VariationsoftheNumberofEmbryosinMicrotusmontebelli
（MILNE−EDWARDS）＊
YukibumiKANEKO BioIogicalLaboratory，FacultyofEducation，KagawaUniversity，Takamatsu，Japan
ABSTRACT．−Thedistributionofembryosinuterin￠horns，SeaSOnalchangesofbody Weightinpregnaptfbmales，andrelationbetweenthenumberofembryos，bodysize and age of pregnant fbmales，micro−habitat and season were describedin肋rotusmonlebelli．Number of specimens，Which were co11ected on ricefield，ricefieldin fallowandyoungplantation，WaS84．Therewasnostatisticallysignificantdeviation fromarandom distributjonofembryoswithintheuterus．Theaveragebodyweight
Ofpregnantfbmalessubtractingtheweightofembryoswasnotsignificantlydi鮎rent
duringayear．Theaveragenumberofembryosincreasedfromclass6to8andde− CreaSed fi．orn class9tollinage−grOupOfpregnantfヒmales．Therewasatendency
that theaverage bodyweight ofpregnantftmalesincreasedwiththeincreaseofthe
numberofembryosatthesameclassin age−grOup．Thenumberofembryosinthe SameWeightofpregnantftmalesonleveeswaslargerinSeptemberthaninMay．A
COmParison between two neighboring Octoberpopulations showed that the number
Ofembryosinthesamらbodyweightwaslargeronricefieldinfallowthaninyoung
plantation．Itissuggestedinthisvolethatthedifferences offoodqualitybrought aboutthevariationofthenumberofembryosofthesameageandbodysizeofpreg− nant fbmalesindi庁brentmicro−habitatsinthesameseasonand／orindi飴rentseasons in thesamemicro＿habitat． IntroductionIthasbeenwe11knownthatthenumberofembryosorlittersizevaries和ithseason，
year，body size，POPulation density，▲habitat，altitude andlatitude．Severalpapers
havebeenpublishedinMicrotusn−Omwi1dpopulationsinEuropeandNorthAmerica
onth占variationofthenumber極relationtoage，SeaSOn，year，habitat，bodysizeand
latitude（JAMESON，1947；BEE正et∼al．，1957；BEER＆MACLEOD，1961；KoTT＆
RoBINSON，1963；NEGt）＄＆PINTER，1965；TAST，1966；EvANS，1973）．IntheJapanese
丘eld vole，Microtus montebelli，however，ftw works have been carried out on the
Variationofthenumberofembryos，eXCePtthefbllowlngPaPerS：thenumberchanged
WithseasonduringayearinalocalityinJapan（MIYAOetdl．，1966；SHIRAISHI，1967；
ABE，1974；KANE竺0，1976）andwithhabitatsinneighboringOctoberpopulationsin
ayear（KANEKb，1978b）．
ThepresentpurposeofthispaperistOdescribethedistributionofembryosin
uterinehorns，SeaSOnalchangesofbodyweightinpregnantftmales，andtherelations
betweenthenumberofembryos，bodysize−andageofpregnantfbmales，micro−habitat
andseasonintheJapanesefieldvoie，Microtusmontebelli（MILNE−EDWARDS）．
＊ContributionNo．40fromtheBiologlCalLaboratory，FacultyofEducation，KagawaUniversity．哺乳動物学雑誌 Vol．7，No．3
122
Materials and Metbods
Inthisstudy84specimens oftheJapanesefield volewere taken on rice丘eld，
ricefieldin fallow and young plantationin the fbllowlngfivelocalitiesinJapan：
Morioka，Iwate Pref．；Ry6zu，Niigata Pref．；Iwakura，Kyoto Pref．；Jinsekiand
Kurome，HiroshimaPref．；and Fukuoka，FukuokaPref．The specimens caught at
Iwakura，JinsekiandKuromewerethesameaspreviouslydescribedinKANEKO（1976，
1978a＆b）．Thespecimenswerecapturedbysnap−traPSWithpumpkinseedsasbait．
Al1fbmaleswereweightedandtheirbodysizesweremeasured．Thenumberofvisible
embryosinbothhornsoftheuteruswascountedwiththenakedeye，andthetotal
weightofembryoswasmeasuredincludingthefbetalmembranesanduterus．Body
weightsubtractingtheweightofembryoswas■uSedasthatofeachpregnantfbmale．
Theclassinage−grOuP Ofpregnantftmaleswasdeterminedbythemorphology
ofoccipitalprocessandridges，Whichwasdescribedinthepreviouspaper（KANEKO，
1978a）．Sincetherewasagoodcorrelationbetweenbodyweightandthelengthofhead
andbody（KANEKO，1978a），bodyweightofpregnantftmaleswasusedasbodysize．
R（さSⅥ1ts and】Discussionl）Distributionofembryosinuterinehorns
Thenumberofvisibleembryosin40volesrangedn・OmtWOtOnineinthe1971−
72collectionsofIwakura，Kyoto．Theincidenceofunilateralpregnancywas7．5％
（3／40）and two specimens wereleft side pregnancies and one was right side．The
PrOPOrtionofbilateralpregnancieswithequalnumbersofembryosinbothhornswas
15．0％（6／40）．Therewasnosignificantdeviationn・Omarandomdistribution（Poisson
distribution）ofembryoswithintheuterus，aCCOrdingtoax2testasshowninTablel
Tablel．Thedistribution ofembryosbetweenleft（Lt）andright（Rt）uterinehornsof 助川助∫椚♂〝Jg∂d〃atIwakum，Kyoto，197ト72． Difrbrenceinnos． No．ofvolesofembryosbetween Distribution betweenhorns Observed
（total） Expected no．（∽） borns Lt＞Rt Rt＞Lt （6） ′LU 5 1 7 0 1 1 1 2 0 つJ 4 2 0ノ 00 3 0 5 2 0 1 1 0．590 0．308 0．379 0．474 2．200 0．011 ズ2＝3．962 O l つー3 4 5 00 5 4 0 1 7 ‘U つJ O O x，＝difftrencebetweentheobservedandexpectednumberofanimals．Variance（sり＝0．653・
（0．25＜p＜0．5）．ThesamefindingwasobtainedinM．montebelliinFukuokaPre仁
bySHIRAISHI（1967）andinM．penn叩IvanicusandM．ochrogasterbyCoRTHUM（1967）．
D Seasonalchangesofbodyweightinpregnantftmales
Inthe40specimensofIwakura，Kyoto，1971−72，thesma11estweightofpregnant
atIwakura，Kyoto，1971−72．
Month N Range（g） Meanj＝SE（g）
May Sept． Nov． Jan． Mar． 32．5−38．9 29．9−41．5 27．7−38．5 22．6−29．2 26，0−37．1 7 4 7 3 ￠U l 3（5．13土．98 35．93土．83 32．26土1．50 25．27士2．00 33．04±1．25
ftmaleswas22．6ginJanuary，andthelargestwas4l．5ginSeptember（Table2）．The
averagemonthlyweightoftheftmalesvariedftom25gto36g．Theaverageweight
WaS nearly equalthrough ayear exceptJanuary，When
Althoughtheaveragenumberofembryosdi鮎redstatisticallyamongmonthsexcept
between March and May（KANEKO，1976），the averageweightwas not significantly
di飴rent among six months．The mean bodylength ofpregnant fbmales did not
Change seasona11ythroughthebreedingseasonin ClethrionomysandbrsoniandApo−
ゐ∽〟∫α曙e〃fe〃∫（MIYAO，1969）．
3）Thenumberofembryosagainstclassesinageqgroupofpregnantfemales
Thevisibleembryosappearedatclass6inage−grOuPintheIwakurapopulation，
Kyoto，1971−72．The average number ofembryosincreased ftom class6to8and
decreasedftomclass9tollduringayear（Table3），thoughthedifftrencebetween
Table3．Thenumber ofembryos under various classesin age−grOupin Microtus montebelliatIwakura，Kyoto，1971−72．The statisticaldifference between Class8andclass6was O．2＜p＜0．3and the difftrence between other classes exceptclassllwasp＞0．6． No．indi鮎rentnumberclassesofembryos n p ．1 u eS rO SS瑠 a e l Cag No．ofembryos （Mean±SE） 2 3 4 5 6 1 1 2 7 1 1 4 8 9 Total 12 4．6土．4 2 9 5．6士．8 1 4 5．8士1．2 6 5．2±．5 1 4 ‘U 7 00 q／ O 1 1 1 2 1 2 つん 2 2 1
Classesexceptclassllwasstatisticallyinsignificant・NEGUS＆PINTER（1965）demon−
StratedinM・mOntanuSthatthelittersizeincreasedgraduallythroughthefifthcon−
SeCutivelitteranddecreasedthefbllowlnglitter．
Acomparisonofthenumberofembryos‘atthesameclassinage−grOuPShowed
that thelarge number occurredin September；While the smallone was fbundin
January（Fig・1）・Therefbre，thenumberofembryosatthesameclassinage−grOuP
Ofpregnantftmaleschangedwithseasons．
4）Relationshipbetweenthenumberofembryosandweightofpregnantftmales
InFig・2thenumberofembryosisplottedagalnSttheincreaseofbodyweight
哺乳動物学雑誌 Vol．7，No．3 124 。T 困刷冊SE脚 nU・ム 臼 ● ○
9 8 7 る 5 4 3 2 1
SOと芯≦山］○。OZ b ● ● 吐 血 血 nU O ・△ 03 4 5 る 7 8 9101112
ÅGE−GROUPFig．1．Scatter diagram of the relationship between classin age−grOup and the numberofembryosinMicrotusmontebelliatIwakura，Kyoto，197ト72． 0 9 8 7 d 5．4 3 2 1 SO≧垂烹〓〇．〇Z 0」AN，tr＝−0．占d4） ム仙AR．＝「＝†0．る65】 ロ八AY け＝†0．140） ●SEPT．け＝†0．d35） O NOV．け＝二0．089j ● ロ ●●臼 ●● 鎗 附● ●ム血● 00 （）A o dコ 0 ム 8 8 0 30 BODY WEJGHT（g） 40 Fig．2．Scatterdiagramoftherelationshipbetweenthenumberofembryosandbody WeightofpregnantfemalesinMicrotusmontebelliatIwakura，Kyoto，1971−72．
Bodyweight subtractingtheweight ofembryoswasusedasthat ofpregnant
females．The coe伍cient ofcorrelation（r）in each month was shownin this 負gure．
goodrelationbetweentheweightofmothersandthenumberofembryosina11the
SamPles during ayear（r＝0．647）．There was not so apparent correlation between the twoin each month（Fig．2），eXCePtin September（0．01＜p＜0．02）．Heavier
ftmaleswithlargenumberofembryoswereobservedinSeptember，Whilelighterftmale
WithsmallnumberofembryosoccurredinJanuary．
bryos under various classesin age−grOupin MicrotLES montebelliatIwakura， Kyoto，1971−72． Mean＊bodyweight（g）indifferentnumber Classesofembryos
Classes
111 age−grOuP Mean士SE 8 9 2 3 4 5 6 7 26．0 29．1 32．00土．97 40．3 35．01土1．85 39．1 34．98土2．23 35．72±．68 38．5 33．7 34．6 34．7 35．6 2夕．2 ′b 7 8 0ノ O 1 1 1 33．0 22．6 31．3 37．4 36．3 33．9 35．4 38．2 38．5 ＊NumberofspecimenswasgiveninTable3．thesame classin age−grOuP（Table4）．Therewas atendencythatthe averageinq
creasedwiththeincrease ofthenumber ofembryos atthe sameclassinage−grOuP・
AccordingtoBEERetal．（1957），therewasasteadyincreaseinnumberofcorporawith
increaseillbodylenBth．Furthermore，they showed thatthe totalintrauterineloss
variedinverselywiththesizeofftmalesinM．pennsylvanicusandC．gqpperi．There−
fbre，the number ofembryos atthe same classin agergroupincreasedwith thein−
CreaSeOfbodysize．
5）Thenumberofembryosindi鮎rentmicro−habitats
The number ofembryos changed seasonallyinfourdifrerentmicro−habitats on
cultivatedland atIwakura，Kyoto，197ト72（Table5）．The di飴rence of average
number ofembryos between di鮎rent micro−habitats was thelargestin September
amongfivemonths，thoughthedi鮎rencewasstatisticallyinsignificantin September
（0．1＜p＜0．2）．In Septemberthe vegetation ofleveesinfallowlandswascomposed
mainly ofPob）gOnum thunbergii，Commelina communis and Cherus microiria，and
Gb）Cinemaxwasplanted on severallevees．Threepregnantftmalesweretakenon
fallowlandinJanuary（Table5），When fallowland was densely overgrown with
Withering Solidqgo altissimain abotlt2m height，and young Soli‘わgo altissima
Table5．Thenumber ofembryosin variousmicro−habitatsin difrbfent mOnthsin MicrotusmontebelliatIwakura，Kyoto，1971−72．Each尽gurerepresentsnumber Ofembryosofeachpregnantfemale≡collectedineachmicro−habitat．：Number inparenthesesshowstheaveragenumberofembryos．
Agricultul−al road‥‥D Fallowlandsdensely Grass−grOWnlevee Levee
inね1lowユand ユnrlCe ‥．B 丘eld…．C 0VergrOWllWjth ふ痛血酢＝油盆山那 ‥‥A 5，6（5．5） 4，5（4．5） 5 6，7，9，9（7．8） 5，5，5，5，5， 7 5，6，9，9（6，0） 4，4（4．0） 4，4，4（4．0） May 4，5，7（5．3） Sept． Nov． 2，4（3．0） Jan． 2，2，3（2．3） March 5 3，4，4，5，5， 5 5（4．3）
Vol．7，No．3 哺乳一動物学雑誌
126
sproutedatthebaseofthewitheredroots・
FromTable6，itislikelythattheaveragenumberofembryosindi能rentmicro−
habitats on ricefieldinfallowdidnot so greatlyvaryin May，June，August and
Octoberinsevera1localitiesinJapan．
A8C l ● l O ム︵ソ 8 7．〇 5．4 3
S2∝山王山L〇．〇Z
A ∞ム Ol ム○● ムム息ム血 130
40
BODY WE）GHT（g）
20
50
Fig．3・Scatterdiagramoftherelationshipbetweenthenumberofembryosar）dbodyweightofpregnantfemalesinMicrotusmontebelliindifferentmicro−habitatsat
Iwakura，Kyoto，1971−72．A；fallowiand densely overgrownwith Sb肋如 altissima，B：Grass−grOWnleveeinfallowland，C：Leveeinricefield・Open
symboIsrepresentspecimensinSeptember，andsolidonesshowspecimensin
May． 7 ′0 5 ．4 3 つ‘ SO＞出血王山 ﹄〇．〇Z 00 0 000 ■l 00● ○ ● ●● ● ● ● ● ●20 30 40 50
BODY WEtGHT（g） Fig．4．Scatterdiagramoftherelationshipbetweenthenumberofembryosandbody weightofpregnantfemalesin凡打crotusmontebelHinneighboringOctoberpopu− 1ations，HiroshimaPref．，1972．Opencirclesrepresentfemalesonricefieldinfallowandsolidonesshowfbmalesinyoungplantations．
コ ト．刊？寸 仇 M．刊﹁寸 p−ひ唱 Z 国S¶ロ召∑ 0−qd−亀ぎ JO∽払p佃銘 打︶ コ ︵∞．巴∽ゼゼ 寸．刊∽．れ ゼゼゼべれM m．刊○這 ぷp
鋸
Sき○竜∽OS月︶岩岳d点J月日nZ pdOh ︻空n︸ち0叫払く ︵巴 ︵？寸︶れhM や ︵﹁己れれれ れ音へ寸れMれM ︵？示︶ り．れれ寸 心小人ひJ rq︶ ．sOh−q己ごOJOq己2＝払巴璧ゐ﹄華 ．︸眉q遥占JO叫∈竜dOひ︸8−ちじ書宍喜騙苫垂論ビd ざ．豊 トゼれれ ︵∞．寸︶ れーれ．れへれれれ寸 ︵竺寸︶り㌦．れれ寸寸 ︵∞．巴 トドぜ．れれ叫れれ p日月き0コd u叫0りAひ﹂ ︵U︶ く芭00琶 pロdlき○∃虎 口き○誌・S∽已0 ︵巴 ∽りS∽d︼叫 L≡き星g払 ﹂J診○とOS宕p p白虎lき○コd﹄ ︹く︶ やU−qd↑ M卜巴 さ羞＝d琶nづ広 Nト仇︷ −月○︸00 3ひ∽月ヽ ︻トのー ︺sコ地コV コZ叉亘 Mトヘ︻ へ告nトムd∑ ぷ○叫旨∑ 重石○苫 き叫1日OJ哺乳動物学雑誌 Vol．7，No．3
128
Sincethenumberofembryosatthesameclassinage−grOuPlnCreaSedwiththe
increase ofbodyweight（Table4），COmParisons ofthe numberin the same weight
Weremadeamongdifftrentmicro−habitats（Fig．3）．Thenumberinthesameweight
OnleveewaslargerinSeptemberthaninMay（B＆CinFig．3）．Itfb1lowsthatthe
number ofembryosin the same weight ofpregnant ftmales collectedin the same
micro−habitatchangedwithseasons．
Inthepreviouspaper（KANEKO，1978b），habitatdi飴rencesbetweenneighboring
Octoberpopulationsbroughtaboutthevariationoftheaveragenumberofembryos，
throughbothpopulationswereconsideredtobethesameinagecomposition・Figure
4showsthatthenumberofembryosinthesamebodyweightislargeronricefieldin
fallowthaninyoungplantation．
Summaringthesefindingsabovementioned，COmParisons ofthe same size and
age showedin Microtus montebellithat the number ofembryos changed primarily
Withthedi飴renttypesofmicro−habitatinthesamecollectingperiodandwithseasons
inthesamemicro−habitat．AlthoughEvANS（1973）showedinM．agrestisthatthere
WerenOSignificantdifrbrencesinpotentiallittersizesbetweenyoungplantationand
limestone grassland，NEGUS＆PINTER（1966）revealedin M．montanus that small
dietary supplements ofeither sprouted wheat or acetone−ether extracts ofsprouted
Wheatelicitedimprovedreproductiveperfbrmance・Furthermor？，theydemonstrated
thattheimprovementresultedprimarilyinmorefiequentpostpartummaitings and
lower rates oflitterloss．Therefbre，itis suggested that the di鮎rences of food
qualitybroughtaboutthevariationofthenumberofembryosinthesameageand
Size ofpregnant ftmalesin diffbrent micro−habitatsin the same season and／orin
di飴rent seasonsin the samemicro−habitat．Acknowledgement
IwishtothanktoDr．Y．FuJIMAKIforreadingandcriticizlngthemanuscript・
摘 要 金子之史：ハタネズミにおける胎児数の変化 水田，水田休耕地および君命造林地で採集した84個体のハタネズミ妊娠雌について，胎児の左右 子宮内での分布，妊娠雌の体重の季節的変化，および胎児数と妊娠雌の体重・年令，捕獲された微小 棲息場所，捕獲時期との関係を調査した。 1．子宮内の胎児の分布のし方ほ，ポアソソ分布から統計的に有意な差を示さなかった。 2．胎児体重を差し引いた妊娠雌の体重は，1年間で有意な差は示さなかった。 3．平均胎児数は妊娠雌のage−grOup6から8にかけて増大し，9から10にかけて減少したが，統 計的に有意のある差ではなかった。 4．同一クラスのage一許Oupの妊娠雌で比較すると，胎児数の増加は妊娠雌の平均体重の増加に伴 なっていた。 5．畦で捕獲された同一体重の妊娠雌における胎児数は，5月よりも9月の方が大きかった。 6．同時期に採集した近接した2個体群内の同一体重の妊娠雌の胎児数ほ，水田休耕地の方が若令 造林地よりも大きかった。棲息場所の異なった採集時期，および同一時期に採集した異なった微小接点場所，によってもたらさ れる食物の質のちがいによって変化することが示唆された。
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