〟α椚椚αJ5ね‘め21:1−13(1996) ⑥theMammalogicalSocietyofJapan
Agevariationofthethirduppermolarin
且bi励e几Om〝g gm王統ff
YukibumiKANEKO 凡−(}/(融・‘イ/ム止≠正一り・.品(・J小・‘1J■Jご‘れ、‘J//州.ん桝冊【㌻′∼′nl叫、・.れ血川(7/J′′1丁(し力外川 Fax,0878−36−1652Abstract.A study was made of age variationin the size and
enamelpatternsofthethirduppermolarof99Eothenoクク砂SSmithii
specimensfromJapan.Nosignificantagevariationwasfoundin
eitherthefrequency ofthepatterns,Or thewidth ofthe dentine
COnfluentspacebetweenthesecondandthethirdtriapgles・Deep
lingualreentrantfolds,Ontheposteriorloop,appearlnSpeCimens where the condylobasallength(CBL)is of22−24mm,then the
patternwith a shallowreentrantfoldincreasesin frequencyin
largerCBLclasses.Thedepthoftheinnerfoldshowedthesame
tendency as the changesin the patterns.A significant associa− tion,however,between five enamelpatterns and age classes,
dependsonclassificationaccordingtoCBLorbodyweight.This
provedinsignificantinfiveCBLclasses,butsignificantinthree
CBL orbody weight classes.A gradualtransitionin the age
variation of the posteriorloop patterns was found among Eo−
thenon砂S SpeCies which have rootless molars throughoutlife.
The simple enamelpattern form slgnificantlylnCreaSedin fre−
quency with advancing agein E.andeYSOniand E.shanseius, resemblingClethrionoクク砂Sgla柁OlusandC.n4舟canus;Ontheother hand,inEプ聯Ius,E.inez,E.eva,E.chinenめE.wardi,E.custos andE.proditornoagevariationwasfoundontheposteriorloop, thusresemblingMicYVtuSPennsylvanicus.E.smithiishowsalittle
agevariationintheenamelpatterns,thevariationofwhichisof
anintermediatetype. Keywords:ageVariation,enamelpattern,EothenonサSSmithii,Sizeofmolar, thirduppermolar. WithregardtothephylogenyoftheArvicolidae,BauchauandChaline(1987), andChalineandGraf(1988),COnSideredthat,basedonacomparisonofmolarstructures,theocclusalenamelpatternsofthethirduppermolartendedtovary
fromsimpletomorecomplexforms.ThegenusClethriononv)Sdevelopsmolar rootswith advancing age,Whereasthe genusEothenonys develops no roots. Thetwogenera,however,reSembleeachotherinmanyothercharactersofthe skulland dentalmorphology(Hinton1926,Kaneko1990,1992),andintheir karyotypes(Yoshida et al.1989).Throughthe ontogenetic process of C・ glanolus(Zejda1960),C.ndbcanus(Abe1982)andE.andeYSOni(Miyao1966, Kitahara1995),alargeproportionofmolarschanges from complex enamel肋∽研α/Sf〟め′ 21:1996
PatternStOSimplerforms.Noagevariationwasfound,however,Onthesame molarinE.smithii(Tanaka1971).Tanaka,s(1971)resultsf6rE,Smithiimay
havebeenbiasedbecause ofhisrelativelysmallsample group ofspecimens COllectedduringjustoneperiodoftheyear,Whenfullyadultanimalsmayhave
been absent.
Thepurposeofthisstudy,therefore,WaStOreeXaminetheagevariationin bothsizeandenamelpattern ofthethirduppermolars ofE.smithii,andto
COmparetheresultswiththoseofotherEothenonwsspecies.
MATERIAIJSAND]Ⅵ:ETHOS
Atotalof99specimensofE.smithiiwerecollectedatMinoura,Toyohama District,Kagawa Prefecture,Japan,(34002′30′′N,133037′30”E).Specimensin
each ofthe12monthsweresampled at one period duringtheyears1977−80
(Kaneko1989).Thecollectingsite for this studywaslessthan50kmfrom Tanaka’s(1971)siteonthesameisland,Shikoku.Fivemeasurementsofthe
thirduppermolarandthecondylobasallength(CBL)weretakenfromcleaned
Sku11s,these were:tOtallength(TM3L),anteriorlength(AM3L),pOSterior length(PM3L),the width of dentine confluent spaces between the first and
SeCOndtriangles(WDC)andthedepthofthethirdlingualreentrantfoldorthe
posteriorroop(DRF,Fig.1).Toothdimensionsweremeasuredtothenearest 朋界雷一・幣・ヾ小︶ 舶摘㌧笥即Y小.・.J〟境野じ
桑票し
摺
用
只野.. 只京.小〃卜=》
()ハ嘲Fig.1.Measurements taken(1eft),and enamelpattems(AO−A3and Pl−P4)on the third upper molar according to Tanaka(1971).TM3L,AM3L,PM3L,WDC and DRF are ex−
Kaneko,Age ua7iation〆the molarinEothenomyssmithii
0.01mmusingastereo−microscope(Nikon,SMZ−10)withanobjectivemicro− meter(Kogaku,minimuminterval=0.05mm).TheCBLwasmeasuredtothe nearestO.1mmwithadialcaliper(minimuminterval=0.05mm).
Tanaka(1971)definedtheenamelpatternsformedontheocclusalsurface
bytheenamellamellae onlyin figures;however,inthis study more precise criteria havebeen used.Four patterns(AO−A3)in the shape ofthe dentine
COnfluentspacesbetweenthesecondandthirdtriangleswererecognized.In AO,thelamellaedonotformtwotriangles,butawidedentineconfluentspace instead.AlandA2areintermediatepatternsbetweenAOandA3(AIshows asmallerprotrusionoftheenamellamella,andA2alargerprotrusion).InA3 thelamellaeformtwocompletetriangles.Fourotherpatterns(Pl−P4)were Observedontheposteriorloop,OrOnthefourthsalientangle.InPlthepattern iscomplex,Withthreereentrantfoldsonthelingualside,Withthethirdfold
exceedingthetransverseline atthe anterior edge ofthesalientangle ofthe
POSteriorloop.P2isintermediatebetweenpatternsPlandP3.P3hasthree Salientangleswithastraight−Sidedposteriorlooponthelingualside.InP4 thepatternissimplewithtworeentrantfoldsonthelingualsideandwithout COnCaVityontheposteriorloop.Enamelpatternswereobservedontheright Orleftmolarunderastereo−microscopewitha X201ens. Inthisstudy,CBLwasusedasanapproximateindicatorofage,becauseit COrrelatespositivelywithageasdefinedbyrootdevelopmentinClethrio
ndbcanus(Kaneko1990).As there have been no reportsindicating sexual
differencesin either size or enamelpatterns,both males and females were COmbinedforanalysis.
RESULTS
AsCBLincreased,bothtotallength(TM3L)andanteriorlength(AM3L) increasedsignificantly(r=0.661,P<0.001andd.f=97inTM3L;r=0.676,P< 0.001,d.f=97in AM3L).As CBLincreased,the posteriorlength(PM3L) increased untilCBL reached22.5mm whereit reached asymptote,thougha
significant regression coefficient was calculated throughout the size of CBL
(γ=0.305,0.001<♪<0.01,d.′=97,Fig.2).
The depth ofthe reentrant fold ofthe posteriorloop(DRF)was nearly
COnStantagainstCBL=22−25mmanddecreasedslightlyinCBLslargerthan25
mm.WiththeincreaseofCBL,Widthofthedentineconfluentspaces(WDC) remained almost constant.Regression coefficients between CBL and DRF, and between CBL and WDC wereinsignificant(r=−0.198,n.S.,d.f=97in DRF;r=−0.057,n.S.,d.f=97inWDC,Fig.3).
Theaveragelength(X),Standarddeviation(SD),andcoefficientofvaria− tion(CV)ofthesefivemeasurementsaretabulatedforfivesizeclassesofCBL.
AverageTM3LandAM3LincreasedcontinuouslyasCBLincreased,Whereas
PM3L ceased toincrease from the CBL=22.O mm class onwards.Thecoeffi−
CientofvariationisgreaterinPM3LthaninTM3LandAM3Lforeachsize
肋椚桝αJSf〟め′ 21:1996 00 0 008 0 00() 0 ◎ O ′. (〉 血 TM3L 0 0 0 g 茸。‰冨 ∴、∼ざ、}・ 1.5 0 0 0 AM3L 0 0 ◎‰○ 0 0 ∵∵叫. ◎ 卵S 。。 00 &0 0g 0︵ 。 亀。。 ○ ○ ▼ 000 VOβ 0 0 β 〆叩。。。
00 0 0
0 00 00
PM3L 0 0 0 0 0000000
さ_ 0 0 21 22 23 24 25 26 C B L(mm) Fig.2.PlotsofTM3L,AM3LandPM3Lagainstthecondylobasallength(CBL). ◎ 0 l ㍉.∵ 亀 ㌍◎0 0‰縞。。 0恥○ ◎ 0 こ豆モ土ミ・一二隠21 22 23 払 25 26
C B L(m) Fig.3.PlotsofWDCandDRFagainstthecondylobasallength(CBL).Kaneko,Age variation d the mohlrinEothenomyssmithii
WDCwasslightlylongerinthe20mmCBLclass.Thecoefficientofvariation
wasgreaterwasDRFinthe25mmCBLclass(Tablel).
The association of enamelpatterns betweenleftand right third upper
molarswastestedusingtheGLteStWithWilliams’adjustment(SokalandRohlf 1973).Noindependence was shown between right andleft molarsin the patternsofthedentinespacesbetweenthesecondandthirdtriangles(AO−A3; Ga。j=35.66,P<0.005,d.f=9,Table2),Orbetweenrightandleftpatternsofthe posteriorloop(Pl−P4;Ga。5=43.88,P<0.005,d.f=9,Table2)・Similarlyno
associationwasshownbetweenthepatternsofthedentineconfluentspacesand
thoseoftheposteriorloop(AO−A3andPl−P4,Gadj=6.56,n.S.,d.f=9,Table3)・ Consequently,theleftthirduppermolarwasusedforfurtherstudies・In an analysis of the width of dentine spaces(WDC)and the enamel
patterns(AO−A3)betweenthesecondandthirdtrianglesforeachofthefivesize classesofCBL,theO.05−0.10mmWDCclassandpatternA2predominatedin everyCBLsizeclassover22.Omm,andaverageWDCremainedalmostcon− stantthroughoutthesizeclasses(Fig.4).Noassociationwasfoundbetweenthe enamelpatternsandthefiveCBLsizeclasses(Ga。j=18.73,n.Sリd.f=12,Table 4).Whenthefrequency ofthe patternswasdividedinto threebodyweight classes,aSdefinedbyTanaka(1971),OrbyCBL(20.5一,23.0−,and 25.0−mm Tablel.FivemeasurementsofthethirduppermolarsofEothenonwssmithii. Five CBL classes(mm) 20.5− 22.0− 23.0− 24.0− 25.0− 5 19 21 37 17 1.508 1,720 1.769 0.150 0.082 0.119 9.946 4.757 6.661 1.37 1.50 1.55 1,75 1.86 1.99 1.832 1.908 0.093 0.102 5.064 5.368 1.63 1.76 2.00 2.08 X 孟Ⅳ仙Ma L l h 3 ︶ 1.190 1.215 0.080 0.087 6.679 7.170 1.04 1.03 1.36 1.41
蔓
r ︶ 若槻仙 0.974 1,068 1.122 0.060 0.060 0.073 6.181 5.572 6.534 0.93 0,97 1.00 1.08 1.20 1.25 r 、∴二 O ︶∵
0.739 0.752 0,076 0.087 10.303 11.510 0.56 0.60 0.90 0.93 0.564 0,730 0.748 0.144 0.071 0.073 25.497 9.760 9.818 0.43 0.57 0.61 0.80 0.88 0.90 Confluent X O.094 0.050 0.064 0.056 0.059 width SD O.026 0.028 0.029 0.032 0.032 (WDC) CV 27.766 55.800 44.479 57.271 54.422 (mm) Min. 0.05 0.00 0.00 0.00 0.00 Max. 0.12 0.10 0.11 0.12 0.10 Depth of X O.170 0.223 0.221 0.217 0.130 reentrant SD O.082 0.076 0.088 0.083 0.087 fold CV■ 48.177 34.187 39.638 38.199 67.154 (DRF) Min. 0.10 0.07 0.06 0.05 0.00 (mm) Max. 0.30 0.40 0.36 0.38 0.30肋∽∽αJ5′〟め′ 21:1996
廿
25‘)− N=17 2ん0− N=37qlQ2mm
WDC
CBL
(m巾) Fig.4.FrequencydistributionsofWDCmeasurements andenamelpatternsAO−A30fthe dentinespaceforfiveCBLclasses.Aclosedtriangleshowstheaverage,andSDindicates thestandarddeviationoftheaverage.Kaneko,Age variation〆the mohlrinEothenomyssmithii
258−
N=17
︼ 7∵晒
ーーl■二■ニー二云
●
−
慧
20.5−
N=5
CBL
(mm)GIG2 α3(払mm
DRF
Fig.5.FrequencydistributionsofDRFmeasurementsandenamelpatternsPトP4forfive CBLclasses.Aclosedtriangleshowstheaverage,andSDindicatesthestandarddeviation Oftheaverage.ルね∽∽αJ5蝕め′ 21:1996
Table2.A test ofindependence for frequencies of the enamel
patterns(AO−A3andPトP4)betweentherightandleftmolarsof g(7〃圧りれり町5封〃〟/㌦. The left Confluentpatternsofthe2ndand3rdspaces AO AI A2 A3 Total Theright a t O 1 2 3 0 A A A A T 3 4 9 3 9 1 6 1 9 2 1 6 0 9 1 1 0 2 9 1 2 5 6 0 0 4 2 6 1 1 Patternsofposteriorloop PI P2 P3 P4 Theright PI P2 P3 P4 Total 2 9 5 3 9 4 3 1 9 0 5 2 1 8 1 1 4 0 0 0 4 3 1 5 7 4 3 0 4 3 2
Table3.A test ofindependence for frequencies between the
enamelpatterns(AOLA3andPl←P4)ofEothenonwssmithii. Confluentpatternsbetweenthe2ndand3rdspaces AO AI A2 A3 Total Posterior Pl 1 5 loop P2 0 7 patterns P3 1 6 P4 0 1 Tota1 2 19 8 4 9 1 2 6 2 2 0 3 2 1 6 1 1 4 3 1 9 4 4 8 3 9
based on Table4),nO Significant association was shown between the two
dimensions(Gadj=8.13,n.S.,d.f=9forCBL;Ga。j=4.83,n.S.,d.f=6forbody weight,Table5).Thus,thevariationofthepatternofthedentineconfluent SpaCeS(AO−A3)isindependentofage. Inananalysisofthedepthofthereentrantfold(DRF)andoftheenamel patternsoftheposteriorloop(Pl−P4)foreachCBLsizeclass,patternSPl−P3
appearedwithsimilarfrequenciesinCBLclassesfrom22mmto24mm,aSdid
DRF,Withalmostthesameaverage,thoughwithinawiderangebetweenO.05 andOA5mm(Fig.5).Inthe25mmCBLclass,aVerageDRFbecameslightly Shallower,andpatternP4appearedforthefirsttime,andP2becamethemostfrequent pattern,No associ?tion was fらund between the patterns of the
posteriorloopandthefiveCBLsizeclassesatthe5%1evel(Gadj=19.36,0.05<
P<0.1,d.j:=12,Table4).However,Whenthepatternsoftheposteriorloop wereclassifiedintothreebodyweightclasses,aSdefinedbyTanaka(1971),and
助neko,Age yaYiation qf the molarinEothenomyssmithii
Table4.A test ofindependence for frequencies between the
enamelpatterns(AO−A3and Pl−P4)andfive CBLsize classes of
励〃zβ乃0クワ砂ざ5研グfカグブ. Confluentpatternsbetweenthe2ndand3rdspaces AO AI A2 A3 Total Five 20.5− CBL 22.0− classes 23.0一 (mm) 24.0− 25.0 Total 0 3 3 8 2 6 1 3 ハU 4 0 2 9 1 1 1 5 4 9 3 2 ﹁⊥ l l 1 6 5 ∩プ 1 7 7 9 1 2 3 1 9 Posteriorlooppatterns PI P2 P3 4 P O T t a Five 20.5− CBL 22.0− classes 23.0一 (mm) 24.0− 25.0− Total 1 3 1 7 2 4 1 1 1 4 2 4 7 3 8 4 1 3 2 2 3 7 4 8 1 5 9 1 7 7 9 1 2 3 1 9
Table5.A test ofindependence for frequencies between the
enamelpatterns(AOuA3andPl−P4)andthreebodyweightclasses Orムー〃れリハり町・ゞ封〃/仙/∴ ConfluerltpatternSbetweenthe2ndand3rdspaces AO AI A2 A3 Total 3 2 4 9 1 1 2 1 3 6 1 1 9 5 5 9 6 2 9 3 1 8 2 4 1 6 Posteriorlooppatterns PI P2 P3 a t O T 4 P Body lO.0− Weight 20.0− (g) 30.0− Total 9 5 5 9 6 2 9 3 6 5 4 3 4 3 1 0 4 2 1 3 3 8 7 8 1 byCBL(20.5−,23.0−and25.0−mmbased onTable4),Significantassociations Werefoundforbothdimensions(Gadj=17.69,P<0.01,d.f=6,forCBL;Ga。j= 15.34,P<0.025,d.f=6,forbodyweight,Table5). Whenthetotal(TM3L),anterior(AM3L),andposterior(PM3L)1engthsof
thethirduppermolarwereexaminedinrelationtothefourenamelpatternson
theposteriorloop(Pl−P4)foreachofthefourCBLsizeclasses(Fig.6),itwas foundthataveragePM3LstendtodecreasefromPltoP4atthe24and25mm CBLclasses,WhereasaverageAM3Lincreasedslightly.10 肋∽∽αJ5′〃め′ 21:1996
0
等・こ=
CB◎ βド巾岬噛 P−Tb CBヒ25.0− ○ mm mm 1.0 2.1 P・⊥Ⅶ・b CBlj払.0− ‘ ﹂M三< 」 M ∑ ‘Ll.1 CBL芦23.0− 0 CBし;受3.0− CBL戎3(ト だギb。堰。
。握。
CBL言22β− か 2 つ︻ ヒ B C CBヒ22.0− 0 8馳 P十P βホb 8 1.7 0 1 l PIP2 P3 P4 PIP2 P3 P4 P1 P2 P3 P4 Fig・6・MeasurementsofTM3L,AM3LandPM3LagainstenamelpatternsPl−P4forfour CBLclasses.Aclosedtriangleshowstheaverage,andaverticallineindicatesthestandard deviationoftheaverage.11 Kaneko,Age variaiion〆themolarinEothenomyssmithii
DISCUSSION
InrevisingthetaxonomicpositionofEothenoクク砂S Smithii,Tanaka(1971) firstshowedthatno agevariationwas found onthe enamelpatterns・Ofthe
dentine confluence between the second and third triangles and the enamel
patternsofposteriorlooponthethirduppermolar.Sixty−Sixspecimensused byTanaka(1971)werecollectedattheendofJulyat1700monMt.Tsurugi, TokushimaPrefecture,Shikoku,Japan.Thebreedingseasonofthisvoleisat itspeakduringJulywhenTanaka(1971)collectedspecimens,andfullyadult VOlesrepresentedonlyasmallproportionofthepopulation(KanekoandMorii 1976).Inthisstudy99specimenscollectedthroughoutayearwereexamined,
andtheseincludedalargerproportion(26%)ofold adultswithbodyweight
heavierthan30g,thanin Tanaka,s(1971)sample(16%).Inthe specimens COllectedforthisstudy,however,theposteriorloopofthethirdmolarshowed SOme age Variation,aS Shown by theincrease of the frequency of P2,the appearenceofP4(thesimpleform)andtheshallowerreentrantfold(DRF)in thelargestCBLsizeclass(Figs.3and5,andTablel).
Table6.Therelationshipbetweenenamelpatternsandskullsizesinspeciesof励theno〝り′S
(datafromKaneko1990,1992).
Posteriorlooppatterns* Species
Size(mm) Type6 TypeP2 TypeP3 TypeP4
E.ざゐα乃5g才〟ざ 12.2− (トM3) 13.卜 Ga。j=52.02 14.0− ♂.′=12 15.2− ♪<0.05 16.1− Total 0 2 6 2 ﹁⊥ 1 1 1 3 9 8 亡U 7 2 0 2 3 6 0 1 1 1 3 5 6 1 0 ハU 2 1 6 3 9 6 7 1 1 1 2 1 8 且オ乃gZ ll.3← (トM3) 12.2− Ga。j=6.28 13.ト d′=9,乃.∫. 14.0− Total 0 2 8 2 2 1 0 4 2 3 9 1 1 1 7 0 5 3 2 3 丘、.e〃α 11.3− (Ⅰ−M3) 12.2− Gadj=5.74 13.ト d′=6,邦.∫. 14.0− Total 0 2 9 0 1 1 3 2 0 1⊥ 6 1 2 3 3 7 9 1⊥ 0 1 2 5 且ク聯J〟S 21.0− (CBL) 22.0− G8dj=8.94 23.0− d′=15,乃.ざ. 24,0− 25.0 26.0− Total 2 3 5 6 5 6 7 2 3 4 6 00 6 8 5 3 *Type6hasthreesalientanglesonthelingualside,aShortposteriorloop,andaconfluent dentalisthmusbetweentriangles.ExceptforType6,alltypesappearinginKaneko(1990,・ 1992)werefollowedinthepresentclassification. n.s.:nOn−Slgnificant.
12 肋∽∽αJS≠〟め′ 21:1996
Astatisticallyslgnificantassociationwasfoundbetweentheposteriorloop
patterns(Pl−P4)andthree CBLclasses(20.5−,23.0−and25.0−mm,Table4), becausetheDRFwasnearlyconstantthroughoutthe22r24mmCBLclassesbut decreasedonlyinthelargest25mmCBLclass(Fig.3aムdTablel).Further− more,ateStbetweenpatternsPl−P4andthethreebodyweightclasseswould besignificant(Table5),becausebodyweightcorrelatessiginificantlywithCBL (r=0.911,P<0.001,d.f=97)andindividualswithaCBLofmorethan25mm COrreSpOndwiththoseofbodyweightsofover30g.Asageincreases,thepatternoftheposteriorlooptendstobecomesimple
inClethriononvLSglareolus(Zejda1960)andC.nLfbcanus(Abe1982),Whichhaverootedmolarsin olderindividuals.Thesimpleformincreasesinfrequency
from the root ratio exceeding63%in C.glayeolus and32%in C.ク頑canus
(Zejda1960,Abe1982).Duetothelossofthethirdreentrantfoldwithage,the
proportionofthe simple formincreasesin C.gh17mlus and C.ndbcanus.In COntraSt,ageVariationwasnotobservedontheloopinMicrotuspennsyluanicus, Whichremainsrootlessthroughoutlife(Oppenheimer1965). AmongtenspeciesofEothenon砂Shavingrootlessmolarsthroughoutlife, agradualtransitionisfoundintheagevariationoftheposteriorlooppattern: inE.andeYSOnitheratioofthesimpleformoftheposteriorloopincreaseswith advancingage(Miyao1966,Kitahara1995),andinE.shanseiustheproportion OfthesimpleformincreaseswithincreasingCBL,thoughsamplesfromdiffer− entpopulationswerepooled(Table6),reSemblinginthisrespectClethriononws glareolusandC.rt4bcanus.Incontrast,ageVariationhasnotbeenobservedin
either Einez,eVa,プ聯Ius,Chinenめ,u)aYdi,CuStOS,Or Pnditor,they thus resemble Micntuspenn即Iuanicus,thoughsamplesfromdifferentpopulations WerepOOled(Table6andunpublisheddata).Thus,E.smithiiisanintermedi− atetypebetweenthesetwogroupsshowingalittleagevariation. InE.smithii,theposteriorlengthofthethirduppermolar(PM3L)ceased togrowwithage,thoughboththetotalmolarlength(TM3L)andtheanterior length(AM3L)increasedwithage(TablelandFig.2).ThegrowthofTM3L, therefore,isrelatedtothatoftheanteriorlength.However,WhenPM3Lwas plottedagainstthefourposteriorlooppatterns(Pl−P4,Fig.6),theposterior lengthtendedtobecomerelativelyshorterfromPltoP4asCBLincreases.In C,ndbcanus,thesimpleform(P4)increasesgreatlyinfrequency(Abe1982),aS thelengthdecreaseswithadvancingage(Abe1973).Itissuggested,therefore, thattheincreaseinthefrequencyofP4isduetoashorteningofPM3Lwith
advancing age.Furthermore,in E.smithiiit appears that PM3L does not
decreaseprominentlywithadvancingage(Fig.2),becausetheincrea?einthe frequencyofthesimpleP4patternisrelativelylowerthanin C.rゆcanus. Acknou)ledgements:MyspecialthanksareduetoDr.K.Maeda,NaraUniver−
SityofEducation,forreadingadraft,andtoanonymousrefereesforreading andimprovingthefinalmanuscript.
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