Male harassment on female colour morphs in Ischnura elegans (Vander Linden): testing two frequency-dependent hypotheses (Zygoptera: Coenagrionidae)

2005·1.1 MB·English

by  Van Gossum, H

Odonatohgica34(4):407-414 December I.2005
Maleharassmenton femalecolourmorphs
inIschnura elegans(Vander Linden):
testing twofrequency-dependent hypotheses
(Zygoptera: Coenagrionidae)
H.Van Gossum¹’*,L.DeBruyn¹’² andR.Stoks³
1EvolutionaryBiologyGroup,UniversityofAntwerp,Groenenborgerlaan171,B-2020Antwerp,
Belgium;—Fax:[phone];- Phone;[phone];— e-mail:[email protected]
2 InstituteofNatureConservation,Kliniekstraat25,B-1070Brussel,Belgium
3LaboratoryofAquaticEcology,UniversityofLeuven(KULeuven),De Beriotstraat32,B-3000
Leuven,Belgium
ReceivedFebruary 14,2005/Revised andAcceptedMarch23, 2005
InmanyZygopteraspp, 9 9occurindifferentcolourmorphs,withonemorphcol-
ouredlikethe d(andromorph),whiletheothersarenot(gynomorph).Twohypotheses
have been proposedtoexplainfrequency-dependentharassment of9 morphs.Ac-
cordingtothefirst, 6dshouldprefer themorefrequentofthe2 9morphs(learned-
-materecognitionhypothesis). Accordingtothesecond, d6 should preferandro-
morphsmoreiftheir frequencyrelative to ddincreases,but notsoforgynomorphs
whichalwaysshouldbeattemptedtomatewith onencounter(mimicry hypothesis).
Here,it isreportedonare-analysis ofearlier publisheddataonmorph-specifichar-
assmentforI.elegans,which allows examination ofthe2proposedhypotheses.The
datawerecollected in 8 insectaries with differentratios of d d and 9 morphs.As
reportedearlier,dharassmentishighestonthemostcommon9 morphsupporting
thelearned-mate recognitionhypothesis.Theratioofandromorphsto ddhad no
morph-specificeffectsinamountsofdharassment,wherefore thedatasuggestrejec-
tion ofthemimicryhypothesis.
INTRODUCTION
Coexistenceof multiple femalecolourmorphs withinaspecies isacharacter-
isticofmanydamselfliesand dragonflies (e.g. CORBET, 1999;FINCKE,2004).
Typically, oneofthefemaledamselflymorphs (andromorph) iscoloured, andin
somespecies also patternedlikethemale,whiletheothermorphs (gynomorphs)
*Correspondingauthor
408 H. vanGossum, L.de Bruyn&R.Stoks
aredifferent(e.g. CORBET,1999).Despite understanding ofthemodeofinher-
itance in several damselfly species (JOHNSON, 1964, 1966;CORDERO 1990;
ANDRES& CORDERO, 1999),and molecularand experimental support for
thepolymorphism tobeunderselection(ANDRES etah,2000, 2002;SIROT&
BROCKMANN,2001;WONG etah, 2003), considerabledebateremainsabout
the underlyingmechanismsinvolvedin themaintenanceof multiple femalecol-
our morphs (e.g. FINCKE, 1994;CORDERO etah, 1998;SHERRATT,2001;
VANGOSSUM etah,2001a,2001b; SIROTetah, 2003). Amathematicalmodel
(SHERRATT,2001),a verbalmodel(FINCKE,2004)andempirical studies(e.g.
MILLER&FINCKE. 1999;VANGOSSUM etah,2001b)indicatethat frequen-
cy-dependent sexualharassmentby malesmay contributetothe maintenanceof
multiple femalecolourmorphs. However, theexactnature of thefrequency-de-
pendent selectionis being debatedandtwo hypotheses havebeenproposed.
Accordingtoone view,maleswillprefer tomatewiththemostcommonfemale
morph intheirpopulation andwill change this matepreference according to fe-
male morph frequency (ratio of andro- to gynomorphs); i.e. the learned-mate
recognition hypothesis orLMR (MILLER& FINCKE, 1999).FINCKE(2004)
recentlyelaboratedonherideaandsuggestsconspecific males,orevenheterospe-
cific malesandfemalesalso mayaffect a male’scapability ofpartnerselection.
In short, theamount of amaleharassmenttowardafemalemorph may depend
on theratioofthis morph to thenumberof distractorspresentin apopulation
(FINCKE, 2004). These may differbetween populations because, forexample,
phenotypically similarlooking heterospecificindividualsmaynotbepresentinall
populations ofaspecies. Insupportofthe LMRseveralstudiesindeedshowed
thattheratioofandro-to gynomorphs inapopulation was a good predictor of
malemate choicefor femalecolourmorphs(MILLER&FINCKE, 1999;VAN
GOSSUM etah, 1999;VANGOSSUM etah,2001a, 2001b).
Second, accordingotherresearchers, andromorphs are consideredtobefunc-
tionally malemimicswithmalesbeing incapable ofdistinguishingbetweenthese
femalesandothermalesatleastundersomeconditions(e.g.ROBERTSON, 1985;
HINNEKINT, 1987). In this context SHERRATT (2001) developed a math-
ematical modelto explain the maintenanceof multiple femalecolourmorphs.
SHERRATT(2001) assumes thatthefrequencyofharassmentonandromorphs
is positively correlatedwiththeratioof andromorphs tomales(inthiscasemales
couldbe considereddistractors) inthepopulation(the so-calledmimicryhypoth-
esis). Hence, amale’sability torecognize afemaledepends onhowmanyandro-
morphs andconspecific malesarepresentinapopulation. Forgynomorphs such
a relationship isnot tobeexpected andthismorph shouldalwaysbematedwith
on encounter (SHERRATT2001).
UndersomecircumstancestheLMRandthemimicry hypothesesmaygive iden-
ticalpredictions (see further),wherefore priorstudiesindicatingsupportforthe
LMRmustbe questioned. Canthemathematicalmodelby SHERRATT(2001)
Male harassmentonfemalecolourmorphsinIschnuraelegans 409
explainthecurrentlypublished empiricaldataintheliterature?Today, oneofthe
besttestsoftheLMR remainsthestudyby VANGOSSUM etal.(2001a). They
firsttested malesfrom the field in simple binary choicetests. Afterthesetests,
themales,whichhadattemptedto matewithandromorphs, wereplaced togeth-
erinaninsectary along withtwenty gynomorphs, whilethosemales, whichhad
attempted to mate withgynomorphs, were placed together, along with twenty
andromorphs. Theauthors viewedthis as aperiod inwhichmaleswerelikely to
switch theirmate choice basedon the ratioof the femalemorphs. However, it
is important to notethat by caging malestogetherwith femalesthe ratioofan-
dromorphs to maleswas also radicallyaltered(Thomas Sherratt,pers.comm.).
Ifmales are kept with gynomorphs, thenit isreasonableto suggest thatmales
mightbegin to learn thatanything male-likewas unprofitable. Thus, theprefer-
enceforgynomorphs, following aperiod in aninsectary withonly gynomorphs
andmalesmightwellbebecausethesignificant majorityofmalessimplyleamto
avoidmatingwithanything male-like,ratherthanlearntorecognize gynomorphs
aspotential mates. Thereported preferenceofmalesforandromorphs mayarise
as a consequenceof theperceived ratioof andromorphs to malesas predicted
by Sherratt’smodel.Toconclude, previous work isconsistentwithbothhypoth-
eses, socannot discriminatebetween them(T. Sherratt, pers. comm.). Here, we
re-analysed thedataofanearlierpublished study on thefrequencyofmalehar-
assment onthetwo femalecolourmorphs ofI. elegans(VAN GOSSUM etal.,
2001b). Whilein the original study morph-specific degrees ofharassmentwere
only examinedin the light of variationin the ratioof andro- to gynomorphs
(LMR),herewealsoexplore theeffectsof variationintheratioofandromorphs
to males(mimicryhypothesis).
METHODS
To distinguish between the LMR and themimicry hypotheseswere-analysedthe data ofVAN
GOSSUM etal.(2001b).The experimentmanipulatedmorphfrequencies,densitiesandsexratioin
insectariestoevaluate theireffectsontheamountofmaleharassmentonandro-and gynomorphs.
Sincenootherdamselflyspecieswerepresentintheexperimentalcages,thedistractorsetforthemales
reduces tothetwofemalemorphsand othermales (cf. FINCKE,2004). Insectarieswerefilledwith
notyetsexuallyactive,field-caughtanimals randomlydistributed amongeightoutdoorinsectaries
(3x3x2.5m). Insectariescontained smallponds,sufficient food andsuitable vegetation.
Table 1
Ratioofandro-(A)togynomorph(G) femalesand ratioofandromorphs(A)tomalesin theeight
insectaries examinedforthisstudy
Insectary
Ratio 1 2 3 4 5 6 7 8
A-toG-females 0.25 0.25 0.25 0.25 1.5 4.0 4.0 4.0
A-females tomales 0.1 0.2 0.4 0.4 1.0 0.4 0.4 1.6
410 H.vanGossum, L.deBruyn&R.Stoks
Incontrasttothe situation inmany otherdamselflyspecies,ratios ofandro-to gynomorphsare
highlyvariableinnaturalpopulationsofI.elegans,withsomepopulationsdominatedbygynomorphs
and others dominated byandromorphs(PA_RR, 1965; VAN GOSSUM etal., 1999;SANCHEZ-
-GUILLENetal.,Inpress).Therefore,ratiosofandro-to gynomorphsappliedinthestudybyVAN
GOSSUMetal.(2001b)fellwithintherange observedin nature.Across insectaries,theratio ofan-
dro-to gynomorphs(rangingfrom0.25to4) andtheratioofandromorphstomales(rangingfrom
0.1to1.6)(Tab.I)wereuncorrelated (N=8,r2=0.58, p=0.135).Populationdensityintheinsectaries
rangedfrom 15to30individuals andsexratiowerefemale-ormale-biased. Densitieswereinaccord-
ancewith densityestimations fromnaturalpopulations(MOORE, 1964; HINNEKINT, 1987).
Maturationin I.eleganstakes onaveragesixdays(CORDEROetal., 1998).Theexperimentstart-
edwhen animalswere 15daysold,assuringtheywereall sexuallymature.Maleapproachestowards
females wererecorded dailyfrom 8h00until 18h00Mid EuropeanSummer Timeonthreeconsecu-
tive days(5,6and 7 August).Duringeach ofthesedays observationsessionsoffifteenminuteswere
rotated amonginsectaries,resultinginthemonitoringofalltheinsectarieseverytwo hours.Thero-
tationsequence wasrandomlydetermined ateachsamplingdate.Observations onmaleharassment
weremadefromoutsidetheinsectaries withthenakedeyeandbyusingbinoculars (Opticron, 10x42).
Firstfocuswasonmale approachand only secondontheapproachedfemalemorph,ensuringthat
theobservedfemale encounterfrequenciesareunbiased bymorph.
Tomaintain constant densitiesand frequenciesofmales and female colour morphs, each time
anindividual died itwasreplacedby adummy.Adummywasanew,livingdamselflyidentical in
morph/sextothe animalthatdied, andwasfreetointeract with theotherindividuals intherespec-
tiveinsectaries. Duringthethreeconsecutive daysfivedamselflies diedand werereplacedwith newly
caughtsexuallymaturedamselflies. Threeoftheseweremales,whichadaptvery fastbehaviourally
tonewmorphfrequencies(MILLER&FINCKE 1999;VAN GOSSUMetal.2001a;MILLER&
FINCKE, 2004).Thetwoother animalswereandromorphsininsectaryeight,aninsectary withsix-
teenandromorphs.Evenifthedummyfemaleswould have behaved differently,it isthereforehighly
unlikelythattheybiasedourresults.
Differencesin maleharassment between femalecolourmorphswereanalysedwith mixedmodel
regression(SAS8.0, proc MIXED;LITTELL etal., 1996).Theamountofmale harassment was
log10transformed in ordertomeettheassumptionsofthestatistical analysis.We used insectaries as
oursampleunitsandaveragedthefrequencyofmaleharassmentoneach femalemorphperinsectary
perday.Thisresultedincoupledscoresofboth morphsforeachinsectary.Wetookthisintoaccount
by nestingmorphswithin insectaries. Because foreachinsectary the sameanimals (withfive excep-
tions,seeabove)wereinvolved duringall three days,weconsidered successivedailyobservations of
maleharassment in each insectaryasrepeats.Colour morph,and respectively theratioofandro-to
gynomorphsortheratioofandromorphstomalesweretreatedasindependent,fixed effects.Aback-
ward stepwisemodel selectionwasappliedin ordertoobtain the final model (CRAWLEY, 1993).
Thedegreesoffreedom ofthefixedeffectsF-testswereadjustedforstatistical dependenceusingthe
Kenward-Rogerformula(KENWARD&ROGER, 1997;SCHAALJEetal.,2001).
RESULTS
Examinationof the LMR indicateda significant interactionbetweencolour
morph andratioofandro-togynomorph femalesinfluencingtheamountofmale
harassmentonfemalemorphs (F , =5.02, p =0.036) (Fig. 1). Whenthe ratio
22
andromorph/gynomorph increases, thenumberofmalematingattemptsonan-
dromorphsincreases[log (mating attempts) =0.010+0.052A/Gratio]andde-
l0
creasesforgynomorphs [log (matingattempts) =0.180 0.017A/Gratio].Theless
10
frequent morph wasalways subjected to fewermatingattempts compared tothe
Maleharassment onfemalecolourmorphsinIschnuraelegans 411
mostcommonfemale
morph (Fig. 1). Ex-
ploring whether the
ratio ofandromorphs
to males(mimicry hy-
pothesis) influences
the amount of male
harassment on fe-
male morphs indicat-
ed no morph-specific
effects(F, =0.08,p
22
=0.782). Aslight ten-
dency was revealed
that females were
harassed less when
there were more an- Fig. 1.Effectofratioofandro-togynomorphfemales onthefrequen-
dromorphs relative cyofmorph-specific maleharassment(malematingattempts).Dots
to conspecific males representtheactualobserved dataaveragedperfemalemorphperday.
=3.95,p=0.060), Totalnumber ofdotsis 24for eachfemalemorph(8insectarieswith
observations repeatedfor3 days).
but such changes in
amount ofharassment wereidenticalforandro-andgynomorphs.
DISCUSSION
Our results didsupport the LMR. In concordance with earlierfindings on
Enallagma andIschnura(MILLER & FINCKE, 1999; VAN GOSSUM et ah,
2001a, 2001b) andromorphs weresubject tomoreharassmentunderalldensities
and sex ratios ifthey constitutedthe majority femalemorph, butreceivedleast
harassmentwhengynomorphs weremorecommon.Results fromfieldstudiesin-
dicatethatmalesare more attractedto gynomorphs (e.g. ROBERTSON, 1985;
FINCKE, 1994),whichareusually thecommonestmorph (McKEE etah,2005).
Furthermore,FORBES (1994) showedapreference forandromorphswhenthey
were thenaturally occurring majoritymorph.
Interestingly, SHERRATT(2001) assumedthatgynomorphs are notexpected
tobeharassedinafrequency-dependent way, i.e.shouldreceiveauniformamount
ofharassmentirrespective oftheirfrequency ifcompared toandromorphs or if
compared to males.Indeed, basedontheobservationthatgynomorphs are not
mimicsanymaleinsearchofafemalethatencountersagynomorph shouldalways
attemptto matewithher(SHERRATT, 2001). As a consequence,andromorphs
areneverexpected toreceivemoremaleharassmentthangynomorphs. Nonethe-
less, our results indicate thatandromorphs do receive moremaleharassmentif
they are themorecommon femalemorph compared togynomorphs.
412 H. vanGossum,L. deBruyn& R.Stoks
Further, ourresults show thattheamount ofmaleharassment didonly slight-
ly depend ontheratioofandromorphs tomales.Interestingly, maleharassment
on femalesdecreasedslightlywhentheratioofandromorphs to malesincreased,
andthis so identicalforandro-andgynomorphs. Thisis incontrary toexpecta-
tionsunderthemimicryhypothesis (SHERRATT, 2001)wheregynomorphs are
expected to haveequal amounts ofharassmentunderallfrequencies andandro-
morphs toreceive moreharassment thangynomorphs when they arerelatively
commoncompared to males. Currently, thereisonly oneotherstudythatcan be
usedtoevaluatethemimicryhypothesis.Inanexperiment conductedincaptivity
withIschnura ramburi(SeL), andromorphs thatwereconfined to a cageathigh
operational sex ratio(oneandromorph andthreemales) matedsignificantlymore
thanthoseina cageatlowoperational sexratio(twoandromorphs andtwomales)
(SIROT& BROCKMANN,2001).Gynomorphs showednodifferenceinmating
frequency underthe differenttreatments (SIROT& BROCKMANN,2001). In
thatcase, theresults forgynomorphs wereconsistentwith thepredictions under
the modelby Sherratt, but theresults forandromorphs were opposite to those
predicted underthemodel. Nonetheless, theauthorsarguedthatsince thedam-
selfliesin thisexperiment were confinedto smallenclosures potentiallythemale
mimicry ofandromorphs was broken by theirinability to getaway from males
(SIROT et ah, 2003). We believe thatgynomorph behaviourin the smallcages
likelyalsodifferedformthenaturalsituation.Indeed,innatureandromorphs use
moreopenhabitatand donotfly large distances, butgynomorphs hidemorein
thevegetationandflylongerdistances(VANGOSSUM etal.,2001b).Therefore,
we consideritmorelikely thattheresults forgynomorphs inthesmallcages de-
viate from thesituationin anaturalpopulation. Lastly, inmost damselflypop-
ulations operational sex ratios are strongly male-biased(e.g. CORBET, 1999).
Therefore, maleswilloutnumberandromorphs byfaratthemating site, indicat-
ing thatdifferencesinandromorph to maleratioswillbe verysmallandmightbe
hardto detect(at leastby humanobservers). Incontrast, relativefrequencies of
bothfemalemorphs maybemuch easier to detectandused as a proximate cue
fordifferentialmaleharassment.
Together, we thinkthatthe mimicry hypothesis (SHERRATT, 2001) cannot
provide explanation for the empirical findings on the studied Ischnuraspecies.
Following the resultsofourre-analysis neithertheharassmentpatterns foran-
dromorphs, nor thosefor gynomorphs do support the expectations under the
mimicry hypothesis. Thelearned-materecognition hypothesis, onthe contrary
is fullysupported, andappearsmostconsistent withall current fieldandexperi-
mentalwork (seealso FINCKE,2004).
ACKNOWLEDGEMENTS
WethankT.SHERRATTandananonymousrefereeforcommentsand remarksonprevious ver-
MaleharassmentonfemalecolourmorphsinIschnuraelegans 413
sionsofthemanuscript.H.V.G.andR.S.arepostdoctoralfellowsoftheFund forScientificResearch
Flanders (FWO).The experiment describedwasperformedinaccordancewiththe lawsandethical
guidelinesofFlanders (AMINAL-Flanders).
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