Anim. Behav .,1968,16, 385-391

THE SOCIAL FACILITATION OF PREENING BEHAVIOUR INDROSOPHILA MELANOGASTER

BY KEVIN CONNOLLYDepartment of Psychology, Sheffield University

The preening, or cleaning, behaviour of arthro-pods has been investigated by Heinz (1949) .Working with Sarcophagi, Heinz has describedthe preening movements in terms of a number ofelements and investigated some of the conditionswhich release this behaviour . Similarly Connolly& Weidmann (unpublished) have described thepreening behaviour of Drosophila in terms of anumber of separate behavioural elements. Thecategories of preening so described were asfollows. The flies preen front, middle and hindpairs of legs, head, head and first pair of legs(in rapidly alternating sequence), wings, abdo-men, thorax, proboscis, genitalia . It is of coursepossible to subdivide these categories further, forexample, preening upper and lower surfaces ofthe wings, the eyes or the antennae . These,however, are contained in wing and head preen-ing respectively and it was not considerednecessary to separate them for the purposes ofthe investigation described in this paper .

Various external agents have been shown toaffect the preening behaviour of dipterous flies,for example dust, excess moisture, heat, finehairs and so forth. It is clear that these move-ments serve to keep the animal clean and freethe sensory surfaces from contaminants . How-ever, it may be that preening serves other func-tions in addition to keeping the animal clean .Weidmann (1950) obtained evidence of an in-crease in preening whenever a male Drosophilainterrupted his courtship behaviour. On thebasis of these observations Weidmann suggestedthat preening may also serve as a displacementactivity. One question which is raised by Weid-mann's finding is whether social contacts otherthan courtship also serve to increase the amountof preening. The possibility that preening mayserve as an index of responsiveness to othermembers of the species was therefore investi-gated .

Experiment IMethods

The flies used were wild type Drosophilamelanogaster (Pacific strain) maintained as alaboratory stock . They were raised on agar-

385

molasses media at a temperature of 25+1°Cand on a light-dark cycle of 12 hr .

aFig . 1 . Diagram of apparatus used in study of preening . (a)plugs in position, closing entry points. (b) sliding door sep-arating the two chambers.

The apparatus used is shown in Fig. 1. Itconsisted of two chambers each 2 cm x 2 cm x0 .6 cm separated by a sliding Perspex door .Each of the chambers had an entry point whichcould be sealed by a plug turned from Perspexrod. When in position the plug fitted flush withthe inner wall of the chamber. The apparatuswas made entirely from white Perspex exceptfor the lid which was made from clear Perspex .The lid was held in position by screws and couldbe removed for cleaning purposes.

Into one of the chambers ten white eye mutantsof one sex were introduced through a funnel,and into the other chamber one experimentalfly of the same sex. The sexes were kept separatein order to reduce the interference from court-ship behaviour . This was achieved completelyin the case of females, males however showedcourtship responses to other males . The preeningof the experimental animal was observed over a3-min period. At the end of this 3-min period thedoor separating the two compartments wasdrawn back and the fly, previously isolated, was

386

ANIMAL BEHAVIOUR, 16, 2-3

then in a group with ten other flies of the samesex. A further 3-min observation followed . Inthis way each animal served as its own control .White-eye mutants were used so that the experi-mental animal could be easily and clearly dis-tinguished in the group situation without thenecessity of its being artificially marked.

Preening was measured using a serial time-event recording device . When the fly began topreen the experimenter depressed a microswitchon a finger panel . This displaced the path of apen on a multi-channel pen recorder and activ-ated a print out counter . The counter was pulsedfrom a 10 per sec pulse generator so that boutsof preening were timed to the nearest 0 .1 sec .When a bout of preening ended the experi-menter lifted his finger from the switch whichopened the relay on the pen and switched theprint out counter from count to print and reset .The part of the body preened was also recordedserially on a tape recorder. In this way frequencyof preening, bout length, interbout intervaland part of body preened for each bout wasrecorded . Fifteen males and fifteen femaleswere measured in this way.Results

The mean number of bouts and the mean timespent preening for males and females alone andin the group situation is shown in Table I .

There is a significant increase in preening in thegroup situation, this being the case for bothmale and females measured either in terms oftime spent preening or number of bouts ofpreening, Wilcoxon test P<001 . The increase inpreening is greater for females than males .Comparing the two conditions, males show anaverage increase in the group situation of 4 .3bouts and 7 sec whilst the comparable increasefor females is 5 .8 bouts and 18 .4 sec. This differ-ence is probably accounted for by the courtshipbehaviour shown by most of the males towardsother males in the group situation . Courtshipand preening are incompatible, at least in a

temporal sense, therefore, within a limited ob-servation period, time taken up by courtshipresponses reduces the time available for preening .

Comparing the data obtained for males andfemales in the condition where a fly is alone inone chamber there is a sex difference . Femalesspend significantly more time preening, Mann-Whitney U-test P<0 01, though there is nodifference in the number of bouts of preeningbetween males and females . The mean boutlength is therefore greater for females than formales.Table II shows the mean bout lengths for

male and female flies alone and in the groupsituation. There is a decrease in mean boutlengths for both males and females in the groupsituation, this decrease is significant in the caseof the females at the P<0 05 level, Wilcoxon test .The difference between the males in the twoconditions is approaching significance, P<01>0 .05 .

Figures 2 and 3 show the preening of bothmales and females, in the two conditions,divided up into the various elements . It is clearfrom the histograms that an increase in the fre-quency of preening occurs among all the elementsexcept for head-lst leg (H1) in the case of thefemales. This needs however to be considered inthe light of the mean frequency for head (H)

Table I . Mean Number of Bouts and Mean Time Spent Preening for Malesand Females Alone and in Group Situation

preening since the two are clearly correlated .When this is taken into account the directionof change is entirely consistent .Discussion

Preening serves to keep the fly clean and thesensory surfaces of the body free from contam-inants. However, it does seem improbable thatthe increase in preening is due to the fly becom-ing dirty or contaminated since it has only beenin the company of others for 3 min. Otherexperiments have shown that when a fly ismade dirty by dusting it with finely powderedchalk it preens that part of the body to a higher

MALE

FEMALE

alone

in group alone

in group

Bouts of preening 7 . 3

11 .6

10.4

16.2

Time spent preening (sec) 20 . 9

27.9

47. 5

65.9

CONNOLLY : PREENING BEHAVIOUR IN DROSOPHILA

38 7

Table II. Mean Bout Length of Males and Females Aloneand in Group Situation

degree which has been dusted . If the whole ofthe animal is dusted by shaking chalk all overit then it will preen predominantly its head(most of the attention being devoted to the eyes),its first pair of legs and its wings . Figures 2 and 3show clearly that the increase in preening isdistributed amongst all the behavioural elements .If this increase resulted from the animal becom-ing dirty then a general increase over all theseelements would not be expected, particularlyover such a short time scale .

The likelihood of an animal becoming dirtyor contaminated in the group situation increaseswith the time spent in the group. From this onewould expect more preening to occur towardsthe end of the test period . This is not the casehowever within the observation time employed .The mean number of bouts of preening in thefirst min is 3 . 9 and in the third 4 .0 ; this differ-ence is not significant .

The decrease in mean bout length in the groupas compared with the individual situation furthersuggests that the behaviour is not serving prim-arily to clean the animal . A fly artificially made

dirty in the manner reterred to above showsgreatly lengthened bouts, single bouts of over30 sec having been recorded.

In the group situation flies come into physicalcontact with each other. Most frequently theparts of the body involved in such contacts arethe legs, which represent the perimeter guards ofthe body. Detheir (1963) points out that the legsof insects are particularly rich in tactile senseorgans, the sensilla trichodea, and that thesethin hollow extensions are sensitive to deflexion .If these sensory projections are displaced byphysical contact the fly may preen them backinto position. Experiment II was designed toinvestigate this possibility .

Experiment IIIf sensory hairs are displaced by physical

contact and preening serves to reposition themthere should be some relationship between con-tact and preening. It was hypothesized that ifthe increase in preening resulted from suchcontacts then there should be a significantpositive correlation between the number ofcontacts and the amount of preening shown by afly .Methods

Twenty flies, ten male and ten female, fromthe same stock and reared under the sameconditions as those in experiment I were used .The apparatus and technique employed werealso the same with the addition of a furtherchannel on the pen recorder used for recordingwhen contacts occurred. In this way it was poss-ible to record the frequency of contacts, wherein an experimental record they occurred andtheir relationship to other aspects of the animal'sbehaviour .Results

The number of contacts and their relationshipto preening, measured as number of bouts andas time spent preening, are shown in Table III .The correlation between number of contactsand bouts of preening is 0 .27 whilst that betweencontacts and time spent preening is 013,Spearman's Rho . These give t values of 1 .33and 0 .55 respectively, both correlations arenon-significant .

The total number of contacts between experi-mental flies and others in the groups was eighty-five, but only three of these occurred when anexperimental fly was preening. This suggests thatpreening reduces the probability of contactoccurring. Flies walking round the vertical walls

Flynumber

MALE

alone

in group

FEMALE

alone

in group

1 3 .4 2 . 3 9 .5 3 .7

2 1 .4 1 .9 5 .9 5 .4

3 4 . 1 3 .3 2 . 3 2 . 2

4 2 . 1 2 .9 6 . 5 11 . 7

5 6 .0 2 .2 2 . 7 2 . 8

6 1 .5 1 . 3 2 . 3 2 .4

7 2 .6 2.2 4 . 8 4 .0

8 2 .3 2 .4 4 . 1 2 . 8

9 3 .8 2 . 1 2 .4 2 .0

10 0 1 .2 3 .0 2 . 8

11 14 .2 2 .0 3 . 1 2 .4

12 3 .0 2 . 7 3 i 2 .4

13 1 . 8 2 . 3 2 .4 2 .9

14 7 . 3 19 6 . 1 10 . 7

15 2 .2 1 .2 4 . 1 3 .2

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ANIMAL BEHAVIOUR, 16, 2 -3

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BEHAVIOURAL ELEMENTSFig . 2. Histogram of males preening ; in isolation, hatched area ; in groupsituation, solid area. The code for the different behavioural elements is : L lfirst pair of legs, 1.2 middle pair of legs, L 3 hind pair of legs, W wings, Hhead, Hl head and rst pair of legs in rapid alternating succession, Ababdomen, Th thorax, Pb proboscis .

BEHAVIOURAL ELEMENTSFig. 3 . Histogram of females preening ; in isolation, hatched area ; in groupsituation, solid area . Code as in Fig. 2.

of the cell were frequently seen to detour round mental flies' legs coming into contact with thea fly which was preening and very rarely came legs of other flies. Despite this however the in-into physical contact with it. A little over 60 crease in leg preening is not proportionatelyper cent of the contacts resulted from the experi-

increased, as can be seen from Figs 2 and 3 . In

N

CONNOLLY : PREENING BEHAVIOUR IN DROSOPHILA

389

the isolated situation 75 per cent of the timespent preening was devoted to the legs, in thegroup situation 73 per cent .

The results therefore do not support thehypothesis that the increase in preening is afunction of physical contacts .

DiscussionThe results of experiments I and II suggest

that the increase in preening in the groupsituation is not due, at least not entirely, to theanimal cleaning itself or preening back intoposition sensory hairs displaced by contact. Thegeneral increase in preening over all parts of thebody, coupled with the decrease in mean boutlength does not seem compatible with the be-haviour being directed specifically towardscleaning. Similarly, if preening resulted from thedisplacement of sensory hairs following contactnot only would a significant correlation beexpected but also much of the increase in preen-ing should be confined to that part of the bodyin contact, namely the legs .An alternative explanation would be that

preening serves as a signalling device facilitatingthe spacing of animals and reducing the prob-ability of accidental contact in a group situation .Sexton & Stalker (1961) have shown thatDrosophila paramelanica adopts a uniformspacing pattern at high population densities inan experimental chamber. This they suggest mayserve to reduce competition for food or ovi-position sites . They also describe an avoidanceresponse exhibited by flies when they are within1 to 5 mm of each other, this response enablinga fly to control its immediate surrounding areaby extending its legs. These `fending off' move-ments were observed in experiments I and IIbut always towards the end of the 3-min observa-tion period when the flies had settled down andwere showing relatively little movement . The`fending off' movements were exhibited bystationary flies towards the occasional move-ments made by one of the others and only whenthe moving fly was very close . The techniqueemployed by Sexton & Stalker allowed a con-

siderable time for a population to settle downbefore measurements were made . It is probabletherefore that in a quiescent population adequatespacing and signalling are accomplished by these`fending off' movements. This hypothesis con-cerning the signalling function of preening gainssupport also from the fact that very few contactsoccurred when flies were preening .

If preening serves a signalling function thenthe presence of other animals has to be detectedin some way. As Tinbergen (1964) points outmovements and postures which convey inform-ation seem on the whole to be perceived visually .The compound eye of Drosophila, which is avery sensitive detector of movement, would bean excellent transducer for this purpose . Ol-factory and vibratory stimuli may also beinvolved, and indeed this seems probable since`fending off' movements are made withoutcontact and when movement is minimal. Experi-ment III was designed to investigate the role ofvision in detecting the presence of other flies .

Experiment IIIIf the presence of other animals is detected

primarily through the visual modality then flieswith impaired visual acuity will be inefficientunits in a communication channel. For suchanimals one would predict a smaller increase,or no increase, in preening. Sex-linked whiteeye mutants whose visual acuity is known to bereduced (Kalmus, 1943), were used as experi-mental flies .

MethodsThe apparatus and techniques used were as in

experiments I and II . The experimental flies,ten males and ten females sex-linked white eyemutants (w), were raised in the standard labor-atory conditions described above. The fliesmaking up the group situation were normal wildtype Drosophila ; this enabled the experimentalanimal to be clearly distinguished .

ResultsThe results of the experiment are shown in

Table IV.

Table III. Number of Contacts and Their Rdatiooship to Amount of Preening

Fly number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Number of contacts 5 3 6 5 5 6 3 2 2 3 5 6 4 5 4 5 3 5 5 3

Number of bouts 95591422171319158491213121217108

Total time spent preeningto nearest second 34 24 10 16 52 76 33 31 46 24 18 26 57 32 25 48 22 48 29 19

390

Table IV. Mean Number of Bouts, Time Spent Preeningand Bout Length for White Eye Mutants Alone and in

Group Situation

ANIMAL BEHAVIOUR, 16, 2- 3

The decrease in preening in the group situ-ation is significant (Wilcoxon test P<0 05) .The difference in mean bout length between thetwo situations is not significant.Discussion

There is no evidence to suggest that the differ-ence between the wild type flies and the whiteeye mutants is due to any factor other than im-paired visual acuity. In the isolated situation theamount of preening shown by the mutant flies iscomparable with that shown by the wild type(Tables I to IV) . It therefore seems improbablethat the mutant gene has any pleiotropic effectson the nervous system which might affect preen-ing behaviour . The decrease in preening, meas-ured either as number of bouts or time spentpreening, suggests that vision is the principalsensory modality for detecting the presence ofothers, beyond the immediate space around theanimal's body at least. The mutant flies do show`fending off' movements ; so other sensorymodalities are probably involved in detectingflies in close proximity .

Sexton & Stalker (1961) are careful to pointout that the uniform spacing adopted by bothmale and female groups of D. paramelanicacannot be interpreted as evidence for territor-iality_ Indeed it appears to have certain char-acteristics which distinguish it from the territ-oriality exhibited by vertebrates ; there is nodefence of a specific area and the flies do notactively seek contact, other than sexual contact .However, if an animal does not show territor-ialism it may still possess an `individual distance'within which the presence of cogeners, if allowedat all, may result in a greatly increased mortality .The psychosomatic symptoms described byChitty (1959) as `shock disease' appear to resultentirely from crowding. Similarly Christian(1960) has shown how crowding mice leads toadrenal hypertrophy and a concomitant in-crease in mortality. Relatively little informationis available regarding the effects of populationdensity on insects . Pearl et al . (1927) have shown

that life span in Drosophila is related to popu-lation density . Within limits, the higher thepopulation density the shorter the life span . Thismay reflect a . number of variables, food avail-ability, accumulation of toxins and so forth .Although there is no direct evidence, as yet, of`shock disease' in populations of Drosophila itis unlikely that food shortage is responsible forthe reduction in longevity since Loeb & Northrop(1917) have shown that flies reared on a meagrefood supply (glucose-agar media) lived as longas flies reared on a complete unrestricted diet .The work of Pearl et al. and investigations

such as that reported by Naylor (1959), showingthat Tribolium adopt a uniform spacing patternsuggests that mechanisms controlling spacinghave considerable biological advantage .

Huxley (1923) pointed out that many signal-ling movements resemble incomplete versions ofmovements which have some other function .The preening movements observed in the groupsituation appeared qualitatively indistinguishablefrom those made by a fly when alone in a cell .However, the decrease in mean bout lengthshown by the wild type flies might be interpretedas suggesting that those movements were in-complete, incomplete at least in terms of theireffectiveness as cleaning movements . By contrastit is interesting to note that the white eye mutantsdo not show any significant reduction in meanbout length .

Summary1 . The preening, or cleaning behaviour of

Drosophila melanogaster is briefly described .2. The presence of other flies was found to

increase the amount of preening . This increasewas not restricted to preening particular partsof the body and did not appear to be due to fliesbecoming dirty .

3. The increase in preening was , shown notto be a function of physical contacts betweenflies .4. White eye mutants with severely impaired

visual acuity show no increase in preening in thegroup situation . Vision appears to be the sensorymodality largely responsible for detecting thepresence of other animals .

AcknowledgmentsThe author would like to thank Miss C .

Bryan and Miss L. Brown for valuable assistancein conducting the experiments, Dr U. Weidmannfor much helpful discussion and the KittayFoundation for a generous research grant whichgreatly facilitated this work .

Alone In group

Bouts of preening 7 8 5 .9

Time spent preening (sec) 21 . 8 14 . 4

Bout length (sec) 2 . 25 2 . 12

CONNOLLX: PREENING BEHAVIOUR IN DROSOPHILA

REFERENCESChitty, C. (1959). A note on shock disease . Ecology,

40, 728-731 .Christian, J . J. (1960) . Adrenocortical and gonodal

responses of female mice to increased populationdensity . Proc. Soc . exp. Biol. Med., 104, 330-332.

Detheir, V. G. (1963) . The Physiology of Insect Senses .London : Methuen .

Heinz, H-J. (1949) . Vergleichende Beobachtungen fiberdie Putzhandlungen bei Dipteren im allgemeinenand bei Sarcophagi carnaria L . im besonderen.Z. Tierpsychol., 6, 330-371 .

Huxley, J. S. (1923). Courtship activities of the red-throated diver (Colymbus stellatus Pontopp) :together with a discussion on the evolution ofcourtship in birds . J. Linnaean Soc ., 35,253-293 .

Kalmus, H . (1943) . The optomotor responses of some eyemutants of Drosophila. J. Genet ., 45, 206-213 .

Loeb, J. & Northrop, J. H. (1917) . On the influence offood and temperature upon the duration of life .J. Biol. Chem., 32, 103-121 .

Naylor, A . (1959) . An experimental analysis of dispersalin the flour beetle, Tribolium confusion . Ecology,40, 453-465 .

Pearl, R., Miner, i. R. & Parker, S. L. (1927). Experi-mental studies on the duration of life XI . Densityof population and life duration in Drosophila.Am. Nat., 61, 289-318 .

Sexton, O. J. & Stalker, H . D . (1961) . Spacing patternsof female Drosophila melanogaster . Anim . Behav.,9, 77-81 .

Tinbergen, N . (1964) . The evolution of signalling devices .In Social Behaviour and Organisation amongVertebrates (ed. by W. Etkin). Chicago: Univ .Chicago Press .

Weidmann, U . (1950) . Untersuchungen zur Ethologievon Drosophila. Die Balz-und Putzhandlungen .Unpublished dissertation. Zurich University .

(Received 6 July 1967 ; revised 1 January 1968 ;Ms. number: 766)

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