A Geometric Morphometric Analysis ofthe Medial Tibial Condyle of African

HominidsADAM D. SYLVESTER*

School of Life Sciences, College of Medical, Veterinary and Life Sciences, University ofGlasgow, Glasgow, G12 Q88, United Kingdom

ABSTRACTAlthough the hominid knee has been heavily scrutinized, shape varia-

tion of the medial tibial condyle has yet to be described. Humans, chim-panzees, and gorillas differ in the shape of their medial femoral condylesand in their capacity for external and internal rotation of the tibia relativeto the femur. I hypothesize that these differences should be reflected inthe shape of the medial tibial condyle of these hominids. Here I use geo-metric morphometric techniques to uncover shape differences between themedial tibial condyles of humans, chimpanzees, and gorillas. Humans aredistinguished from the other two species by having a much more oval-shaped medial tibial condyle, while those of chimpanzees and gorillas aremore triangular in outline. Gorillas (especially males) are distinguished byhaving more concavely-curved condyles (mediolateral direction), which isinterpreted as an effect of heavy loading through the medial compartmentof the knee in conjunction with differences in the degree of arboreality.Anat Rec, 296:1518–1525, 2013. VC 2013 Wiley Periodicals, Inc.

Key words: knee; Homo; Pan; Gorilla; biomechanics

Animal movement continues to be of keen interest tobiologists and paleontologists alike because locomotionprovides access to the key resources of food, water, safety,and potential mates. The articular surfaces of long bonesare particularly useful for understanding locomotor adap-tations because their size and shape are related to jointfunction, meeting the requirements for articular strength,mobility, and stability during normal locomotion (Currey,1984; Swartz, 1989; Godfrey et al., 1991; Hamrick, 1996).Strength is the magnitude and frequency of loading ajoint can withstand without suffering damage and is par-ticularly important because irreparable damage canimpede joint function and hinder locomotion (Hamrick,1996). Joint mobility is the potential range of motion of ajoint, and stability is the ability to withstand motions out-side normal kinematics or that disrupt joint integrity(Hamrick, 1996).

The hominid knee is especially well-studied because ofthe information the joint provides for identifying bipedsin the fossil record and for understanding the evolution ofhominin bipedalism (see Ward, 2002). Features of boththe distal femur and proximal tibia have been used to dis-tinguish bipedal humans from quadrupedal non-human

apes (Thompson, 1889; Preuschoft, 1971; Tardieu 1981,1983), fossil hominins from other extinct primates, andfor taxonomic or locomotor distinctions among fossil homi-nins (Stern and Susman, 1983; Senut and Tardieu, 1985;Zihlman, 1985; Tardieu, 1986, 1999; McHenry andBerger, 1998). Compared to chimpanzees and gorillas, dis-tinctive features of the human distal femur include adeep patellar groove with a projecting lateral lip (Le GrosClark, 1947), distally flattened femoral condyles (Heipleand Lovejoy, 1971), and a high bicondylar angle (Sternand Susman, 1983). A higher bicondylar angle moves theknee closer to the midline of the body, making balancing

*Correspondence to: Adam D. Sylvester, Department of HumanEvolution, Max Planck Institute for Evolutionary Anthropology,Deutscher Platz 6, Leipzig, Germany D-04103. E-mail: Adam.Sylvester@glasgow.ac.uk

Received 31 July 2012; Revised 12 June 2013; Accepted 19June 2013.

DOI 10.1002/ar.22762Published online 19 August 2013 in Wiley Online Library(wileyonlinelibrary.com).

THE ANATOMICAL RECORD 296:1518–1525 (2013)

VVC 2013 WILEY PERIODICALS, INC.

on a single support foot easier (Preuschoft, 1971). Thedeep patellar groove prevents patellar subluxation/dislo-cation that can result from the contraction of the quadri-ceps muscle in combination with a valgus knee (Lovejoy,2007). The distally flattened femoral condyles increasethe contact area between tibial and femoral condyles atknee angles near full extension (Kettlekamp and Jacobs,1972; Maquet et al., 1975; Maquet, 1976). All of these fea-tures are accommodations to habitual bipedal locomotionon a relatively extended lower limb.

Differences in knee function (and by extension locomo-tor behavior) are also reflected in other aspects of mor-phology. In humans, the medial and lateral menisci arecrescent-shaped and each attaches to the tibia via two lig-aments, one ligament for the posterior horn of the menis-cus and a separate ligament for the anterior horn(Tardieu, 1986). In chimpanzees and gorillas, a single lig-ament attaches a ring-shaped lateral meniscus to thetibia. The single attachment allows the meniscus to trans-late more freely in an anteroposterior direction whichfacilitates significant external and internal rotation of thetibia at the knee (Tardieu, 1986). The medial meniscus ofthe non-human apes has two attachment points, as mod-ern humans do, making it relatively immobile.

Coupled with the difference in lateral meniscus attach-ment is a difference in the anteroposterior curvature ofthe lateral tibial condyle. Humans are characterized byrelatively flat lateral tibial condyles, whereas the chim-panzee and gorilla lateral tibial condyles are convexlycurved (Thompson, 1889; Trinkaus, 1975; Tardieu, 1983).Organ and Ward (2006) argued that the convex curva-ture of chimpanzee and gorilla lateral tibial condyles pro-vides greater stability throughout the range of kneeflexion-extension. The convex lateral tibial condyles ofchimpanzees and gorillas, paired with the mobile lateralmeniscus, are likely related to their greater capacity forinternal and external rotation at the knee. Tardieu(1986) demonstrated via manipulation of cadaveric limbsthat chimpanzees have a much larger range of internal/external rotation at the knee (�30–40 degrees more).Research on human and chimpanzee knees indicatesthat this rotation occurs about a longitudinal axis thatruns through the medial compartment of the knee paral-lel to the shaft of the tibia (Tardieu, 1986; Freeman andPinskerova, 2005).

Although both the distal femur and aspects of the lat-eral tibial condyle have been scrutinized for functionaland behavioral differences between humans and non-human apes, the medial tibial condyle has received lessattention. Sylvester and Organ (2010) examined theanteroposterior and mediolateral curvature of the medialtibial condyle, but did not find evidence for differencesbetween humans, chimpanzees, and gorillas or an effectof body mass on curvature. Difference in the range oflongitudinal knee rotation and medial femoral condyleshape between human and non-human apes suggests,however, that there may be differences in the shape ofthe medial tibial condyle. Here I use geometric morpho-metric techniques to determine whether shape differen-ces exist between the medial tibial condyles of human,chimpanzees, and gorillas. I test the null hypothesisthat medial tibial condyles are the same shape in allspecies and examine any shape differences in light ofsome of the known differences in knee function andmorphology.

MATERIALS AND METHODS

Sample and Data Collection

The sample consists of tibiae from 135 African homi-nid specimens curated as parts of the Hamann-ToddOsteological Collection (Cleveland Museum of NaturalHistory), the William M. Bass Skeletal Collection (TheUniversity of Tennessee), and the Ta€ı chimpanzee skele-tal collection (Max Planck Institute for EvolutionaryAnthropology) (Table 1). All specimens are free frompathology and skeletally adult. Polyvinylsiloxane molds(President Jet Regular, Coltene-Whaledent) of the proxi-mal tibia of specimens that are part of the Hamann-Todd Collection were prepared (Galbany et al., 2006),and molds were scanned using a NextEngine laser scan-ner. Tibiae that are part of the Ta€ı chimpanzee skeletalcollection were scanned using a Breuckmann optoTop-HE white light surface scanner. Both scanners directlyproduce triangulated mesh surface models. The whitelight scanner produced surface models with vertices thatwere on average less than 0.3 mm apart, and the laserscanner produced models with vertices that were lessthan 0.2 mm apart. Human tibiae from the William M.Bass Skeletal Collection were scanned on a GE Light-speed Computed Tomography 16-slice scanner (100 kVp,150 mA, Filter 5 “Body Filter”) with isometric cubic vox-els (0.625 mm). Image stacks were manually segmentedin commercially available software (AvizoVR , VisualizationSciences Group, Burlington, MA) resulting in surfacemodels of the bones (Sylvester et al., 2008). All surfacemodels of the proximal tibiae were imported into a vir-tual workspace (Geomagic StudioVR , Geomagic, Inc., Mor-risville, NC) and the medial tibial condyle was trimmedfrom the rest of the bone surface along the articular mar-gin (Fig. 1).

The process of trimming the articular surface fromthe rest of the tibia was performed two additional timesfor 12 of the tibiae (two of each sex for each species)over a period of weeks. The geometric morphometricanalysis described below was performed on replicatesand all Procrustes distances between replicates werefound to be smaller than the smallest Procrustes dis-tance between specimens (entire sample). On average,replicate Procrustes distances were smaller by morethan an order of magnitude (average Procrustes distancebetween replicates was 0.008 compared to 0.100 betweenspecimens).

Analysis

To quantify the three-dimensional shape of the medialtibial condyle, 504 sliding semi-landmarks were distrib-uted across each joint surface using custom software

TABLE 1. Sample

Species Males Females

Homo sapiensBass 17 13Hamann-Todd 10 14

Pan troglodytesMax Planck Institute 6 6Hamann-Todd 16 20

Gorilla gorillaHamann-Todd 13 20

HOMINID MEDIAL TIBIAL CONDYLE 1519

written for MatlabVR (MathWorks, Inc., Natick, MA) fol-lowing Gunz et al. (2005). Seventy-one of these pointswere placed along the articular margin and the remain-ing 433 were placed on the articular surface (Fig. 2). Alllandmarks were slid iteratively along tangent planes(surface landmarks) and curves (articular margin land-marks) to minimize the bending energy of the thin-platespline interpolation function between each specimen andthe updated Procrustes average (Gunz et al., 2009). Iconverted specimens to shape coordinates by carryingout generalized Procrustes superimposition, whichremoves information about location and orientation andscales all specimens by centroid size (Rohlf and Slice,1990). Shape variation was summarized using principal

component analysis and average medial condyles werecreated for each species and sex within species.

Randomization tests were used to test for differencesbetween means shapes of species and sexes within spe-cies (Sokal and Rohlf, 1995). The randomization testscompared the Procrustes distances between the meanshapes of two groups (i.e., species or sexes) to a distribu-tion of Procrustes distances. This distribution was cre-ated by pooling all individuals from the two groupsbeing compared, and then randomly assigning individualspecimens to one of two groups that had sample sizes ofthe original groups. The Procrustes distance betweenthe means of the randomized groups was calculated, andthe randomization procedure was carried out 10,000times for each comparison. This procedure tests whetherthe difference between the means of the two groups isgreater than would be expected by random chance.

To confirm differences in curvature that were found inthe geometric morphometric analysis, two angle meas-urements were collected on average male and femalespecies medial tibial condyle surfaces created from thegeometric morphometric analysis. To do this, each aver-age condyle shape was oriented by fitting the main por-tion (excluding the eminence) to a horizontal plane. Ananteroposterior transect across the articular surface wasextracted following Sylvester and Organ (2010). Insteadof extracting the contour at 50% of mediolateral widthas Sylvester and Organ (2010) did, the transect waspositioned to pass through the deepest (most inferior)point of the central portion of the articular surface. Anangle between two lines was then measured on the con-tour. The first line passed through the deepest point andthe most superior point on the anterior portion of thecontour, while the second line passed through the deep-est point and the most superior point on the posteriorportion (Fig. 3). An analogous process was used toextract a mediolateral transect contour and measure amediolateral angle. This contour was taken perpendicu-lar to the anteroposterior transect and passed throughthe most superiorly projecting point on the intercondylareminence (Fig. 3).

To determine whether body mass influences the shapeof the medial tibial condyle, the principal componentscores resulting from the geometric morphometric analy-sis of the entire sample were regressed against superoin-ferior femoral head diameter (used as a proxy for bodymass, Sylvester and Organ, (2010)) within species. Noneof the regression analyses revealed a statistically signifi-cant relationship and as a result are not reported.

RESULTS

The first two principal components of shape space sep-arate the three species examined here, and randomiza-tion tests indicate significant shape differences betweenspecies pairs (Fig. 4 and Table 2). Humans are well sepa-rated from the other two species along the first principalcomponent, while the separation of the chimpanzees andgorillas along the second component is less dramatic.Together the first two principal components account for�44% of the total shape variance. The first principalcomponent, which accounts for 33% percent of the shapevariation, describes a shape transition from an oval-shaped medial tibial condyle (found in humans) to amore triangular-shaped medial tibial condyle in the non-

Fig. 2. Medial tibial condyle with 504 sliding semi-landmarks (Pantroglodytes).

Fig. 1. Medial tibial condyle trimmed from the rest of the proximaltibia (Pan troglodytes).

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human apes (Fig. 5). The second component describes achange in the angle between the main portion of the con-dyle and the lateral portion of the articular surface thatcovers the intercondylar eminence (best viewed fromeither an anterior or posterior perspective) (Fig. 5). Ingorillas, the lateral portion of the articular surface thatcovers the intercondylar eminence rises more steeplyaway from the main portion of the condyle thus forminga more acute angle between these two portions of thearticular surface.

Tests for differences in mean shape do not indicatedifferences between human males and females, but doindicate shape differences between the sexes withinchimpanzees and gorillas (Table 3). When male andfemale average medial tibial condyles are aligned withinchimpanzees and gorillas, subtle shape differences canbe appreciated that relate to the curvature across thecondyle. The male gorilla joint surface appears morecurved in both anteroposterior and mediolateral direc-tions as compared to the female joint surface. The cen-tral portion of the male gorilla condyle falls below thatof the female, while rising above it on the edges of thecondyles (Fig. 6). The angle measurements take from

the transect profiles on the average male and femalecondyles confirm both of these trends. Among gorillas,the angle is smaller for the average male in both themediolateral and anteroposterior directions (133.6 and168.6 degrees in mediolateral and anteroposterior direc-tions, respectively) compared to the average female(134.9 and 171.3 degrees in mediolateral and anteropos-terior directions, respectively). The chimpanzee sexes arenearly identical in their mediolateral angle (male 5 141.4degrees, female 5 141.2 degrees), while in the anteropos-terior direction the average male angle is 170.5 degreesand the average female angle is 172.0 degrees. Althoughthe randomization test did not indicate statistically signif-icant shape differences between the human sexes, theyfollow a pattern of the measured angles similar to thegorilla pattern. The human male angles are 138.5 degrees(mediolateral) and 168.1 degrees (anteroposterior), whilethe human female angles are 141.7 degrees (mediolateral)and 168.6 degrees (anteroposterior).

DISCUSSION

Known differences in knee morphology and functionbetween humans, chimpanzees, and gorillas providepotential explanations for the shape differences in themedial tibial condyle observed here. Tardieu (1981, 1983)demonstrated a fundamental shape difference betweenthe distal femora of humans and non-human apes usinga ratio of maximum anteroposterior and mediolateraldimensions. In humans, the anteroposterior and medio-lateral lengths are nearly equal, while in chimpanzeesand gorillas the distal femur has a much smaller antero-posterior dimension compared to its mediolateral width.Tardieu (1986) also demonstrated that chimpanzees (andpresumably gorillas) have a much larger range of inter-nal/external rotation at the knee compared to humansacross the entire range of knee flexion.

Separate lines of evidence suggest that internal/exter-nal rotation at the hominid knee occurs about a longitu-dinal axis passing through the medial compartment ofthe knee. In chimpanzees and gorillas, the lateral menis-cus of the knee has a single attachment point, allowingit to move more freely in an anteroposterior directioncompared to the relatively stable medial meniscus,which has two ligamentous attachment points (Tardieu,1986). This suggests that the axis of rotation is located inthe more stable medial compartment. Churchill et al.(1998) loaded human whole lower limb anatomical sam-ples and tracked the relative motion of the tibia andfemur, finding that internal/external rotation indeedoccurs about an axis through the medial tibial condylethat is roughly parallel to the anatomical axis of the tibia.More recently, magnetic resonance imaging studies of thehuman knee have demonstrated that during internal/external rotation, the lateral femoral and tibial condylesundergo much greater relative motion, compared to a sta-ble medial compartment (Freeman and Pinskerova, 2005),again demonstrating that the axis of internal/externalrotation passes through the medial tibial condyle.

First Principal Component

The human medial tibial condyle is more oval-shapedand anteroposteriorly elongated while those of the chim-panzee and gorilla are mediolaterally expanded (relative

Fig. 3. Contours extracted to measure anteroposterior and medio-lateral angles. A: Anteroposterior transect (black line) was takenthrough the deepest point (most inferior) of the central portion of thearticular surface, and mediolateral transect (black line) was taken per-pendicular to the anteroposterior transect through the most superiorprojection of the intercondylar eminence. B: Anteroposterior angle:Black line represents the anteroposterior contour across the condyle.The two gray lines represent the lines used to measure the angle, run-ning from the most inferior point on the articular surface to the mostsuperior points on the anterior and posterior portions of the condyle.C: Mediolateral angle: Black line represents the mediolateral contouracross the codyle. The gray lines are the same as for the anteroposte-rior angle.

HOMINID MEDIAL TIBIAL CONDYLE 1521

to their anteroposterior dimension) and approach a dis-tinctly triangular shape. This shape difference relativeto humans can be explained in terms of providing anadequate area of support for the medial femoral condyle.The human medial femoral condyle is anteroposteriorlyelongated (relative to mediolateral width) compared tochimpanzees and gorillas. In the human knee, themedial femoral condyle is supported not only directly bycontact with the medial tibial condyle, but also indirectlythrough medial meniscus (Messner and Gao, 1998). As aresult the human medial tibial condyle may need to besimilarly elongated in order to provide an area of sup-port below the femoral condyle. The distinctly triangularshape of the chimpanzee and gorilla medial tibialcondyle may reflect the need to maintain contact andprovide an area of support for the medial femoral con-dyle through a larger range of internal/external kneerotation.

The chimpanzee knee can accommodate 40 degrees ofcombined internal and external rotation (Tardieu, 1986).This rotation creates a more obtuse angle between thesagittal plane anteroposterior axes of the medial femoraland tibial condyles. While the curvatures of the medialfemoral and tibial condyles are different, resulting in arelatively small area of direct contact between the carti-

lage of the articular surfaces; the congruity of the surfa-ces is dramatically increased by the presence ofthe medial meniscus. The medial meniscus covers �60%of the medial condyle in humans (Clark and Ogden,1983) and transmits up to 50% of the load which passesthrough the medial compartment (Messner and Gao,1998). Loss of the meniscus has been shown to decreasethe contact area between the femur and tibia by 30–50%(Fukubayashi and Kurosawa et al., 1980; Baratz et al.,1986; Ihn et al., 1993). Thus contact area between thetibial and femoral condyles, including that mediated bythe meniscus, is not limited to the central portion ofarticular surfaces. The triangular shape of the medialtibial condyle in chimpanzees and gorillas likely providesa medial expansion of the condyle necessary to providesupport to the meniscus and medial femoral condyle asit rotates relative to the tibia (Fig. 7).

Second Principal Component

The second principal component of shape spacedescribes a difference in the angle between the main(horizontal) body of the medial tibial condyle and theportion of the condyle that extends onto the intercondy-lar eminence. This trend is confirmed by the angle mea-surement taken on mediolateral transects. The gorillamedial tibial condyle, in which the angle is more acute,is distinguished from a more obtuse angle in human andchimpanzee medial tibial condyles. This difference ispossibly related to load transmission through the medialcompartment of the knee in conjunction with require-ments for internal/external rotation. The medial femoralcondyles of gorillas and chimpanzees are mediolaterallywider than the lateral femoral condyles to accommodate

Fig. 4. Principal component analysis in shape space. PC1 and PC2explain approximately 44% of the sample variance. The shape differ-ences associated with the principal components are plotted as sur-face deformations of the mean shape (2 standard deviations in eitherdirection). Shapes along PC1 are superior in view; those along PC2

are from posterior perspective. Open circles 5 Homo sapiens females;Closed circles 5 Homo sapiens males; Open diamonds 5 Pan troglo-dytes females; Closed diamonds 5 Pan troglodytes males; Open trian-gles 5 Gorilla gorilla females; Closed triangles 5 Gorilla gorilla males.

TABLE 2. P-Values for Randomization Tests for Dif-ferences Between Means

Comparison Procrustes distance

Human-Chimp <0.001Human-Gorilla <0.001Chimp-Gorilla <0.001

1522 SYLVESTER

the greater load borne by the medial compartment of theknee during quadrupedal walking (Preuschoft, 1971; Pre-uschoft and Tardieu, 1996). In humans, loads throughthe medial and lateral femoral condyles are nearly equaland this equal load transmission is reflected in the equalmediolateral dimensions of the human medial and lateralfemoral condyles (Preuschoft, 1971). The differentialloading in the non-human ape knee, combined with thegreater body mass of gorilla, must result in higher loadsthrough the medial compartment as compared tohumans. Male chimpanzees, however, approach the bodymass of female gorillas (Smith and Jungers, 1997), andyet the female gorilla medial tibial condyle forms a moreacute angle relative to the rest of the condyle comparedto male chimpanzee. This suggests that body mass is notthe only factor affecting the chimpanzee and gorilla mor-phologies. The more obtuse angle of the chimpanzee kneemay reflect a greater capacity for internal/external rota-tion at the knee associated with greater arboreal behav-iors. Doran (1996, 1997) reports a much greater level ofarboreality in chimpanzees (33–68% arboreal) comparedto gorillas (2–13% arboreal), including more quadruma-nous climbing. Remis (1995, 1998) reports higher levelsof arboreality among lowland gorilla at the Bai HokouStudy Site, but cautions that these gorillas were not fullyhabituated and difficult to see on the ground, precluding

accurate reconstruction of percent time in the trees.Because the intercondylar eminence rises into the inter-condylar notch of the femur, the steep rise of this featurein gorillas may provide greater stability against medio-lateral translation (Blackburn and Craig, 1980; Tardieu,1981) that may occur during some activities, while sacri-ficing capacity for internal/external rotation. In humans,the stereotypically low mediolateral loading during walk-ing (Chao et al., 1983) and running (Cavanagh andLafortune, 1980) may not require an eminence that risesas steeply away from the rest of the condyle.

Fig. 5. Average right medial tibial condyle shapes. Images above species names are superior viewswith anterior at the top of the image and medial to the left. Images below the species names are posteriorviews with medial to the left of the image.

TABLE 3. P-Values for Randomization Tests forDifferences Between Mean Sex Shapes Within

Species

Comparison Procrustes Distance

Human 0.4970Gorilla 0.0060Chimpanzee 0.0046

Fig. 6. Average right male and female medial tibial condyle shapes.Left column of images: Gorilla gorilla; right column: Pan troglodytes.Dark gray mesh surfaces are male average surfaces and light graysurfaces are female average surfaces. A 5 anterior; M 5 medial.

HOMINID MEDIAL TIBIAL CONDYLE 1523

Sex Shape Differences Within Species

Aligning the average male and female gorilla medialtibial condyles demonstrates that the middle portion ofthe male condyle falls below that of the female while theedge of the male articulation rises above the female.This indicates that the medial tibial condyle of the malegorilla is more concave than that of the female, a findingconfirmed by the angle measurements. A similar patterncan also be seen in the chimpanzee medial tibial condyle,but mainly in an anteroposterior direction. The anteriorand posterior edges of the male chimpanzee articularsurface rise above the female chimpanzee articular sur-face, while the middle portion of the male chimpanzeearticular surface dips below the female surface. Thissuggests a greater degree of curvature in the anteropos-terior direction. The flatter surfaces of the female medialtibial condyles, compared to their male counterparts,may reflect both differences in body mass as well as dif-ferences in locomotor behavior. In both chimpanzees andgorillas, females are both more arboreal (Doran, 1996;Remis, 1999) and smaller than males (Smith andJungers, 1997). While it is not possible to tease apartthe relative contribution of locomotor and body sizedimorphism on the shape dimorphism of the gorilla andchimpanzee medial tibial condyles, it is perhaps informa-tive that the human sexes do not display statisticallysignificant differences in shape. Human body size dimor-phism approaches that of chimpanzees (Smith andJungers, 1997), but differences between the humansexes in knee kinematics during locomotion have notbeen found consistently (Malinzak et al., 2001; Ferberet al., 2003). This suggests that differences in knee mor-phology of male and female chimpanzees and gorillasmay be driven more by differences in locomotor behavior,particularly the degree of arboreality, and less by differ-ences in body size.

CONCLUSION

Here I document shape variation in the medial tibialcondyle that distinguishes modern humans, chimpan-zees, and gorillas. The major source of shape variationdescribes the outline (from superior view) of the articu-lar surface. Humans have a more oval-shaped medialtibial condyle, while the non-human apes have a moretriangular-shaped condyle. This difference likely reflectsthe greater capacity for internal/external rotation at theknee in the non-human apes and differences betweenthese species in the shape of the medial femoral condyle.The second component, which distinguishes gorillas fromchimpanzees and humans, is interpreted as being aproduct of greater load transmission in the medial com-partment of the knee of gorillas, along with lower arbor-eality among gorillas compared to chimpanzees. Sexdifferences in shape within the non-human ape speciesare not directly attributable to body mass, but insteadmay indicate differences in the degree of arboreality.

ACKNOWLEDGEMENTS

The author would like to thank M. Mahfouz and J.Organ for access to tibia scans, B. Latimer, L. Jellema,L.M. Jantz, and R.L. Jantz for access to specimens intheir care, and P.A. Kramer for critiques of earlier ver-sions of this manuscript. The author would also like tothank the three anonymous reviewers for their critiquesand comments that improved the work presented here.

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Fig. 7. The effect of medial femoral condyle shape and internal/external rotation at the knee on medial tibial condyle shape. Upperrow 5 Femoral and tibial knee components in H. sapiens. Lower row5 Femoral and tibial knee components in P. troglodytes. Column A:Outlines of distal right femora from an inferior view, with the medialcondyles shaded gray. Column B: Idealized outlines of the medial fem-oral condyles. Column C: Condyle outlines rotated to maximal internaland external rotation following the experimental data in Tardieu (1986).Column D: Shape of medial tibial condyles that would contain themedial femoral condyle throughout the range of external and internalrotation.

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