Social organization of the colonial spider Leucauge sp. in the Neotropics: vertical stratification within colonies

2010·6.3 MB·English

by  Maxence Salomon

I:'
2010.TheJournalofArachnology38:446-451
Social organization ofthe colonial spider Leucauge sp. in the Neotropics:
vertical stratification within colonies
MaxenceSalomon, CarlySponarski',AllenLarocque-andLeticiaAviles: Department ofZoologyand Biodiversity
ResearchCentre, UniversityofBritish Columbia, Vancouver, B. C. V6T 1Z4, Canada. E-mail:
[email protected]
Abstract. Afirststeptoward understandingwhysocialityhasevolvedinaparticulartaxonomicgroupistoestablish
comparison points by studying the organization ofdifferent social systems. Weexamined the social organization and
spatialdistributionofindividualsincoloniesoftheundescribedcolonialspiderLeucaugesp.(Araneae:Tetragnathidae).
Thesocialorganizationofthisspecieswastypicalofacolonialspecies,withspidersmaintainingindividualterritories(orb
webs)withinascaffoldingofsharedsupportlinesmaintainedbythegroup.Furthermore,weobservedasize-dependent
verticalstratificationofspiderswithincolonies,withlargespidersoccupyingthehighestpositions,followedbymedium,
extra-smallandsmallindividuals,aspacingpatternthatwasconsistentacrosscoloniesofallsizes.Spiderscapturedand
consumedpreyindividuallyanddisplayedterritorialbehaviorsinvolvingwebdefense.Thisstudyprovidesanewexample
of a colonial spider species that shows a distinctive within-group spatial distribution. We discuss possible reasons
underlyingthisspecies’spatialarrangementinthecontextofsocialevolution.
Keywords: Aggregation,groupliving,orbweb,sociality,spatialdistribution
Knowledge about the social organization of particular dependlargelyonthetradeoffsbetweenthebenefitsandcosts
species provides key insight into the mechanisms and of communal living. In colonial spiders, benefits of group
cpornodvietniontosibnevoglvoeoddinmotdheelevoslyusttieomnsofforsoctihaelitsyt.uSdpyidoefrssohcaivael slihvairnegdinscillukdefrdaecmreewaosrekd p(eUretczapi&ta Hsiilekbeirnve1s9t9m7e)n,tidnuceretaoseda ii
evolution because they encompass a large range of social accessibilitytoareasofhighpreyavailability(e.g.,openspace
phenotypes (see Aviles 1997; Uetz & Hieber 1997; Lubin & over bodies ofwater) otherwise out ofthe reach ofsolitary
i
Bilde2007forclassicandrecentreviews).Thefewspeciesthat spiders (Buskirk 1975a; Smith 1983), enhanced predator
expresssocialtendenciesfallintotwobroadcategoriesdefined warning(Uetzetal. 2002)andincreasedpreycapturesuccess ^
b‘cyoloinnidailv’idaunadls‘’coloepveerlatiovfe’co(Aovpielreasti1o9n97;aUnedtzg&rouHpiebsetrru1c9t9u7r)e.: dprueeytotothe‘ripcroocxhiemt’ityfrboemtwoeneeniwnedibvidtoualthweebnsextthat(Umeatzyc1a9u8s9;e 1.:
Colonial(orterritorialpermanent-social)speciesaregenerally Whitehouse&Lubin2005).However,colonialityalsoinvolves
orb-weaving spiders that spin individual webs and form costs,suchasincreasedvulnerabilitytopredatorsorparasites
permanentgroupswithlowdispersalrates.Acoloniallifestyle and competition for local resources including food and web
ischaracterizedbythegroupingofindividualwebsthatserve space(Buskirk 1975a, b; Uetz&Hieber 1997; Rayor&Uetz
as foraging or multi-purpose territories depending on the 2000).
species, and which are usually maintained by single individ- In this study, we document the colonial structure of a
uals. Group membersengage in individual activities on their recently discovered and undescribed neotropical spider,
webs (e.g., foraging, brood rearing), and cooperation is Leucauge sp. (referred to as Plesiometa sp. in Aviles et al.
usually limited to the maintenance ofshared framework silk 2001). This orb-weaving species has previously been catego- <
thatjoinsthedifferentwebs(butseeFernandezCampon2007 rizedas‘colonial’byAvilesetal.(2001),basedonlimiteddata i
foranexampleofcooperativeforaging). Conversely,cooper- aboutitssocialorganization.Toaddressthisshortcoming,we )
ative (or non-territorial permanent social) spiders form examined the social organization of Leucauge sp. colonies, I
permanent groups on communal webs without any spatial focusing on group composition, colony architecture and the 1
separationbetweengroupmembers,andindividualscooperate social dynamics involved in territory maintenance and *1
in various activities such as prey capture, web maintenance foraging. Spatial structuring within colonies may reflect a b
and parental care. Although both social structures have need for individuals to maximize resource acquisition and
evolvedindependentlymultipletimes,thesetwoorganization- survival in a group-living situation where competition and
aslocTihaslecihteysomc(eiLsaulbironerpg&raensiBezinalttdieod2ni0s0ot7if)n.catgreovuolputoifonianrdyividpuaatlhswamyasy bteo ppinrrdeeildviaimtdiiunoaanlrsypwroiebtsshseiurnrveactomilooannyisessi,umgpwgoeessetteisnctgoendasttsrhpaeaitnhitayslp.oatBrharesaseinsdgetmohenanttotuhoref , (ji
j
characterized by various attributes, such as spatial arrange- distribution of Leucauge sp. individuals within a colony is p
ment, temporal pattern, behavioral interactions and genetic non-randomwithrespecttospidersize,aproxyforageclass.
relationships. The characteristics of these social attributes METHODS
Currentaddresses: 'FacultyofEnvironmental Design, Universityof Studyarea.—Weconductedthisstudyin lateAugust2005 .jj.l
Calgary,Calgary,A.B.T2N 1N4,Canada;"DepartmentofBiology, intheJatunSachaBiologicalReserve(01°04'S,n°36’W,elev.
McGillUniversity,Montreal,Q.C. H3A IBl,Canada. 400-440m), in theNapo Provinceofeastern Ecuadorin the r
446
I
SALOMONETAL.—COLONIALITYINLEUCAUGESP.SPIDERS 447
—
Figure 1. a)Leiicaiigesp.aboveastreaminthelowlandrainforestofEcuador:severalvisibleorbwebs(O)arejoinedtogetherbyframework
silk(F)andanchoredtothevegetationwithlongsupportlines(S).b)Adultmaleandc)adultfemaleLeucaugesp.spideronanorbweb(scale
bars= 10mm).Photocredits:a)andc)L.Aviles;b)A.Larocque.
Amazonbasin.Thereservecomprises2200haof70%primary historyinspiders(Jakobetal. 1996).Wegroupedspidersinto
and30%secondarytropicalrainforestinatransitionalregion foursizeclassesbasedontheoveralldistributionofobserved
between the lower Andean slopes and the Amazonian bodylengths: large(‘L’, averagelength: 11-12mm), medium
lowSltaunddys(sJpaectiuens.S—acThhaeFoournbd-awteiaovnin2g009s)p.ider Leucauge sp. 4(‘Mm’m,)7.-9Lmsmp)i,derssmawller(e‘S’a,du5l-t6fmemma)leasndanedxtrmaa-lsemsa,llM(‘XsSp’i,de3r-s
(Araneae: Tetragnathidae) is a territorial permanent-social weresub-adultsand both S and XS spiderswerejuveniles of
speciesfoundinneotropicalrainforests.Thisspecieswasfirst variousinstars.Wemeasuredaspider’sheightfromtheground
characterized in Ecuador by Aviles et al. (2001) and bears asthedistancefromthesurfaceofabodyofwaterdirectlybelow
significant morphological resemblance to Leucauge argyra a colony to the center ofthe spider’s body. Spider location
(Walckenaer 1841) (= Plesiometa argyra: Platnick 2009). withinthecolonywascategorizedason(1)anorb,(2)asupport
Leucaugesp. spidersareblack withaprominent whitepatch strandwithinthesilkframework,(3)adraglineproducedbythe
ontheabdomenoutlinedwithsilverbandsandredcoloringon spiderandattachedtothewebcomplexor(4)plantsubstrateat
theirprosoma(Fig. lb,c).Theyspinindividualorbwebsthat theperipheryofacolony(usuallytheundersideofaleaO-We
mayreachcloseto 1 mindiameterand areusuallyfound in collectedthesedataondayswithoutrain.
riparianhabitatsoftherainforestabovecreeksorotherbodies For analysis, we first checked the data for normality and
ofwaterinside the forest (personal observation; Avileset al. heteroscedasticity and applied transformations where appro-
2001). Colonies consist of clusters of individual orb webs priate.Weexcludedthreecolonieswithfewerthanfivespiders
joinedtogetherthroughaframeworkofsharedsilkscaffolding from the analyses (see Fig. 2). To examinedifferences in the
thatisanchoredtothevegetationonthebanksofa bodyof distribution of spider sizes among colonies we used a log-
waterwith thick silk threads (Fig. la). Their phenology and likelihoodratiotestwithWilliam’scorrectionforsmallsample
dispersal patterns are unknown, although we have observed sizes (Sokal & Rohlf 1995). We used a logistic regression
spiDdaetrsaocfolallelctaigoencalnadsseasnailnysAius.g—ustT.his study consisted oftwo dmiosdterlibuttioonsteswtitthhien ceoflfeocntieso.fWceoloansysesssiezde tohne rsepliadteironsshiizpe
parts:apopulationsurveyofLeucaugesp.colonies(n-22)at betweenaspider’ssizeanditslocationwithinacolonyusinga
thestudysiteandbehavioralobservationsofasinglecolony. log-likelihoodratiotestwithWilliam’scorrection. Finally,we
Population survey: We collected the following population used ageneral linearmodel (GLM) toexaminetheeffectsof
dataovertwodays:(1)numberofspiderspercolonyand,for colony size and spider size on a spider’s height above the
each spider therein, (2) spider body size, (3) height of the ground(log-transformed), usingaveragesforeachspidersize
spider from the ground and (4) location within the colony. classwithineachcolonyto avoidpseudoreplication. Wealso
Spiderbodysize, measuredwith a ruleras total bodylength assessedtherelationshipbetweencolonysizeandtheaverage
(distance from the front of the prosoma to the tip of the heightfromtheground(log-transformed)ofallspiderswithin
abdomen), reflects both developmental stage and feeding acolonyusingaGLM and Spearman’srankcorrelation.
^
448 THEJOURNALOFARACHNOLOGY
i
1.6-
(
Numberofspiderspercolony
colFoingieusr)e.2T.h—eDpilsatirnibveurttiiocnalolfineLeiuncdaiciaitgeesstph.emceodlionayncsoilzeosny(«siz=ean2d2 0^ X,S S^ IV^l L
thedashedverticallinethemeancolonysize. Spidersize
—
Figure 3. DistributionofLeucaugesp.spiders’heightsfromthe
stuBdeyhacvoinosriaslteodbsoefrvtahteioonbss:erTvhaetiobnehoafviaorsailnglceomcpoloonneyntforof10thhe sgmraoluln;dS(i=.e.,smtahlel;waMter=sumrefadcieu)ma;sLaf=unclatrigoen).ofBobxopdlyotssizesh(oXwSm=edexitarnas-
per day (0700-1700 hours) on two consecutive days. Our (thicklines),means(filledsquares),25thand75thpercentiles(bottom I
purpose was to record foraging activities and territorial andtopofboxes)and 10thand90thpercentiles(capoflowerand
interactions among colony members. The focal colony was upperwhiskers). Lettersinsidetheboxesdenotedifferencesbetween
chosenforeaseofaccess,mediancolonysizeandthepresence body-size classes based on Bonferroni-adjusted pairwise contrasts
ofall age classes (21 adults, subadults andjuveniles). Itwas (seeResults).
locatedon a sunny tree fall overa small creek. Weobserved
thecolonycontinuouslyover10h,andforeachindividualweb tion).Individualsmaintainedindividualterritories(orbwebs),
inthecolonywerecorded(1)allsuccessfulpreycaptureevents although they were commonly seen moving between webs
andtheidentityofthespider(s)involvedinacaptureand(2) whiletravellingtootherpartsofthewebcomplex,andsome
thenumberofterritorialinteractionsandtheidentityofboth spiders did not own a web. When disturbed, spiders would
the intruding and resident spider. Wecontinuously observed typically flee along shared support silk threads to common
interactionsbetweenspidersuntiltheyledtoaclearoutcome retreatsin thevegetation at the end ofthe anchorpoints to
(e.g.,stayversusretreat).Toincreaseoursamplesize,wealso the substrate. Web owners regularly maintained their orb
monitoredpreycaptureatanothercolonyofsimilarsize. We webs (i.e., as the need arose), and both spiders that owned
estimated prey size (total body length) visually and recorded webs and web-less spiders maintained the support silk
the prey’s taxonomic order. Prey biomass (dry weight) was framework. Web building was most common in the early
calculated from total body length using insect-order specific morningand evening. —
regressionmodelsdevelopedbySage(1982). Foranalysis,we Spatialarrangementwithincolonies. Therewassignificant
combinedpreysizeandbiomassdatafrombothcolonies. heterogeneity acrosscolonies in the proportion ofspiders of
RESULTS — dG^iafdfjer=en7t8.s3i9z,ePcla=ss0e.s02(,ladrfge=, 5m4e)d.iHuomw,evsemra,llthaenddisetxrtirbau-tsimoanllo;f
Social organization and colony architecture. Colonies spidersizesacrosscolonieswasindependentofcolonysize(x^
ranged in size from 2-27 spiders, with a median group size = 6.43, df = 3, P = 0.09) except that extra-small spiders
of19(Fig. 2).Mostcoloniescontainedmultipleadultfemales, tendedtobemorecommonthansmallonesinlargecolonies
adult males, sub-adults and juveniles. A colony typically (X^ = 5.67, P = 0.02). Large-andmedium-sizedspiderswere
consistedofacollectionofindividualorbwebsarrangedina more likely to occupy an orb, and small and extra-small
three-dimensional pattern and connected by a framework of spiders were more likely to be found on the framework, a
silk forming a web complex (Fig. la). Orb webs were draglineortheundersideofperipheralleaves(Gadj = 120.58,
organized intomultiplenon-horizontal planesorientedeither df= 9, P< 0.0001).
at the same angle or at different angles from each other A spider’s vertical position within a colonywas positively
(differences in orientation < 90°). Neighboring webs were correlated with its size class = 22.54, P < 0.0001;
closelyarrayedand oftenfaced similardirections(somewere Fig. 3). Large spiders assumed the highest positions in a
as close as 10 cm apart). The horizontal dimensions of a colony(meanheight±SD: 1.10±0.35m,wl= 108),followed
colonialwebcomplexwere usuallyproportionaltothewidth by medium-sized spiders (0.76 ± 0.26 m, «m = 70). Small
and topology of the body ofwater above which they were spiders occurred closest to the bottom of the web complex
placedandrangedfrom0.5-3m.Webcomplexeshadadense (0.43 ±0.22m,«s=73)whereasextra-smallspidersclustered
three-dimensionalcorewith flattenededgesconnectingtothe in betweenmediumandsmallspiders(0.63 ± 0.28 m,Hxs —
vegetationatthewater’sedge(Fig. la),andcolonyheightwas 114),apparentlylackingindividualorbwebs.Therefore,from
proportional to the number ofspiders (see ‘Spatial arrange- highest to lowest, the spiders from each size class were
mentwithincolonies’). distributedasL> M > XS> Swithinacolony,withadults
Web size (i.e. orb diameter) scaled positively with spider closer to the top andjuveniles closer to bottom ofthe web
size: larger spiders occupied larger webs (personal observa- complex. Bonferroni-adjusted contrasts revealed significant
1
SALOMONETAL.—COLONIALITYINLEUCAUGESP.SPIDERS 449
pairwisedifferencesbetweeneachsizeclass{P<0.05)except (a)Preysize
between XS and S (P = 0.38; see Fig. 3). The correlation 1-
betweenheightfromthegroundandspiderbodysizeheldfor 0.8
colonies of all sizes as there was no significant interaction
between spidersizeandcolonysize(^3,68 = 0.94, P = 0.43). 0,6
Furthermore,theaverageheightatwhichspidersofeachsize 0.4
class occurred across colonies did not vary with colony size 0.2H
(F,68=2.82,P=0.10).However,theoverallheightfromthe 0
(g^ro1,u17nd=of5.s2p5i,derPs i=n e0a.c0h3),colsoungygesitnicrnegastehdatwictohlocnoileosnygrsiozwe Bo6dylen8gth (m10m) 16
vertically. This relationship was stronger when the three
tshmeaTlealrnerasilttyocsroiilsalo(nrib^eesh=awv0ii.to7hr1.,f—enwWe=re2t2ho,abnsPefri=vvee0ds.p02i0d00e2r)as.ttweemrpetsincblyudeeidghitn q0Q0I. 0.81--(b)Preybiomass
differentweb-lessspiders todisplaceweb-holdingindividuals 0.6-
(“attacks”)onsixdifferentwebswithinthefocalcolony.Most
oftheseattacks(16/20,80%)tookplaceonwebslocatedinthe 0.4-
center of the web complex and involved adult spiders. 0.2-
Responses to attacks followed an escalating pattern of 0-
a1g9o7n5ibs;tiHcobdegheav&iorUettyzpic1a9l9l5y).seUepnoinnacpolpornoiaaclhspbeyciaens i(nBtursukdierrk, 0 2 4 6 8 D1r0y1m2ass14(m1g6) 18 20 22 24 26
resident spiders positioned at the hub ofthe orb web would —
typically orient towards the intruder. The resident spider Figure 4. DistributionofpreycapturedbyLeiicaiigesp.spiders
would then either contract all eight legs simultaneously, intermsofa)sizeandb)biomass.
resultingina rhythmicpulsinglasting 5-10s that forced the five different spiders captured one prey each and six spiders
intruder to halt and brace itself, or pluck the web by captured two to four prey each. Spiders occupying webs
rfoerpceeadtedtlhyejienrtkriundgerwetobbrraadciei iwtistelhft(hoen ftrwoontoclecgassitohnastjearlksos lporceaytecdapitnurtehse,ceanntdertohfosaecaotlotnhyespeecruirpehder1y1hoaudto1f22o3u(t47o.f8%23)
caused intruding spiders to fall out of the orb web). If (52.2%)preycaptures.Weonceobservedaspidersuccessfully
intruderspersistedandapproachedcloser,theresidentspider stealing a prey item captured by another spider; no other
winoualdonreu-sohn-oountetoenfcacoeunttheerinltarsutdinegraftrothme<per1ipshetory3o^ftsh,ewwietbh conflictoverpreybetweenspiderswasobserved.
legsandpedipalpsentangled in ablurofactivity. Aftersuch DISCUSSION
anencounter,theloserwouldretreatashortdistanceandthe Thisstudypresentsthefirstempiricalevidenceofacolonial
victorwouldscrambletothehuboftheorb. Residentspiders social organization in the recently discovered neotropical
most often won these encounters, repelling 85% (17/20) of spiderLeucaugesp. (Avileset al. 2001). Incolonies from the
attackers and maintaining their ownership ofa web. Spiders lowland rainforestofEcuador, spidersmaintained individual
not occupying orbs were generally tolerant ofeach other as orb web territories within a framework of shared silk and
closeapproachandeventouchingwasobservedonframework engaged in individual prey capture. Our population survey
linesofthewebcomp—lexwithoutanyagonisticinteractions. showed that the spatial distribution of individuals within
Foraging behavior. In the focal colony, we observed 23 coloniesisverticallystratified,supportingourhypothesisofa
preycaptureeventsby 11 differentspiderson 10differentorb non-random spatial distribution. Colonies were vertically
webs.Inthesecondcolonyweobservedanadditional23prey stratified according to spider size so that large spiders
captureevents. Mostpreyenteredthewebcomplexfromthe positionedthemselvesclosesttothetop,medium-sizedspiders
side; a few entered from the bottom. Spiders captured and werefoundbelowlargeones,andsmallspidersoccupiedwebs
consumed prey individually without cooperating. In both that were closest to the bottom of the colony. Extra-small
colonies, preyweremostlydipteranswitha fewhymenopter- spidersoccurredwithintheverticalrangeofsmallandmedium
ans and lepidopterans, and ranged from 1-15 mm in length spiders,astheyusuallydidnotownawebbutinsteadlivedas
(dry mass = 0.17-25.87 mg) with a majority ofprey being floatersinthecolony’sframework(i.e.,anorb-lesssilkmatrix
small(median length = 2mm; median drymass = 0.32 mg; within the colony’s web complex). This non-random spatial
Fig.4). Fewpreywere>10mminlength(focalcolony:2/23, distribution of individuals within colonies suggests that the
8.7%; second colony: 3/23. 13.0%), but these accounted for social organization of Leucauge sp. follows a hierarchical
46.7% (focal colony) and 72.5% (second colony) ofthe total arrangement, which may be explained in at least three non-
prey biomass. Because these data represent a small prey mutuallyexclusiveways.
samplingeffort,theexactshapeofthedistributionsshouldbe First, this size-dependent spatial distribution may result
consideredwithcaution,whereastheoverallpatternisrobust. fromenvironmentalopportunitywithinahabitat.InLeucauge
Prey capture occurred throughout the day: 13/23 (56.5%) sp.colonies,largerspidersspinlargerorbwebs,aswithother
captures took place between 0800-1200 hours and the colonial (e.g. Metabus gravidus Pickard-Cambridge 1899
remainder (43.5%) took place between 1300-1700 hours; no (Buskirk 1975a); Metepeira incrassata Pickard-Cambridge
preywerecaptured between 1200-1300. In the focal colony. 1903 (Rayor & Uetz 2000) and usually solitary orb-weaving
450 THEJOURNALOFARACHNOLOGY
species (e.g., Cytiosa spp. Menge 1866 (Miyashita 1997); prey availability is higher or prey are more profitable. This
Leucauge markma (Taczanowski 1881) (Eberhard 1988); may be especially true in tropical forests, where insect
Nephila clavipes (Linnaeus 1767) (Higgins & Buskirk 1992)). abundance, diversity and size vary with the height above
The highest positions in a colony may better accommodate ground,evenatsmallspatialscales(Stork&Blackburn 1993;
large orb webs, causing size-related variation in space Basset et al. 2001). For example, Buskirk (1975a) found
occupation. Likewise,habitatparameterssuchastopography importantspatialdifferencesininsectspeciescompositionand
andtheavailabilityandorientationofwebattachmentpoints abundance associated with the distance above and around
may affect spacing patterns within spider colonies. For streams occupied by Metabus gravidus spider colonies in
example, in Metabusgraviduscolonies, theaverage heightof tropical riparian habitats. Therefore, Leucauge sp. spiders
individual orb webs depends on the water current of the positionedatdifferentverticallocationswithincoloniesmay
streambeneathacolonyandthedistancebetweenthestream have access to different insect prey communities. In
banks (Buskirk 1975a). Further research is needed to populations ofLeucauge venusta (Walckenaer 1841) spiders
determine how the physical characteristics of habitats from southern Mexico, the vertical distribution ofwebs on
occupied by Leucauge sp. colonies affect their spatial coffee plants is correlated with prey size and availability;
organization. large spiders build webs at an average height of 150 cm
Alternatively,thespatialstratificationofindividualswithin where prey are larger than at ground level, whereas small
Leucaugesp.coloniesmaybetheresultofinteractionsamong spidersbuildwebsclosetothegroundwherepreyaresmaller
colony members. The spatial arrangement of co-occurring but more abundant (Henaut et al. 2006). If rainforest
spiders within a habitat is commonly thought to reflect habitats occupied by Leucauge sp. colonies have a similar
competition between individuals for the occupancy of spatial distribution of prey, large prey would be more
profitable locations (Wise 1993). Along these lines, Herber- common at the top of colonies compared to the bottom,
stein (1998) showed through a manipulative experiment that whichlargeadultspiderslivingclosetothetopmaycapture-
competitionforhabitatspacebetweentwoco-occurringspecies more easily than smaller subadult or juvenile spiders.
ofweb-buildinglinyphiidspidersleadstoverticalstratification Likewise, juveniles living closer to the bottom may have
of species within the habitat. Likewise, competition among accessto small butabundantprey.
conspecifics may produce spatial structure within a colony. Anotherpossibleexplanationforthegreaterintrinsicvalue
Leborgne&Pasquet(1987)showedthatthespatialorganization of high positions within a colony invokes the architectural
ofZygiellax-notata(Clerck1757)spiderslivinginaggregations propertiesofacolonialwebcomplex.Spiderslivinghigherup
is density-dependent. At high densities, cohabitation between in a web complex may enjoy more architectural stability
spiders of different sizes involved modulations in web size because they are less susceptible to sources of physical
because the presence oflarge spiders with large webs caused disturbance such as seasonal changes in water levels that
smallerindividualstospinsmallerwebs. maydestroywebs.
Similar competitive interactions correlated with individual Differencesinthetimingofwebbuildingbetweenspidersof
differences in age and size may affect spatial structuring in different sizes may also create heterogeneity in the spatial
group-living species. In the colonial species Metepeira positioning of individuals. Rayor & Uetz (2000) found
incrassata, spiders distribute themselves in a size-dependent evidenceforasequentialweb-buildingpatterncorrelatedwith
pattern (Rayor & Uetz 1990). Large females compete for spiderageand sizeinthecolonial spiderM. incrassata,with
primepositionsclosetothecoreofthecolonythataffordthe larger individuals securing prime web sites sooner at the
best protection from predators, whereas smaller immature expense of smaller ones. Similarly, the different positions
spidersliveclosertotheedgeofthewebcomplexwhereboth occupiedbyLeucaugesp.spidersmaydependontheirageand
prey and predators are more common. This spatial arrange- web-building abilities. We found that individuals actively
ment reflects a tradeoffbetween the foraging and protective defendtheirwebsagainstintruders,suggestingthattheymay
requirements of different age classes and results in a also compete temporally to secure favorable positions.
hierarchical distribution across different parts of the web Individuals may then shift positions as they grow to
complex based on competition for specific environmental progressively occupy more competitive locations. Further
conditions.InLeucaugesp.colonies,individualsmaycompete researchisneededtoexaminethishypothesisinmoredetail.
forthehighestlocations,andlargerspidersmaydominatedue Spatial positioning ultimately depends on compromises
to their size advantage, just as in M. incrassata colonies between the foraging, protective and structural costs and
(Rayor&Uetz2000).Todetermineifthespatialarrangement benefits procured by different locations within ofa colonial
of spiders within Leucauge sp. web complexes is based on webcomplex. Forexample,locationswithahigherincidence
competitive interactions, we would need to conduct field oflargeinsectsmayalsobemoreexposedtopredatoryinsects,
manipulationsofspidersizecompositionwithincoloniesand from which larger spiders may be better protected and thus
environmental conditions (e.g., predation pressure) at differ- afford to live in. Conversely, small spiders may settle where
entheightsabovetheground. thepreyandpredatorfaunamaybemoresuitable.
Whywould the highest positions in a webcomplex bethe Itisnotknownwhethercolonialityprovidesanyindividual
mostcovetedones?Onelikelyexplanationisbasedonthefact fitnessbenefitstoLeucaugesp. spiders,suchasreducedweb-
that groups of colonial spiders are commonly viewed as building costs. In a congeneric species found in secondary
‘foragingsocieties’thatformtoincreaseindividuals’foraging forestsofcentralCostaRica,Leucaugemariana,spidersform
potentials(Whitehouse& Lubin2005). Ina foragingsociety, localaggregationsofadultsduringthedryseasonandtendto
individualsmaycompeteforlocationswithinacolonywhere live solitarily the rest of the year (W. G. Eberhard, pers.
SALOMONETAL.—COLONIALITYINLEUCAUGESP.SPIDERS 451
comm.; Valerio & Herrero 1977). In these aggregations, Henaut, Y.,J.A. Garcia-Ballinas&C. Alauzet. 2006. Variationsin
individualorbslackthesurroundingtanglelinesthatsupport webconstructioninLeucaugevenusta(Araneae,Tetragnathidae).
sthhear^evdebsuapnpdoratresisloklidsltyraanndsc.horTehedretfoorteh,e stuhbestmraatienthbernoeufgiht HerJboeurrsntaelin,ofMAr.aEc.hn1o9l9o8g.yW34e:b234p-l2a4c0e.ment in sympatric linyphiid
siinlvko(kVeadlefroirog&roHueprrleirvoing19i7n7)t.hese colonies is an economy of Higsignpitinedsre,-rssLpe.(cEAi.rfia&cchcRno.imEdp.ae,tBiuAtsrikaoinnre.ka.eA)1c:9t9a2i.nOdeAicvoitldrouaagpl-ibcfuaoirl1ad9gi:ni6gn7g-p7rd1ee.cdiastioornsexrheivbeiatls
In summary, this study documents the colonial social differenttacticsfordifferentaspectsofforagingbehaviour.Animal
organizationofLeucaugesp.spiders.Weshowedthatcolonies Behaviour44:485-499.
follow a size-dependent spatial arrangement with a positive Hodge, M.A. & G.W. Uetz. 1995. A comparison of agonistic
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