specimens : hindlimb bones were measured in 83 individuals among the five 
 species , and also , 26 individuals were dissected in order to weigh the hindlimb muscle mass . 

we used only male specimens of m. 
 itatsi and m. sibirica and omitted female specimens , because of 
 their strong sexual dimorphism .
in particular , it was pointed out that the feeding ecology 
 of the female of m. itatsi differed from that of the males .
there is a possibility that the female of m. 
 itatsi does not use the aquatic resources .
both male and female specimens of the 
 other three species were used , since their both sexes also depend on the aquatic habitat 
 [ 3 , 20 , 26 ] .
muscle mass and skeletal length measurements : muscle mass was recorded 
 since it is considered to be proportional to the maximum power generated by the muscle 
 . the hindlimb muscle nomenclature used in the 
 present study and
equivalent names used in previous studies are listed in supplementary table 2 [ 9 , 14 , 16 , 21 ] .
all 
 carcasses had been frozen at 20c until dissection . during dissection , the left - side 
 hindlimb muscles were exposed .
some of the muscles were fused and could not be divided ; so 
 28 muscle groups were used for the measurements ( table 
 1table 1.muscles and muscle groups examined in this study and their abbreviationsabbreviationscontained musclesillpsiliopsoasgsfctflgluteus superficialis , gluteofemoralis 
 ( caudofemoralis ) , tensor fasciae lataegmpigluteus medius , piriformisgpacgluteus profundus , iliocapsularisoeobturator externusoigegemelli , obturator internusqfquadratus femorisrfrectus femorisv3vastus lateralis , vastus medialis , vastus 
 intermediusbfsttebiceps femoris , tenuissimus , semitendinosussmcrsemimembranosus cranialissmcasemimembranosus caudalissarsartoriusglagracilispecpectineusaddadductortcrehltibialis cranialis , extensor hallucis longustcatibialis caudalisfi3peroneus longus , peroneus digiti quinti , peroneus 
 brevisedlextensor digitorum longusglatgastrocnemius caput latelarisgmedgastrocnemius caput medialisplaplantarisfhlflexor hallucis longusfdlflexor digitorum longuspoppopliteuspesbextensor digitorum brevis , flexor digitorum brevis , 
 quadratus plantae , lumbricales , interosseus , calcaneometatarsalissolsoleusmuscular names were reffered to the previous studies [ 9 , 14 , 16 , 21 ] . ) .
the presemimembranosus muscle was not used in the analysis , because it was 
 absent in some specimens of n. vison .
then , adipose and connective tissues were removed before the muscles 
 were weighed to the nearest 0.001 g using an electronic balance ( ux420h , shimadzu corp . , 

eleven osteological characteristics were measured to the nearest 0.1 mm using 
 calipers ( fig .
the abbreviations are listed in table 2 . and table 2table 2.bone measurements used in the analysisabbreviationmeasurement definitionsflmaximum length of the femur from the upper rim of the 
 femoral head to the medial condyle parallel to the shaft.fgt*mediolateral breadth from the femoral head to the 
 greater trochanter.fapdanteroposterior diameter in the mid - shaft of the 
 femur.fmldmediolateral diameter in the mid - shaft of the 
 femur.feb*biepicondylar breadth of the femur.tl*length of the tibia from the anterior rim of the 
 medial condyle to the anterior rim of the talar trochlear articulation parallel to 
 the shaft.tsl*length from the anterior rim of the medial condyle to 
 the distal extension of the tibial tuberosity ( spine ) parallel to the shaft.tapdanteroposterior diameter the mid - shaft of the 
 tibia.tmldmediolateral diameter the mid - shaft of the tibia.pl*maximum length of the pelvis from the posterior end of 
 the ischium to the anterior end of the iliac wing.il*length from anterior rim of acetabulum to distal end 
 of iliac wing.the bone measurements
( 2013 )   for these measurements , except for fgt , pl and 
 il .
the measurements were using definitions from a previous study , except for fgt , il and pl .
the fgt measurement was 
 defined as the breadth of the femoral head to the greater trochanter of the femur , in order 
 to obtain clear examination of lever arm length of the gluteus superficialis muscle in the 
 lateral direction .
the il and pl pelvis measurements were taken in addition to the existing 
 measurements described by samuels et al . .
muscular names were reffered to the previous studies [ 9 , 14 , 16 , 21 ] .
( 2013 )   for these measurements , except for fgt , pl and 
 il .
definition of aquatic tendency : aquatic tendency was defined using the 
 dietary data of each species based on the weight ratios of fish remnants in total feces 
 weight in the four species , except for e. lutris [ 4 , 6 , 18 , 22 , 37 ] .
the percentages of consumed fish and aquatic tendencies were as 
 follows ; m. sibirica , 0.0% ; m. itatsi , 17.7% ; n. 
 vison , 23.9% ; and l. lutra , 81.1% .
since e. 
 lutris lives their entire lives on the sea surface , we considered their aquatic tendency to be 100% .
we defined the terms  aquatic  ,  semi - aquatic  and  terrestrial  as follows .
aquatic 
 animal : species that possess the ability to live on and/or in the water , e.g. e. 
 lutris .
semi - aquatic animal : species that feed on fish and show terrestrial 
 adaptation , e.g. l. lutra , n. vison and m. 
 itatsi . terrestrial animal : species that are completely adapted to terrestrial 
 habitats and never feed on fish , i.e. they do not possess the ability to hunt fish , e.g. 
 m. sibirica . statistical analysis : statistical tests were performed to examine the 
 relationships between hindlimb morphological characteristics and ecological aquatic tendency 
 in mustelidae .
the tests included interspecific pair - wise comparisons , principal components 
 analysis ( pca ) and partial mantel tests .
the muscle masses were divided by the geometric 
 means ( gm ) calculated from obtained 27 masses of muscle groups , except for sol , since 
 e. lutris lacks sol .
the bone measurements were divided by the gm of 
 femoral length ( fl ) , tibial length ( tl ) and pelvic length ( pl ) .
analysis of variance ( anova ) 
 was conducted to detect significant differences in the muscular and skeletal measurements 
 between the five species .
homogeneity of variance was tested using the bartlett s test for 
 post - hoc comparisons .
pair - wise comparisons were made using the games  howell method to 
 reveal the interspecific statistical significance between the muscle and bone measurements . 

the relative positions of the species were visualized in morphological space using pca and 
 by drawing biplots with the first ( pc1 ) and second ( pc2 ) principal components .
the variables 
 used in the pca were the scaled measurement values calculated from the respective gm values . 

seven skeletal measurement variables were used in the pcas to avoid deficient values ; these 
 were fl , fgt , feb , tl , tsl , pl and il ( table 2 ) . 

the partial mantel tests were performed to estimate the functional and morphological 
 correlations between hindlimb morphology and the degree of the aquatic tendency in 
 mustelidae , after phylogenetic effects had been removed .
we prepared euclidean distance matrices of the five mustelids for 
 morphology , ecology and phylogeny .
the morphological distance matrices were prepared with 
 pc1 , pc2 and pc3 from the pca results of the muscle and bone data .
the ecological distance 
 matrix was compiled from sine - transformations of the aquatic tendency ratios described 
 earlier .
the phylogenetic distance matrix was constructed of molecular data obtained from 
 the sequence homology of the cytochrome b gene   ( table 3table 3.extracted data of cytochrome b gene patristic distances for 
 1,140 bp in the 5 species that were applied to the partial mantel test ; from marmi 
 et al .
sibirica22.122.118.76.4the accession numbers of national center for biotechnology information for samples 
 used in making the phylogenetical distance matrix from cytochrome b 
 analyses by marmi et al . 

are as follows : m. sibirica , ab026108 , ab051242 , ab051243 ; m. 
 itatsi , ab026104 ; n. vison , ab026109 , af057129 ; l. 
 lutra , af057124 , x94923 ; e. lutris , ab051244 , af057120 , 
 x94924 . ) .
the accession numbers of national center for biotechnology information for samples 
 used in making the phylogenetical distance matrix from cytochrome b 
 analyses by marmi et al . 

are as follows : m. sibirica , ab026108 , ab051242 , ab051243 ; m. 
 itatsi , ab026104 ; n. vison , ab026109 , af057129 ; l. 
 lutra , af057124 , x94923 ; e. lutris , ab051244 , af057120 , 
 x94924 .
muscle mass measurements : the mean values , standard errors and 
 interspecific significant differences of the 28 muscle weight measurements that were scaled 
 by geometric mean are shown in table 
 4table 4.mean values , standard deviations and interspecific significant differences of 
 measurements for geometric mean scaled muscle massesgroupsenhydra lutrislutra lutraneovison visonmustela itatsimustela 
 sibiricameansdvs.meansdvs.meansdvs.meansdvs.meansdvs.illps1.750.321.230.14is1.590.252.030.20l1.820.31lgsfctfl3.160.32lvis1.750.09e1.640.20e1.730.11e1.820.19egmpi4.070.38lvis2.230.14ev1.790.14el1.930.17e1.980.28egpac0.420.090.340.05s0.320.04s0.270.03s0.220.02lvioe0.990.15lvis0.560.05eis0.500.04e0.450.06el0.410.05eloige0.230.06vis0.230.01vis0.410.08el0.410.04el0.380.06elqf0.080.03lis0.180.01e0.140.040.180.03e0.240.05erf1.300.20lvis1.900.14e1.990.20e2.090.07e2.200.27ev32.130.41vis2.970.35s3.350.08eis3.610.09ev3.930.26elvbfstte5.020.46s5.620.34s5.530.47s5.980.576.660.40elvsmcr*1.950.14li2.620.25e2.310.312.300.19e2.550.34smca1.160.11lvis2.480.18ei2.260.16ei3.130.22elv3.140.52esar*2.100.391.700.151.840.252.120.322.110.31gla*1.030.19lvs1.540.22e1.810.29e1.410.081.560.18epec0.440.060.350.060.330.040.270.120.350.05add1.980.16lvis2.670.22e2.670.23e2.620.29e2.610.19etcrehl*1.140.481.430.141.270.061.220.091.230.11tca0.610.320.330.050.300.040.250.060.260.08fi31.290.09lvis0.770.11e0.820.06e0.770.04e0.700.05eedl0.760.120.640.100.560.030.590.040.500.05glat1.130.11vs1.230.081.410.10e1.390.151.590.24egmed2.390.13lis2.050.10eis1.680.491.710.09el1.760.15elpla*1.130.141.210.051.100.121.130.081.170.05fhl0.540.05lvi0.910.06e1.030.12e0.910.07e0.910.31fdl*0.260.050.280.050.300.050.340.050.260.05pop0.510.02lvis0.300.04e0.340.02es0.290.04e0.240.03evpesb*1.010.161.100.201.040.110.890.210.880.24sol0.000.00lvis0.170.03e0.250.05es0.160.01e0.140.02evmuscle groups are defined in table 1 .
asterisks indicate no significant differences 
 between the 5 species for that muscle group ( p>0.05 ) .
b ) 
 vs. , species that showed significant differences in univariate anova tests at the 
 p<0.05 level using games - howell s tests post hoc procedure ( e , enhydra lutris .
, there were significant differences between species for the muscle 
 groups , except for smcr , sar , gla , tcrehl , pla , fdl and pesb .
interspecific pair - wise 
 comparison analyses also revealed significant differences in the relative mass of the muscle 
 groups between species ( table 5table 5.factor loadings of muscle masses in pcaabbreviationspc1pc2pc3illps0.130.650.47gsfctfl0.880.360.04gmpi0.920.220.16gpac0.780.420.09oe0.960.010.02oige0.590.090.63qf0.750.130.27rf0.880.140.05v30.900.240.11bfstte0.680.460.39smcr0.570.170.53smca0.870.210.04sar0.080.720.15gla0.640.220.08pec0.520.030.04add0.730.070.18tcrehl0.110.760.25tca0.700.340.05fi30.920.110.12edl0.770.130.11glat0.680.220.10gmed0.710.030.39pla0.010.450.35fhl0.640.410.21fdl0.220.240.54pop0.920.030.20pesb0.280.760.15sol0.750.440.18ev13.283.702.04pve47.4413.227.30cpve47.4460.6667.96the abbreviations are defined in table 1 . 

there were significant differences between e. lutris and at 
 least one other species for 18 muscle groups ( gsfctfl , gmpi , oe , oige , qf , rf , v3 , bfstte , 
 smcr , smca , gla , add , fi3 , glat , gmed , fhl , pop and sol ) ( table 4 ) .
some muscle groups showed significant differences 
 between e. lutris and all four other species .
the masses of 
 gsfctfl , gmpi , oe , fi3 and pop muscle groups were significantly larger in e. 
 lutris than in the other four species , but those of rf , smca , add and sol were 
 significantly smaller .
in the case of sol , this is because it does not exist in e. 
 lutris .
notably , in four muscle groups ( gsfctfl , rf , add and fi3 ) , the only 
 significant difference was between e. lutris and the other four 
 species .
asterisks indicate no significant differences 
 between the 5 species for that muscle group ( p>0.05 ) .
b ) 
 vs. , species that showed significant differences in univariate anova tests at the 
 p<0.05 level using games - howell s tests post hoc procedure ( e , enhydra lutris .
not all muscle groups in e. lutris were significantly different to all 
 four other species .
for example , the gmed muscle group was significantly larger in 
 e. lutris than in l. lutra , m. itatsi 
 and m. sibirica , but not significantly different to that of n. 
 vison .
some other muscle groups were smaller in e. lutris than 
 in some of the other species ; qf , smcr , gla and fhl muscle groups were significantly smaller 
 than those of l. lutra ; oige , v3 , gla , glat and fhl were significantly 
 smaller than those of n. vison ; oige , qf , v3 , smcr and fhl were 
 significantly smaller than those of m. itatsi ; and oige , qf , v3 , bfstte , 
 gla and glat were significantly smaller than those of m. sibirica .
there were also significant differences between the muscle masses of the four other 
 species .
for example , the masses of 19 muscle groups in l. lutra were 
 statistically significantly different from at least one other species ( illps , gsfctfl , gmpi , 
 gpac , oe , oige , qf , rf , v3 , bfstte , smcr , smca , gla , add , fi3 , gmed , fhl , pop and sol ) .
the 
 mass of gmpi in l. lutra was larger than in n. vison , but 
 that of oige was smaller .
also , l. lutra had significantly larger oe and 
 gmed muscle mass than m. itatsi , but smaller illps , oige and smca muscle 
 groups .
furthermore , in comparison to m. sibirica , l. lutra had larger 
 gpac , oe and gmed muscle groups , but smaller illps , oige , v3 and bfstte muscle groups . for n. vison
, 16 muscle groups showed statistically significant 
 differences from at least one other species ( gsfctfl , gmpi , gpac , oe , oige , rf , v3 , bfstte , 
 smca , gla , add , fi3 , gmed , fhl , pop and sol ) .
v3 and smca were significantly smaller than 
 those of m. itatsi ; gpac , pop and sol were significantly larger than in 
 m. sibirica ; v3 and bfstte were significantly lower than those of 
 m. sibirica .
m. itatsi and m. sibirica indicated statistically 
 significant differences from at least one species in 17 muscle groups ( illps , gsfctfl , gmpi , 
 gpac , oe , oige , qf , rf , v3 , smcr , smca , add , fi3 , gmed , fhl , pop and sol ) and 18 muscle 
 groups ( illps , gsfctfl , gmpi , gpac , oe , oige , qf , rf , v3 , bfstte , smca , gla , add , fi3 , glat , 
 gmed , pop and sol ) , respectively .
however , the only significant difference between 
 m. itatsi and m. sibirica was for gpac , which was 
 significantly larger in m.itatsi .
no exclusive significant differences were observed between l. lutra and 
 the three mustelidae species , except for e. lutris in a comparison between 
 e. lutris and the others .
although no significant differences were 
 detected , l. lutra tended to have relatively larger distal hindlimb muscles 
 ( tcrehl , pla and pesb ) than those of the other species .
pc1 accounted for 47.4% of the variance and primarily separated e. lutris 
 from the other species ( fig .
numbers in parentheses 
 represent the percentage of the variation explained by the component . , enhydra lutris . 

pc1 was negatively 
 correlated with oe , gmpi , fi3 , pop and gsfctfl , but it was positively correlated with v3 , rf 
 and smca .
pc2 accounted for 13.2% of variance in the dataset and primarily separated 
 l. lutra and n. vison from the other species .
pc2 was 
 negatively correlated with pesb and tcrehl , but it was positively correlated with illps and 
 sar .
table 6table 6.the p values of partial mantel test of muscle massesmo vs. ecmo vs. phec vs. phmo vs ec con .
. shows the results of the partial mantel test using the ecological , 
 phylogenetic and morphological matrices prepared from pca scores of the muscular weights . 

the coefficient of correlation between morphology ( pc1 ) and ecology was 0.70 
 ( p<0.05 ) .
bone length measurements : there were significant differences in fl and pl 
 bone length measurements between the five species ( table 7table 7.mean values , standard deviations and interspecific significant differences of 
 geometric mean scaled bones measurementsitemsenhydra lutrislutra lutraneovison visonmustela itatsimustela sibiricanmeansdvs.nmeansdvs.nmeansdvs.nmeansdvs.nmeansdvs.fl130.810.01lvis100.920.01evis101.010.01elis261.050.01elvs241.090.01elvifgt130.310.01lvis100.280.01evis100.260.00elis230.230.01elv240.230.01elvfapd130.100.01vis50.090.00vi100.070.00elis240.080.01elv240.080.00evfmld130.140.01lvis50.100.01evis100.080.01el240.080.00el240.080.00elfeb130.240.01vis100.230.01vis100.200.01elis230.190.01elvs240.180.01elvitl130.930.01lvis100.990.02evis101.060.01els261.060.01els241.040.01elvitsl130.440.02is100.440.02vis100.420.01lis260.390.02elvs240.380.02elvitapd130.100.01vis50.100.01vis90.080.01el240.080.01el240.080.01eltmld130.070.01vis50.070.00vis90.060.00el240.060.00el240.060.00elpl131.320.02lvis101.100.03evis100.940.01elis260.900.01elvs240.890.01elviil130.590.03lvis100.490.02e100.490.01e260.480.01e230.480.01ethe abbreviations of measurements are defined in table 2 .
c ) vs. , species that 
 showed significant differences in univariate anova tests at the 
 p<0.05 level using games - howell s tests post hoc procedure ( e , 
 enhydra lutris .
fl was shorter in species that possessed the higher aquatic tendency 
 ( e. lutris and l. lutra ) , being the shortest in 
 e. lutris .
in contrast , pl was longer in these species with more aquatic 
 tendency and was longest in e. lutris .
c ) vs. , species that 
 showed significant differences in univariate anova tests at the 
 p<0.05 level using games - howell s tests post hoc procedure ( e , 
 enhydra lutris .
s , 
 mustela sibirica ) the percentage of variation explained by pc1 was 84.4% , which separated the samples into 
 the following four plots on the basis of descending scores ; e. lutris , 
 l. lutra , n.vison and lastly both 
 mustela(fig .
numbers in 
 parentheses represent the percentage of the variation explained by the component . 
 ,
table 8table 8.factor loadings of bone measurements in pcaabbreviationspc1pc2pc3fl0.980.020.00fgt0.960.060.13feb0.940.170.23tl0.930.220.10tsl0.750.620.22pl0.990.130.01il0.870.380.30ev5.910.620.22pve84.398.883.10cpve84.3993.2796.37the abbreviations of bone measurements are defined in table 2 .
both fl and tl correlated negatively with pc1 , whereas fgt , feb , tsl , pl and 
 il correlated positively .
the percentage of variation explained by pc2 was 8.9% , and a 
 separation of plots was found between e. lutris and both n. 
 vison and l. lutra .
m. itatsi and m. 
 sibirica had intermediate scores and were not separated from other species .
numbers in 
 parentheses represent the percentage of the variation explained by the component . 
 , enhydra 
 lutris .
table 9table 9.the p values of partial mantel test of bone measurementsmo vs. ecmo vs. phec vs. phmo vs. ec con .
. shows the results of the partial mantel tests of the ecological , phylogenetic 
 and morphological matrices .
the coefficient of correlation conditioned on the phylogeny 
 between the morphology ( pc1 ) and the ecology was 0.89 ( p<0.05 ) .
several studies have reported on the swimming motion of e. lutris [ 17 , 36 , 40 ] , but no studies have described sequence data of the 
 swimming motion and the movements of each hindlimb joint .
howell   studied the hindlimb skeleton of e. lutris and 
 predicted that its submerged swimming motion differed from that of pinnipeds .
he suggested 
 that the swimming motion of e. lutris includes placing the feet 
 horizontally to the rear with the soles up , one either side of the tail and oscillating them 
 in the sagittal plane , in a motion similar to that of whale flukes .
he suggested that the 
 swimming motion of e. lutris is characterized by dorsoventral undulation of 
 the trunk and by movements of both hindpaws in the dorsoventral direction , with the soles 
 facing dorsally and/or caudally .
the gsfctfl , gmpi , 
 oe , fi3 and pop muscle groups are proportionately approximately two times larger in 
 e. lutris than in the other species , but the rf , smca and add are 
 significantly smaller in e. lutris than in the other species ( table 4 ) .
these results indicate that e. 
 lutris utilizes gsfctfl , gmpi , oe , fi3 and pop more powerfully than the other 
 four species since muscle mass is proportional to work capacity .
therefore , we suggest that during submerged swimming , the power - stroke 
 phase of e. lutris comprises abduction of the femur by the gsfctfl and gmpi 
 muscles , medial rotation of the crus by the pop group , eversion movements of the ankle and 
 also abduction of the fifth toe or extension of interdigital web by the fi3 group , in the 
 fi3 group the peroneus brevis and the peroneus longus enable abduction of the fifth toe , and 
 the peroneus digiti quinti extends the interdigital digit .
the gluteus superficialis , 
 gluteofemoralis and tensor fasciae latae muscles in the gsfctfl group are fused in 
 e. lutris [ 16 , 17 ] .
this muscle group covers the lateral side of the hip 
 joint ; it attaches the lateral femoral ridge of the inferior border of the greater 
 trochanter to the upper border of the lateral femoral condyle .
therefore , gsfctfl may act as an abductor of the hip joint . in 
 addition , it is pointed out that the piriformis muscle of e. lutris acts as 
 an abductor on femur .
movement of these three 
 muscles at the hip , knee and ankle joints is not required for running on land .
therefore , 
 the greater mass and power of these muscles in e. lutris suggests that 
 these movements of the hindlimb are used for the swimming motion of this species .
the other mustelids are equipped with large muscles in the distal part of the hindlimbs 
 ( tca , edl , gmed and pesb ) , according to their aquatic tendency ( table 4 ) .
however , the evolutionary specializations observed in 
 e. lutris were not observed in m. sibirica , m. 
 itatsi , n. vison or l. lutra .
the hindlimb 
 morphology of terrestrial and semi - aquatic mustelids is regarded as a continuum .
the north 
 american river otter ( lontra canadensis ) swims using dorsoventral movement 
 of the trunk , whereas seals swim using lateral movement of the trunk .
it is possible that none of the five species examined in this study 
 swim using lateral movement of the trunk .
the swimming motion of m. itatsi 
 and m. sibirica has not yet been studied , but dunstone described the 
 swimming motion of n. vison , which used all four limbs with either 
 diagonally opposite legs simultaneously under the body .
we suspect that the swimming motion of both of these mustelids may be similar 
 to that of n. vison , as the hindlimb muscle distribution morphologically 
 resembles in these species .
therefore , the results of our study suggest that 
 the semi - aquatic animals investigated in this study swim with a motion similar to 
 terrestrial running .
taylor   compared the roughness of iliac crest of 
 e. lutris with that of harbor seal ( phoca vitulina ) and 
 l. canadensis quaritatively , and noticed more aquatic species held 
 rougher crest .
he suggested that the sartorius muscles ( sar ) , which originate from the 
 crest , for adducting the femur were augmented in size with the increase in importance of the 
 hindlimbs as a paddle from his observations .
this aquatic adaptation tendency referred to by 
 taylor was not observed in any of the five species examined in our result , which showed no 
 significant differences .
although , the mean sar value in e. lutris ( mean 
 value 2.10 ) was proportionately larger than that of l. lutra ( mean value 
 1.70 ) , the mean values in other three mustelids : n. vison , 1.84 ; m. 
 itats , 2.12 and m. sibirica , 2.11 , were relatively larger than 
 that in l. lutra .
these data show that the aquatic tendency is not 
 associated with the size increasing of sar that was proposed by taylor , at least not in mustelidae .
various studies have shown femoral shortening in aquatic or semi - aquatic animals [ 28 , 29 , 33 , 35 , 38 ] .
samuels et al .   reported that femoral shortening brought the paddling 
 limb closer to the body and thus reduced induced drag during the recovery stroke when 
 swimming . in our study , we also found femoral shortening ( lower fl values ) in e. 
 lutris and l. lutra species which had higher aquatic tendency 
 ( table 7 ) .
additionally , it is interesting to 
 note the significant difference in fl between m. itatsi ( mean value 1.05 ) 
 and m. sibirica ( mean value 1.09 ) , which hunts fishes but it does not well 
 adapt to swimming , although both species diverged each other about 1.6 million years ago 
 .
smith and savage   suggested that more 
 aquatically adapted animals possess smaller gluteus and larger ischiopubic muscles , which 
 include the semimembranosus , biceps femoris , semitendinosus and tenuissimus after comparing 
 the iliac length in proportions to the pelvic bones among a marten ( martes 
 sp . ) , a river otter ( lutra sp . ) and a seal ( phoca sp . ) . 

however , our results do not support this suggestion since e. lutris had the 
 largest relative mass of gluteus muscles , and it and l. lutra also had 
 smaller relative mass of the semimembranosus , biceps femoris , semitendinosus and tenuissimus 
 muscles ( table 4 ) .
in addition , the il length 
 value of e. lutris ( mean value 0.59 ) was significantly much larger than 
 that of the other four species ( mean value ; l. lutra and n. 
 vison , 0.49 ; m. itatsi and m. sibirica , 0.48 ) 
 ( table 7 ) .
there are morphological 
 differences , since the body of seal moves bilaterally , but that of both e. 
 lutris and l. lutra moves dorsoventrally when they swim . the difference between the results of smith and 
 savage and ours was caused by following reason : e. lutris developed their 
 gluteal muscles and became to aquatic ; however , seals adopt another development for 
 aquatic . in conclusion , our muscle and bone measurement analyses revealed that the hindlimb 
 morphology of the mustelids has obvious functional and morphological relationships with 
 their ecology rather than with their phylogeny ( tables 
 6 and 9 ) .
we could confirm that 
 e. lutris had unique hindlimb morphological characteristics compared to 
 the other mustelidae . in our study , we defined the aquatic tendency of each species from 
 ecological information .
based on this information , we defined e. lutris as 
 having full aquatic tendency , whereas the other four species had semi - aquatic or fully 
 terrestrial tendencies .
furthermore , the muscle and bone analyses indicated morphological 
 differences associated with hip joint abductors between e. lutris and the 
 four other species that were categorized as terrestrial or semi - aquatic species .
therefore , 
 we suggest the hindlimb of e. lutris clearly reflects their complete 
 aquatic lifestyle . this hindlimb structure has resulted in e. lutris 
 acquiring a more abductable femur , as it evolved away from the other semi - aquatic mustelidae 
 and their need to partly survive on land .
the e. lutris abductable femur 
 may enable the species to develop maneuverability when they swim by raising the center of 
 mass near to the body axis , the same as the shortening of femur .
we conclude that e. 
 lutris is a complete aquatic animal , possessing differences in the proportions of 
 the hindlimb muscles compared with those in other semi - aquatic and terrestrial mustelids . 

the semi - aquatic species share similar muscle proportions with terrestrial rather than with 
 aquatic species .
 although the sea otter ( enhydra lutris ) is a complete aquatic species , 
 spending its entire life in the ocean , it has been considered morphologically to be a 
 semi - aquatic animal . 
 this study aimed to clarify the unique hindlimb morphology and 
 functional adaptations of e. lutris in comparison to other mustelidae 
 species . 
 we compared muscle mass and bone measurements of five mustelidae species : the sea 
 otter , eurasian river otter ( lutra lutra ) , american mink 
 ( neovison vison ) , japanese weasel ( mustela itatsi ) and 
 siberian weasel ( m. sibirica ) . in comparison with the other 4 species , 
 
 e. lutris possessed significantly larger gluteus , popliteus and 
 peroneus muscles , but smaller adductor and ischiopubic muscles . 
 the popliteus muscle may 
 act as a medial rotator of the crus , and the peroneus muscle may act as an abductor of the 
 fifth toe and/or the pronator of the foot . 
 the bundles of the gluteus superficialis muscle 
 of e. lutris were fused with those of the tensor fasciae latae muscle and 
 gluteofemoralis muscles , and they may play a role in femur abduction . 
 these results 
 suggest that e. lutris uses the abducted femur , medially rotated crus , 
 eversion of the ankle and abducted fifth digit or extended interdigital web as a powerful 
 propulsion generator . 
 therefore , we conclude that e. lutris is a complete 
 aquatic animal , possessing differences in the proportions of the hindlimb muscles compared 
 with those in other semi - aquatic and terrestrial mustelids . 
MATERIALS AND METHODS
RESULTS
DISCUSSION
Supplementary Material