RosallaMartin AlejandroRodriguez MiguelDelibes
Localfeedingspecializationbybadgers(Melesmeles)
inamediterraneanenvironment
Abstract Acaseoflocalfeedingspecializationinthe Europeanbadger(Melesmeles),a carnivorespecieswith morphological,physiologicalandbehaviouraltraits pro pertoatrophicgeneralist,isdescribed.Forthefirst time,wereportamammalianspecies,theEuropeanrab bit(Oryctolaguscuniculus),asthepreferredpreyof bad gers.Secondarypreyareconsumedaccordingtotheir availability,compensatingfortemporalfluctuationsin theabundanceofrabbitkittens.Wediscuss how both predator(littleabilityto hunt)andprey(profitability and predictability) features, mayfavourtheobservedspecial
ization, aspredicted byforaging theory.Badgersshowa trendtospecializeondifferentpreyin differentareas throughoutthespeciesrange.Itissuggestedthatchanges inpreyfeatures canreverse thebadgerfeeding strategy atthepopulationlevel.Suchdynamicbehaviouralre sponsesmakedifficulttolabelbadgersasgeneralistsor specialists atthespecieslevel.
Keywords Feedingstrategy.Localspecialization
Preyfeatures.Meles
Introduction
Oneoftheecological meaningsof“trophicspecializa tion”expressestheconsumer’sconsistentreductionin dietwidthinrelationtotheavailabilityofpreytypesin itshabitat(Murdoch1969;Schoener1971;Morse1980). Thisdefinition,whenappliedatthespecieslevel,often impliessomekindofgeneticallybaseddifferentiationin morphologicalorphysiologicaltraitswhichmaybere flectedinrestrictedfeedingperformance(e.g.Sanderson
1991).However,atthepopulationlevel,evolutionary changeinbehaviourwithoutapparentchangeinmorpho logicalorphysiologicaladaptationmayaccountfordif ferencesintheuseoftrophicresourcesamongallopatric populations (Futnyma 1986; Futuyma and Moreno
R.Martin.A.Rodrfguez()•M.Delibes
EstaciónBiológica deDoñana,CSIC,Avda.MariaLuisas/n,
41013Sevilla,Spain
1988).Thus,allopatricpopulationsof generalistspecies (those whose diet variesaccording toprey availability) mayspecialize bymeansofbehavioural responses tothe differentprey abundancetheyexperience.Changein consumer behaviourisoneofthefactorsinvokedbyFox andMorrow (1981) toexplainmultiple localspecializa tionsondifferent plantspecies ofmanyherbivorous in sects.Theseauthors stresstheimportance ofgeographi calscaleformakinginferences aboutecological proper ties(e.g.thedegreeofspecialization) ofindividuals, populations orspecies. In vertebrates, the phenomenon ofallopatricpopulationsspecializingon different prey hasreceived little attention withrespect toother varia tionin intraspecificspecializationalsooccurringwithin populations, asrelated toindividual, sexor age differ ences infeeding behaviour (Partridge andGreen1985). Informationisespecially scarceinmammals and,within thisclass,incarnivores.
Inthispaperwedescribeacaseoflocalfeeding spe cializationina populationofEuropeanbadger(Meles meles).Thedietofthebadgerhasbeenextensively stud iedin northernand centralEurope,whereearthworms (Lumbricus spp.)areusually itsstaple prey(e.g.Skoog
1970; Ashby andElliot 1983; Henry 1983; Lups etal.
1987), complemented by other animal and vegetable foods. Preference for earthworms does not seem to be
consistent acrossthewholeofthebadger’srange,andit hasbeenfoundthat otherfoodtypespredominateunder differentecologicalconditions,e.g.fruits(Kruukand de Kock1981),wheat(SkinnerandSkinner1988)andava riety of vegetable matter and domestic refuse (Harris
1984). Adecrease in the importance of earthworms in the diet of badgers has also been related to temporal changesinagricultural practices whichinvolveadecline in earthwormavailability(KruukandParish1985). Moreover, either aminor role or the absence ofearth wormsfrom thediethavebeenreported inthefewpa persdealingwiththe foodhabitsofbadgersinmediter raneanenvironments(e.g.Ciampaliniand Lovari1985; Pigozzi 1991; Rodriguez andDelibes 1992).Inmostof thesestudiesitisgenerally supposed thatpreyaretaken
accordingtotheiravailability,byvirtueofwhichthe species isusuallyclassifiedasatrophicgeneralist(e.g. Hanskietal.1991).
Ontheotherhand,KruukandParish (1981)demon stratedthatthebadgeractsasafeedingspecialistin earthwormsinagriculturalareasofScotland.Thesecond studyreportingspecialization,carriedout inaplacewith stronghumaninfluence(northernItaly),showsaconsis tentcomsumptionofcultivatedolivefruits(Olea eur opaea) throughouttheyear(KruukanddeKock1981). Herewewillshowhowbadgersselectadifferentprey, theEuropeanrabbit(Oryctolaguscuniculus),inalocali tyfreeofagriculture,nearthesouthwesternborderofthe speciesrange.Wewillalsodiscusshowrabbitavailabili tyfitspredictedconditionsunderwhichalocalspecial izationisfavoured(i.e.whenthepreferredpreyremains abundant,accesibleandpredictableovertime;Schoener
1971;Stephensetal.1986;Begonetal.1990).
Materialsandmethods
Studyarea
ThestudywasconductedbetweenSeptember1977andOctober
1978 at theDoflanaBiological Reserve,located onthecoastal
plain,westoftheGuadaiquivirRiverdelta,south-westernSpain
(36°59’N,6°45’ W).Thesiteisasandy,flatarea,separated
mateswas testedbycomparisonwiththeactualdryweightsofre mainsfrom56samples.Estimatesanddryweightsweresignifi cantlycorrelated(r0.978,P<0.00l), thusvalidatingtheproce dure.
Resultsarepresentedaspercentageoccurrenceperpreyitem, andasestimatedfreshbiomassingested.Forthelatter,weights werecalculatedusingcorrectionfactorslargelybasedonthoseof Lockie(1961)formartens(Martes martes),andlaterusedby RodriguezandDelibes(1992)forthebadger.
Theagesofthe rabbitseatenwerecalculatedfromthesizeof theirincisors(asfoundinfaeces)byuseofaregressionlinebe tweenrabbit weightandincisor width(M.DelibesandJ.Cal derón,unpublishedwork). Teethwereassignedtooneofthe followingclasses: (1) kittens, rarely emerged from the burrow (<80g);(2)young,abletoleavetheburrow(80—300g);and(3) adult orsubadult(>300g).
Sincenodataexistonearthwormdistributionandabundance insouthernSpain,formalinsampling(Raw1959)wascarriedout intwenty-four0.6x0.6mquadratstoallowtheestimationofearth wormavailability(12quadratsinthedryhabitat,and12inthehu midone).Samplingwasundertakenintherainyseason.No direct estimatesofpreyavailability, otherthanofearthworms,wereob tained.Dataonthedistributionorabundance ofotherpreygroups arederived fromtheliteratureandfromtheobservationsofthe authors.
To allowanalysisoftemporalvariationsinfooditems,four seasonsweredefinedusingclimaticcriteria(Font1983):spring
(March-May), summer(June-August), autumn(September-No vember),andwinter(December-February).Seasonalfooddiversi tywasestimatedbytheShannonIndexandz2testswereusedto compare observedandexpected valuesof preyoccurrence be tweenthetwohabitattypes.
fromtheseaby alineoflagoonsandamobiledunesystem,and
fromtheriverbyextensivemarshes.Themarshandmost ofthela goonsdryoutbetweenlatespringandearlyautumn.Theclimate ismediterranean,withhumidinfluenceduetoprevailingoceanic winds.Inthestudyyeartheannualmeantemperaturewas16.5°C andtheannualrainfallwas 599mm.Theclimateismarkedlysea sonal,withmild,wetwinters,andhot,drysummers.Duringthe studyyear 73%oftherainfalloccurredinthemonthsbetweenOc tober andFebruary.
Xerophyticscrubland(Halimium,Cistus,GenistaandRosma rinus) coversmostofthe studyarea.Low-lyingmorehumidsites, interspersed within the dry scrubland, contain denser shrubs,
mainlyErica, Calluna, Ulex andRubus.Thesegeneraalsoappear inpatchesnearthelagoons.Therearescatteredcorkoaks(Que rcus suber),andsparsestandsofpine(Pinus pinea).Scrubland graduallyopensouttotheborderofthefloodareas(marshand la goons),viaamosaicofbracken, rushandpastures.Inland,dry scrublandhasbeenreplacedbyapineplantation.Thus,we distin guishedtwogeneraltypesofhabitat,accordingtothedegreeof soilmoisture:dry(xerophyticscrublandandpinestands)andhu mid(densescrublandandpastureland).Agricultureisabsentand humandisturbanceisminimal.Furtherdetailsonthestudyarea
Results
Foodspectrum
Thecategoriesofpreyfoundinbadgerfaecesarelisted inTable1.Insectsandlagomorphsarethemostcommon items,occurringin100%and63%ofthesamples, re spectively.Frogs,lizards,mushrooms,fruitsandsmall mammalsoccurinabout10%,whereastheremaining seventypesofpreyoccurin5% orlessofsamples.Esti matesofthebiomassconsumedyieldadifferentresult,
Table 1Prey categories found in badger faeces during a 12- monthperiodat Doflana,Spain
Percentage Percentage Correction Percentage occurrence dryweight factorbiomass
canbefoundinAllieretal.(1974).
Lagomorphs Smallmammals Birds
63.0144.944371.79
7.921.05220.85
1.880.40610.91
withlagomorphs (exclusivelyrabbits)beingby farthe dominantfoodsource(71%oftotalbiomass).Insectsac countforonly15%. Withthenotableexceptionofam phibians (5%), the remaining prey areof little signifi canceintermsofbiomass.
Alargeproportionoftherabbits(84.3%ofallindivid
uals)arekittens,and13.6%oftheyoungage-class.Only
7rabbitsoutof331wereofadultsize.Almosteveryspe
ciesofinsectivore,rodent,smalllizard,snakeand am phibianlivinginthestudyareawaseatenonsomeocca sion.BadgersalsofedonInsectaofat least21 different families. Reptile eggs included those of lizards, snakes andtortoises.Besidesrabbits,the otherspeciesthatwere eatenfrequentlywerePelobates cultripes(Amphibia), Gryllotalpagryllotalpa(Orthoptera),larvaeofthe beetle Melolontha papposa, andadults ofScaritesoccidentalis andGeotrupessericeus(Coleoptera).Bees’wax,beesand waspswerefoundinonly2.5%ofthesamples.Birds(Fu lica atra, Athene noctua, domestic hen) were probably takenascarrion.Thespeciesrichnessof birds, fungiand plantsintheDofianaareaispoorlyrepresentedinthediet ofthebadger.The major proportion of non-animal bio massiscontributed bythemushroom Rhizopogon rose olusandberriesofCoremaalbumandRubusulmifolius.
Noremainsofearthworms werefound.Thisisconsis
tent with their low abundance in the area: earthworms were absent from formalin samples from thescrubland and pineplantations(dryhabitats),whilstsamplesfrom pastures near lagoons and marsh borders (humid habi
47
Percentageof occurrence
Fig. 1Seasonal relative importance of food types eaten by badgers(1 rabbits, 2 amphibians,3 other vertebrate, 4 insect larvae,5insectimagos,6 fruitsandfungi)
Table2Seasonalvariationsinestimatedpercentagebiomassin gestedbybadgersatDoflana(nsamplesize).Diversitycalculated fromShannonIndex
(Fig.1),beingeatenfrequentlyandalwaysreachingmore
theadultrabbitsweretakeninearlyautumn.Smallsam- piesize(n=5)isprobablyresponsibleforthelackofrab bitremains inscats collected during February and No vember. Complementary foods vary greatly. In spring, insectsaccountforalmostallthebiomassotherthanthat of rabbits. In winter, fewer prey items arefound, with frogsandinsectsbeingalmosttheonlysecondaryfoods. Incontrast,insummerandautumn,whentheimportance ofrabbitsinthedietdecreases, alotofalternative prey aretaken.Theseresultsarereflectedinastrongnegative correlation between rabbit percentage biomass and monthly dietary diversity when rabbits are present (r=
—0.961,P<0.000, n=l0).UsingHutcheson’sstatistic(Zar
1984)wefoundthattrophicdiversitydifferedsignificant lybetweenpairsofseasons(t-test,P<0.001) withtheex ceptionofthewinter-springpair.
Diversityindex 0.342 0.560 0.705 0.295
Seasonal changes intheavailability ofyoung rabbits arerelatedtotheirtemporalbreedingpattern.Rabbitsare abundant inthestudyarea.Birthscanoccurthroughout the year (Valverde 1957, 1967), but are concentrated stronglyintheFebruarytoJuneperiod(DelibesandCal derón 1979; Soriguer 1984). These authors found little signof reproductiveactivityin rabbitfemalescollected duringtheothermonths(Fig.2).
Themonthly percentage ofrabbitbiomass inthediet of badgers and arabbit kitten availability index (mea
suredasthemonthlyproportion offemale rabbitsshow ingreproductive activity)shownosignificant correlation (r—O.009,P=O.978;seeFig.2).
Fig. 2Themonthlypercentagesof rabbitkittensas ingested biomass(linewithsolidcircles)comparedwiththepercentageof pregnantrabbitfemales,anestimateofkittenavailability.Rabbit datafromDoliana(1 year,broken line;Delibes andCalderón
1979),and westernSierra Morena, south-west Spain (2 years,
solidanddottedlines;Soriguer1984)
Morethan95%of biomassisderivedfromthreeprey categoriesinspringandwinter,sixin summer,andeight inautumn(Table2).Sixofthesemain foodsareeaten throughouttheyear. Aside from the relativetemporal consistencyofrabbitsinthedietofthebadgers,theim portanceofotherpreyvariesseasonally.
Preydistributionandthespatialvariation inthefoodof badgers
Amajor determinant of the local distribution of badger preyatDoñanaiswateravailability.Habitat-dependent differencesin rabbitlitteravailabilityhavealso been found: burrowsarediscontinuously distributed, aridcon centrateinmoist pasturesneartheborders ofmarshesand lagoons (Rogers andMyers 1979). Amphibians arealso linkedto moisthabitats,whilstmostreptiles(e.g. Psammodromus algirus or Acanthodactyluserythrurus) aremoreabundantondry,sandysubstrata.Therefore we havegroupedsamplesfromhumid(marshborderandla goons,n=135)anddry(pinesandscrubland,n=130)habi tats, and compared the number of occurrences of each preyclass.2testsshowsignificantdifferencesinthepro portionsofthemainfooditems:rabbitsareeatenmoreof teninwethabitats(P.<O.005),asareamphibia(P<O.005). However,a high proportion ofsamples (38%) from the dryhabitatsalsocontainrabbitremains.Bothinsectlarvae (P<O.OOl)andreptiles(P<O.05)occurmoreoftenthanex pectedindrysites.Occurrenceofinsectimagosandfungi wasnotrelatedtothishabitatclassification.
mammalsareeitherabsentinitsdiet(Ciampaliniand
Lovari1985)ortakenonlyinsmallquantities(Harris
1984;RodriguezandDelibes1992).Mammalsequalto orgreaterthantherabbitinsizeareofteneatenascarri on(KruukandParish1981;Kruuk1989).Incontrast, liverabbitkittensareeatenfrequentlyinthestudyarea,
andtodothisitisnecessarytodigthemfromthebur row.Thisbehaviourisconsistentwiththeobservationby Villafuerte(1994)thatattributed26%dugrabbitburrows inthestudyareatobadgerpredation.Thus,herewealso reportforthefirsttimebadgersactivelysearching for mammalianprey.
Thebadgerbehaves asarabbitspecialist,atleastdur ingtheyearofstudy.Rabbitbreedingalmostceasesbe tweensummerandearlyautumn(DelibesandCalderón
1979;RogersandMyers1979)when bothdroughtand hightemperaturesprecludethegrowthofgrass(Myers
1971).InsouthernSpain,rabbitsbreedinginsummer
haveonlybeenreportedinareasassociatedwithinigat
edcrops(SoriguerandMyers1986).Thus,at Doñana,it
ispossiblethatrabbitreproductioninthedryseason wouldonlyoccurnearareaswherewaterreservessus
taingreengrassontheirshores.Inspiteofthesespa tio-temporalfluctuationsinavailability,badgersshowa markedpreferenceforrabbitkittens,whichareeaten commonly,evenwhenandwherescarce.
Asidefromrabbitkittens,otherfooditemsseemto betakenaccordingtotheirseasonalabundance.Wegive
threeexamples:
1.Alltheadultrabbitremainsoccurredinearlyau
tumnsamples,whenmyxomatosisreachesits annual
peak.
2.Subterraneanlarvaearemainlyconsumedin spring, whentheiraccesibilityincreases(Pigozzi 1989).
3.Thebulkofsamplescontainingremainsofthefrog Pelobates cultripes werecollectedinsummer;during thisseason,afterreproduction, greatgroupsofPelobates bury themselvesinthesoilatdepthsoflessthan20cm (Valverde1967)andarethuseasilydugforbybadgers.
Thebadgerfeedingpatternischaracterized bya selectedpreyitembeingtakeninlargequantities,and severalsecondarypreyitemsbeingconsumedopportu
nistically,thuscompensating forfluctuationsinthe availabilityofthemainprey.Theconstantincreasein fooddiversityfromwintertoautumn,followingthean nualdecreaseinrabbitreproduction,isconsistentwith thisviewoftheroleofsecondarypreyitems.Itisthe samepatternfoundintheothertwostudiesreporting feedingspecializationby badgers(Kruukand Parish
1981;KruukanddeKock1981).
Somebadgertraits,likedentition,seemtobeadapted to anomnivorous(i.e.plantplusanimal)diet(Neal
1986).Thegutanatomyissuitableforprocessingvege
tablematter(Starketal,1987)but,asexpectedfora car
nivore,thisisaccomplishedinaninefficientway(Kruuk
Discussion
Thisisthefirststudytoreportamammalian species as the staple food of the European badger. In most areas
andParish1985).Furthermore,thebadgerisaquite heavymustelid,needingtoingestadailyquantityof foodestimatedcloseto500g(Henry1983).Thus,mor phologicalandphysiologicaltraitsmaycausebadgersto
needahighdailyfoodintake(over5%bodysize),pref erablyofanimalmatter,althoughitcanbereplaced bya greater quantity of vegetables. Such high food intake maybeeasily obtained byswitching feeding behaviour tothemostabundant preyineachseason;infact,thisis thepattern found inmany studies. This being so,what arethe conditionsthatmayfavourlocalfeedingspecial ization?Oneofthemmaybeintrinsic tothebadger,and theothertwomaybeintrinsictoitsprey.
1.Forthebadger,besidesmorphophysiological con straints, there isanadditional oneofabehavioural na ture: thespecies’ relative inability tohunt(Neal 1986),
showinga typicalforagingbehaviourclosetothatof harvestingby hervibores(Kruuk1989).Therefore,its potentialfoodspectrumisreducedto preysharingthe followingthreefeatures (Kruuk1989):(a)littleabilityto escape (inthepresent study,rabbitkittens areunableto leavetheirburrows, birdsweretakenascarrion,reptiles wereprobablyeatenduringtheirnocturnalperiodof in activity,andamphibians areeasytocatch);(b)smallsize (excludingcarrion,in Dofianapreyweightalwaysfalls below300g);and(c)abundance withinasmallarea(i.e. apatchydistribution) tobalancesmallpreysizeandhigh energyneeds.
2.Amongseveralequallyclumpedprey, profitability seemstobe animportantfactorexplainingpreference. Animal prey are preferable to vegetable material, the bulk of which remains largely undigested (Kruuk and Parish 1985; Neal 1986). Adecrease in the weight of badgersresultingfromthesubstitutionof barleyfor earthwormsintheirdiethas beendescribed(Kruukand Parish1983).Energycostsduetoforagingeffortinclude locomotion toforaging areasanddigging behaviour(the latter only for underground prey,like rabbit kittens or soillarvae).Locomotion costsarelikelytoberelatedto fooddispersion. Soclumpedfoodresources neartobad gerrestingsitesareexpected tobepreferred. Diggingis anexpensiveway toobtainfoodandwouldbeuneco nomical if the energy reward was insufficient (Pigozzi
1989).Theexcavationofa rabbitdenatDonanawould provide anaverage of320gofhighly profitable animal food(averagerabbitlittersize=4;DelibesandCalderón
1979); 60%oftheirestimated dailyrequirement (Henry
1983).Moreover,rabbitburrowstendtobeclumpednear humidsites,increasing thechance offinding morethan
oneinasingleboutofforaging, andthussatisfying the whole daily requirement. Results indicate that thedig
ging of rabbit burrows takes place troughout the year,
even when other prey (e.g. insects, frogs), which are
presentatground surface, areabundant.Thesoft,sandy soilinthestudyareafacilitates digging.
3. Besidesprofitability,preypredictability maybe necessaryforspecialized foraging. Thepredictable loca tionofrabbitdensonsandyridgesnearbothlagoonsand marshborders(RogersandMyers1979)wouldleadbad gers to specialize through both area-restricted foraging and through acquired memory ofthelocation ofrabbit dens,bothsuggestedas mechanismsforspatiallearning (Mellgren andRoper 1986). Individuals living nearhu
49
midsites probably alsogainenough experience tofind youngrabbitsinthedryseasonwhentheyarescarce.In dividualslivinginthedryhabitatsatDoflanahavefewer rabbit kittens available inthewetseason, andprobably veryfew,ornone,inthesummer-autumnperiod.Asthe importance ofrabbits inthedietisrather highthrough outtheyear,eveninthedryhabitat subsample, wepre dict that badgers living there will have greater ranges thatthoselivinginhumidhabitatsinordertoencompass somehumidpatcheswithintheirrange.An alternative prediction is that badgers living in dry habitats make longjourneystowardsareasof rabbitburrowrichness, particularlyinthedryseason.
Anyhow,badgerlocalspecializationisprobablyrevers ible(FoxandMorrow1981):reversal shouldoccurby switchingtootherpotentialfoodsifrabbitkittensbecame scarcedowntoathreshold equivalent tothenetenergy gainfrom opportunistic foraging. Thus, supporting Fox andMorrow’sconclusionsforothertaxa,onthescaleofa species’wholerange,feedinggeneralismmaybethesum oftemporary(reversible,asafunctionofthechangingop timalbehaviourwithtime)specialistpopulations.
Acknowledgements Thisworkhasbeensupportedbytheproject PB-87/0405oftheDirecciónGeneral deInvestigaciónCientIficay Técnica.Personal supporttoARhasbeenprovided bygrants from CSICandJunta deAndalucla.Acriticalreadingofthemanuscript waskindlymadebyH.Kruuk.WealsothankH.l.Griffithsfor commentsand forimprovingthe English.
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