterresistance to making a response when stimuli

T

C M F P (F-test)

Preceding black dish 4 . 3

4.4 4.6 4 . 4 Ns

Preceding white panels 4 .4

4.0 4.6 4 . 0 NS

Preceding dishes at side 4 . 0

3.7 4.4 4 . 3 NS

Main effect T

30 trials 48 trials P (F-test)

Preceding black dish 5 .0 3 . 9 < 0 .001

Preceding white panels 4 . 5 3 . 8 < 0 .01

Preceding dishes at side 4 . 6 3 . 5 < 0 .001

Main effect S

BALB/c Porton P (F-test)

Preceding black dish 4 . 1 4 . 0 NS

Preceding white panels 4 . 3 4 . 3 NS

Preceding dishes at side 4 .3 4 .4 NS

ARCHER: TESTOSTERONE AND PERSISTENCE IN MICE

associated with feeding have been changed, inthe first case because of greater precision in thefeatures which have to be matched beforemaking a feeding response and in the secondbecause there is greater resistance to changingthose features which have to be matched beforefeeding can occur . A more wide-ranging com-parison of the effects of testosterone with thoseof increased training trials ('overtraining')might provide a useful way of further investi-gating the attentional processes involved inthe two phenomena, since it is knownthat overtraining affects variables such asreversal learning, generalization and extinction(Sutherland & Mackintosh 1971 ; Mackintosh1974) .

It was found that mice of the BALB/c strainshowed longer distraction times than did thoseof the Porton strain in all three tests . Thissupported the observation that BALB/c micewere the more `emotional' and difficult to handle ;in an open-field study (Archer 1977), mice ofthis strain were found to remain immobile forlonger (mean time approximately 29 s) after abell had sounded than Porton mice did (meantime 5 s) . These observations suggest that thestrain differences in runway distraction timesrepresent a general difference in emotional re-activity or responsiveness to stimuli. They alsoprovide further evidence that a difference inoverall `emotional reactivity' would producechanges in runway performance different fromthose found between Ts and Cs (see also Archer1974) . This is important for eliminating an effecton emotional reactivity as a possible explanationfor the testosterone effect (see also Archer 1973,1976a, b, for evidence against this possibilityin chicks).

The three main effects found in the presentstudy, those of testosterone, of a greater numberof training trials, and of strain, all appeared tooccur relatively independently of one another,since there was only one (out of a possible 15)first-order interactions . The main effects can,therefore, be regarded as fairly general robusteffects in the animals used . Clearly, it would beof further interest to obtain more conclusiveevidence concerning the influence of circulatinglevels of testosterone on distraction times inmice, and to extend this type of investigation toother mammalian species .

AcknowledgmentsThis work was supported by a grant from theMedical Research Council of the United

487

Kingdom to Professor R . J. Andrew. I thankProfessor Andrew for his helpful commentsthroughout this work, for his comments anddiscussion of earlier drafts of this paper, and fortechnical assistance . I also thank Mr D . Hichinfor carrying out the analysis of variance .

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male and female chicks, following different dosesof testosterone. Anim. Behav., 20, 741-750 .

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Andrew, R. J . (1975) . Effects of testosterone on thebehaviour of the domestic chick. I . Effects presentin males but not in females . Anim. Behav ., 23,139-155 .

Andrew, R . J . (1976). Attentional processes and animalbehaviour . In : Growing Points in Ethology (Ed .by R. A. Hinde & P. P. G. Bateson) . CambridgeUniversity Press .

Andrew, R. J . & Rogers, L . (1972) . Testosterone, searchbehaviour and persistence. Nature, Lond., 237,343-346.

Archer, J. (1973) . Effects of testosterone on immobilityresponses in the young male chick . Behav. Biol.,8, 551-556 .

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Archer, J . (1976a) . Emergence tests in testosterone-treated chicks . Physiol. & Behav., 16, 513-514.

Archer, J. (1976b) . Testosterone and fear behavior inmale chicks . Physiol. & Behav., 17, 561-564.

Archer, J. (1976c). Biological explanations of psycho-logical sex differences. In : Exploring Sex Dif-ferences (Ed. by B. B . Lloyd & J. Archer), Londonand New York: Academic Press .

Archer, J . (1977). Sex differences in the emotionalbehaviour of laboratory mice . Br. J. Psychol., 68,in press.

Bartke, A., Steele, R . E., Musto, N . & Caldwell, B. V.(1973) . Fluctuations in plasma testosterone levelsin adult male rats and mice. Endocrinol., 92,1223-1228 .

Brain, P. F. & Nowell, N. W. (1971). Isolation versusgrowing effects on adrenal and gonadal functionin albino mice. I. The Male . Gen. Comp . Endocrinol.16, 149-154.

Hutt, C. (1972). Males and Females . Harmondsworth :Penguin .

Levy, J. V . & King, J. A. (1953) . The effects of testos-terone propionate on fighting behavior in youngmale C57BI/10 mice. Anat. Rec., 117, 562-563 .

Mackintosh, N. J. (1974) . The Psychology of AnimalLearning . London and New York : AcademicPress.

Purvis, K. & Haynes, N. B . (1974) . Short-term effects ofcopulation, human chorionic gonadotrophininjection and non-tactile association with afemale on testosterone levels in the male rat. J.Endocr., 60, 429-439.

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Rogers, L. J. (1974) . Persistence and search influencedby natural levels of androgens in young and adultchickens . Physiol. & Behav ., 12, 197-204 .

Rogers, L. (1976) . Male hormones and behaviour. In :Exploring Sex Differences (Ed. by B. B . Lloyd &J. Archer). London and New York : AcademicPress .

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Siegel, S . (1956) . Non parametric Statistics for the Be-havioral Sciences. New York : McGraw-Hill .

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(Received 29 March 1976 ; revised 12 July 1976 ;MS. number : 1521)