The ontogeny of the chin: an analysis of allometric and biomechanical scaling

The ontogeny of the chin: an analysis of allometric andbiomechanical scalingN. E. Holton,1,2 L. L. Bonner,1 J. E. Scott,2 S. D. Marshall,1 R. G. Franciscus2 and T. E. Southard1

1Department of Orthodontics, The University of Iowa, Iowa City, IA, USA2Department of Anthropology, The University of Iowa, Iowa City, IA, USA

Abstract

The presence of a prominent chin in modern humans has been viewed by some researchers as an architectural

adaptation to buttress the anterior corpus from bending stresses during mastication. In contrast, ontogenetic

studies of mandibular symphyseal form suggest that a prominent chin results from the complex spatial

interaction between the symphysis and surrounding soft tissue and skeletal anatomy during development.

While variation in chin prominence is clearly influenced by differential growth and spatial constraints, it is

unclear to what degree these developmental dynamics influence the mechanical properties of the symphysis.

That is, do ontogenetic changes in symphyseal shape result in increased symphyseal bending resistance? We

examined ontogenetic changes in the mechanical properties and shape of the symphysis using subjects from a

longitudinal cephalometric growth study with ages ranging from 3 to 20+ years. We first examined whether

ontogenetic changes in symphyseal shape were correlated with symphyseal vertical bending and wishboning

resistance using multivariate regression. Secondly, we examined ontogenetic scaling of bending resistance

relative to bending moment arm lengths. An ontogenetic increase in chin prominence was associated with

decreased vertical bending resistance, while wishboning resistance was uncorrelated with ontogenetic

development of the chin. Relative to bending moment arm lengths, vertical bending resistance scaled with

significant negative allometry whereas wishboning resistance scaled isometrically. These results suggest a

complex interaction between symphyseal ontogeny and bending resistance, and indicate that ontogenetic

increases in chin projection do not provide greater bending resistance to the mandibular symphysis.

Key words: Mandibular symphysis; growth and development; Homo.

Introduction

During mastication, the anthropoid mandible is subjected

to high repetitive loads, resulting in a predictable pattern

of mechanical stresses and strains (e.g. Hylander, 1984,

1985). Variation in load magnitude and frequency associ-

ated with variability in dietary properties, as well as non-

dietary paramasticatory behaviors, has been used to explain

variation in mandibular form across a wide range of extant

and fossil primate taxa (Hylander, 1984, 1985; Ravosa, 1990,

1996a,b; Daegling & Grine, 1991; Ravosa & Simons, 1994;

Vinyard & Ravosa, 1998; Daegling, 2001; Daegling et al.

2009, 2014). In particular, there has been considerable

emphasis placed on the interaction between masticatory

loads and mandibular symphyseal morphology. While the

mandibular symphysis experiences a combination of

mechanical stresses during mastication, the curvature of the

symphyseal region contributes to particularly high lateral

transverse bending stresses (i.e. wishboning), especially

along the lingual aspect of the symphysis (Hylander, 1984).

Resistance to lateral transverse bending across anthropoids

is evident in architectural features of the mandibular sym-

physis such as thicker lingual cortical bone and the presence

of superior and inferior transverse tori, which aid in resist-

ing increased tensile stresses along the lingual symphysis

(Hylander, 1984; Daegling & McGraw, 2009; Panagiotopou-

lou & Cobb, 2011). Moreover, allometric scaling of symphy-

seal dimensions during ontogeny and across static adult

comparisons suggests that variation in symphyseal form

maintains functional equivalency with regard to lateral

transverse bending loads (e.g. Hylander, 1985; Vinyard &

Ravosa, 1998; Daegling, 2001).

In a similar fashion, the unique morphology of the mod-

ern human mandibular symphysis, i.e. the presence of a

prominent chin, has also been viewed as an architectural

adaptation to buttress the anterior corpus from bending

Correspondence

N. E. Holton, Department of Orthodontics, The University of Iowa,

Iowa City, IA 52242, USA. T: +319-384-4786; F: +319-335-6847;

E: [email protected]

Accepted for publication 3 March 2015

© 2015 Anatomical Society

J. Anat. (2015) doi: 10.1111/joa.12307

Journal of Anatomy

stresses during mastication (DuBrul & Sicher, 1954; White,

1977; Daegling, 1993; Dobson & Trinkaus, 2002; Gr€oning

et al. 2011). Although in vivo data for human mandibular

strains do not exist, it is generally accepted that strain data

from other anthropoids are applicable to humans as well,

albeit with regard to a derived mandibular form. Following

Daegling (1993), the combination of a reduction in mandib-

ular length and a wide dental arch in modern humans

would have lessened the relative severity of lateral trans-

verse bending stresses at the mandibular symphysis. This

would result in a relative increase in vertical bending in the

coronal plane producing compressive forces along the alve-

olar region and tensile forces along the symphyseal base.

Thus, if resistance to mechanical stress underlies the derived

morphology of modern human symphyseal form, this

would suggest that a prominent chin serves as a key func-

tional adaptation to resisting vertical bending stresses dur-

ing mastication.

Recent biomechanical assessments of mandibular symphy-

seal form in archaic and recent Homo indicate that there is

a general lack of consensus regarding the relationship

between functional loading of the anterior corpus and the

development of the chin. As such, whether a prominent

chin in modern humans can be explained as a function of

symphyseal loading remains unresolved. In their compara-

tive functional analyses of the mechanical properties of the

mandibular symphysis, both Dobson & Trinkaus (2002) and

Gr€oning et al. (2011) found that resistance to vertical bend-

ing stress was maintained across the range of mandibular

symphyseal form in later genus Homo, suggesting that a

prominent chin in modern humans may act to resist altered

patterns of mechanical strains associated with evolutionary

changes in mandibular form. Consistent with Daegling’s

(1993) model, both Dobson & Trinkaus (2002) and Gr€oning

et al. (2011) also found that there was a general trend for

modern humans to be less resistant to wishboning stresses

when compared with archaic Homo. Dobson and Trinkaus

(2002) however, were unable to document a significant dif-

ference in lateral transverse bending resistance between

Neandertals and modern humans.

In addition to comparisons of function and mandibular

symphyseal form across later genus Homo, both Gr€oning

et al. (2011) and Ichim et al. (2006) examined whether the

presence or absence of a chin in three-dimensional models

of modern human mandibles affected symphyseal strain dis-

tributions using finite element analysis. Gr€oning et al.

(2011) concluded that a modern human mandible with a

chin was better at resisting dorso-ventral shear, lateral

transverse bending and vertical bending when compared

with a non-chinned model. Ichim et al. (2006), on the other

hand, found that there were no differences in symphyseal

strains in their chinned and non-chinned models leading

them to conclude that the chin is likely unrelated to the

functional demands of mastication. This latter result is con-

sistent with a recent analysis by Daegling (2012), who was

unable to document a meaningful correlation between chin

prominence and symphyseal bending moment arm lengths

in modern human samples.

In contrast to the potential mechanical influences on the

development of the modern human mandibular symphysis,

others have suggested that a projecting chin may be the

result of differential patterns of upper and lower facial

prognathism during human evolution (Hrdli�cka, 1911;

Waterman, 1916; Bolk, 1924; Weidenreich, 1936; Bigger-

staff, 1977; Cartmill & Smith, 2009). As such, the degree to

which a prominent chin may act to resist mechanical stresses

during mastication may simply be a secondary consequence

of differential jaw growth and associated changes in sym-

physeal form (e.g. Gr€oning et al. 2011; Daegling, 2012),

rather than being a primary causal mechanism. Indeed,

although a bony chin may act to increase resistance to

bending, mandibular strains in humans are likely low to

begin with as a result of an evolutionary reduction in the

size of the masticatory apparatus and an increased reliance

on technocultural adaptation and extraoral processing.

Moreover, for a given size, the human mandible has signifi-

cantly more cortical bone throughout its corpus (including

the symphyseal region) when compared with mandibles of

other apes, which do experience prolonged, forceful masti-

cation (Daegling, 2007, 2012). As such, the human mandible

has more cortical bone than is likely necessary to resist the

relatively low stresses incurred during routine masticatory

loading.

With regard to non-functional influences, it is well estab-

lished that ontogenetic variation in chin prominence in

modern humans is tied to larger craniofacial growth

dynamics that affect the spatial relationship between the

mandible and surrounding soft and hard tissue anatomy.

During early postnatal development, for example, the posi-

tion of the mental region is influenced by soft tissue (e.g.

the tongue and suprahyoid muscles) and spatial constraints

in the pharyngeal region, which, in turn, are affected by rel-

ative positions of the facial skeleton and cervical vertebral

column (Coquerelle et al. 2013a,b).

Chin prominence is further associated with variation in

ontogenetic patterns of mandibular rotation during later

postnatal development (Bj€ork, 1969; €Odegaard, 1970a,b;

Lavergne & Gasson, 1976; Bj€ork & Skieller, 1983), where

increased chin prominence is part of a larger suite of fea-

tures associated with a pattern of forward mandibular rota-

tion (Fig. 1). In contrast, reduced chin prominence is more

closely associated with a pattern of backward mandibular

rotation. Variation in mandibular rotation and associated

correlated symphyseal form are tied to the complex interac-

tion of mandibular posture, dentoalveolar development

and the direction of growth of the condylar cartilage

(€Odegaard, 1970a,b; Lavergne & Gasson, 1976; Buschang &

Gandini, 2002; Araujo et al. 2004; Buschang et al. 2013).

In addition, variation in chin prominence has also been

tied to differential anterior-posterior dimensions of the


The ontogeny of the chin, N. E. Holton et al.2

dentoalveolar complex and the lower border of the mandi-

ble both during ontogeny (Marshall et al. 2011) and across

broader ranges of population variation (Scott et al. 2010;

Scott, 2014).

While variation in the degree of chin prominence is

clearly influenced by differential growth and spatial con-

straints in the facial skeleton, it is unclear to what degree

these developmental dynamics influence the mechanical

properties of the symphysis. That is, a spatial model of chin

development does not rule out the possibility that an onto-

genetic increase in chin prominence increases resistance to

symphyseal bending stresses. Therefore, to further our

understanding of the functional implications of chin promi-

nence on the geometry of the mandibular symphysis, we

examine whether ontogenetic changes in chin development

in a longitudinal cephalometric sample result in a relative

increase in symphyseal bending resistance. To accomplish

this, we first assessed whether ontogenetic variation in

measures of symphyseal bending resistance is correlated

with mandibular symphyseal shape. If chin prominence is a

response to symphyseal bending loads during development,

then we would predict that increased bending resistance

will be associated with ontogenetic increases in chin projec-

tion. Conversely, reduced chin projection early in develop-

ment should correspond to a decrease in relative bending

resistance. Secondly, we examined the allometric scaling of

symphyseal bending resistance to determine whether there

is an ontogenetic increase in resistance relative to bending

moment arm proxies during development.

Materials and methods

We collected 292 longitudinal cephalometric observations from a

total of 37 individuals (19 male and 18 females) selected from the

Iowa Facial Growth Study located at The University of Iowa Depart-

ment of Orthodontics. This growth study, which began in 1946,

consists of individuals predominantly of northwest European ances-

try who resided in or around the Iowa City, IA area. Children were

at least 3 years of age at enrollment with records taken quarterly

until age 5 years, bi-annually from 5 to 12 years, and annually from

12 to 18 years. Final records were taken once during early adult-

hood. The subjects included in our analysis were selected from the

larger growth study sample based on the completeness of the indi-

vidual radiographic sequences as well as the quality of the lateral

cephalograms with respect to the variables of interest. Measure-

ments were collected at nine different observations beginning at

3.0–4.0 years of age through 20+ years of age (Table 1).

We collected symphyseal cortical bone parameters from lateral

cephalometric films by first tracing the external cortical symphyseal

surface using DOLPHIN IMAGING software. Because the developing per-

manent dentition obscured the endosteal surface of the posterior

aspect of the mandibular symphysis in the younger age groups, we

were unable to identify the border of the endocortical surface in

this region. It was therefore necessary to model the symphysis as a

cross-section through a solid beam (Fig. 2) following the methods

used by Dobson & Trinkaus (2002). All tracings were uploaded into

IMAGEJ 1.45, scaled and rotated such that the mandibular plane (i.e.

the lower border of the mandibular corpus from menton to gon-

ion) was oriented horizontally.

Next, using the shape of the external cortical surface, we mea-

sured symphyseal bending resistance using second moments of area

(e.g. Daegling & McGraw, 2001; Organ et al. 2006; Fukase & Suwa,

2008; Ant�on et al. 2010). These values are a function of the amount

and distribution of bone relative to specified axes about which

bending is hypothesized to occur (Fig. 3). First, we calculated sec-

ond moments relative to the X- and Y-axes (i.e. parallel and perpen-

dicular to the mandibular plane, respectively). Ixx is a measure of

resistance to vertical bending stresses relative to the horizontal axis

(X in Fig. 3). We then calculated Iyy as a measure of resistance to

wishboning stresses relative to the vertical axis (Y in Fig. 3). Addi-

tionally, for a given symphyseal cross-section there is an axis about

which the second moment of area will be maximized (X0 in Fig. 3)

A B

Fig. 1 Examples of two 13-year-old male subjects who exhibit variation in mandibular growth resulting in morphological differences in mandibular

symphyseal form. The individual in (A) is characterized by a pattern of forward mandibular rotation, while the individual in (B) exhibits backward

mandibular rotation. Forward mandibular rotation is associated with greater vertical growth at the mandibular condyle and with a brachyfacial

morphology (i.e. relatively shorter vertical facial dimensions, flat mandibular plane, reduced gonial angle, etc.). Backward mandibular rotation is

associated with greater posteriorly oriented growth at the mandibular condyle. Individuals with greater posterior mandibular rotation exhibit a do-

licofacial morphology (i.e. relatively longer anterior vertical facial dimensions, steep mandibular plane, larger gonial angle, etc.).


The ontogeny of the chin, N. E. Holton et al. 3

and an orthogonal axis about which the second moment of area

will be minimized (Y0 in Fig. 3). Thus, we calculated Imax as a mea-

sure of maximum resistance relative to the X0 axis and Imin as a mea-

sure of minimum bending resistance relative to the Y0 axis. Secondmoments of area were calculated using MOMENTMACRO for IMAGEJ

(http://www.hopkinsmedicine.org/FAE/mmacro.htm).

Although it is desirable to calculate biomechanical properties

from actual cortical bone cross-sections rather than external con-

tours (i.e. a hollow beam model vs. a solid beam model), there is a

very strong correlation between second moments of area calculated

from both cortical bone and external bone contours. Stock and

Shaw (2007), for example, found correlations in the range of 0.84–

0.98 in various postcranial skeletal elements. Similarly, there is a

high range of correlations (0.83–0.96) when using both methods to

calculate second moments for the mandibular symphysis in CT scan

images (N.E. Holton, unpublished data). The consistently high corre-

lations between second moments of area calculated using both

methods indicate that the results of our analysis are unlikely to be

affected by the use of external contours vs. actual cortical bone

cross-sections. However, this precludes the examination of poten-

tially meaningful developmental changes in symphyseal cortical

cross-sectional area and regional differences in cortical bone

thickness.

To assess the relationship between bending resistance and man-

dibular symphyseal shape, we collected a series of k = 8 mandibular

landmarks (Fig. 4), which were superimposed using generalized

Procrustes analysis. We then used multivariate regression analysis to

examine the relationship between mandibular shape and our inde-

pendent variables. First, we examined the relationship between

mandibular shape and centroid size to illustrate ontogenetic varia-

tion in the mandibular symphysis. Next, we examined the relation-

ship between mandibular shape and bending resistance values

scaled to moment arm proxies to determine whether shape varia-

tion associated with an increase in relative bending resistance mir-

rored the ontogenetic increase in chin prominence.

Wishboning resistance was scaled to mandibular length (pogon-

ion-articulare) and vertical bending resistance was scaled to bigo-

nial breadth (measured from posterior-anterior cephalograms),

which served as a proxy for the vertical bending moment arm. The

magnitude of vertical bending stress is, in part, a function of width

of the coronally oriented region of the anterior corpus. As such,

other researchers have used measures such as bicanine or bimolar

distances to scale estimates of vertical bending resistance (e.g. Dae-

gling, 2001; Dobson & Trinkaus, 2002; Ant�on et al. 2010). Although

dental casts are available for the subjects used in our analysis, we

were unable to take transverse measures of the dentition during

the mixed dentition phase. As such, we selected bigonial breadth

(e.g. Holton et al. 2014) as a transverse measure, which was avail-

able for all subjects. All geometric morphometric analyses were con-

ducted using MORPHOJ (Klingenberg, 2008–2010).

We examined the ontogenetic allometry of symphyseal second

moments of area using reduced major axis regression. Second

moments of area, which are measured in mm4, were converted to

their fourth roots and log-transformed. We then regressed log-

transformed second moments against log-transformed symphyseal

bending moment arm proxies for each individual ontogenetic tra-

jectory. To assess the ontogentic scaling of measures of bending

resistance, we calculated the mean of the individual regression

slopes and the 95% confidence intervals (e.g. Lammers & German,

2002).

Finally, because there is sexual dimorphism in masticatory func-

tion (Helkimo et al. 1977; Raadsheer et al. 1999; Kovero et al. 2002;

Table 1 Sample composition.

Age, years Female, n Male, n Total, n

3.0–4.9 14 17 31

5.0–6.9 19 17 36

7.0–8.9 17 19 36

9.0–10.9 18 18 36

11.0–12.9 18 18 36

13.0–14.9 18 17 35

15.0–16.9 17 16 33

17.0–18.9 11 8 19

20.0+ 15 15 30

Total n 147 145 292

A B

C D

Fig. 2 Lateral cephalograms of a subject at 6

years of age (A) and at 17 years of age (B).

External symphyseal contours in the same

subjects are outlined in (C) and (D).



http://www.hopkinsmedicine.org/FAE/mmacro.htm

Kiliardis et al. 2003) that manifests during adolescence (e.g. Ingerv-

all & Minder, 1997), we examined growth allometries in the mixed-

sex sample and in the individual male and female samples.

We tested for significant differences in male and female growth

allometries using mixed model ANOVA. Specifically, we compared

least-squares regression slopes by testing for the interactive effects

of sex and moment arm length on second moments of area.

Results

Correlated variation between mandibular shape and cen-

troid size (P < 0.001) is illustrated in Fig. 5A. Smaller man-

dibular centroid sizes (left) are associated with a vertically

oriented mandibular symphysis and a relatively flat labial

symphyseal surface in the midsagittal plane. This is further

associated with a more posteriorly oriented mandibular

ramus and relatively wider gonial angle. As centroid size

increases, the mental protuberance along the labial border

becomes more prominent due to the relative posterior dis-

placement of infradentale and B-point along with a relative

anterior displacement of pogonion, menton and genion.

Additionally, the mandibular ramus becomes more verti-

cally oriented and is associated with a relatively narrower

gonial angle and flatter mandibular plane.

With regard to symphyseal bending resistance, scaled Ixx,

Iyy, Imax and Imin were all significantly correlated with man-

dibular shape (P < 0.001). Since morphological variation in

the mandibular shape associated with Ixx and Imax was virtu-

ally identical, only the results for Ixx are illustrated in Fig. 5.

Additionally, due to similarity in the results between Iyy and

Imin, only the results for Iyy are presented. Variation in resis-

tance to vertical bending was correlated with mandibular

symphyseal shape such that decreased resistance was associ-

ated with a relative decrease in symphyseal height and a

relative increase in the prominence of the chin (Fig. 5B). In

contrast, increased symphyseal resistance to vertical bend-

ing was correlated with an increase in symphyseal height

and a flatter labial symphyseal border due to the anterior

projection of B-point relative to pogonion. The pattern

reflected in the correlation between Ixx and mandibular

shape is in contradistinction to the pattern of ontogenetic

development of the mandibular symphysis (Fig. 5A).

Resistance to lateral transverse bending was correlated

with the relative anterior-posterior dimensions of the man-

dibular symphysis (Fig. 5C). Decreased lateral transverse

bending resistance was associated with a reduction in sym-

physeal depth along both the labial and lingual borders of

the symphysis (i.e. at pogonion and genion), whereas the

more superior aspects of the symphysis (infradentale and B-

point) were relatively stable. As such, reduced lateral trans-

verse bending resistance was associated with reduction in

the prominence of the chin. Conversely, greater resistance

to lateral transverse bending was associated with greater

symphyseal depth resulting from an anterior displacement

of pogonion and a posterior displacement of genion. Cou-

pled with the relatively stable superior symphyseal region,

increased lateral transverse bending resistance was associ-

ated with greater prominence of the chin.

With regard to ontogenetic scaling of cortical bone

parameters (Table 2, Fig. 6), the mean slopes for Ixx and Imax

Fig. 3 Axes used to calculate second moments of area. The X- and Y-

axes are oriented parallel and perpendicular to the mandibular plane,

respectively. Ixx is calculated relative to the X-axis and Iyy is calculated

relative to the Y-axis. X0 and Y0 are the axes for the major and minor

principal axes, respectively. Imax is calculated relative to X0 and Imin rel-

ative to Y0.

Fig. 4 Landmarks used to quantify mandibular shape. 1 = infraden-

tale; 2 = B point; 3 = pogonion; 4 = menton; 5 = genion; 6 = man-

dibular orale; 7 = gonion; 8 = articulare.



relative to bigonial breadth (slope = 0.71 and 0.75, respec-

tively) scaled with negative allometry. In both cases, the

upper limit of the 95% confidence intervals fell below isom-

etry. In contrast, the mean slopes for Iyy (slope = 0.96) and

Imin (slope = 0.98) approached an isometric relationship

with mandibular length and the confidence intervals for

both bivariate comparisons spanned isometry. It is of note

that there was a considerable variation in regression slopes,

with individual slopes spanning the range from negative to

positive allometry for all measures of bending resistance

(Table 2).

The individual male and female samples exhibited the

same scaling patterns as the combined sample. Ixx and Imax

scaled with negative allometry relative to bigonial breadth

in both the males (slope = 0.73 and 0.76, respectively) and

females (slope = 0.72 and 0.74, respectively). With regard to

Iyy and Imin, both males (slope = 1.01 and 1.01, respectively)

and females (slope = 0.91 and 0.91, respectively) tend to

scale isometrically relative to mandibular length. With

regard to patterns of sexual dimorphism in ontogenetic

scaling, the results of our mixed-model ANOVA comparing

least-squares regression slopes indicated that there were no

significant differences between males and females

(Table 3).

Discussion

If a prominent chin is a structural adaptation that increases

resistance to symphyseal bending stresses, then correlated

variation between bending resistance and symphyseal

A

B

C

Fig. 5 Wireframe images illustrating mandibular shape variation (black wireframes) correlated with centroid size (A), scaled Ixx (B) and scaled Iyy(C). The gray wireframes represent the mean shape configuration.

Table 2 Mean RMA slopes, confidence intervals and ranges of variation in mean slopes.

n Slope 95%CI Slope range

Combined

Ixx vs. bigonial breadth 37 0.71 0.53–0.90 0.42–1.12

Iyy vs. mandibular length 37 0.96 0.72–1.19 0.49–1.52

Imax vs. bigonial breadth 37 0.75 0.56–0.93 0.43–1.15

Imin vs. mandibular length 37 0.98 0.72–1.16 0.49–1.52

Male





Female







shape should mirror allometric changes in the symphysis

during ontogeny. In the present study we examined onto-

genetic changes in bending resistance of the mandibular

symphysis, first to determine whether there is a correlation

between increased bending resistance and ontogenetic

changes in chin development, and secondly to examine

ontogenetic scaling of bending resistance relative to bend-

ing moment arms.

Collectively, the ontogenetic changes in symphyseal

shape documented in our multivariate regression analysis

generally reflect previously established symphyseal changes

resulting from a typical pattern of anterior mandibular

rotation during development (e.g. Bj€ork, 1969; €Odegaard,

1970a,b; Lavergne & Gasson, 1976; Bj€ork & Skieller, 1983).

During ontogeny, there is a significant increase in chin pro-

jection resulting, in part, from an anterior displacement of

the lower symphysis. Increased chin projection is further

associated with a relative reduction in symphyseal height, a

more vertically oriented ramus and a relatively flatter man-

dibular plane.

An ontogenetic increase in chin prominence is also corre-

lated with a relative posterior positioning of the superior

alveolar region (e.g. Chen et al. 2000), which has been

shown to result from differential anterior growth between

the mandibular and maxillary region of the facial skeleton

(You et al. 2001; Marshall et al. 2011). During ontogeny,

the lower border of the mandible exhibits increased ante-

rior growth relative to the maxilla as the result of a supe-

rior-inferior gradient of growth cessation in which the

lower regions of the facial skeleton cease growing later

than the more superior regions (Buschang et al. 1983; En-

low & Hans, 1996; but see Bastir et al. 2006). However,

although increased anterior mandibular growth is evident

in the lower symphyseal region, developmental and func-

tional integration between the maxillary and mandibular

alveolar regions due to occlusal interlocking (You et al.

2001; Marshall et al. 2011) restricts the anterior growth of

the mandibular alveolus, resulting in a relative posterior

positioning of the superior symphysis during growth. Thus,

increased chin projection results from a suite of morpholog-

ical changes in the mandible associated with differential

A B

C D

Fig. 6 Bivariate relationship between second moments of area and bending moment arm proxies. Males (gray circles) and females (open circles)

exhibit the same scaling relationships for vertical bending resistance (A and B) and lateral transverse bending resistance (C and D).

Table 3 Mixed model ANOVA results.

Comparison F P

Ixx vs. bigonial breadth

Sex 1.718 0.191

Bigonial breadth 22.471 < 0.001

Sex*bigonial breadth 1.842 0.179

Iyy vs. mandibular length

Sex 0.163 0.687

Mandibular length 48.456 < 0.001

Sex*mandibular length 0.154 0.695

Imax vs. bigonial breadth

Sex 1.157 0.283

Bigonial breadth 27.329 < 0.001

Sex*bigonial breadth 1.246 0.265

Imin vs. mandibular length

Sex 0.939 0.333

Mandibular length 19.110 < 0.001

Sex*mandibular length 0.986 0.322



growth and complex spatial constraints that begins early in

development (e.g. Coquerelle et al. 2013a,b) and continues

through postnatal ontogeny and into adulthood (e.g. Bj€ork,

1969; €Odegaard, 1970a,b; Lavergne & Gasson, 1976; Bj€ork &

Skieller, 1983; Scott et al. 2009; Marshall et al. 2011; Scott,

2014).

With regard to the relationship between vertical bending

resistance and symphyseal shape, our sample exhibited a

pattern that largely contrasted with ontogenetic changes

in chin projection. The results of our geometric morpho-

metric analysis indicate that a relative increase in vertical

bending resistance is associated with a morphological pat-

tern seen during early ontogeny, i.e. a relatively taller sym-

physis along with a flatter labial surface. Reduced relative

vertical bending resistance, on the other hand, was associ-

ated with a relatively shorter symphysis and a prominent

chin, as seen in adults. This result is consistent with the

ontogenetic scaling of Ixx and Imax relative to the vertical

bending moment arm (i.e. bigonial breadth). In both cases,

vertical bending resistance scaled with negative allometry,

indicating that resistance to vertical bending stress relative

to bigonial breadth decreases during development. Thus,

in contrast to previous studies that have found that a

prominent chin in modern humans may be important for

resisting vertical bending stresses when compared with

archaic Homo (e.g. Daegling, 1993; Dobson & Trinkaus,

2002; Gr€oning et al. 2011), our results suggest that

increased chin prominence during modern human ontog-

eny is associated with reduced resistance to vertical bend-

ing stresses. As such, although the morphology of the

mandibular symphysis affects the mechanical environment

of the anterior corpus, our results suggest that the develop-

ment of a projecting chin is likely independent of the need

to resist masticatory stresses.

The reduction in vertical bending resistance during devel-

opment is likely due, in part, to ontogenetic changes in

mandibular symphyseal orientation, which is thought to

have a significant influence on the ability to resist bending

stresses during mastication. For example, the oblique orien-

tation of the symphysis in non-human anthropoid primates

is well suited to counter relatively greater wishboning stres-

ses (e.g. Hylander, 1985; Vinyard & Ravosa, 1998; Daegling,

2001). Similarly, a more vertically oriented symphysis in

modern humans relative to Neandertals (Daegling, 1993;

Dobson & Trinkaus, 2002; Nicholson & Harvati, 2006;

Gr€oning et al. 2011) has been argued to reflect changes in

the biomechanical environment of the anterior corpus dur-

ing mandibular loading. However, due to differential ante-

rior growth of the alveolar region and lower symphysis

during ontogeny (You et al. 2001; Marshall et al. 2011), the

modern human mandibular symphysis, which is vertically

oriented at younger ages, becomes more obliquely oriented

during development (e.g. Coquerelle et al. 2013a,b;

Fig. 5A). As a result, the distribution of symphyseal bone

relative to the vertical bending axis may not be as well

suited to resist vertical bending stresses during later devel-

opment.

Whereas our results suggest that a prominent chin does

not increase resistance to bending stresses during develop-

ment, Gr€oning et al. (2011) found that the absence of a

chin in their model of an adult modern human mandible

resulted in greater mechanical loads in the anterior corpus,

including an increase in vertical bending stresses. It is impor-

tant to consider, however, that the modeled mandibular

symphyseal variation used by Gr€oning et al. (2011) may not

realistically reflect the overall pattern of correlated mor-

phology associated with variation in chin prominence in

modern humans. For example, individuals with a less pro-

jecting chin, both during development and across static

adult comparisons, are typically characterized by an increase

in symphyseal height dimensions (Bj€ork, 1969; Bastir & Ro-

sas, 2004) that likely act to increase resistance to vertical

bending stresses. Holton et al. (2014) recently documented

that mandibles with increased vertical bending resistance

were also characterized by increased symphyseal height

dimensions and a pattern of increased posterior mandibular

rotation. In spite of the increased resistance to vertical

bending, this pattern of mandibular form is commonly asso-

ciated with reduced chin prominence (e.g. Bj€ork, 1969;€Odegaard, 1970a,b; Lavergne & Gasson, 1976; Bj€ork & Skiel-

ler, 1983; Bastir & Rosas, 2004). As such, examining the func-

tional consequences of altering chin prominence in a finite

element model (e.g. Ichim et al. 2006; Gr€oning et al. 2011)

while not accounting for correlated variation in symphyseal

height (among other features), may not accurately repre-

sent the actual relationship between patterns of functional

loading and mandibular form, at least with regard to

within-modern human comparisons.

Whereas an increase in vertical bending resistance was

associated with reduced chin prominence and thus con-

trasted with ontogenetic changes in symphyseal shape, an

increase in the projection of the chin was associated with

greater resistance to wishboning stresses. This pattern, how-

ever, did not track ontogenetic changes in chin prominence.

Rather, relative wishboning resistance was associated with

variation in symphyseal depth dimensions resulting from

morphological changes along both the labial and lingual

symphyseal borders. The lack of association between rela-

tive wishboning resistance and ontogenetic changes in the

mandibular symphysis suggests that at least some of the

variation in wishboning resistance is independent of ontog-

eny and that correlated morphological variation in aspects

of symphyseal shape is established early in development

and maintained through adulthood (e.g. Fukase & Suwa,

2008). Indeed, during ontogeny both Iyy and Imin scaled with

isometry in the combined sample and in the individual male

and female samples. This indicates that during development

the mandibular symphysis does not exhibit an increase in

wishboning resistance, at least relative to mandibular

length.



In spite of the average isometric relationship between

wishboning resistance and mandibular length there was,

nevertheless, a considerable range of variation in individual

regression slopes with both Iyy and Imin. An examination of

the range of slope values shows that there were individuals

who scaled with negative allometry, whereas others scaled

with positive allometry (a wide, albeit narrower, range was

also documented for Ixx and Imax). The variability in individ-

ual ontogenetic allometries may suggest that the distribu-

tion of symphyseal cortical bone may be less constrained

by functional loading and therefore influenced to a

greater degree by variables unrelated to symphyseal stres-

ses. This result may be due, in part, to the use of a 20th

century sample that subsisted on relatively processed diets.

If increased masticatory function (e.g. tougher diets that

require greater intraoral processing) has a greater influence

on symphyseal cortical bone, then it is possible that other

samples (e.g. hunter-gatherer populations) may exhibit rel-

atively lower ranges of within-sample morphological vari-

ability.

In contrast to our results, other studies indicate that wish-

boning may have a significant influence on symphyseal cor-

tical bone. We recently documented, for example, that

wishboning resistance was correlated with in vivo bite force

magnitude and estimated wishboning forces modeled from

data-derived computed tomography scans of living human

subjects (Holton et al., 2014). Furthermore, additional stud-

ies have documented that cortical bone along the lingual

symphysis is characterized by greater thickness (Fukase,

2007; Fukase & Suwa, 2008) and density (Schwartz-Dabney

& Dechow, 2003) relative to the labial aspect of the symphy-

sis. Given that tensile strains along the lingual symphysis are

predicted to be greater than compressive labial strains dur-

ing wishboning (Hylander, 1984), the pattern of symphyseal

cortical bone thickness and density is consistent with the

predicted osseous response to wishboning stresses.

Despite the results of these studies, there are reasons

to think that wishboning of the mandibular symphysis is

an unimportant loading regime during mastication in

modern humans. First, the modern human mandible is

reduced in length relative to archaic Homo. This has the

effect of reducing the length of the wishboning moment

arm and, therefore, theoretically should reduce wishbon-

ing stresses in the symphyseal region (Daegling, 1993).

Moreover, the human mandible does not exhibit the

same degree of curvature as other anthropoid mandibles

and therefore estimated lingual stresses in the human

symphysis are only around 1.5 times greater than labial

stresses (Hylander and Johnson, 1994). This is in contrast

to anthropoids such as baboons, in which lingual stresses

are estimated to be as high as 5.0 times greater than

labial stresses (Hylander, 1984, 1985; Hylander & Johnson,

1994).

In addition to mandibular geometry, humans do not exhi-

bit the same masticatory muscle recruitment patterns that

are associated with wishboning in other anthropoids. Wish-

boning stresses in anthropoid primates are associated with

the recruitment of the balancing-side deep masseter muscle

late during the power stroke (Hylander & Johnson, 1994;

Hylander et al. 1998; Vinyard et al. 2008). In humans,

however, the balancing-side deep masseter reaches peak

activity earlier in the chew cycle (e.g. van Eijden et al.

1993). Ultimately, a more thorough assessment of symphy-

seal cortical bone geometry and in vivo functional data in

humans (e.g. masticatory force production, muscle activity

patterns) will be necessary to resolve potential incongruities

between different datasets (e.g. ontogenetic allometries,

population variation in cortical bone properties, modeled

mandibular stresses, etc.).

Conclusions

Our results indicate that the ontogenetic development of

the chin does not result in an increase in resistance to sym-

physeal bending stresses relative to moment arm proxies. In

the case of vertical bending, an ontogenetic increase in chin

prominence was associated with decreased bending resis-

tance. Moreover, although relative symphyseal depth was

correlated with wishboning resistance, this was indepen-

dent of ontogeny.

We note that while we have examined symphyseal form

relative to bending moment arm lengths, a better under-

standing of the effects of masticatory function on mandibu-

lar symphyseal form would benefit from a thorough

examination of ontogenetic scaling of masticatory force

production and modeled symphyseal stresses in humans. In

the present study, we only considered bending resistance

relative to bending moment arm lengths (e.g. Daegling,

2012). Although symphyseal bending is, in part, a function

of bending moment arm length, there are of course other

variables such as masticatory adductor force that are

needed to estimate bending forces during mastication (e.g.

Vinyard & Ravosa, 1998; Holton et al. 2014). In the present

cephalometric study, we were unable to assess how bend-

ing resistance scales with regard to masticatory force pro-

duction and symphyseal bending forces during human

ontogeny. Indeed, the ontogenetic scaling of these vari-

ables is currently unknown. It is possible, for example, that

vertical bending resistance scales isometrically with vertical

bending force to maintain functional equivalency during

ontogeny as documented in papionin primates (Vinyard &

Ravosa, 1998). If symphyseal bending resistance in humans

scales isometrically with estimated symphyseal bending

forces, then our conclusions would need to be reconsidered.

Ontogenetic analyses of masticatory loads and symphyseal

bending forces in human samples are needed to fully

resolve this issue. Nevertheless, our analysis provides impor-

tant data that furthers our understanding of the complex

interaction between symphyseal form and jaw function dur-

ing development.



Acknowledgements

The authors thank the Editor, Associate Editor and reviewers for

their valuable comments and suggestions. We also thank Terris Wil-

liams for assistance with data collection.

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The ontogeny of the chin: an analysis of allometric and biomechanical scaling

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