A microanalysis of wheel running in male and female rats

A microanalysis of wheel running in male and female rats

Phvstology& Behawor, Vol 43, pp 625--630 Copyright©PergamonPress plc, 1988 Printedin the U S A 0031-9384/88$3 00 + 00 A Microanalysis of Wheel Runni...

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Phvstology& Behawor, Vol 43, pp 625--630 Copyright©PergamonPress plc, 1988 Printedin the U S A

0031-9384/88$3 00 + 00

A Microanalysis of Wheel Running in Male and Female Rats ROELOF EIKELBOOM 1 AND REBECCA MILLS 2

D e p a r t m e n t o f Psychology, Queen's Umverslty at K m g s t o n , Ontarto, Canada R e c e i v e d 27 July 1987 EIKELBOOM, R AND R MILLS A mtcroanalysts oJ wheel runnmg m male and female rats PHYSIOL BEHAV 43(5) 625-630, 1988 --The pattern of ad lib wheel running was studied m adult Sprague-Dawley rats Wheel turns per 20 seconds were recorded for 5 days Females ran more than males but both ran chiefly at mght, with a peak at the beginning and a dechne to low levels by daytime Log-survivor plots showed running occurred m distract episodes separated by long periods of nonrunnlng Both the mghtly decrease and the sex difference m running were due to changes m duration of episodes and the running speed Imtlatlon of runmng, as reflected in the length of nonrunnmg periods was s~mdar m both sexes and remained constant over the night Running was d~scussed m terms of independent initiation and termination factors and ~ts parallels to eating patterns Wheel running

Rat

Activity

Sex &fferences

C~rcadmnrhythms

Runmng patterns

Log-survivor

of widely separated episodes or meals, each composed of a number of eating bouts [2,3] One of the questions this raises is how the distinction can be made between the interruptions which separate bouts w~thln an episode and the periods which separate episodes Whde some have used arbitrary intervals of noneatlng to define the end of a meal, more recent work has used log-survwor plots to make this deterruination [2,3] If the interruptions between bouts are shorter than the periods separating eptsodes, then the log-survivor plot of all nonrespondlng intervals wdl be concave and show a &stlnct change in slope (called the breakpoint) at the tune that best distinguishes mtraep~sode interruptions from mtereplsode periods [4] Using this technique the mterval of noneatmg derived to define the end of a meal is approximately 6 minutes [2] Tradmonally, wheel running has been recorded m relatively large blocks of a half hour or more [7-9, 18, 21-23], which do not provide the detad necessary to determine the fine-grained temporal pattern of th~s behavior A few researchers have used higher temporal resolution (smaller tune bms) to look at running, but they used arbitrary intervals of nonrunnmg (periods when no wheel turn was recorded), that ranged from 2 5 sec to one mmute, as their definmon of running termination [11-13, 20] This approach does not &stlngmsh between bouts and episodes In the present experiment running wdl be analyzed using log-survwor plots, first to confirm that running occurs m episodes, and second to provide an objective definmon for the termination and initiation of such running episodes Once lmtmtlon and ternamatlon of runnmg episodes have been defined, it wdl be possible to look at how th~s behavior ~s controlled Is there a relationship between the duration of

T H E fact that rats and other rodents will spontaneously run m wheels has long been of interest [1, 15-17] Wheel running appears to be inherently rewarding to rats as they wdl perform a variety of operant responses to gain access to a wheel [5, 10, 14] It is stdl unclear why wheels are so attractive to rats, or how wheel running compares to other appetitive behaviors This may be because wheel running itself has generally not been the focus of earlier work Rather running has been used either as an index of arousal or to study endogenous rhythms For example, female rats are known to run more than males and show a 4 or 5 day cycle which peaks on the mght of estrus [24] Since R~chter's [16] work ~t has been known that runnmg occurs in episodes, largely at mght, but the temporal pattern of this behavior has not been systematically explored Analysis of the temporal pattern of running may provide clues to how this behawor is controlled, how &fferences m running are produced, and how it parallels other appetlt~ve behaviors If temporal patterns are to be studied, then initiation and termlnatton must be carefully defined Behawors do not start, occur continuously, and then stop Rather they are subject to many short interruptions These short interruptions need to be dffferentmted from the longer periods of nonrespondlng which occur when the behawor stops Unfortunately, &scusslons of behawor patterns are comphcated by an inconsistent use of terms such as bouts, epmodes and intervals In this paper a bout is defined as a continuous burst of the behavior m question, at least to the temporal resolution used An episode of behawor is defined as a cluster of bouts, the mltmtlon and termmatlon of which is preceded, and followed, by a longer period when the behavior does not occur For example, ad hb eating m the rat consists

1Requests for reprints should be addressed to Dr R Elkelboom, Department of Psychology, Queen's Umverslty, Kingston, Ontario, Canada K7L 3N6 ~Present address Department of Psychology, Umverslty ot Alberta

625

626

EIKELBOOM AND MILLS

T N

Male

Female

x

"U p, t~ W 0 I--

@ 0

J

0

dark

hght dark Three Hour Blocks

hght

FIG 1 Mean (_SE) number of wheel turns m three hour blocks Ior female and male rats averaged over five days

episodes of running and periods of nonrunnlng ° If running behavior changes, as it does over the night, what is responsible for this change 9 Are episodes shorter when total running decreases, or are intervals of nonrunnlng longer 9 How does male and female running behavior differ 9 Over the night females run more than males [24] but the nature o f this difference has not been speofied Do females lmtlate running more frequently, run for longer periods, or run faster than males 9 In the current experiment male and female rats were compared to determine the nature of these differences in running behavior METHOD

5ubject~ Twelve male and 12 female Sprague-Dawley rats (Charles River Canada), 250-275 g and 200-225 g respectively, were housed individually with ad lib access to Punna Lab Chow and tap water throughout the expenment The ammal room was maintained on a 12 12 light-dark (LD) cycle and kept at a temperature of 21+2°C Males and females were stud~ed separately, males in the early winter and females about 2 months later

Matertal~ The twelve running wheels, constructed in the Psychology workshop, consisted of standard, wire-mesh cages (24x 17x 20 cm) each with an attached wheel (11 cm wide, 34 cm diameter) The cages were mounted on a three-tiered metal rack, so the rats could hear each other, but only m the wheels could they see the other ammals Using magnetic reed switches the wheels were interfaced w~th a Mm~-Mitter Co data collection system run on a Zemth 151 computer using version 2 3 of the Dataquest III software The number of wheel turns was recorded automatically m 20 sec bins

Procedure Ammals spent at least 2 weeks m the colony and were given ad hb access to the actw~ty wheels for at least a ten day habituation penod prior to the start of data collection For the following five days, the number of wheel turns per 20 sec bm was recorded During these five days, dlsrupt|ons of the ammals for the admmistration of food and the refilling of water bottles were as brief and infrequent as possible, only occurring during the light part of the cycle

Data Analyst Intervals of nonrunnlng were defined as consecutive bins w~th zero counts between bins with at least one wheel turn Bouts of running were defined as consecutive nonzero count bins between zero-count bins For each bout of running, both the duration and number of wheel turns were determined Log survivor plots of both the intervals of nonrunning and of the bouts of running were graphed for each animal A straight hne plot would tmply that termination intervals or bouts were equally likely at any time, ~ e , its termination was randomly determined A convex plot would suggest there was a " t y p i c a l " interval or bout length, a duration at which intervals or bouts were most likely to terminate A concave plot would suggest the intervals came from two distract classes or d~stnbutlons, which could be separated at the breakpoint In this study breakpomts were denved by visual inspection of graphs of lndwidual ammals, ernng on the longer side to minimize the number of m~sasslgned intervals [19] An episode of running, the unit used in subsequent analyses, was defined as a cluster of running bouts preceded and followed by an interval of nonrunnmg longer than the breakpoint (I e , a nonrunning period) Thus through the log-survivor analys~s the ongmal behavior record of 20 sec bins consisting of intervals of nonrunnlng and bouts of run-

W H E E L R U N N I N G PATTERNS

627 Male Rat 5

1000

Male Rat 7

!.. unn'"°ut" ]

[] Non-runnlng

I

lOO

[]

1

1

0

[



I

I

I

l

[

30

i,

60

0

,

30

I

I

60

T,me (M,n) FIG 2 Log survivor plots of nonrunnlngintervals and of running bouts for two representative ammals summed over five nights Arrows indicate the breakpomts for nonrunnmg intervals, two for rat 5 and one for rat 7 Note the log-surwvor plot for running bouts ~s a straight hne

nlng could be reclasslfed into episodes of running separated by periods of nonrunnlng While log-survivor plots defined initiation and termination of running behavior, running speed was more difficult to derive Before it could be determined the actual duration of running within an episode had to be calculated This was done by subtracting from the episode duraUon the total duration of intervals of nonrunnmg occurring between bouts within the episode The wheel turns per minute was then the number of wheel turns within the episode divided by the actual running time for the episode RESULTS

Mean (over 5 days) wheel turns summed into 3 hour blocks for the day and night were determined for each animal and then averaged over males and females (Fig l) Females ran more than males throughout the night but neither sex ran extensively d u n n g the day A Sex by Block analysis of variance (ANOVA) revealed that both mare effects and the interaction were significant [Sex, F(1,22)=30 3, p < 0 001, Blocks, F(7,154)= 118 0, p < 0 001, and Interaction, F(7,154)= 13 l, p < 0 001] A second Sex by Block ANOVA using the four nighttime blocks revealed that only Sex, F(1,22)=29 8, p < 0 001, and Block, F(3,66)=47 7, p < 0 001, effects were significant This suggests that while females ran more than males the overall nighttime patterns were similar As the amount of daytime running was negligible only nighttime running was analyzed further Episodes of running and periods of nonrunnlng which were completed m the night were Included m the subsequent analyses Log SUl"Vlvor plots of all nonrunnlng intervals were graphed for each animal and were concave (see examples in Fig 2) For 14 of the animals there was only one distract breakpoint in the graph, but for the other l0 two breakpolnts

were evident The first breakpoint in these cases was s~mdar to that of ammals having only one and thus used m all subsequent analyses From Table 1 it is evident that the nightly number of nonrunnlng intervals and the breakpoint were slmdar for males and females In contrast to the concave plots for nonrunning intervals, the log-survivor plots of running bouts showed uniformly straight lines (see Fig 2) Global descnptlons of nightly running were determined for each male and female using individual breakpomts and averaged over sex (see Table l) The duration of nonrunnlng periods and the number of runnmg episodes was found to be similar for males and females However, duration of running episodes, number of wheel turns per episode and running speed were elevated for females relative to males Correlations between episodes of running and a number of other measures were calculated for each animal Table 2 shows the mean correlation, standard error, and the range for male and female rats There was no correlation between the duration of running episodes, and either the preceding or following period of nonrunnmg As might be expected, the duration of an episode of running was highly correlated with the number of wheel turns It ~s evident from Fig l that wheel running changed over the night, decreasing from an initial high to a much lower level by the end of the night To determine the nature of this change, running within each 3 hour block was analyzed for male and female rats Individual breakpomts as determined from total night runmng were used in these analyses (Prehminary analysis suggested that these breakpomts did not vary over the mght ) The duration of episodes of running and periods of nonrunnlng, the number of wheel turns per episode, the number of running episodes, and running speed were determined individually for the four blocks for each animal, and then analyzed using Sex by Blocks ANOVAs F~gure 3 shows that the duration of the penods of non-

628

EIKELBOOM

AND MILLS

TABLE 1 MEANS AND STANDARD ERRORS OF WHEEL RUNNING MEASURES FOR THE 12 HR DARK PERIOD IN MALE AND FEMALE RATS

Number Intervals of Nonrunnlng

First Breakpoint (mm)

Nonrunning Period (mm)

Number Episodes of Running

Running Episode (mm)

Wheel Turns per Episode

Running Speed (W T / rain)

Female (SE)

90 9 50

5 00 0 42

21 45 1 20

13 0 05

34 18" 2 64

947 0* 88 0

33 65* 09

Male (SE)

91 9 5 1

4 56 0 41

22 68 1 15

15 1 09

23 81 2 87

408 7 68 1

20 15 118

I

n=12 for each sex *p<0 05, independent t-test comparing males and females

70-

TABLE 2 AVERAGE CORRELATIONS AMONG EPISODES OF RUNNING AND NONRUNNING IN MALE AND FEMALE RATS Running to Preceding Nonrunnmg Female (SE) Range High Low Male (SE) Range High Low

Running to Following Nonrunnmg

60

Running Time to Number of Wheel Turns

086 052

045 030

972 004

421 - 191

271 - 089

986 939

- 077 049

004 047

962 013

250 - 252

319 - 209

995 836

n=12 for each sex

r u n n i n g did n o t c h a n g e o v e r t h e night a n d w a s s l m d a r for m a l e s a n d females, all F ' s < 1 By w a y of c o n t r a s t , Fig 3 also s h o w s t h a t the d u r a t i o n o f e p i s o d e s o f r u n n i n g d e c r e a s e d o v e r t h e night. F ( 3 , 6 6 ) = 2 9 74, p < 0 001, a n d was l o n g e r for f e m a l e s t h a n m a l e s . F ( 1 , 2 2 ) = 4 25. p < 0 06, but t h a t the sex difference r e m a i n e d c o n s t a n t o v e r t h e night, I n t e r a c t i o n F < 1 As m i g h t b e e x p e c t e d f r o m t h e high c o r r e l a t i o n between duration of the episode and number of wheel turns (Table 2), t h e results w e r e similar for t h e A N O V A o f w h e e l tllrns T h e n u m b e r o f r u n n i n g e p i s o d e s i n c r e a s e d gradually o v e r the night, F ( 3 , 6 6 ) = 2 97, p < 0 05, see Fig 4, d u e possibly to t h e d e c r e a s i n g d u r a t i o n of r u n n i n g e p i s o d e s T h e decline in n u m b e r o f r u n n i n g e p i s o d e s in the last t h r e e h o u r s for m a l e s a p p e a r s to b e a c o n s e q u e n c e o f the fact t h a t t h e y s t o p p e d r u n n i n g c o m p l e t e l y at t h e e n d o f t h e m g h t a n d o f t e n h a d long p e r i o d s o f n o n r u n n m g b e f o r e m o r n i n g T h e Sex effect a n d I n t e r a c t i o n a p p r o a c h e d slgmficance, F ( 1 , 2 2 ) = 3 94, p < 0 1. F( 3,66) = 2 54, p < 0 1, re spect~vely, w~th m a l e s s h o w i n g m o r e e p i s o d e s o f r u n n i n g t h a n f e m a l e s , w h o did n o t s h o w t h e d r o p at t h e e n d o f the n i g h t F i g u r e 5 s h o w s the n a t u r e o f the d i f f e r e n c e s m r u n n m g s p e e d T h e r u n n i n g s p e e d s h o w e d a c h a n g e o v e r t h e night,

Running ~1x ---$ FMalmealR:Nnol2gRunnlng ~Male Non-Running • Female

50 ~; = O ,~ m ~"~ r~

40 30 20

10

"i

1

2

3

4

N , g h t t l m e 3 Hour B l o c k s

FIG 3 Mean I_+SE) duration ol running episodes and nonrunnmg periods m male and female rats for each quarter of the mght (three hour blocks), averaged over five mghts

F ( 3 , 6 6 ) = 5 9 50, p < 0 001, with f e m a l e s r u n n i n g f a s t e r t h a n m a l e s , F ( 1 , 2 2 ) = 6 8 94, p < 0 001, a sex d i f f e r e n c e w h i c h dec r e a s e d t o w a r d s t h e e n d o f the m g h t , F ( 3 , 6 6 ) = 4 20, p < 0 01 DISCUSSION A t a m a c r o s c o p i c level the r e s u l t s o f this e x p e r i m e n t are consistent with prewous research For both males and f e m a l e s , e x c e p t for s o m e r u n n i n g s h o r t l y b e f o r e t h e d a r k period, d a y t i m e r u n n i n g w a s w r t u a l l y n o n e x i s t e n t in the p r e s e n t e x p e r i m e n t T h e p r e d o m i n a n c e o f r u n n i n g at night is c o n s i s t e n t w i t h m a n y o t h e r a p p e t i t i v e b e h a v i o r s in t h e rat w h i c h o c c u r chiefly at m g h t A s e x p e c t e d , f e m a l e s r a n m o r e t h a n m a l e s t h r o u g h o u t t h e n i g h t F r o m t h e analys~s o f t h r e e h o u r b l o c k s it a p p e a r s t h a t d e s p a e t h e d~fferences m a m o u n t

W H E E L R U N N I N G PATTERNS

629 50 Males

W 0 '13 0

4-

|

_m

LU O)

_=

Females

40 4P

3.

_c

l: I=

30

hO.

= E -!

I.-

Z

0 J=

-1

Female

--~

Male

20

]:

"t 10

I

I

I

p

1

2

3

4

Nighttime 3 Hour Blocks

FIG 4 Mean (_SE) number of running episodes m male and female rats for each quarter of the mght (three hour blocks), averaged over five mghts of running the nighttime pattern for both sexes is similar From these wheel running totals there appear to be two vanables needing further exploration, the changes over the course of the night and the male-female difference It has long been documented that wheel running occurs in episodes [16] The log-survivor analysis of intervals of nonrunning confirms this in that running occurs in short bouts clustered together in episodes, separated by longer penods when running does not occur For both male and female rats this analysis revealed a similar initial mean breakpoint of around 5 minutes suggesting that this period of nonrunning provides a good definition for the end of an episode of running A similar interval has been used to define the end of a meal [2,3], suggesting that common motivational processes might be involved in terminating these behaviors In some rats there appeared to be a second longer breakpoint While the nature of this second breakpoint is unclear, it may be that not all nonrunning intervals are similar In some of these intervals the rat may be eating, sleeping or engaging in some other behavior Investigations Involving simultaneous monltonng of more behaviors and video recording of the animals throughout the night are planned to explore this possibility In contrast to the analysis of the intervals of nonrunnlng, the analysis of the individual bouts of running revealed, in all cases, straight log-survivor plots This means that, at least when using 20 sec bins, once a rat starts running there is no time at which there is a change in the probablhty that it will stop If bouts were of a more or less constant, or had a typical length, then the log-survivor curves would have been convex with a sharp decline around the time of this typical bout The straight line implies that there is no predictable bout duration, it appears to be determined at random The running bout distribution using 20 sec bins appears similar to the pattern of running bouts described by Premack and his associates using higher temporal resolution [11-13]

I

I

F

I

1

2

3

4

Nightt,me 3 Hour Blocks

FIG 5 Mean (_+SE) running speed for male and female rats for each quarter of the mght (three hour blocks), averaged over five mghts

The relation between periods of nonrunnlng and running episodes was addressed through correlational analysis In feeding there appears to be no correlation between intervals of eating and noneatlng [2], and the same appears true of wheel running There was an essentially zero correlation between episode s of running and either the preceding or followlng period of nonrunnlng (see Table 2) This makes depnvat~on or satiation theories for the occurrence of this behavior unlikely under conditions of ad lib access Since running occurs in episodes, differences in the number of wheel turns must be reflected at the level of episodes Wheel turns may be increased because running is initiated more frequently, because running episodes are longer, or because animals run faster within the episodes Initiation of running IS reflected in the duration of the nonrunning periods, shorter durations mean more frequent initiation (Number of episodes is not a good measure of initiation as it is a function of the duration of both nonrunning periods and running episodes ) Termination of running is directly related to the duration of running episodes, longer durations imply termination is less likely In looking at the differences in total wheel running It is remarkable that none of the differences are due to changes in initiation The periods of nonrunning are stable over the night and equal for males and females In contrast, running termination occurs more quickly as the mght progresses and there is a clear sex difference with females running longer Speed of running shows similar changes, it decreases by the end of the night and is reduced in males relative to females The duration of running episodes and running speed seem to vary together and may reflect a single factor Combined they suggest that running becomes less " i n t e n s e " or "interestlng" over the night and may be less "interesting" to males At this point it is unclear what is responsible for these

630

EIKELBOOM

c h a n g e s o v e r t h e night, or for t h e sex d i f f e r e n c e s , a n d t h e s e are clearly tssues for f u r t h e r l n v e s h g a t i o n s T h e p r e s e n t r e s u l t s suggest r u n n i n g is c o n t r o l l e d by t w o i n d e p e n d e n t factors, a n i m t i a t l o n f a c t o r w h i c h a p p e a r s relah v e l y stable a n d a t e r m m a t i o n f a c t o r w h i c h , if it i n c l u d e s a r u n n i n g s p e e d c o m p o n e n t , is r e s p o n s i b l e for m o s t o f the v a r i a n c e s e e n m this e x p e r i m e n t Similarly in feeding b e h a v ior, t h e meal size a p p e a r s m o r e flexible t h a n m e a l tnttiatlon [6] Since o t h e r v a r i a b l e s are k n o w n to influence r u n n i n g , s u c h as the e s t r o u s cycle [24], a n d food d e p r i v a t i o n [1], It w o u l d be i n t e r e s t i n g to d e t e r m i n e if t h e s e c h a n g e s are also p r o d u c e d t h r o u g h effects o n e p i s o d e t e r m i n a t i o n T h e p a t t e r n o f w h e e l r u n n i n g a p p e a r s to b e v e r y s i m d a r to that seen with eatmg Running and eating both occur almost c o m p l e t e l y at night w~th the p e a k o c c u r r i n g s o m e time after the h g h t s are t u r n e d off a n d t h e n d e c h n m g w h e n t h e lights are t u r n e d on Like eating, r u n n i n g o c c u r s in e p i s o d e s separ a t e d b y longer p e r i o d s o f n o n r u n n l n g , with e a c h e p i s o d e b e i n g m a d e u p o f s m a l l e r b o u t s of r u n n i n g T h e a v e r a g e

AND MILLS

b r e a k p o i n t for r u n n i n g w a s a little less t h a n 5 m i n u t e s , a n interval similar to t h a t d e f i n m g the e n d o f a meal [2] A s wtth eating [2], the c o r r e l a t i o n s b e t w e e n e p i s o d e s of r u n n m g a n d e i t h e r p r e c e d i n g or f o l l o w m g p e r i o d s o f n o n r u n n m g w e r e n o t significant T h e c o n c l u s i o n o f this s t u d y , t h a t r u n n i n g b e h a v ior is u n d e r t h e c o n t r o l o f t w o relatively i n d e p e n d e n t factors, a n initiation a n d a t e r m i n a t i o n f a c t o r , h a s also b e e n s u g g e s t e d for feeding [25] T h e s e similarities are i n t e r e s t i n g as eating is e s s e n t i a l for s u r v i v a l while w h e e l r u n n i n g is not It suggests f u r t h e r c o m p a r i s o n s of t h e s e v e r y different app e t m v e b e h a v i o r s m i g h t p r o v e profitable

ACKNOWLEDGEMENT This research was tunded by a grant from the National Soence and Research Councd of Canada to R Eikelboom. who ~s a career scxentlst of the Ontario Mimstry of Health. Health Research Personnel Development Program

REFERENCES 1 Baumelster. A . Hawkins. W F . Cromwell. R L Need states and actlwty level Psychol Bull 61 438--453 1964 2 Castonguay. T W . Kaiser. L L . Stern. J S Meal pattern analysis Artifacts. assumptions and lmphcatmns Brain Res Bull 1743%443 1986 3 Chfton. P G . Popplewell. D A . Burton. M J Feeding rate and meal patterns in the laboratory rat Physlol Behav 32 36%374. 1984 4 Fagen. R M . Young. D Y Temporal patterns of behaviors Durations. intervals, latenoes, and sequences In Colgan. P W . ed Quanutatlve ethology Toronto John Wiley & Sons. 1978 7%114 5 Kagan J . Berkun. M The reward value of running activity J Comp Physml Psychol 47 108. 1954 6 Levltsky. D A Collier. G Effects of diet and deprivation on meal eating behavior m rats Physml Behav 3 137-140. 1968 7 Mondon. C E . D o l k a s . C B . S l m s . C . R e a v e n . G M Spontaneous running actlwty m male rats Effect of age J Appl Physlol 58 1553-1557. 1985 8 Peng. M T Jmng. M J . Hsu. H K Changes m running-wheel actw~ty eating and dnnkmg and their day/mght d~stribut~ons throughout the hfe span of the rat J Gerontol 35 33%347. 1980 9 Peng. M T . Kang. M Clrcadmn rhythms and patterns ot running-wheel activity, feeding and drinking behaviors of old male rats Physml Behav 33 615--620. 1984 10 Premack. D Reverslbdlty of the reinforcement relation Science 136 255-257. 1962 11 Premack. D . Kmtsch. W A descnptlon of free responding m the rat Learn Motw 1 321-336. 1970 12 Premack. K . Schaeffer. R W Distributional properties ot operant-level locomotmn m the rat J Exp Anal Behav 5 8% 95 1962 13 Premack. D . Schaeffer. R W Some parameters affecting the dlstnbuuonal properties of operant-level running m rats J Exp Anal Behav 6 473-475. 1963

14 Premack, D Schaeffer, R W , Hundt, A Reinforcement ot dnnklng by running Effect of fixed ratio and reinforcement time J Exp Anal Behav 7 91-96 1964 15 Reed. J D Spontaneous activity of ammals A review of the literature since 1929 Psychol Bull 44 393-412, 1947 16 Richter. C P Animal behavior and ,nternal drives Q Rev Blol II 30%343. 1927 17 Shirley M Spontaneous actlvlty Psychol Bull 26 341-365. 1929 18 Shyu. B C Anderson. S A Thoren. P Spontaneous runnlng wheels A microprocessor assisted method for measunng physiological parameters dunng exercise in rodents Acta Physxol Scand 121 103-109 1984 19 Slater. P J B . Lester. N P Minimizing error m sphttmg behavlour into bouts Behavlour 79 153-161. 1982 20 Sterner. M . Katz R J Carroll. B J Detailed analysis of estrous-related changes in wheel runmng and self-stimulation Physlol Behav 28 201-204. 1982 21 Stephan F K Phase shifts of circadian rhythms in actwlty entrained to food access Phys~ol Behav 32 663-671 1984 22 Stephan. F K Coupling between feeding- and hght-entramable circadian pacemakers in the rat Physlol Behav 38 537-544 1986 23 Stewart. K F . Rosenwasser. A M . Adler. N T Interactions between nocturnal feeding and wheel running patterns m the rat Physlol Behav 34 601-608. 1985 24 Wang. G H The relation between spontaneous' activity and the oestrous cycle in the white rat In Comparative psychology monographs vol 2. series 6 Baltimore Wdhams and Wdklns. 1923 25 Wemgarten H P Stimulus control of eating Implications for a two-factor theory of hunger Appetite 5 147-158. 1984