interaction of hunger and sexual motivation in the male rat: a time-sharing approach

Anim. Behav., 1979, 27, 8 87-896

INTERACTION OF HUNGER AND SEXUAL MOTIVATION IN THEMALE RAT: A TIME-SHARING APPROACH

BY RICHARD E. BROWN* & D . J. McFARLANDAnimal Behaviour Research Group, University of Oxford, Oxford OX1 3PS

Abstract. To examine the interaction of hunger and sexual motivation, male rats were given sexualbehaviour tests under 0, 24, and 48 h of food deprivation . Each male was placed with an oestrous-induced female and allowed to copulate until sexual exhaustion . Food was freely available throughoutthe test period . Mount and intromission latencies increased under both levels of food deprivation, butneither ejaculation frequency nor any of the other fourteen measures of sexual behaviour were alteredunder deprivation conditions . There was a decreased latency to start eating, an increase in time spenteating, and an increase in the amount of food eaten with increased food deprivation . The frequency offeeding bouts increased, but the duration of feeding bouts was constant over deprivation conditions .Under food deprivation, animals shifted from sexual behaviour to feeding more frequently than whenthey were non-deprived . These shifts occurred primarily during periods of low sexual arousal . Themajority of feeding occurred during the post-ejaculatory intervals . Bouts of sexual behaviour were notusually interrupted by bouts of feeding. This pattern suggests that feeding and sexual activity occur ona time-sharing basis with sexual behaviour as the dominant activity .

Hunger has been found to have little effect onsexual behaviour in male mammals . Jarmon &Gerall (1961) tested different groups of maleguinea pigs under 3, 24, or 49 h of food depriva-tion. Their tests lasted for 10 min and, using acomposite sexual behaviour score, no deficitswere found in the food deprived groups . Sachs(1965) deprived male rats of food for 0, 1, 2, or3 days. He found that hunger increased intro-mission latency and mount frequency in a testwhich was continued until the end of the firstpost-ejaculatory interval. The number of intro-missions required for ejaculation, ejaculationlatency, inter-intromission interval, and post-ejaculatory interval were not affected by fooddeprivation of up to 3 days. When food depriva-tion was continued for up to 9 days, Sachs (1965)still found an increase only in intromissionlatency and mount frequency. In a third study,Sachs & Marsan (1972) recorded sexual beha-viour until the end of the second post-ejaculatoryinterval in different groups of male rats whichwere food deprived for 0, 1, 2, 4, or 6 days . Inthis study, both eating and sexual behaviourwere analysed. The mount and intromissionlatencies showed an increase with length of fooddeprivation, while intromission frequency andpost-ejaculatory intervals were not affected byfood deprivation. These authors report thatmales spent little time eating before the firstintromission, and did not eat during a copulatory*Present address : Department of Psychology, DalhousieUniversity, Halifax, Nova Scotia, Canada B3H 4J1

887

series. Eating occurred primarily during thepost-ejaculatory interval, but did not prolongthis. These authors further suggest that :

`We may view the male's response tendenciesas being hierarchically organized but withsexual behaviour having normal predomi-nance over feeding and being incompatiblewith it. The initiation of one of these activitiestends to suppress the other. In the context ofBeach's (1956) two-factor theory, activationof the SAM (Sexual Arousal Mechanism) orthe CEM (Copulation Ejaculation Mechanism)inhibits the tendency to eat. Discharge of theCEM by ejaculation releases inhibition,permitting the food deprived male to eat untilthe SAM is again aroused sufficiently to reim-pose inhibition of eating (Sachs & Marsan1972, pp . 48-49) .McFarland (1974) has discussed three approa-

ches to the study of the interactions between themotivational states which produce alternatingsequences of behaviour. These approachesexamine the causes of transitions from one set ofmotivated behaviours (say, sexual behaviour) toa set of behaviours having a different underlyingmotivational system (for example, feeding) . The`competition' or `inhibition' paradigm statesthat the ongoing behaviour ceases because thecausal factors for the second behaviour increaseand inhibit the ongoing activity, thus causing ashift in behaviour . One activity suppresses theother. The chaining' or `disinhibition' paradigmstates that the ongoing behaviour ceases due to a

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ANIMAL BEHAVIOUR, 27, 3

decrease in the causal factors for that behaviourso that the second behaviour is disinhibited . The`time-sharing' paradigm states that the motiva-tion for one behaviour is consistently dominantover the other and controls the onset of thesecond behaviour by disinhibition and thetermination of this behaviour by inhibition .

To obtain a behaviour sequence A-B-A, thecompetition model states that the causal factorsfor B must be greater than those for A before ashift will occur ; when the causal factors for Aare again greater than those for B, A will inhibitB. The chaining model states that when thecausal factors for A decrease, B is disinhibiteduntil its motivational level is lower than that ofthe causal factors for A, at which time B willdisinhibit A . The time-sharing model states thatA is the dominant behaviour and disinhibits Bwhen its causal factors decrease ; when thecausal factors for A increase again, B is inhibitedand A occurs .

Behavioural transitions occur by competition(inhibition) when a manipulation of the causalfactors for the second-in-priority behaviouralters the duration and transition time of thedominant behaviour. If manipulation of thecausal factors underlying the second-in-priorityactivity does not affect the duration or transitiontime from the dominant activity, the shift is dueto disinhibition. Time-sharing occurs when thedominant behaviour determines both the initia-tion and the duration of the bouts of the sub-dominant activity.The quotation given above from Sachs &

Marsan (1972) has confused the competitionand time-sharing paradigms . Does the activationof one system, either feeding or sexual behaviour,suppress the other (competition) ; or, doessexual motivation disinhibit and inhibit feedingin a time-sharing model? The interaction ofhunger and sexual motivation in the male ratappears to be an optimal system for the study ofthis problem . Sexual behaviour occurs in dis-tinct ejaculatory series with post-ejaculatoryintervals between each series . Within eachejaculatory series, there are a number of mountbouts separated by time out periods (Sachs &Barfield 1970; Brown 1978).

In a test to sexual satiation, a male rat willachieve about seven ejaculations (Beach &Jordan 1956 ; Larsson 1956; Brown 1974; Karen& Barfield 1975). Sexual arousal levels arethought to fluctuate during the course of thesatiation test. In general, the first ejaculationtakes longer than the second and requires more

intromissions. Intromissions then occur at arapid rate until the sixth or seventh ejaculatoryseries when there is a sharp increase in latencyto ejaculate . These changes are thought to be dueto a `warm-up' and an `exhaustion' of the arousalmechanism (Brown et al . 1974). The post-ejaculatory intervals (PEIs) increase in durationafter each ejaculation, and this is thought to bedue to a decrease in arousal level which causesincreases in the relative refractory period of thePEI. Within an ejaculatory series there is thoughtto be a brief refractory period after each intro-mission (Sachs & Barfield 1976). Time outs afterintromissions are significantly longer than thosefollowing mounts (Brown 1978) .

There are thus four periods when sexualarousal is `low' during a satiation test : (1) duringthe first ejaculatory series, due to the `warm-up'effect ; (2) during the final ejaculatory series, dueto the `exhaustion' effect; (3) during the relativerefractory period of the post ejaculatory interval ;(4) during the brief refractory period followingan intromission.

Feeding, like sexual behaviour, also occurs inbouts. The length of a feeding bout has beendefined differently by researchers using dif-ferent methods to study feeding behaviour .(See e .g. Levitsky 1970.)

The present study records eating and drinkingas well as sexual behaviour in male rats tested toa criterion of sexual satiation . The patterning ofeach behaviour under 0, 24, and 48 h of fooddeprivation was recorded in order to investigatethe nature of the transitions from one activity tothe other.

MethodSubjects

Seven male Long-Evans hooded rats (OLACstrain) born at Oxford University were used assubjects. These rats were reared in unisexualgroups of littermates from weaning at 25 days ofage. They were housed in Polypropylene cages(NKP) with woodshavings as bedding . Maleswere given sexual experience at about 120 days ofage and had achieved between two and fiveejaculations before the experimental sessionswere started. During the experiment eachsubject received three sexual exhaustion testsspaced between 14 and 20 days apart . Exhaus-tion tests were given under 0, 24, and 48 h offood deprivation to the subjects in random order .Each subject was thus its own control for theanalysis of changes in sexual behaviour due tofood deprivation. Following the exhaustion

BROWN & McFARLAND : INTERACTION OF HUNGER AND SEXUAL MOTIVATION

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tests, each subject was again tested under thethree hunger conditions in random order foreating patterns with no female present . Thesefood only tests each lasted 100 min . Stimulusfemales in the sexual behaviour tests were Long-Evans hooded rats which were the same age asthe males and had been ovariectomized whenabout 90 days of age. Oestrus was induced withintramuscular injections of 50 µg oestradiolbenzoate (Intervet) in oil 54 h before testing,and 0.5 mg progesterone (Intervet) in oil4 to 6 h before testing . Females were tested forbehavioural oestrus by being placed with a studmale for a short period before the satiation test .All rats were housed in the same room on a14 :10 hour light :dark cycle with lights off at13.00 hours. Tests were conducted between14.00 and 19.00 hours under dim white light .Animals were fed with Dixons diet [FFG(M)]food pellets ad libitum except before the fooddeprivation tests . Water was always available .

Apparatus and ProcedureTesting was carried out in a rectangular glass

arena measuring 24 x 44 x 25 cm. Males wereplaced into this arena and 5 min later the femaleand a measured amount of food pellets wereintroduced simultaneously . Testing was con-tinued until an exhaustion criterion of 30 minwithout an intromission, or 1 h without anejaculation, was met . Mounts, intromissions andejaculations were recorded on an event recorderwhich rotated at 1 mm/s . Ultrasonic vocaliza-tions at 22 kHz were monitored using a HolgateMk V ultrasonic receiver and the occurrence ofthe ultrasounds was recorded manually on theevent recorder. Feeding was recorded each timethe male picked up a food pellet and chewed it .Drinking was recorded when the rat was obser-

ved licking the water tube which was availablein the test arena.

In order to make a detailed analysis of sexualbehaviour, a complete analysis of mounts,intromissions, and mount-bouts was conducted(see Sachs & Barfield 1976) . The behavioursrecorded are depicted in Fig. 1 and are definedas followsMount latency (ML) . The time from the

introduction of the female until the first mount .Intromission latency (IL). The time from the

introduction of the female until the first intro-mission .

Ejaculation latency (EL). The time from thefirst mount to an ejaculation ; this is called anejaculatory series .

Ejaculation frequency (EF) . The total numberof ejaculations the male achieved before thecriterion of sexual satiation was met .Mount frequency (MF). The total number of

mounts which occur in an ejaculatory series, i .e .the number of mounts without intromissions,plus mounts with intromissions, including theejaculatory mount .

Intromission frequency (IF) . The number ofmounts with intromission in each ejaculatoryseries, including the ejaculatory thrust .

Inter-mount-interval (I1VII) . The time in sec-onds between each mount in an ejaculatoryseries .

Inter-intromission-interval (III). The time bet-ween each intromission in an ejaculatory series .

Mount-bout-frequency (MBF) . The number ofbouts of mounts in each ejaculatory series . Amount bout was defined as a series of mounts

Fig. 1 . An example of the recorded sexual behaviours of a male rat andtheir temporal patterning . See text for explanation of abbreviations .

I Mount ? Intromission

MF, IF S f t I

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MBP I I i I i I

Time out I_ I

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8 90


with or without intromissions in which the IMIwas 10 s or less (Brown 1978).

Mounts per mount bout (M per MB) . Numberof mounts in each mount bout .Intromissions per mount bout (I per MB).

Number of intromissions in each mount bout .Mount-bout-period (MBP) . The time from the

first mount of one mount bout to the first mountof the next mount bout .Time-out (TO). The time from the last

mount of one mount bout to the first mount ofthe next mount bout .

Post-ejaculatory interval (PEI). The time fromthe ejaculation to the first mount of the nextejaculatory series. There is a 22-kHz post-ejaculatory song during the PEI which divides itinto an absolute refractory period (time fromejaculation to the end of the song) and a relativerefractory period (time from the end of the post-ejaculatory song to the first mount of the nextejaculatory series) (Karen & Barfield 1975) .

Feeding and drinking behaviour were recordedeach time they occurred . For the analyses in thispaper, a feeding bout was defined as the timewhen the rat was actively holding a food pelletand chewing it . Thus, if a rat picked up a foodpellet and put it down without chewing it, noeating was recorded . If he picked up the food,chewed for 3 s, then put down the food for 5 s,and picked it up and chewed for 20 s, feedingbouts of 3 and 20 s length were recorded . Thefeeding behaviours recorded were as follows

Eat latency. The time from the introductionof the food until the first bout of feeding .

Bout frequency. Number of feeding boutsduring the test.

Bout duration . Duration of each bout offeeding .

Time eating. Total time spent eating in thetest ; or in the first 100 min of the test for com-

parison with food eaten in the absence of thefemale .

Food eaten (g). The amount of food eatenduring the test period. Drinking patterns were notanalysed .

Data AnalysisBecause of the changes in each measure of

sexual behaviour during the exhaustion test ; andbecause of differences in the ejaculation fre-quency for individual rats, scores were analysedfrom the first three and last three ejaculatoryseries for each exhaustion test. In each ejacula-tory series, the differences between groupmedians were analysed using the Friedman twoway analysis of variance (Siegel 1956) . Signifi-cant differences were examined using the post-hoc test discussed by Rodger (1971) . Feedingdata were also analysed using the Friedman test .Comparisons of feeding behaviour during sexand no sex tests were done using the Wilcoxonmatched-pairs signed-ranks test (Siegel 1956) .

ResultsSexual Behaviour

Table I shows the median and range for ML,IL, and EF under each food deprivation condi-tion. There were no significant differences in anyof these measures between deprivation condi-tions, although IL did increase linearly acrossdeprivation levels .

Figure 2 shows the median scores for thefrequency measures of sexual behaviour duringthe first and last three ejaculatory series for eachfood deprivation condition . There were nodifferences due to food deprivation levels in anyejaculatory series for any of these five measures(MF, IF, MBF, M per MB, or I per MB) .Figure 3 shows changes in six temporal measuresof sexual behaviour for each ejaculatory series(EL, IMI, III, MBP, TO and PEI). Only one

Table I. Mount Latency, Intromission Latency, and Ejaculation Frequency Under Each DeprivationCondition

Measure Median (range) x 2 (2 df) P

Hours of food deprivation0 24 48

Mount latency (ML) 52(19-234) 71(25-207) 47(22-508) 3 .71 0.20 < P < 0 . 1Intromissionlatency (IL) 64(19-294) 118(25-291) 145(51-290) 3 . 71 020 < P < 0 .1Ejaculation

0 . 64frequency (EF) 7(3-9) 7(5-10) 7(4-8)


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test between deprivation levels reached the0005 level of significance (MBP on the secondejaculatory series) and three reached the 0 .10level of significance (TO on the second and Nthseries, and III on the first series) .

22

16

8

2

10

6

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14

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4

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2

1

1 .2

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2 3 N-2 N-1 NEjaculatory series

Fig . 2 . Median mount frequency (MF), intromissionfrequency (IF), mount bout frequency (MBF), number ofmounts per mount bout (M per MB), and number ofintromissions per mount bout (I per MB) in the first andlast three ejaculatory series after 0, 24, and 48 h of fooddeprivation .

There were, therefore, no consistent effectsdue to food deprivation of up to 48 h on any ofthe measures of frequency or of temporalpatterning of sexual behaviour . Only IL showeda linear increase due to food deprivation levelsand this is consistent with the results of Sachs

60

30

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25

140

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20

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1200

600

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900

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1

2 3 N-2 N-1 NEjoculatory series

Fig . 3 . Median inter-mount intervals (IMI), inter-intro-mission intervals (III), mount bout periods (MBP), timeout (TO), ejaculation latencies (EL), and post-ejaculatoryintervals (PEI) in the first and last three ejaculatory seriesafter 0, 24, and 48 h of food deprivation .

o 0 M depriveda24 "048 "

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(1965) and Sachs & Marsan (1972) for malesdeprived of food for up to 48 h. Alteration of thecausal factors for feeding thus did not modifythe frequency, duration, or the temporal pattern-ing of sexual behaviour .

Feeding BehaviourFigure 4 shows the median scores for the

latency to the first bout of feeding, the time spenteating in the test period, and the amount of foodeaten. In each case there is a significant changedue to food deprivation : the latency to eat isdecreased (x 2 = 11 . 14, P<0 .01); the timespent eating (x2 = 11 . 14, P<0 .01) and theamount of food eaten are increased (X2 = 10 .57,

1600

800

200

1200

600

200

12

6

2

I G

I II

III

0 24

48Hours food deprived

Fig . 4. Median latency to start eating, time spent eatingduring the test and amount of food eaten after 0, 24, and48 h of food deprivation .

P<0.01). Post-hoc tests indicate that eatinglatency is longest at no food deprivation, signifi-cantly shorter after 24 h of food deprivation,and still shorter . after 48 h of deprivation . Fortime spent eating and amount eaten the lowestscores occurred at no food deprivation, andincreased significantly after both 24 and 48 h offood deprivation, but the amounts eaten underthese two levels did not differ. Although theduration of the sex and feeding tests ranged from108 .6 to 141 .4 min, there were no significantdifferences in test lengths between the threedeprivation conditions (x 2 = 3 .43), nor wastest length affected by the criterion used todetermine sexual exhaustion .

In order to examine more closely the changesin eating patterns due to food deprivation, thepattern of feeding bouts from the first 100 min ofeach sexual satiation test was compared withthat of the 100 min of eating when no female waspresent. Figure 5 shows that the time spenteating in the first 100 min increased significantlyover deprivation levels, when the female waspresent (Sex) and when no female was present(No Sex) (X2 = 12.286 for both conditions,P<0.01). Post-hoc tests show that significantlymore time was spent eating after 24 h of fooddeprivation than after no deprivation, and sig-nificantly more eating occurred after 48 h ofdeprivation than after 24 h, under both Sex andNo Sex conditions . At all three deprivationslevels, rats spent significantly more time eatingin the No Sex condition that in the Sex condition(Wilcoxon tests : T = 1,0 and 0, for the 0-,24-,and 48-h deprivation conditions, respectively) .This increase in eating may, however, have beenaffected by test order .

The frequency of feeding bouts also increasedsignificantly over the three deprivation condi-tions whether a female was present (x2 = 12 -29,P < 0 .01) or absent (x2 = 11 . 64, P < 0.02). Inboth Sex and No Sex conditions, post-hoc testsindicate that there were more feeding bouts after24 h of food deprivation than after 0 h, and moreafter 48 h than after 24 h . There were no dif-ferences in feeding frequency at any of the threedeprivation levels between tests where femaleswere present (Sex) and tests where no femaleswere present (No Sex) (Wilcoxon test, N = 7,T = 10 .5, 13 and 10 for 0-,24-, and 48-h depri-vation conditions, respectively) .

The duration of feeding bouts showed nosignificant differences under the three depriva-tion conditions for the Sex test (x 2 = 3 . 71). Inthe No Sex test, however, feeding bout duration

2000

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893

did differ due to deprivation conditions (x 2 =6 .0, df = 2, P<0 .05) and post-hoc tests showedthat the longest meals occurred after 24 h offood deprivation and the shortest at no fooddeprivation, with those at 48 h falling in between .Without deprivation, feeding bout durations didnot differ between Sex and No Sex conditions(T = 6) while at both 24 and 48 h of food depri-vation, feeding bouts were longer under No Sexthan under Sex conditions (T = 0 and 1 respec-tively, N = 7, P<0 .05) .

The feeding data thus suggest that the presenceof sex decreased the amount of food eaten bydecreasing the length of feeding bouts and not by

10

10

I -I

Bcut frequency

I1I.0

24

48Hours food deprived

Fig . 5 . Feeding patterns during the first 100 min of testswhen an oestrous female was present (Sex) and when nofemale was present (No Sex) after 0, 24, and 48 h of fooddeprivation .

decreasing the number of feeding bouts. In orderto examine further the length of feeding bouts,the distributions of lengths of feeding boutsunder Sex and No Sex conditions were comparedunder each food deprivation level . With no fooddeprivation, the distribution of bout lengths didnot differ between Sex and No Sex groups(x2 = 9 .91, df = 6) . After 24 h of food depriva-tion, there was a significant difference betweenthese distributions : the Sex condition had moreshort bouts and fewer long bouts while the NoSex condition had fewer short bouts and morelong bouts (x2 = 23 . 73, df = 8, P<0.02) .Similarly, after 48 h of food deprivation, theSex condition showed more short bouts thanexpected, while the No Sex condition showedfewer short bouts (x2 = 26 . 68, df = 11, P<0-01) .

Interaction of Feeding and Mount BoutsFigure 6 shows the mean time spent eating,

and the number of males eating, during the IL andduring the EL and PEI for the first three andlast three ejaculatory series. With no fooddeprivation, very few animals ate before thefirst intromission (IL) or during an ejaculatoryseries (EL) and those that did ate for only short

100 -

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200rn

2 100

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300 -48 Hour deprived

2001-

7 6

2 5

6

1 6 1 6 2 6100

0 Hour deprived

0 1 4 1 3 0 3 0 4

24 Hour deprived

4 6 7 3 7 2 5 1 4

0 6 1 6

1 5

IL

2

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N-2

N-1

NEjaculatory series

Fig. 6. Number of rats eating and mean time they spenteating before the first intromission (IL), during the firstand last three ejaculatory series (solid bars) and duringthe first and last three post-ejaculatory intervals (openbars) after 0, 24, and 48 h of food deprivation .

5 6

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periods of time. As food deprivation levelsincreased, more food was eaten during the ILand the EL. Most of the eating occurred duringthe PEI, and the time spent eating during thePEI showed a J-shaped distribution over thecourse of the test : it was higher after the firstejaculation than after the second and third, andhigher after the last two ejaculations than duringany other PEI . Figure 7 shows the distributionof intromissions and feeding bouts in a singleexhaustion test for a 48 h deprived male whichachieved eight ejaculations.The pattern of feeding during sexual beha-

viour tests suggests that feeding bouts wereinterspersed between mount-bounts and occur-red when the level of sexual arousal was low . Inorder to investigate the relationship betweensexual arousal levels and feeding, some correla-tions were performed . Correlations betweentotal ejaculation frequency (EF) and total timespent eating indicated that feeding was notinversely correlated with overall sexual arousal(r8 = 0.72 (P<0 . 05), - 0 .08 and 0 .32, for 0, 24and 48 h of food deprivation, respectively) .

1600-

Another measure of sexual arousal is theintromission latency (IL). The correlationsbetween the intromission latencies and timespent eating before the first intromission arepositive, but do not reach the level of signifi-cance (re = 0 .50 and .0-61, for 24 and 48 h offood deprivation, respectively) .

In the first ejaculatory series, sexual arousalshows a `warm up' period, and it was found thatthe length of the III in the first ejaculatory serieswas correlated with the amount of time spenteating, under the 24- and 48-h food deprivationconditions (r8 = 0.625, P<0.05, N = 14) .

The length of the post-ejaculatory interval isalso a reflection of the male's level of arousal, asPEI increases as exhaustion approaches. Whenthe length of the PEI is compared with theamount of food eaten there is a positive correla-tion (rs = 0.86 (P<0 .05), 0 .61 and 0 .21 for0, 24 and 48 h of food deprivation, respectively) .Since it has been shown that the absolute refrac-tory period is constant as exhaustion occurs, andthe relative refractory period increases (Karen& Barfield 1975) the time spent eating during

Eat

4

Fig . 7 . Cumulative frequency of intromissions (IF), cumulative time spenteating (Eat) and cumulative time spent drinking (Drink) for a single ratafter 48 h of food deprivation. The solid bars represent the length of each ofthe eight ejaculatory series . The open spaces between these bars indicate theduration of the post-ejaculatory intervals . Eating during sexual behavioursequences occurred only in the first and eighth ejaculatory series . Most of theeating occurred during the PEIs .


895

each PEI was correlated with the relative refrac-tory period. These were also all positive (r 8 =0 .79 (P<0 .05), 0.75 (P<0 .05) and 0 .21, for0, 24 and 48 h of food deprivation, respectively).Pooling the results for 24 and 48 h deprivationtests gives a highly significant correlation bet-ween length of the relative refractory period andtime spent eating (r s = 0 .90, P<0 .01) .

DiscussionDo the results of the interaction between hungerand sexual motivation support a competition, achaining, or a time-sharing model of motiva-tional interaction? Since manipulation of thecausal factors for feeding (hours of food depriva-tion) did not alter the frequency nor the temporalpattern of sexual behaviour, it must be con-cluded that hunger at up to 48 h of food depriva-tion did not compete with sexual motivation . Thefact that the intromission latency increased dueto food deprivation and the time spent eatingbefore the first intromission also increased withfood deprivation, however, indicates that somecompetition may have occurred .

The time-sharing model would predict that, ifsexual behaviour was the dominant motivationand hunger was subdominant, feeding shouldoccur during those periods when sexual motiva-tion is at a low level . This occurs after eachejaculation during the post-ejaculatory interval(PEI) and also during the brief post-intromissionrefractory periods during each ejaculatoryseries. Interacting with these refractory periodsare the `warm up' and `exhaustion' of sexualbehaviour, which indicate lower sexual arousallevels on the first and last ejaculatory series thanon the intermediate series .

Available time. Bouts of eating occur betweenbouts of sexual behaviour . As the causal factorsfor eating (food deprivation) are increased, moreeating occurs during the sexual behaviour test .The competition model would suggest that thelonger periods of eating should prolong theintervals between bouts of sexual behaviour.Two results suggest that such competition is notoccurring : first there are no increases in temporalmeasures of sexual behaviour due to fooddeprivation ; second, feeding bouts do notincrease in length over deprivation levels in thesex tests . This suggests that as the males becomemore food deprived, they use more of the`available time' for eating. It is possible thatthere is more than enough time available betweenejaculatory series for the male to satisfy hishunger even when 48 h food deprived .

General sexual motivation and amount ofeating were positively correlated, indicating thatrats which copulated less did not spend moretime eating than those which copulated a lot . Asthe rats become more hungry, i .e. as the causalfactors for eating were increased, there was moretime spent eating. Before the first intromissionoccurred, the more hungry rats spent more timeeating and thus may have prolonged the IL bycompetition . During the warm-up period on thefirst ejaculatory series there was also a significantcorrelation between time spent eating and thelength of the intervals between intromissions ;but there is no evidence that the time spenteating here produced longer III's through com-petition with sexual arousal .

During the refractory periods, the morehungry rats fill more of the available time byeating. Even after 48 h of food deprivation,however, feeding is a subdominant activity anddoes not `compete' for time with sexual beha-viour during the refractory periods . If hungerwere increased by more than 48 h of food depri-vation, competition for time might occur duringall periods when sexual arousal was low .

Time-sharing has been demonstrated in feed-ing and drinking situations in doves (McFarland& Lloyd 1973 ; McFarland 1974) and in pigs(Sibly & McCleery 1976); and in the courtship ofthe three-spined stickleback (McFarland 1974 ;Cohen & McFarland 1979). In all of these casesthe relationship between the alternative activitiesis symmetrical, i .e. behaviour A may be domi-nant and control bouts of the subdominantbehaviour B, or B may be dominant and controlsubdominant. bouts of A, depending upon themotivational strength of A and B . In the presentcase, it appears that sexual behaviour is alwaysdominant over feeding behaviour, even afterconsiderable food deprivation . We had hoped touse the dominance boundary method (McFar-land & Sibly 1975 ; Sibly & McCleery 1976) as atool in the analysis of sexual motivation, butthis method requires a symmetrical dominancerelationship .In all other time-sharing situations, the

dominant control system permits bouts ofsubdominant behaviour at periodic intervals,but the mechanism controlling the temporalorganization of these bouts remains unknown .In the present case, however, we have a goodidea what the mechanism might be like, since wehave a model that accounts for the temporalorganization of the dominant behaviour (Brownet al . 1974). We suggest that future research into

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the interaction of feeding and sexual behaviourshould concentrate upon the details of thetransitions between feeding and copulatorybehaviour, in relation to a time-sharing model .

AcknowledgmentsThis research was supported by a NationalResearch Council of Canada Post-DoctoralFellowship and a Grant-In-Aid of Researchfrom the Sigma Xi Fund of Canada to R.E.B .Preliminary results from this study were pre-sented at the Durham meeting of the Associa-tion for the Study of Animal Behaviour inJune, 1977 .

REFERENCESBeach, F . A . 1956 . Characteristics of masculine `sex

drive' . In : The Nebraska Symposium on Motivation(Ed. by M . R . Jones), pp 1-32 . Lincoln, NebraskaUniversity of Nebraska Press.

Beach, F. A. & Jordan, L. 1956. Sexual exhaustion andrecovery in the male rat. Q. J. Exp. Psychol.,8,121-133 .

Brown, R. E . 1974. Sexual arousal, the Coolidge effectand dominance in the rat (Rattus norvegicus) .Anim. Behav ., 22, 634-637.

Brown, R . E . 1978. Measuring copulatory rate : A mountbout analysis. Submitted for publication.

Brown, R. E., Freeman, S . & McFarland, D . J. 1974 .Towards a model for the copulatory behaviour ofthe male rat. In : Motivational Control SystemsAnalysis (Ed. by D . J . McFarland), pp. 461-510 .London and New York : Academic Press.

Cohen, S. & McFarland, D. 1979 . Time-sharing as amechanism for the control of behaviour sequencesduring the courtship of the three-spined stickle-back (Gasterosteus aculeatus) . Anim. Behav ., 27,270-283 .

Jarmon, H. & Gerall, A . A. 1961 . The effect of fooddeprivation upon the sexual performance of maleguinea pigs . J. comp. physiol. Psychol., 54, 306-309 .


Karen, L. M. & Barfield, R. J . 1975 . Differential rates ofexhaustion and recovery of several parameters ofmale rat sexual behavior. J. comp. phystol.Psychol., 88, 693-703 .

Larsson, K. 1956. Conditioning and Sexual Behaviour inthe Male Albino Rat, Stockholm : Almkvist andWiksell.

Levitsky, D. A. 1970. Feeding patterns of rats in responseto fasts and changes in environmental conditions .Physiol. Behav., 5, 291-300.

McFarland, D. J. 1974 . Time-sharing as a behaviouralphenomenon. In : Advances in the Study of Beha-vior, Vol . 5 (Ed . by D . Lehrman, J . S . Rosenblatt,R. A. Hinde & E. Shaw), pp . 201-225 . New YorkAcademic Press .

McFarland, D. J. & Lloyd, I. H. 1973 . Time-sharedfeeding and drinking. Q . J. exp. Psychol., 25,48-61 .

McFarland, D . J . & Sibly, R. M . 1975 . The behaviouralfinal common path . Phil. Trans. Roy. Soc . (B)207, 265-293 .

Rodger, R . S . 1971 . Friedman's Rank Analysis . Unpub-lished manuscript .

Sachs, B. D . 1965 . Sexual behavior of male rats after oneto nine days without food . J. comp physiol .Psychol., 60, 144-146 .

Sachs, B. D. & Barfield, R. J . 1970. Temporal patterningof sexual behavior in the male rat . J. comp. physiol.Psychol., 73, 359-364 .

Sachs, B . D . & Barfield, R . J . 1976. Functional analysis ofmasculine copulatory behavior in the rat . In :Advances in the Study of Behavior, Vol . 7 . (Ed .by J. S. Rosenblatt, R . A . Hinde, E. Shaw & C .Beer), pp. 91-154. New York: Academic Press.

Sachs, D . B. & Marsan, E. 1972 . Male rats prefer sex tofood after 6 days of food deprivation . Psychon .Sci ., 28, 47-49 .

Sibly, R . M. & McCleery, R. H. 1976 . The dominanceboundary method of determining motivationalstate. Anim. Behav., 24, 108-124 .

Siegel, S . 1956 . Nonparametric Statistics for the Beha-vioral Sciences. New York: McGraw-Hill .

(Received 17 May 1978 ; revised 1 December 1978 ;MS. number : 1765)

interaction of hunger and sexual motivation in the male rat: a time-sharing approach

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