Anelosimus oritoyacu, a cloud forest social spider with only slightly female-biased primary sex ratios

2011·3.4 MB·English

by  Leticia Avilés

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2011.TheJournalofArachnology39:178-182
SHORT COMMUNICATION
Anelosimus oritoyacu, a cloud forest social spider with only slightly female-biased primary sex ratios
Leticia AvilesandJessica Purcell: Department ofZoology UniversityofBritish Columbia, 6270University Boulevard,
Vancouver, British Columbia V6T 1Z4, Canada. E-mail: [email protected]
Abstract. WeexaminethesocialcharacteristicsandsexratiooftherecentlydescribedAnelosimusoritoyacuAgnarsson
2006.Wefindthatthisspider,whosenestsoccurontreecrownsandbushesinopenfieldsnearBaeza,Ecuador,livesin
coloniesthatmaycontainfromonetoseveralthousandadultfemalesandtheirprogeny.Itdiffersfrommostothersocial
congenersinthatitoccursatrelativelyhighelevations(1800-1900m)anditsprimarysexratio,2.5femalespermale,isthe
leastbiasedofanyknownsocialspeciesinthegenus.Thelowsexratiobiasmayreflectalowcolonyturnoverratherthan
highgenefiowamongcolonies,asthecoloniesoccurredincomplexesthatwerefewandfarbetween,butappearedtobe
long-lived.Therelativelysmallbodysizeofadultfemalesandawebthatappearstoallowthecaptureofinsectsfromall
directions,combinedwithindividualandgroupforaging,mayallowtheformationoflargecoloniesatanelevationwhere
insects,albeitabundant,areforthemostpartsmall.
Keywords: Ecuador,Theridiidae,cooperation,lifecycle,quasisocial,subsocial
Thegenus AnelosimusSimon 1891 is ofparticular interest in the Nestandwebstructure. A.oritoyacusnestsdifferedfromthoseof
study ofspider sociality because it contains the largest number of mostothersocialspeciesinthegenusinlackingawelldifferentiated
non-territorialpermanent-social(orquasisocial)speciesofanyspider basal basket and extensive superior prey capture webbing, as
genus(Aviles 1997; Agnarsson 2006; Lubin & Bilde 2007). Among depicted, forinstance,forA. eximiusbyYipetal.(2008,fig. 1;see
these,AnelosimuseximiusKeyserling 1884,AnelosimusdomingoLevi alsophotoinAvilesetal.2001,fig.9). Instead,A. oritoyacu'^webs
1963,Anelosimusduhiosus Keyserling 1881, Anelosimusgiuiccimayos consistedofacoreareasurroundingapieceofvegetationandprey
Agnarsson 2006, Anelosimus lorenzo Levi 1979, and Anelosimus capturestrandsrunningawayfromthecore,includinginferiorlyfrom
rupununi Levi 1956 have been the subject ofone to several studies it(Fig. 1),muchlikethenestsofA.rupununi(Aviles&Salazar1999),
(e.g., Fowler & Levi 1979; Rypstra & Tirey 1989; Rypstra 1993; acanopyspecies.AlsoasinA. rupununi,A. oritoyacussilkwasofa
Aviles&Tufino 1998; Marquesetal. 1998;Aviles& Salazar 1999; lightertextureandwhitercoloration than inmostothercongeneric
Avileset al. 2007; Purcell&Aviles2007; Yipet al. 2007). Herewe species.Thiswebstructuremayrefiectthepositionofthewebsontree
reportonanewnon-territorialpermanentsocialAnelosimus,recently crowns,andtheneedtocaptureinsectsflyingfromthesideandbelow
describedbyAgnarsson(2006)asAnelosimusoritoyacu.Weshowthat thenests. A. oritoyacusnests,aswellasthoseofA. rupununi,thus
although this speciesexhibits social organization similar to that of have characteristics that appeara response to the canopy location
other social Anelosimus spiders, it presents some interesting differ- preferredbythesespecies.Thenestsweobservedrangedbroadlyin
ences. Along with A. guaeamayos, A. oritoyacu occurs at what size. At least two nests, but possibly as many as seven, ofthe 55
appears to betheelevational rangelimit forpermanent sociality in recordedcontainedeitherasingleadultfemaleorwhatappearedto
this genus (Aviles et al. 2007) and its sex ratio is the least biased betheclutchofasinglefemale.Themajorityofnests,however,were
amongknownsocialAnelosimus(Aviles&Maddison 1991;Avileset considerably larger. Several nests in the first nest complex seen in
al.2007).Herewepresentabriefaccountofthesizeandstructureof January 2002, for instance, measured on the order of 3^ m in
A. oritoyacus nests and colonies, informal observations on the diameter and probably contained several thousand individuals.
cooperative nature ofits societies, and, given the relevance ofsex Among 20 nests measured (two in 2002, three in 2004, eight in
ratios as indicators ofpopulation structure (Williams 1966; Nagel- 2007, and seven in 2008, from one, two, three, and five different
kerke & Sabelis 1996; Hardy 2002), estimates of its primary and colony complexes, respectively), the smallest measured 13 X 13 X
tertiarysexratios. — 23cmandthelargest,2—05 X 156 X 100cm.
Location of nest complexes seen. Over a period of six years Colonyagestructure. Ofthefiveneststhatwedissected(threein
(January20()2-June2008)welocatedeightareas,allwithina 10km January2002fromtheinitialnestcomplexfound,onein December
radiusofBaeza,Ecuador(0°27'S,77°53'W; 1800-1900melev.),that 2002fromacomplex300mawayfromtheformer,andonein2008
containedfrom 112A.oritoyacunests(median3.5)each,foratotal fromaseven-nestcomplexfound500mawayfromtheoriginalfound),
of55 nest records(Table 1). Whenmorethanonenestwaspresent fourcontainedamixofjuvenileand/oreggsacs,subadult,andadult
withintheseareas,nestsweretypicallyclusteredwithinmetersofone spiders, suggesting that reproduction is not strongly synchronized
anotherinwhatwerefertoas“nestcomplexes.” Distancesbetween within nests (Table2); the remaining nest contained only subadult
identified nest complexes ranged from 25 m to 3.5 km. Most nests malesandfemales(Table2).Wesurveyedtheagestructureofseven
were located on thecrownsoftreesoron bushesgrowingonopen additional nests, both in June-July (2004, 2008) and in December
hillsideso—rroadsides.Thenestcomplexesappearedremarkablystable (2002,2007)andfoundthatadultsandjuveniles/eggsacswerepresent
overtime attwoofthesitesinitiallydiscoveredin2002,nestswere atbothtimes.Takentogether,thesefindingssuggestthatA.oritoyacyu
stillpresentin2007and2008(Table 1).Incontrast,nestcomplexesin eitherhasashortgenerationtimeand/orthatitslifecycleislargely
speciessuchasAnelosimuseximiusrarelylastmorethan2-3years(L. independentofthemildlyseasonalrainpatternsoftheregion(rainiest;
Aviles unpublished data), and those of species such as Theridion May-July;leastrainy:December-February;Neill 1999).Adultmales
nigroannulatumKey.serling1884usuallylastlessthanayear(Avileset were seen overlappingwith adult females in at least eight colonies,
al.2006) suggestingthattheopportunityforintracolonymatingispresent.
178
AVILES&PURCELL—SOCIALSPIDERWITHFEMALE-BIASEDSEXRATIO 179
Table 1.—Locationofnestcomplexesseenandthenumberofseenneststheycontainedatthedateofinspection.Locationcode; BZ-TN =
Baeza-TenaRoad;BZ-LA= Baeza-LagoAgrioRoad;BZ,TNandLA=townsofBaeza,Tena,andLagoAgrio,respectively.KmfromBaeza
shownaftereachlocationcode.
Locationcode Latitude Longitude Elevation(m) Datesseen #Nestsincomplex
BZ-TN8.1 0.497083 77.873861 1822 Jan-02 4
Dec-02 few
Dec-07 several
Jun-08 several
BZ-TN8.1+333m 0.4955 77.876306 1881 Dec-02 2
BZ-TN 1.0 0.46322 77.87662 1866 Dec-02 12
Dec-07 4
Jun-08 3
BZtown Jun-04 1
BZ-TN4.5 0.4729 77.86819 1848 Dec-07 1
Jun-08 2
BZ-LA2.4 0.45157 77.88392 1818 Jul-04 2
BZ-LA2.6 0.451395 77.88954 1823 Jun-08 1
BZ-LA3.0 0.45152 77.88399 1842 Dec-07 8
Jun-08 7
BZ-LA3.0+25m 0.45152 77.88399 1842 Dec-07 4
Jun-08 4
Clutchsizeandsexratio.—In2004, wecollectedeggsacs froma Gunnarsson&Andersson 1992forasolitaryspecieswithbiasedsex
single large colony and used the method described by Aviles & ratios)andfromthe 10:1 sexratiofoundinothersocialAnelosimus
Maddison(1991)tosextheembryostheycontained. Eggsacswere species(e.g.,A.eximiusandA.domingo,Aviles&Maddison 1991;A.
off-whiteincolor,averaged3.9 ± 1.2mmindiameter(n = 5),and gucicamayos, Avilesetal. 2007). Thetertiary sexratio ofadultand
containedbetween16and46eggs(n= 11,median=37,mean=34, subadult spiders from nests collected in 2002 and 2008 similarly
SthEe=nu3m.b1e3)r.oIfncchytroolmogoiscoamlessprceoandtsaionfeidndiinvaitdulaelasetmtbhrryeeosdiwveidicnogunctelelds shSopwiedderasbiizaesaonfdbeitnwsteaerns.t—woWeandmefaisveurfeedmaltehsetloenogntehmoafleth(eTatbilbeia1)-.i-
todeterminewhethertheindividualwasmaleorfemale(n= 130,four patellaonlegpair1(TPl)andlegpair2(TP2),aswellasthesternum
eggsacs.Table2).Asinotherspeciesinthegenus,maleshad22and length(SL)andweightforahaphazardsampleofsubadultandadult
females,24chromosomes(20autosomesplustwosexchromosomes spiderscollectedfromonenestbelongingtotheoriginalcomplexseen
for males, and four sex chromosomes for females). Samples with in2002.Lengthsweremeasuredtothenearest0.1 mmusinganSZH
fewer than three scorable dividing cells were not included in this Olympus dissecting stereomicroscope. We measured weights to the
analysis.Wefoundtheprimarysexratiotobeabout2.5femalestoa nearest0.000Igusinga MettlerToledostandard level balance. The
male(Table3).Thissexratiodifferssignificantlyfromtheexpected average ± SE of each measurement is presented for each instar
1:1 sexratioofsubsocial species(Aviles& Maddison 1991; butsee (Table4). We found that A. oritoyacii males were adult at a size
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Figure 1. Anelosimusoritoyacii?,nestsphotographednear Baeza, Ecuador,and photographsofadultmale(above)andfemale(bottom)
spiders.Notethedifferentscalesofthemaleandfemalephotographs.
s
180 THEJOURNALOFARACHNOLOGY
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Table 2. Colonyagestructurebreakdownandthetertiarysexratio(totalnumberofadultplussubadultfemales/totalnumberofadultplus
subadultmales)forfivedissectedA. oritoyaciicolonies.
Nestsize % Contents(number) Tertiary
Colony Collectiondate (cm) Collected Adm Subm Adf Sub2f Subl f Juvs Sacs sexratio
BZ-TN8.1-1 6Jan2002 45 X 25 X 14 100 27 14 12 47 29 36 0 2.15:1
BZ-TN8.1-2 6Jan2002 40X 18 X 17 100 9 21 16 28 17 51 3 2.03:1
BZ-TN8.1-4 6Jan2002 - 100 32 6 42 28 67 0 4 3.61:1
BZ-TN 8.1 +333-1 17Dec2002 - 100 0 13 0 27 0 0 2.08:1
BZ-LA3.0-7 20Jun2008 - 30 15 11 16 44 76 45 4 5.23:1
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Table 3. PrimarysexratioofAnelosimusoritoyacii,reportedastheproportionofmalesamongdevelopingembryosinfoureggsacs.The
proportionsarecomparedwith1:1and10:1sexratioexpectations(righttwocolumns)usingeitherthebinomialexacttestforeacheggsac(rows
1-4)andthetotalsample(row5)ortheweightedZ-transformmethod(lastrow),whichcombinestheprobabilitiesofthefoureggsacs,witheach
sacweightedbythenumberofembryosscoredtogivemoreweighttomorepreciseestimates,asrecommendedbyWhitlock(2005).
Eggsac Totalembryos Totalscored #ofMales Proportionofmales Pux PXOA
1 24 21 7 0.33 0.09 0.003
2 41 34 8 0.24 0.001 0.02
3 37 31 8 0.26 0.005 0.01
4 46 44 14 0.32 «0.007 «<0.001
Total: - 130 37 0—.28 0.001 0.001
Mean: - 32.5 9.25 — Zs Zs
St. Dev.: - 9.47 3.2 0.04 0.01
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Table 4. Instar measurements for subadult and adult males and females. The mean is shown with the standard error in parentheses.
Measurementsincludetibia+patellaforlegpair I (TPl)andlegpair2(TP2),sternumlength(SL)andweight.
Instar ii TPl (mm) TP2(mm) SL(mm) Weight(mg)
Male
Subadult 6 1.25(0.0224) 1.02(0.0307) 0.7(0.000) 3.53(0.243)
Adult 7 1.86(0.023) 1.39(0.0254) 0.779(0.0149) 3.87(0.167)
Female
FirstSubadult 4 1.43(0.025) 1.16(0.0239) 0.738(0.0125) 3.43(0.330)
SecondSubadult 4 1.69(0.375) 1.36(0.0239) 0.9(0.000) 4.43(0.325)
Adult 14 2.129(0.0266) 1.66(0.0195) 1.04(0.0116) 5.52(0.229)
correspondingtothesecondsubadultfemaleinstar(Table3,Fig. 1), sociality(Aviles 1986, 1993, 1997). Itisinteresting,however,thatA.
suggestingthatmalesmatureoneinstarearlierthanfemales,asisthe oritoyacii sex ratio is the least biased among known permanent
case with other tropical Anelosimus(e.g., Aviles 1986; Avileset al. socialAnelosimus,asotherspeciestypicallyexhibitsexratiosof10:1
2007). (A. eximius, A. domingo: primary sex ratio), 5:1 (A. guacamayos:
Interestingly,A.oritoyaciiappearstoexhibitsignificantlylesssexual primarysexratio),and3:1(A.duhiosus:sexratioamongsubadultsto
size dimorphism than other Ecuadorian social Anelosimus (Fig.2) adults).Aviles(1993)showedthroughcomputersimulationsthatthe
(meanmale:femalebodylengthratio=0.79forA.oritoyacic,0.68for mosthighlybiasedsexratiosarisewhenthedegreeofisolationofthe
A. guacamayos;0.66forA. domingo’,0.65 forA. eximius; total body colony lineages and their rate of turnover (i.e., rate of colony
lengths of 15 to 31 specimens per species measured to the nearest extinctionandreplacement)arethegreatest.Sexratiosthatareonly
0.1mm).ThisisduetoadultA.oritoyaciifemalesbeingrelativelysmall slightlybiasedwouldthusariseifthereweresomedegreeofgeneflow
comparedtofemalesintheseotherspecies(mean±SE,oritoyacic.3.65 among the colonies’ lineages and/or their rate of turnover were
0±.00.71m1mm,m/,;=n=21;l\deoxmiimniguos:\43..8449±±00..0068mmmm,,nn==2115;)g,uwahcialmeaAy.oso:ri4t.oy0a4ci±i Ar.elaotriivteolyyaclioiw,.atWitthheomuotmgeennettiwcedactaanntootasassecsesrtpaoipnulwahtiicohnosftrtuhcetsueretowno
malesarerelativelylarge(oritoyacic2.90±0.19mm,n = 8;eximius: (orcombinationofthesetwo)factorsplaysthemostimportantrolein
3.14±0.08mm,n= \0\guacamayos:2.76±0.12mm,n=4;domingo: determiningthelowsexratiobiasofthisspecies. However,thefact
2.29±0.08mm,n= 12).The—significanceofthispatternisunclear. that A. oritoyacus nest complexes were few and far between does
Conclusionsanddiscussion. Inconclusion,thesize,duration,and suggestthatthelikelihoodthatdispersingmaleswouldfindnestsof
demographic composition ofA. oritoyacii colonies, including their unrelatedfemales(i.e., belongingtoadifferentcomplex)arelowto
female biased sex ratios, are consistent with this being a non- non-existent.Ontheotherhand,thefactthatA.oritoyacusnestsand
territorialpermanentsocialspecieswithcoloniesthatlastformultiple coloniesappearrelativelylong-livedcomparedtothoseofothersocial
generations. The estimated 2.5 females per male primary sex ratio Anelosimussuggeststhat a lowrateofcolony turnovermaybethe
further suggests that some degree of intracolony mating must be parametermostlikelyresponsibleforthelowsexratiobiasobserved,
takingplacein thisspecies,asistypicalofspecieswith thislevelof apredictionthatrequiresfurthertesting.
AVILES&PURCELL—SOCIALSPIDERWITHFEMALE-BIASEDSEXRATIO 181
preliminaryevidencethatfemalesmaycareindiscriminatelyforeach
other’s egg sacs, as we witnessed multiple instances of egg sac
switchingovera24-hperiodinartificiallyestablishedgroups(four)of
fivecolor-codedfemalesandtheirsacs(L.Avilesunpublisheddata).
Aboveandbeyondanybenefitsthatmayarisefromcooperativeprey
capture,offspringfitnesscouldthusbeenhancedbytheavailabilityof
surrogatecaregiversintheeventofthemother’sdeath(e.g.,Joneset
al.2007).Theseareallideasthatwillneedtobeformallyexploredin
futurestudies.
ACKNOWLEDGMENTS
Wethank the Museo EcuatorianodeCiencias Naturalesand the
corporation “Sociedad para la Investigacion y el Monitoreo de la
BiodiversidadEcuatoriana”(SIMBIOE)forsponsoringourresearch
inEcuadorandtheMinisteriodelAmbientedelEcuadorforresearch
permits.Thanksalsoto1.Agnarsson,T.Bukowski,G.Iturralde,W.
Maddison, P. Salazar, and M. Salomon for their help in the field.
Funding was provided by the National Science Foundation ofthe
USA (research grant DEB-9815938 to LA and agraduate research
fellowshiptoJP)andbyaDiscoveryGranttoLAfromtheNatural
— SciencesandEngineeringResearchCouncilofCanada.
Figure 2. Least square means for the total length ofmale and
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