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Polydomy : the organisation and adaptive function of complex
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Robinson, Elva Joan Hilda orcid.org/0000-0003-4914-9327 (2014) Polydomy : the
organisation and adaptive function of complex nest systems in ants. Current Opinion in
Insect Science. pp. 37-43.
https://doi.org/10.1016/j.cois.2014.09.002
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Polydomy:
the
organisation
and
adaptive
function
of
complex
nest
systems
in
ants
Elva
JH
Robinson
Manyantspeciesspreadtheircoloniesbetweenmultiple spatiallyseparatedbutsociallyconnectednests,a
phenomenonknownaspolydomy.Polydomousspeciesare ecologicallyandphylogeneticallydiverse,andoften economicallysignificantasinvasivepests.Benefitsof polydomymayincluderiskspreading,efficientresource exploitationandergonomicfactors.Verylittleisknownabout thecostsofpolydomy;facultativelypolydomousspeciesare goodcandidatesforidentifyingcosts.Analysingpolydomous colonystructureprovidesinsightsintowhichcostsandbenefits aredrivingthecolonyorganisation;forexample,a cross-speciesanalysisofinter-nesttrailnetworksshowsstructural featuresrelatedtolong-distancetransportefficiency.Deeper understandingofpolydomywillshedlightonkeyissuesin evolutionaryandbehaviouralecology,andalsobenefitboth conservationandpestcontrol.
Addresses
DepartmentofBiology,UniversityofYork,Heslington,YorkYO105DD, UK
Correspondingauthor:Robinson,ElvaJH([email protected])
CurrentOpinioninInsectScience2014,5:37–43
ThisreviewcomesfromathemedissueonSocialinsects:the internalrulesofantsocieties
EditedbyNahalieStroeytmeytandLaurentKeller
ForacompleteoverviewseetheIssueandtheEditorial
Availableonline16thSeptember2014
http://dx.doi.org/10.1016/j.cois.2014.09.002
2214-5745/#2014TheAuthors.PublishedbyElsevierInc.Thisisan openaccessarticleundertheCCBY-NC-SAlicense( http://creative-commons.org/licenses/by-nc-sa/3.0/).
Introduction:
what
is
polydomy,
and
why
is
it
important?
Theideaofanantcolonyasacooperativegroupofclosely relatedantsthatlivetogetherinasinglenestmayseem appealinglyobvious;however,manyantsdefythis para-digm for at least part of their life-cycle. Over 150 ant species, sofar,areknowntoinsteadspreadtheircolony acrossmultiplespatiallyseparatedbutsociallyconnected nests. Thisphenomenonis knownaspolydomy(Box 1,
Figure 1) and has evolved many times independently among theants—polydomous speciesare phylogeneti-cally and ecologically diverse. It is likelythat different evolutionary drivershavebeen responsiblefor different origins of polydomy, especially as polydomy occurs in both polygynous and monogynous species (Box 1) [1].
Theprevalenceofpolydomyposesasignificantchallenge tothetraditionalviewofasocialinsectcolonyasa‘factory withinafortress’[2].
Studies of theorganisationof behaviourin socialinsect coloniesareimportantinbehaviouralecology,explaining collectivebehaviourssuchasforaging,divisionoflabour andnestconstruction[3].However,themajorityofthese studies assume that acolony isoperating from asingle nest.Manyprocessesoperatedifferentlyunderthespatial structure provided by multiple nests,so studying these behaviours in the context of polydomy is important to complete our understanding. Taking polydomy into account is essential also for the definition of colony boundaries[1,4–9]andthisinturnisimportantforseveral reasons,bothfundamentalandapplied.Insocialinsects, suchasants,thecolonycanbeseenas thereproductive unit, and thus the unit on which natural selection is operating. To understand the evolutionary ecology of these species,we need to beabletodefineacolonyso weknowatwhatscaleswewouldexpecttosee coopera-tion, intra-colonyreproductive conflictand inter-colony competition.Forexample,inpopulationsthatarehighly polydomousorevenunicolonial(Box1,Figure1),genetic variation betweennests may beso low that individuals helping nestmates are no longer differentially helping theirkin. Without kin-selectionvia differentialbenefits torelatives,theselectionforworkertraitsispredictedto weaken, while selfish reproductive strategies will be selected for[10].Together,theseprocessesshould con-tributetomakingextremepolydomyanunstablestrategy overevolutionarytime[10].Overrathershortertimescales, whenindividualsfrompolydomousspeciesaresampled, knowingcolonyidentityisimportantsothat,inadditionto the nest, the polydomous colony can be included as a groupingfactor[11,12].Relyingonnestalonetoprovide independent replication may give pseudoreplicated or misleading results. For example, genetic differentiation betweennestsofthesamepolydomouscolonycanbelow [13]andifnestssuchastheseweresampledandassumedto be independent, then this could lead to artificially low valuesforwithin-nestrelatedness.
Polydomousspeciesareoftenhighlyecologically success-ful with far-reaching ecosystem impacts, some even becoming invasive pests [14,15
–18]. All of the ant species on the list of the world’s 100 worst invasive speciesarepolydomous[19].Polydomyposessignificant challenges to pest control because of the difficulties of treatingaspatially dispersedcolonythatcanrepopulate
an areafrom a single overlooked nest [20]. These pro-blemsalsoapplytoconservation,fortheoppositereason: anantpopulationmayseemhealthybecausemanynests arepresent,butthesemayrepresentaverysmallnumber ofactualcolonies, leadingto risksassociatedwith small effectivepopulationsize.
In conclusion, understanding polydomy is essential to understandingtheevolutionaryandbehaviouralecology ofants,and foreffectively and accuratelysampling and studying polydomous colonies. This review focuses on recentdevelopmentsinthestudyoftheecologicalcosts andbenefitsofpolydomy,andonhowpolydomouscolony structurerelatestofunction.
Benefits
of
polydomy
Polydomy allows colonies to create new nests without going throughthe high-risk bottleneck of single-queen nestfoundation.Localfoundation ofsociallyconnected nestsprovidesarelativelylow-riskwayofspreadingintoa new area. For polygynous species, this also offers a methodof colony reproduction,because abuddednest orgroupofnestscanlaterbecomesociallyseparatedfrom the‘parent’colonyandfunctionindependently.Indeed, polydomy is likely to have arisen from processes of incompletebuddingor nest migration.Otherecological factorsrelatedtopolydomy,whilenotnecessarilyhaving beendriversoftheevolutionaryoriginofpolydomy,may stillconfercurrentadaptivebenefits.Theseincluderisk spreading, foraging advantages and ergonomic benefits (Figure2).Below,someexamplesofrecentadvancesand gapsinourknowledgeregardingtheadaptivefunctionof polydomyinrelationtothesethreeareasarehighlighted.
Riskspreading
Itseemsintuitivelyobviousthatspreadingacolonyover multiplenestsitesmakesthe colonylessreliantonthe survival of any particular nest. This applies most obviouslytopolygynousspecies,buteveninmonogynous polydomousspecies,sexualbroodisoftentransportedto queenlessnests, making thesurvival of thequeenright nest less critical than it would otherwise be. This has obviouspotentialtobebeneficialifanestsuffers preda-tion or attack by conspecifics [21,22], or indeed pest control attempts [23–25]. However, clear evidence of the adaptive benefit of polydomy in these contexts is lacking. Multiplenests may also beuseful if local con-ditions change, because the inhabitants of a nest that becomesunsuitablecanrelocate[26]toothermore suc-cessful nests. An additional possible risk-spreading benefit to polydomy could be the potential to isolate pathogens or parasites by cutting off contact with an
38 Socialinsects:theinternalrulesofantsocieties
Figure 1
(a) (b) (c)
Current Opinion in Insect Science
Schematicrepresentationofdifferentformsofcolonyorganisation.=antnest;–=socialconnection:thesecouldbeinter-nesttrails,ormorediffuse
movementofindividualsbetweennests.(a)amulticolonialpopulationofmonodomousantcolonies;(b)amulticolonialpopulationofpolydomousant colonies;(c)aunicolonialpolydomousantpopulation.
Box1 Glossary
Monodomy:Anantcolonyishousedinasinglenest,thatis,the nesthousesallqueens,allbroodandatleastthemajorityofworkers (Figure1a).
Polydomy:Anantcolonysimultaneouslyoccupiesatleasttwo spatiallyseparatedbutsociallyconnectednests(Figure1b,c).
Nest:Anesthousesbothworkersandbrood,butnotnecessarilya queen[1].
Socialconnection:Sociallyconnectednestsshareorexchange resources,forexample,foodorworkers.
Unicoloniality:Apopulationofanantspeciesfunctionsasasingle largepolydomouscolony(Figure1c).
Multicoloniality:Apopulationofanantspeciesconsistsofcolonies (monodomousorpolydomous)thatfunctionindependentlyand usuallyinteractagonistically(Figure1a,b).
Polygyny:Atleasttworeproductivelyactivequeensarepresentina colony.Thecolonymaybemonodomousorpolydomous:ifitis polydomous,thequeensmayormaynotbedispersedbetween multiplenests.
Monogyny:Onlyonereproductivelyactivequeenispresentina colony;thecolonymaybemonodomousorpolydomous.
infected nest—although having well-connected nests could have the oppositeeffect of allowing an infection to spread rapidly throughout a colony or unicolonial population [27].
Resourcediscoveryandexploitation
Ant colonies are generally thought of as central-place foragers, but polydomous colonies can have several ‘centres’ towhich foodisretrieved. Theoreticalmodels predictthatthismayreducethetimetodiscovernewfood sources,becauseresourcesaremorelikelytobeclosetoa nestfromwhichforagersaresearching[28,29].However, iflarge-scalerecruitmentisrequiredtoexploitaresource,
thenmodellingpredictsthatpolydomouscoloniesmight be expected to lose out, because their population of potential recruits is dispersed [29]. This cost can be reduced by involving multiplenests in the recruitment process [12,30]or byrecruitingfrom persistentforaging trails [31]. Moderatepolydomy could be aform of dis-covery-dominance trade-off, in which having dispersed nests improves a colony’sability to find new resources (because scouts are spread relatively evenly over the foraging area)but nestsarestilllargeenoughtoprovide enough workers to dominate a resource. As well as decreasing discovery time, polydomy can also decrease the initial food retrieval time. Modelling predicts that Figure2
Potential benefits Potential costs
Monodomy
Single large nest promotes homeostasis & increases chances of survival
• •
•
•
•
•
• Costs of inter-nest transport of e.g. food, workers Spread of infection between nests Reduced long-range dispersal as a result of budding, leading to inbreeding and loss of genetic diversity Per capita productivity is lower in larger nests Limitations on colony growth from constraints on e.g. nest size or foraging trail length
Large single nest attracts predators and parasites
•
•
•
•
•
•
• Reduced variance in inter-nest success by transfer of e.g. food, workers Inter-nest movement extends communication and recruitment for e.g. defence or foraging Relatively even distribution of foragers across home range
Potential to increase colony size irrespective of nest-level constraints Risk spreading between nests
Low-risk local dispersal by budding
Concentrating colony population at one site promotes communication and recruitment for e.g. defence or foraging
Monodomy Monodomy
Polydomy: multiple nests
Polydomy: multiple nests
Polydomy: inter-nest traffic Polydomy: inter-nest traffic
Polydomy
Current Opinion in Insect Science
Summaryofpotentialcostsandbenefitsofdifferentnestingstrategies.Thecostsandbenefitsofpolydomyareseparatedintothosearisingfrom splittingthecolonyintomultiplenestsandthoseassociatedwithinter-nesttraffic,whetherviatrailsormorediffusemovementofindividuals.The potentialcostandbenefitsdonotapplyequallytoallantspecies.
longforagingtrailscanbealimitationoncolonygrowthin monodomousspecies[32];becoming polydomouscould be important to reduce this cost. In heterogeneous environments,polydomouscoloniescanthenredistribute foodfromsuccessfullyforagingneststoothernests[33].
This ability to transfer colony members and food (or specificnutrients)betweennests,andthespatialdispersal of the foraging workforce, should reduce variance in foragingsuccess forpolydomouscolonies.
Nestsizeandcolonysizeergonomics
Formonodomouscolonies, colonysize(numberof indi-viduals) and nest size (physical volume occupied) are closely related. Polydomy allows colonies to break out oftheconfinesofthisrelationship,makingitpossibleto increasecolonysize,evenwhenthereareconstraintson individual nest size. Constraints may be physical, for example,in cavity-dwellingTemnothoraxrugulatus, colo-niesbecomepolydomouswhenthedensityofindividuals inthenestcavityishigh[34],suggestingthisspeciesuses polydomytorespondtocrowdinginnestsitesthatcannot beenlarged.Another cavity-dwellingspecies,the‘turtle ant’Cephalotesrohweri,alsobecomespolydomouswhenit outgrowsitsfirstcavity.Thisspecieshasamorphological castespecialisedto‘plug’anddefendtheentrancestothe cavities.Thedistributionofthesespecialistsamongthe nests of thecolonybalances theirtwo roles:continuing defence of occupied cavities and staking claim to new cavities [35]. In addition to physical constraints, nest
sizemayalsobeconstrainediflargernestsareattractiveto parasitesorpredators[22]orbyergonomicfactors,suchas decreasingproductivitypercapitaasworkernumberina nest increases. This counter-intuitive relationship is known as ‘Michener’s paradox’,and may beaccounted forbyresourcelimitationandincreasednumberof inac-tiveworkersinlargernests[36].Thereispotentiallya
trade-off between the decrease in productivity with increasingworker number, andthebenefitof increased homeostasis (more predictable foraging success, greater chance of survival) as worker number increases [36].
Polydomymayprovideamiddleground,wherenestsare smallenoughtokeepproductivityhigh,andflexibilityin movementoffoodandworkersbetweennestsimproves predictabilityofforagingsuccessandsurvival.However, amongwoodants(Formicarufagroup)atasinglesitein Finland, polydomous species (F. aquilonia and F. poly-ctena)actuallyhadlargerneststhanlocallymonodomous species(F.lugubris,F.rufa)suggestingthattherewasno trade-offbetweennest sizeandnestnumber, atleastat the species level [37]. More data are needed on how ergonomic pressures influence polydomy, and to what extent polydomy can be a way of allowing colonies to growbeyondnestsizeconstraints.
Costs
of
polydomy
Whilepolydomyisawidespreadsocialstructureinants, monodomyappearstobeevenmorecommon,somuchso
thatitisoftenthedefaultexpectationof thosestudying ants.Thismeansthat,whilesomeefforthasbeenmadeto explainthebenefitsofpolydomy,theequallyimportant benefitsofmonodomy(ortherelativecostsofpolydomy) havereceivedlessattention(Figure2).Onepotentialcost of polydomy is that by dispersing resources across multiplenests, they maybecome spread too thinly, for example,dispersedforagersreducingexploitationability [29].Therearealsoenergeticcostsofinter-nesttransport andtheriskofcostlyresourcelossduringtransportation, particularlylossofbroodbeingtransportedtoqueenless nestsinmonogynouspolydomouscolonies(Box1)[38]. Genetic costs may also occur as a result of using local budding as the main means of colony reproduction: specifically, long-distance dispersal ability may be reduced and inbreeding increased, leading to a loss of geneticdiversity locally [39]. Manyspecies of ants are facultatively polydomous, becoming polydomous (or increasing thelevel of polydomy) only at certain times of yearor under certain conditions[40,41
–44].These aregoodcandidatespeciesfor identifyingthecostsand benefits drivingthechoice of onenestingstrategy over another.
Organisational
structure
of
polydomous
colonies
Polydomouscoloniesrangefromthesimplest structure, twoconnectednests,tocomplexesofmanythousandsof nests [45,46]. The social connections between polydo-mouscoloniescanbebroadlycategorisedintothreetypes: first,sharingresources(e.g.food,nestmaterial);second, movement of colony members (e.g. brood, queens, workers, particular task groups); third, sharing infor-mation (e.g. recruiting ants from other nests to food; passingalarmsignals betweennests).Someinteractions fallintomorethanonecategory,forexamplemovement ofrepleteworkerswouldtransferbothfoodresourcesand colony members. These interactions can result in cooperationbetweennests,for example,in exploitinga largefoodresource,andalso potentiallyin competition, for example, over a limited workforce. Another con-sequenceisthatnest-leveldivisionoflabourmayoccur. For example, particular nests within the colony may specialise on rearing brood of a particular stage as a consequenceoflocalthermalconditions[47]oron fora-gingdueto thelocationof stablefoodresources[33].
Even among closely located nests, food sharing is not uniform, indicating that there is structure to inter-nest connections[48].Inter-nestconnectionsareoftenvisible astrails:theseconnectingtrailsare sometimesmapped, thoughrarelyanalysed(butsee[33,49]).Analysingthe
trailnetworkswithinpolydomous coloniesisimportant, becauseitprovidesinsightsinto whichof thecostsand benefitssuggestedabovearedrivingthecolonystructure. Forexample,inthecontextofrisk-spreading,minimising the impact of a predator could suggest dense network
40 Socialinsects:theinternalrulesofantsocieties
connectionstoneighbours,sothatrapidevacuationfroma nest under attack (or rapid recruitment of additional defenders) is possible (Figure 3a). Conversely, if the pressuredrivingpolydomouscolonystructureisto limit thespreadofinfection,aformationclosertotheminimum spanningtree(MST)whichusesthefewestpossibletrails to connect all nests would be expected, as this would allowaninfectednesttobeeasilyisolatedfromthecolony (Figure3b).Analysisofcolonymapsfromsixantspecies showed that colonies generally occupy an intermediate position: whilenothighlydensenetworks,theydohave more trailsthantheMSTwouldpredict,indicatingthat trailcosts(e.g.infectionspread,trailmaintenance)arenot themaindriversofcolonystructure[49].Thepresence
ofextratrailssuggeststhatrobustnessincolonycohesion maybeimportant.Inaddition,inthenetworksstudied, nests did notconnect only to neighbours, but also fre-quentlytodistantnests(Figure3c).Theselong-distance connections greatly increased theefficiencyof the net-work, suggesting that effective information transfer or resourcetransportationhasinfluencedthestructure[49].
Asimilarpatternoffoodsharingbetweendistantnestsis seenin Lasiusneoniger [50].
Ifresource exploitationbenefitsare significantfor poly-domous colonies, then trail structure and nest location should be influenced by food patches, particularly as polydomy is generally associated with the exploitation of clumpedresources [51]. Thereis some evidence for this:inFormicaexsecta,inter-nesttrailsareformed prim-arilyasaside-effectofsharedfoodresources[52],andina
polydomousF.lugubrispopulation,trailsconnectingnests that differed more in their amount of foraging were stronger thantrailsbetweennestswithmoreequal fora-ging,suggestinganimportantfood-redistributionroleto theinterconnection structure [33].Differences in
fora-gingprovisionbetweennestsofthesamecolonycanhave directfitnessimplications,forexampleaffecting ovipos-ition rate in the Argentine ant Linepithema humile [42]. Morework isneededto determinehowthestructureof polydomouscoloniesdevelopsandchangesinresponseto environmental challenges.
Future
directions
Polydomyisanintriguingstrategyofcolonyorganisation, with many possible benefits including risk-spreading, improvedforagingorganisationandreducingtheimpact ofnest-sizelimitationsoncolonygrowth.Therearealso potential coststopolydomy,whichhavebeenrelatively under studied. Studying the structure of polydomous colonies helps to demonstrate what benefits drive the organisationof theconnectionsbetweennests.Areasof interestfor futurestudyinclude:
- Towhatextentisthelevelofselectioninapolydomous species the individual nest, and to what extent the wholecolony?
- Howdoespolydomousorganisationinantscompareto analogous systems in other groups, for example, polydomous termites, or bees and wasps with very highlevelsofinter-nest movement[53,54]?
Figure3
(a) (b) (c)
Current Opinion in Insect Science
Schematicrepresentationofdifferentformsofconnectionstructurewithinahypotheticalpolydomouscolony.=antnest,linesindicateinter-nest
trails.(a)Nestsaredenselyconnected;(b)nestsareconnectedbytheminimumnumberoftrailspossible;(c)thisintermediateformisthemost commonstructureamongrealpolydomouscolonies[49].Nestsconnectmostlytonearestneighbours,butsomelong-distanceconnectionsare
presentalso.
- Dopolydomouscoloniesgainfitnessbenefitsfrom risk-spreading, improved foragingor other ecological con-sequences ofpolydomy?
- Dothebenefitsofrapidresourcediscoveryandretrieval offsetthecostsof laterresourceredistribution? - Howimportantisavoidingergonomicconstraints,such
as decreasing per capita productivity, in promoting polydomy?
- How does polydomy impact onreproductive conflict, particularlyinmonogynouspolydomousspecies? - How does individual-leveltask specialisation interact
withdivisionof labouratthenest level?
- How do polydomous colonies use their nesting structure torespond tolocalchange?
- What drives transitions/regional patterns in faculta-tivelypolydomousspecies?
- How consistent across taxa and environments are organisational featuresof polydomy?
Althoughtherehavebeen manyrecentadditionstoour knowledgeofpolydomy,therearestilllargeand numer-ousgaps inwhatweknowabouthowandwhycolonies distribute themselves between multiple nests. Deeper understanding of this social organisation strategy will shedlightonkeyissuesinevolutionaryandbehavioural ecology,andalso beof benefittoboththeconservation andthecontrolofpolydomous antspecies.
Acknowledgements
EJHRacknowledgesfundingfromTheRoyalSociety.Thefunderhadno roleintheresearchorpreparationofthearticle.EJHRthanksPhillip Buckham-Bonnett,Yi-HueiChen,SamuelEllis,LaurentKeller,Duncan ProcterandNathalieStroeymeytforhelpfulcommentsonearlierversionsof themanuscript.
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