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Sex Estimation Using the Calcaneus – Wharton 1
Abstract
One of the most important aspects of the analysis and identification of human skeletal
remains is determination of the biological profile, which consists of the age, sex,
ancestry, and stature of an individual. Typically, anthropologists analyze the pubic
bone in adult human skeletal remains, which is highly sexually dimorphic due to a
female’s capacity for childbirth. However, when these fragile bones are broken,
damaged, or altogether missing, it is necessary to look to other areas of the skeleton
for sex information. DiMichele and Spradley (2012) developed a method of sex
estimation using four measurements of the calcaneus. For this project, this method
was tested using the left calcanei of modern American skeletal remains from the Texas
State University Donated Skeletal Collection (TXSTDSC). This skeletal sample consists
of 264
individuals (151 males, 113 females) with an average age-at-death of 65.2
years, 91.3% of whom are classified as white. The sectioning points for individual
measurements were able to accurately estimate sex for 76.7–80.9% of this skeletal
sample. Univariate and multivariate equations provided classification rates ranging
from 77.4% to 89.6%. The results of this study further validate that the calcaneus is
sexually dimorphic and can be used to estimate sex in a modern American skeletal
sample.
Sex Estimation Using the Calcaneus – Wharton 2
Acknowledgements
Many thanks to everyone who helped make this project possible…
The Cal U University Honors Program, Mr. Aune, Dr. Fox, Ms. Kim Orslene;
committee members: Dr. John Nass, Dr. Arrigo-Nelson, Dr. Meloy; Dr. Wescott and Sophia
Mavroudas from the Forensic Anthropology Center at Texas State; Cearra, who probably did
something, and
The most thanks to my advisor Dr. Kuba for absolutely everything.
Sex Estimation Using the Calcaneus – Wharton 3
Table of Contents
ABSTRACT
2
ACKNOWLEDGEMENTS
3
TABLE OF CONTENTS
4
INTRODUCTION
5
BACKGROUND
6
METRIC METHODS OF POSTCRANIAL SEX ESTIMATION
6
SEX ESTIMATION WITH FOOT BONES
7
MATERIALS AND METHODS
11
SKELETAL SAMPLE
11
STATISTICAL ANALYSES
14
METHODS AND MEASUREMENTS
12
RESULTS
15
INTER- AND INTRAOBSERVER ERROR
15
SEX ESTIMATION
16
DISCUSSION
18
CONCLUSION
19
LITERATURE CITED
20
Sex Estimation Using the Calcaneus – Wharton 4
INTRODUCTION
An accurate estimation of sex is the foundation of the biological profile. It is
undeniably useful in archaeological studies and essential in identifying human
remains in a forensic context. Additionally, many of the methodologies for estimating
age, ancestry, and stature are also dependent upon an accurate estimation of sex. An
anthropologist will usually look at the skull and the pelvis (or os coxae) for sex
estimation. The morphological features of the pelvis are highly sexually dimorphic
due to a female’s capacity for childbirth. The skull does typically show some sexual
dimorphism, though it can vary between different populations. Nonetheless, i t is
commonly a source of sex information for anthropologists.
Several scenarios may preclude use of standard methods of sex estimation.
Fragmentary, damaged, commingled, or otherwise incomplete human skeletal remains
often pose a challenge to anthropologists. Without complete and well-preserved os
coxae, other methods of sex estimation must be used. It is quite common to be faced
with this issue when dealing with archaeological remains, and though the absence of
complete and observable os coxae is less common in forensic cases, the phenomenon
is still important for any biological anthropologist to consider.
DNA analysis of skeletal remains can provide accurate determinations of sex,
but may not always be a viable option. This method can be costly and time-
consuming. Further, destructive analyses may not be permitted when working with
archaeological remains, and DNA samples could be contaminated. Therefore, it is
necessary to study alternative methods of sex estimation.
Spradley and Jantz (2011) have disproven the once-common belief that the
cranium is second best in providing accurate estimations of sex, leading to a shift in
anthropological research focusing on metric sex estimation with the postcranial
Sex Estimation Using the Calcaneus – Wharton 5
skeleton. This project focuses on a technique used to estimate sex from adult human
skeletal remains using the calcaneus (heel bone; DiMichele & Spradley, 2012). This
method was tested on modern American skeletal remains from the Texas State
University Donated Skeletal Collection (TXSTDSC) to determine the efficacy of this
bone to estimate sex with a variety of statistical methods including use of sectioning
points, univariate, and multivariate equations.
BACKGROUND
Anthropologists analyze the skull or pelvis in adult human skeletal remains.
However, when these fragile bones are broken, damaged, or altogether missing, it is
necessary to look to other areas of the skeleton for sex information.
Metric Methods of Postcranial Sex Estimation
Though established biological anthropologist Bill Bass once stated that “the
skull is probably the second-best area of the skeleton to use for determining sex,”
(Bass, 2005, p. 81) Spradley and Jantz (2011) have disproven this idea with an in-
depth study focusing on many postcranial skeletal elements’ ability to estimate sex. In
this study, standard cranial, mandibular, and postcranial measurements (Moore-
Jansen, Ousley, & Jantz, 1994) from 704 skulls and 639 postcranial skeletons (reported
separately because not all individuals were represented by both a skull and
postcranial skeleton) recorded in the Forensic Data Bank were used to estimate sex
with univariate and multivariate methods. They found that many metrics including
the humerus, radius, clavicle, femur, ulna, and tibia outperformed sex estimation with
the skull.
Further, many other works have demonstrated high accuracy rates using
univariate or multivariate equations, sectioning points, or other metric methods to
Sex Estimation Using the Calcaneus – Wharton 6
estimate sex from vertebrae, long bones, and other elements of the postcranial
skeleton for use in the absence of the os coxae (Albanese, 2013; Bethard, & Seet, 2013;
Holland, 1991; King, Işcan, & Loth, 1998; Marino, 1995; Marlow, & Pastor, 2011;
Safont, Malgosa, & Subira, 2000; Šlaus, & Tomčič, 2005; Steyn, & Işcan, 1999; Tise,
Spradley, & Anderson, 2013; Wescott, 2000).
Sex Estimation with Foot Bones
Many researchers have explored the use of foot bones to estimate sex in
skeletal remains. Several studies show that other foot bones such as the navicular,
talus, and calcaneus are sexually dimorphic.
Salidas, Malgosa, Jordana, and Isidro (2016) tested the navicular bone (one of
the tarsals in the mid-foot) for its use to estimate sex in contemporary Spanish
individuals. They analyzed the naviculars of 231 individuals from several university
skeletal collections in Spain. Eight variables were measured, and it was reported that
all measurements showed significant sexual dimorphism. Binary logistic equations
were created to estimate sex with these measurements and it was found that
maximum width and maximum length of the talar facet; maximum width and
maximum length of the cuneiform surface provided the most accurate results. Overall
high classification rates were shown for all of the equations used to estimate sex,
indicating that the navicular can be used for sex estimation (Salidas, Malgosa, Jordana,
& Isidro, 2016).
A 2003 study by Bidmos and Dayal looked at sex estimation using the talus of
South African whites. They looked at 9 measurements of 120 individuals and created
univariate and stepwise discriminant functions to estimate sex. They found that talus
length gave the highest univariate classification rate (81.7%), and height of the head
of the talus performed very poorly with a classification rate of 57.5%. The
Sex Estimation Using the Calcaneus – Wharton 7
discriminant functions provided accuracy rates between 77.5% and 87.5% (Bidmos &
Dayal, 2003).
Barrett, Cavallari, and Sciulli (2001) studied the talus in prehistoric Native
American skeletal remains from a variety of archaeological sites in the Ohio Valley
region. The “correct” sex was estimated using the os coxae and then compared with
talar measurements. Length, width, and height of the talus were measured for 74
males and 68 females. Each of the measurements was found to be significantly
sexually dimorphic. Using discriminant function analysis with all three variables, the
authors found an overall classification rate of 84.5%. The authors also observed talus
“volume” by multiplying the length, width, and height variables. This volume
measurement was the least sexually dimorphic, but had similar discriminating
abilities. This indicates that while the differences of volume between males and
females are small, they are consistent. All three measurements and “ volume” were
sexually dimorphic and show promise for sex estimation using the talus (Barrett,
Cavallari, & Sciulli, 2001).
Lee et. al (2012) tested the talus in Koreans for its use in sex estimation. Data
was collected from a total of 140 individuals from skeletal collections at Yonsei
University and The Catholic University of Korea. They used nine measurements seen
in previous studies and found similar sexual dimorphism in the talus. Univariate,
multivariate, and stepwise discriminant function equations were created to estimate
sex. They achieved accuracy rates between 67.1% and 87.1%. In addition to their
own mathematical methods, they compared their data to equations from another
study and found that they were not as accurate, supporting the idea that this is a
population specific method (Lee, et al. 2012).
Sex Estimation Using the Calcaneus – Wharton 8
A 2014 study by Mahakkanukrauh et al. looked at the talus of Thai individuals
from the Chiang Mai University Skeletal Collection. Ten measurements of the talus
were taken for 252 individuals (126 males and 126 females). The authors developed
logistic regression equations using the talar measurements and were able to estimate
sex with accuracy rates between 84.5% and 88.2%. They also found that trochlear
breadth and trochlear length were the most sexually dimorphic of their measurements
(Mahakkanukrauh, et al., 2014).
Calcaneus
Steele (1976) was among the first researchers to use the talus and calcaneus
for sex estimation. He took 5 measurements for each left talus and calcaneus from 120
individuals from the Terry Anatomical Skeletal Collection at the Smithsonian. This
collection consists of white and black Americans who died during or before the 1930s.
Discriminant functions from the data resulted in sexing accuracy rates ranging from
79% to 89%. However, Steele questioned whether these discriminant functions would
be accurate for more modern Americans, or if temporal changes would affect the use
of these bones for sex estimation. It was also found that the discriminant functions
are accurate with other ancestral groups, though sectioning points needed to be
altered for better results.
Introna et. al (1996) recreated the study done by Steele, focusing just on the
calcaneus but adding three more measurements. They studied 80 Southern Italian
individuals from skeletal collections at the University of Bari. Their univariate and
multivariate discriminant function equations both provided high accuracy rates with
up to 85% correct classifications. Additionally, data from the Italian skeletal sample
were tested with the equations developed by Steele, but were less accurate for
Sex Estimation Using the Calcaneus – Wharton 9
estimating sex. This stresses the need for population specific information and using
equations developed from the population of the individual being sexed.
DiMichele and Spradley (2012) analyzed the calcanei in 320 modern American
individuals from the Bass Donated Skeletal Collection housed at the University of
Tennessee Knoxville. They looked at four measurements: MXL, LAL, LAW, and PCF
(defined below in Table 2.3). Sectioning points were developed by taking the
weighted average between males and females for each measurement. Additionally, a
discriminant function multivariate equation was created to estimate sex.
Approximately 16% of the skeletal sample used in this study was non-white, with the
remaining 84% of individuals classified as white. Tests for correlations between
measurements and ancestry did show some significant trends, but it was determined
that the calcaneus is not a good estimator of ancestry. The authors also argue that if
the calcaneus is being used for sex estimation, it’s likely one of the most complete
bones available for analysis. Thus, an estimation of ancestry may not be prioritized
(DiMichele & Spradley, 2012)
The sectioning points resulted in classification rates ranging from 80.08 % to
88.10%, and the discriminant function equation was found to correctly assess sex for
88.64% of females and 84.75% of males. It is pointed out that though Spradley and
Jantz (2011) found that there were no differences in estimating sex with the calcaneus
for American whites and blacks, there may be differences among other populations.
Many other studies have also found similar results demonstrating that the
calcaneus is useful for sex estimation (Murphy, 2002; Peckmann, Orr, Meek, & Manolis,
2015; Bidmos & Asala, 2004).
Sex Estimation Using the Calcaneus – Wharton 10
MATERIALS AND METHODS
Skeletal Sample
The Texas State Donated Skeletal Collection (TXSTDSC) is a documented
skeletal collection consisting of individuals who lived and died in the 21 st century.
Donated bodies are first used in studies of human decomposition, and the skeletal
remains are later curated into the skeletal collection. Most of the donations are
individuals from the state of Texas, but bodies are accepted from all over the United
States and around the world.
A total of 264 adult individuals (113 females, 151 males) from the TXSTDSC
were used for this study. These individuals range in age from 18 to 102 with a mean
age of 65 years.
Table 2.1 – Skeletal sample age and sex information
Age
18-34
35-49
50-64
65+
Total
Females
4
9
35
65
113
Males
Total
10
14
8
17
55
90
78
143
151
264
Of the 264 individuals studied, 241 were classified as white, 9 as black, 11 as
Hispanic, and 3 as “other” (Table 2.2). Because of the small amount of non-white
individuals represented in the sample, all individuals were pooled and ancestry was
not used as a factor in this study. Additionally, as DiMichele and Spradley (2012)
found that the calcaneus is not a good estimator of ancestry. If the calcaneus is being
used for sex estimation, it is likely because it is one of the most complete bones, and
an estimation of ancestry may not be prioritized. No further tests for associations
between race and sex were performed.
Sex Estimation Using the Calcaneus – Wharton 11
Table 2.2 – Skeletal sample race information
Race
Black
Hispanic
White
Other
Percent
3.41%
4.17%
91.29%
1.14%
Count
9
11
241
3
Methods and Measurements
Left calcanei were measured for the purpose of consistency, and the right
calcaneus was used for individuals whose left calcaneus was missing. Any calcanei
which were too damaged or appeared to have any pathological conditions were
excluded from this study. Further, if the left calcaneus was excluded due to pathology
or damage, the right side was not used as a replacement since the issue was typically
reflected on both left and right calcanei in the same individual.
Measurements for length and width were taken using Mitutoyo Absolute digital
sliding calipers and measured to the nearest .01mm. Circumference measurements
were taken using a retractable fabric tape measure and rounded to the nearest 0.1cm.
The project investigator was kept blind regarding age and sex until after
measurements were taken.
Measurements taken include maximum length (MXL; Buikstra & Ubelaker,
1994), load arm length (LAL; Steele, 1976), load arm width (LAW; Buikstra &
Ubelaker, 1994; Steele, 1976), and posterior circumference (PCF; DiMichele &
Spradley, 2012), as shown in Figure 2.1 and Table 2.3.
Sex Estimation Using the Calcaneus – Wharton 12
Figure 2.1 – Calcaneus measurements
Table 2.3 – Measurement definitions
Measurements
Adapted From
Definition
Maximum Length
Buikstra &
most anterior point on the superior margin of the articular facet for
(MXL)
Load Arm Length
(LAL)
Load Arm Width
(LAW)
Posterior
Circumference
(PCF)
Ubelaker, 1994
Steele, 1976
Buikstra &
Ubelaker, 1994;
Steele, 1976
DiMichele &
Spradley, 2012
Distance between the most projecting point on the tuberosity and the
the cuboid measured in the sagittal plane and projected onto the
underlying surface.
Defined as the projected line from the most posterior point of the
dorsal articular facet, to the most anterior/superior point of the
cuboidal facet
Distance between the most laterally projecting point on the dorsal
articular facet and the most medial point on the sustentaculum tali
Defined as the minimum circumference of the area between the
posterior point of the dorsal articular facet and most posterior point of
the calcaneus.
Posterior circumference (PCF) was the only measurement which had not been used
multiple times in prior studies. Therefore, DiMichele and Spradley (2012) provide
additional explanation for this metric:
Sex Estimation Using the Calcaneus – Wharton 13
“To properly take this measurement, lay measuring tape flat against the surface
of the bone. Pass the measuring tape around anteriorly to the inner and outer
tuberosity of the calcaneus. Avoid projecting heel spurs located on the inferior
surface of the calcaneus by laying the measuring tape beneath them. In certain
cases, calcaneal tuberosities have been seen to be located at a more anterior
position, in which case it may be appropriate to measure on the posterior side
of the tuberosity, avoiding the feature, in order to properly obtain the
minimum circumference.” (DiMichele & Spradley, 2012, p. 2)
Additionally, cadaver stature (CS), defined as the maximum length of a cadaver
from the base of the heel to the top of the head, was used in some analyses.
Statistical Analyses
Inter- and Intraobserver Error
A sample of 23 individuals were measured in two separate trials by the author
several days apart, and once by an experienced observer to test for inter- and
intraobserver error.
Sex Estimation
Data collected from the TXSTDSC were used to estimate sex with the sectioning points
determined by DiMichele and Spradley (2012). Additionally, sectioning points were
derived from this skeletal sample by taking the weighted average between males and
females for each measurement. If a measurement is below the sectioning point, the
individual is estimated to be a female; if the measurement is over the sectioning point,
the individual is estimated to be a male. Pearson’s correlation coefficients were
calculated in Statistical Analysis Software (SAS) v. 9.04 (SAS Institute Inc., Cary, NC,
Sex Estimation Using the Calcaneus – Wharton 14
USA) to observe the correlations between each measurement, sex, and cadaver
stature.
All four measurements were used to create univariate functions for estimating
sex and a multivariate equation was created using all four variables. The sex variable
was coded as males=1; females=0 to calculate the mathematical equations, and 0.5
was used as a sectioning point for estimating sex. If the result of the equations is more
than 0.5, they are estimated to be a male; if the result is less than 0.5, they are
estimated to be a female.
RESULTS
Inter- and Intraobserver Error
Tests for inter- and intraobserver error show that these measurements are
replicable (Table 3.1). Interobserver error (2.11%) was overall higher than
intraobserver error (0.86%) indicating that different individuals may take
measurements slightly differently. However, intraobserver error was very low,
making it very easy to replicate results with the same observer. In both scenarios,
load arm length (LAL) had the highest error rates, indicating that this measurement is
less reliable than the others. In different individuals, the most anterior/superior point
of the cuboidal facet (a surface of the bone which contains a measurement landmark)
can vary in its location, which may have led to the differences in measurements. A
better-defined metric may clarify the issue to provide for lower error rates in the
future.
Sex Estimation Using the Calcaneus – Wharton 15
Intraobserver
Interobserver
MXL
Table 3.1 – Error rates
0.36
2.23
LAL
LAW
3.65
1.32
1.51
1.06
PCF
Average
1.24
2.11
0.52
0.86
Sex Estimation
The means and standard deviation for each measurement for males and
females are shown in Table 3.2.
Var
MXL
LAL
LAW
PCF
N
148
148
146
150
Table 3.2 - Simple statistics
Males
Mean
87.55
54.87
44.85
113.8
SD
4.83
3.17
2.66
7.3
N
109
108
111
112
Females
Mean
80.21
49.68
40.42
102.0
SD
4.31
2.90
2.33
6.3
Sectioning points developed for the current study were found to be close to
those of the study done by DiMichele and Spradley (2012), who also looked at modern
American individuals. Both the sectioning points from this study and those created by
DiMichele and Spradley (2012) provided high accuracy rates for sex estimation (Table
3.3).
Table 3.3 – Sectioning points and accuracy rates
MXL
LAL
LAW
PCF
Pooled accuracy rates
78.2
79.3
80.5
80.2
Pooled accuracy rates
78.2
80.9
79.8
76.7
Sectioning points from present study
Sectioning points from DiMichele & Spradley
(2012)
83.9
83.8
52.3
51.6
42.6
42.0
10.8
11.0
Sex Estimation Using the Calcaneus – Wharton 16
The Pearson correlation coefficients showed statistically significant
correlations between sex and each measurement. Additionally, cadaver stature also
showed high correlations with each of the measurements, indicating that calcaneal
measurements may be more closely associated with stature than with sex.
Table 3.4 – Pearson correlation coefficients
Sex
MXL
Sex
1.00
LAL
LAW
MXL
LAL
1.00
0.84
0.62
LAW
PCF
CS
0.64
0.66
0.65
0.72
1.00
0.74
0.72
0.70
PCF
CS
0.74
1.00
0.68
0.74
1.00
0.77
0.71
0.67
1.00
Each of the univariate equations provided high classifications for sex
estimation, ranging from 77.43% to 80.86%. All four equations classified males
correctly more often than females, meaning females were misclassified more often.
This could be due to the sample size and the ratio of males to females ( approximately
4:3). With sectioning points set at 0.5, it is assumed that there is an equal likelihood
of an individual being a male or a female. However, this is not the case with the
skeletal sample used in this study.
The multivariate equation was also able to classify sex with high accuracy, and
with similar results for males and females.
Sex Estimation Using the Calcaneus – Wharton 17
Table 3.5 – Univariate and multivariate equations and accuracy rates
l
MXL
0.0523
LAL
LAW
PCF
Constant
Sectioning point
Males classified correctly
(%)
Females classified correctly
(%)
Total classified correctly
(%)
2
-0.0798
3
-0.0976
4
-0.0356
5
-0.0067
-0.0263
-0.0356
-0.0144
-3.8405
0.5
4.6278
0.5
4.6230
0.5
4.2980
0.5
5.4653
0.5
71.56
75.00
76.58
72.32
88.68
81.76
77.43
85.14
80.86
83.56
80.54
86.67
80.53
90.28
89.60
DISCUSSION
Despite low error rates, some clarification on the LAW and LAL measurements
may help inexperienced observers. Because it can be difficult to hold a single
calcaneus in exact anatomical position when in isolation, the exact points from which
to measure are not always clear.
The present project had very similar results to the study done by DiMichele
and Spradley (2012), with high accuracy rates and similar sectioning points.
However, as shown in other studies (Lee et. al, 2012; Bethard & Seet, 2013), metric
methods of sex estimation tend to be population specific. While these sectioning
points are accurate for majority-white American skeletal populations, they may not be
accurate for individuals of other ancestral groups or from different geographic
regions. In the contexts of American forensic anthropology, future studies should look
toward developing standards for other demographics commonly found in the United
States.
Sex Estimation Using the Calcaneus – Wharton 18
The univariate equations also provided high overall accuracy rates. Since
those and the sectioning points only require one simple measurement, these are the
preferred methods compared to using a multivariate equation. While the calcaneus
may often be present in archaeological settings (Waldron, 1987), it may also be
damaged or broken, making it more difficult to get all four measurements needed for a
multivariate equation.
CONCLUSION
This study validates the findings of DiMichele and Spradley (2012), and
demonstrates that the calcaneus is sexually dimorphic and produces high
classifications for sex estimation in a modern American skeletal population.
Like many authors have done in the past, future studies could focus on more
measurements of the calcaneus to determine which metric(s) are the most useful in
estimating sex regardless of ancestry or for different ancestral groups/populations.
Additionally, it may be useful to look into what causes the misclassifications of sex.
Differences in calcaneal metrics could be caused by a variety of factors including body
weight, lifestyle, whether an individual overpronates or oversupinates, or has had a
foot injury. Further, more advanced techniques may be necessary to determine what
can affect sex estimation with the calcaneus.
Sex Estimation Using the Calcaneus – Wharton 19
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Šlaus, M., & Tomčič, Ž. (2005). Discriminant function sexing of fragmentary and
complete tibiae from medieval Croatian sites. Forensic Science International,
147(2-3), 147-152.
Spradley, M. K., & Jantz, R. L. (2011). Sex estimation in forensic anthropology: Skull
versus postcranial elements. Journal of Forensic Sciences. 56(2), 289-296.
Steyn, M., & Işcan, M. Y. (1999). Osteometric variation in the humerus: sexual
dimorphism in South Africans. Forensic Science International, 106(2), 77-85.
Tise, M. L., Spradley, M. K., & Anderson, B. E. (2013). Postcranial sex estimation of
individuals considered Hispanic. Journal of Forensic Sciences, 58(S1), S9-S14.
Waldron, T. (1987). The relative survival of the human skeleton: Implications for
paleopathology. In A. Boddington, A. N. Garland, & R. C. Janaway (Eds.), Death,
decay and reconstruction: Approaches to archaeology and forensic science (pp.
55-64). Manchester, UK: Manchester University Press.
Wescott, D. J. (2000). Sex variation of the second cervical vertebra. Journal of Forensic
Sciences, 45(2), 462-466.
Abstract
One of the most important aspects of the analysis and identification of human skeletal
remains is determination of the biological profile, which consists of the age, sex,
ancestry, and stature of an individual. Typically, anthropologists analyze the pubic
bone in adult human skeletal remains, which is highly sexually dimorphic due to a
female’s capacity for childbirth. However, when these fragile bones are broken,
damaged, or altogether missing, it is necessary to look to other areas of the skeleton
for sex information. DiMichele and Spradley (2012) developed a method of sex
estimation using four measurements of the calcaneus. For this project, this method
was tested using the left calcanei of modern American skeletal remains from the Texas
State University Donated Skeletal Collection (TXSTDSC). This skeletal sample consists
of 264
individuals (151 males, 113 females) with an average age-at-death of 65.2
years, 91.3% of whom are classified as white. The sectioning points for individual
measurements were able to accurately estimate sex for 76.7–80.9% of this skeletal
sample. Univariate and multivariate equations provided classification rates ranging
from 77.4% to 89.6%. The results of this study further validate that the calcaneus is
sexually dimorphic and can be used to estimate sex in a modern American skeletal
sample.
Sex Estimation Using the Calcaneus – Wharton 2
Acknowledgements
Many thanks to everyone who helped make this project possible…
The Cal U University Honors Program, Mr. Aune, Dr. Fox, Ms. Kim Orslene;
committee members: Dr. John Nass, Dr. Arrigo-Nelson, Dr. Meloy; Dr. Wescott and Sophia
Mavroudas from the Forensic Anthropology Center at Texas State; Cearra, who probably did
something, and
The most thanks to my advisor Dr. Kuba for absolutely everything.
Sex Estimation Using the Calcaneus – Wharton 3
Table of Contents
ABSTRACT
2
ACKNOWLEDGEMENTS
3
TABLE OF CONTENTS
4
INTRODUCTION
5
BACKGROUND
6
METRIC METHODS OF POSTCRANIAL SEX ESTIMATION
6
SEX ESTIMATION WITH FOOT BONES
7
MATERIALS AND METHODS
11
SKELETAL SAMPLE
11
STATISTICAL ANALYSES
14
METHODS AND MEASUREMENTS
12
RESULTS
15
INTER- AND INTRAOBSERVER ERROR
15
SEX ESTIMATION
16
DISCUSSION
18
CONCLUSION
19
LITERATURE CITED
20
Sex Estimation Using the Calcaneus – Wharton 4
INTRODUCTION
An accurate estimation of sex is the foundation of the biological profile. It is
undeniably useful in archaeological studies and essential in identifying human
remains in a forensic context. Additionally, many of the methodologies for estimating
age, ancestry, and stature are also dependent upon an accurate estimation of sex. An
anthropologist will usually look at the skull and the pelvis (or os coxae) for sex
estimation. The morphological features of the pelvis are highly sexually dimorphic
due to a female’s capacity for childbirth. The skull does typically show some sexual
dimorphism, though it can vary between different populations. Nonetheless, i t is
commonly a source of sex information for anthropologists.
Several scenarios may preclude use of standard methods of sex estimation.
Fragmentary, damaged, commingled, or otherwise incomplete human skeletal remains
often pose a challenge to anthropologists. Without complete and well-preserved os
coxae, other methods of sex estimation must be used. It is quite common to be faced
with this issue when dealing with archaeological remains, and though the absence of
complete and observable os coxae is less common in forensic cases, the phenomenon
is still important for any biological anthropologist to consider.
DNA analysis of skeletal remains can provide accurate determinations of sex,
but may not always be a viable option. This method can be costly and time-
consuming. Further, destructive analyses may not be permitted when working with
archaeological remains, and DNA samples could be contaminated. Therefore, it is
necessary to study alternative methods of sex estimation.
Spradley and Jantz (2011) have disproven the once-common belief that the
cranium is second best in providing accurate estimations of sex, leading to a shift in
anthropological research focusing on metric sex estimation with the postcranial
Sex Estimation Using the Calcaneus – Wharton 5
skeleton. This project focuses on a technique used to estimate sex from adult human
skeletal remains using the calcaneus (heel bone; DiMichele & Spradley, 2012). This
method was tested on modern American skeletal remains from the Texas State
University Donated Skeletal Collection (TXSTDSC) to determine the efficacy of this
bone to estimate sex with a variety of statistical methods including use of sectioning
points, univariate, and multivariate equations.
BACKGROUND
Anthropologists analyze the skull or pelvis in adult human skeletal remains.
However, when these fragile bones are broken, damaged, or altogether missing, it is
necessary to look to other areas of the skeleton for sex information.
Metric Methods of Postcranial Sex Estimation
Though established biological anthropologist Bill Bass once stated that “the
skull is probably the second-best area of the skeleton to use for determining sex,”
(Bass, 2005, p. 81) Spradley and Jantz (2011) have disproven this idea with an in-
depth study focusing on many postcranial skeletal elements’ ability to estimate sex. In
this study, standard cranial, mandibular, and postcranial measurements (Moore-
Jansen, Ousley, & Jantz, 1994) from 704 skulls and 639 postcranial skeletons (reported
separately because not all individuals were represented by both a skull and
postcranial skeleton) recorded in the Forensic Data Bank were used to estimate sex
with univariate and multivariate methods. They found that many metrics including
the humerus, radius, clavicle, femur, ulna, and tibia outperformed sex estimation with
the skull.
Further, many other works have demonstrated high accuracy rates using
univariate or multivariate equations, sectioning points, or other metric methods to
Sex Estimation Using the Calcaneus – Wharton 6
estimate sex from vertebrae, long bones, and other elements of the postcranial
skeleton for use in the absence of the os coxae (Albanese, 2013; Bethard, & Seet, 2013;
Holland, 1991; King, Işcan, & Loth, 1998; Marino, 1995; Marlow, & Pastor, 2011;
Safont, Malgosa, & Subira, 2000; Šlaus, & Tomčič, 2005; Steyn, & Işcan, 1999; Tise,
Spradley, & Anderson, 2013; Wescott, 2000).
Sex Estimation with Foot Bones
Many researchers have explored the use of foot bones to estimate sex in
skeletal remains. Several studies show that other foot bones such as the navicular,
talus, and calcaneus are sexually dimorphic.
Salidas, Malgosa, Jordana, and Isidro (2016) tested the navicular bone (one of
the tarsals in the mid-foot) for its use to estimate sex in contemporary Spanish
individuals. They analyzed the naviculars of 231 individuals from several university
skeletal collections in Spain. Eight variables were measured, and it was reported that
all measurements showed significant sexual dimorphism. Binary logistic equations
were created to estimate sex with these measurements and it was found that
maximum width and maximum length of the talar facet; maximum width and
maximum length of the cuneiform surface provided the most accurate results. Overall
high classification rates were shown for all of the equations used to estimate sex,
indicating that the navicular can be used for sex estimation (Salidas, Malgosa, Jordana,
& Isidro, 2016).
A 2003 study by Bidmos and Dayal looked at sex estimation using the talus of
South African whites. They looked at 9 measurements of 120 individuals and created
univariate and stepwise discriminant functions to estimate sex. They found that talus
length gave the highest univariate classification rate (81.7%), and height of the head
of the talus performed very poorly with a classification rate of 57.5%. The
Sex Estimation Using the Calcaneus – Wharton 7
discriminant functions provided accuracy rates between 77.5% and 87.5% (Bidmos &
Dayal, 2003).
Barrett, Cavallari, and Sciulli (2001) studied the talus in prehistoric Native
American skeletal remains from a variety of archaeological sites in the Ohio Valley
region. The “correct” sex was estimated using the os coxae and then compared with
talar measurements. Length, width, and height of the talus were measured for 74
males and 68 females. Each of the measurements was found to be significantly
sexually dimorphic. Using discriminant function analysis with all three variables, the
authors found an overall classification rate of 84.5%. The authors also observed talus
“volume” by multiplying the length, width, and height variables. This volume
measurement was the least sexually dimorphic, but had similar discriminating
abilities. This indicates that while the differences of volume between males and
females are small, they are consistent. All three measurements and “ volume” were
sexually dimorphic and show promise for sex estimation using the talus (Barrett,
Cavallari, & Sciulli, 2001).
Lee et. al (2012) tested the talus in Koreans for its use in sex estimation. Data
was collected from a total of 140 individuals from skeletal collections at Yonsei
University and The Catholic University of Korea. They used nine measurements seen
in previous studies and found similar sexual dimorphism in the talus. Univariate,
multivariate, and stepwise discriminant function equations were created to estimate
sex. They achieved accuracy rates between 67.1% and 87.1%. In addition to their
own mathematical methods, they compared their data to equations from another
study and found that they were not as accurate, supporting the idea that this is a
population specific method (Lee, et al. 2012).
Sex Estimation Using the Calcaneus – Wharton 8
A 2014 study by Mahakkanukrauh et al. looked at the talus of Thai individuals
from the Chiang Mai University Skeletal Collection. Ten measurements of the talus
were taken for 252 individuals (126 males and 126 females). The authors developed
logistic regression equations using the talar measurements and were able to estimate
sex with accuracy rates between 84.5% and 88.2%. They also found that trochlear
breadth and trochlear length were the most sexually dimorphic of their measurements
(Mahakkanukrauh, et al., 2014).
Calcaneus
Steele (1976) was among the first researchers to use the talus and calcaneus
for sex estimation. He took 5 measurements for each left talus and calcaneus from 120
individuals from the Terry Anatomical Skeletal Collection at the Smithsonian. This
collection consists of white and black Americans who died during or before the 1930s.
Discriminant functions from the data resulted in sexing accuracy rates ranging from
79% to 89%. However, Steele questioned whether these discriminant functions would
be accurate for more modern Americans, or if temporal changes would affect the use
of these bones for sex estimation. It was also found that the discriminant functions
are accurate with other ancestral groups, though sectioning points needed to be
altered for better results.
Introna et. al (1996) recreated the study done by Steele, focusing just on the
calcaneus but adding three more measurements. They studied 80 Southern Italian
individuals from skeletal collections at the University of Bari. Their univariate and
multivariate discriminant function equations both provided high accuracy rates with
up to 85% correct classifications. Additionally, data from the Italian skeletal sample
were tested with the equations developed by Steele, but were less accurate for
Sex Estimation Using the Calcaneus – Wharton 9
estimating sex. This stresses the need for population specific information and using
equations developed from the population of the individual being sexed.
DiMichele and Spradley (2012) analyzed the calcanei in 320 modern American
individuals from the Bass Donated Skeletal Collection housed at the University of
Tennessee Knoxville. They looked at four measurements: MXL, LAL, LAW, and PCF
(defined below in Table 2.3). Sectioning points were developed by taking the
weighted average between males and females for each measurement. Additionally, a
discriminant function multivariate equation was created to estimate sex.
Approximately 16% of the skeletal sample used in this study was non-white, with the
remaining 84% of individuals classified as white. Tests for correlations between
measurements and ancestry did show some significant trends, but it was determined
that the calcaneus is not a good estimator of ancestry. The authors also argue that if
the calcaneus is being used for sex estimation, it’s likely one of the most complete
bones available for analysis. Thus, an estimation of ancestry may not be prioritized
(DiMichele & Spradley, 2012)
The sectioning points resulted in classification rates ranging from 80.08 % to
88.10%, and the discriminant function equation was found to correctly assess sex for
88.64% of females and 84.75% of males. It is pointed out that though Spradley and
Jantz (2011) found that there were no differences in estimating sex with the calcaneus
for American whites and blacks, there may be differences among other populations.
Many other studies have also found similar results demonstrating that the
calcaneus is useful for sex estimation (Murphy, 2002; Peckmann, Orr, Meek, & Manolis,
2015; Bidmos & Asala, 2004).
Sex Estimation Using the Calcaneus – Wharton 10
MATERIALS AND METHODS
Skeletal Sample
The Texas State Donated Skeletal Collection (TXSTDSC) is a documented
skeletal collection consisting of individuals who lived and died in the 21 st century.
Donated bodies are first used in studies of human decomposition, and the skeletal
remains are later curated into the skeletal collection. Most of the donations are
individuals from the state of Texas, but bodies are accepted from all over the United
States and around the world.
A total of 264 adult individuals (113 females, 151 males) from the TXSTDSC
were used for this study. These individuals range in age from 18 to 102 with a mean
age of 65 years.
Table 2.1 – Skeletal sample age and sex information
Age
18-34
35-49
50-64
65+
Total
Females
4
9
35
65
113
Males
Total
10
14
8
17
55
90
78
143
151
264
Of the 264 individuals studied, 241 were classified as white, 9 as black, 11 as
Hispanic, and 3 as “other” (Table 2.2). Because of the small amount of non-white
individuals represented in the sample, all individuals were pooled and ancestry was
not used as a factor in this study. Additionally, as DiMichele and Spradley (2012)
found that the calcaneus is not a good estimator of ancestry. If the calcaneus is being
used for sex estimation, it is likely because it is one of the most complete bones, and
an estimation of ancestry may not be prioritized. No further tests for associations
between race and sex were performed.
Sex Estimation Using the Calcaneus – Wharton 11
Table 2.2 – Skeletal sample race information
Race
Black
Hispanic
White
Other
Percent
3.41%
4.17%
91.29%
1.14%
Count
9
11
241
3
Methods and Measurements
Left calcanei were measured for the purpose of consistency, and the right
calcaneus was used for individuals whose left calcaneus was missing. Any calcanei
which were too damaged or appeared to have any pathological conditions were
excluded from this study. Further, if the left calcaneus was excluded due to pathology
or damage, the right side was not used as a replacement since the issue was typically
reflected on both left and right calcanei in the same individual.
Measurements for length and width were taken using Mitutoyo Absolute digital
sliding calipers and measured to the nearest .01mm. Circumference measurements
were taken using a retractable fabric tape measure and rounded to the nearest 0.1cm.
The project investigator was kept blind regarding age and sex until after
measurements were taken.
Measurements taken include maximum length (MXL; Buikstra & Ubelaker,
1994), load arm length (LAL; Steele, 1976), load arm width (LAW; Buikstra &
Ubelaker, 1994; Steele, 1976), and posterior circumference (PCF; DiMichele &
Spradley, 2012), as shown in Figure 2.1 and Table 2.3.
Sex Estimation Using the Calcaneus – Wharton 12
Figure 2.1 – Calcaneus measurements
Table 2.3 – Measurement definitions
Measurements
Adapted From
Definition
Maximum Length
Buikstra &
most anterior point on the superior margin of the articular facet for
(MXL)
Load Arm Length
(LAL)
Load Arm Width
(LAW)
Posterior
Circumference
(PCF)
Ubelaker, 1994
Steele, 1976
Buikstra &
Ubelaker, 1994;
Steele, 1976
DiMichele &
Spradley, 2012
Distance between the most projecting point on the tuberosity and the
the cuboid measured in the sagittal plane and projected onto the
underlying surface.
Defined as the projected line from the most posterior point of the
dorsal articular facet, to the most anterior/superior point of the
cuboidal facet
Distance between the most laterally projecting point on the dorsal
articular facet and the most medial point on the sustentaculum tali
Defined as the minimum circumference of the area between the
posterior point of the dorsal articular facet and most posterior point of
the calcaneus.
Posterior circumference (PCF) was the only measurement which had not been used
multiple times in prior studies. Therefore, DiMichele and Spradley (2012) provide
additional explanation for this metric:
Sex Estimation Using the Calcaneus – Wharton 13
“To properly take this measurement, lay measuring tape flat against the surface
of the bone. Pass the measuring tape around anteriorly to the inner and outer
tuberosity of the calcaneus. Avoid projecting heel spurs located on the inferior
surface of the calcaneus by laying the measuring tape beneath them. In certain
cases, calcaneal tuberosities have been seen to be located at a more anterior
position, in which case it may be appropriate to measure on the posterior side
of the tuberosity, avoiding the feature, in order to properly obtain the
minimum circumference.” (DiMichele & Spradley, 2012, p. 2)
Additionally, cadaver stature (CS), defined as the maximum length of a cadaver
from the base of the heel to the top of the head, was used in some analyses.
Statistical Analyses
Inter- and Intraobserver Error
A sample of 23 individuals were measured in two separate trials by the author
several days apart, and once by an experienced observer to test for inter- and
intraobserver error.
Sex Estimation
Data collected from the TXSTDSC were used to estimate sex with the sectioning points
determined by DiMichele and Spradley (2012). Additionally, sectioning points were
derived from this skeletal sample by taking the weighted average between males and
females for each measurement. If a measurement is below the sectioning point, the
individual is estimated to be a female; if the measurement is over the sectioning point,
the individual is estimated to be a male. Pearson’s correlation coefficients were
calculated in Statistical Analysis Software (SAS) v. 9.04 (SAS Institute Inc., Cary, NC,
Sex Estimation Using the Calcaneus – Wharton 14
USA) to observe the correlations between each measurement, sex, and cadaver
stature.
All four measurements were used to create univariate functions for estimating
sex and a multivariate equation was created using all four variables. The sex variable
was coded as males=1; females=0 to calculate the mathematical equations, and 0.5
was used as a sectioning point for estimating sex. If the result of the equations is more
than 0.5, they are estimated to be a male; if the result is less than 0.5, they are
estimated to be a female.
RESULTS
Inter- and Intraobserver Error
Tests for inter- and intraobserver error show that these measurements are
replicable (Table 3.1). Interobserver error (2.11%) was overall higher than
intraobserver error (0.86%) indicating that different individuals may take
measurements slightly differently. However, intraobserver error was very low,
making it very easy to replicate results with the same observer. In both scenarios,
load arm length (LAL) had the highest error rates, indicating that this measurement is
less reliable than the others. In different individuals, the most anterior/superior point
of the cuboidal facet (a surface of the bone which contains a measurement landmark)
can vary in its location, which may have led to the differences in measurements. A
better-defined metric may clarify the issue to provide for lower error rates in the
future.
Sex Estimation Using the Calcaneus – Wharton 15
Intraobserver
Interobserver
MXL
Table 3.1 – Error rates
0.36
2.23
LAL
LAW
3.65
1.32
1.51
1.06
PCF
Average
1.24
2.11
0.52
0.86
Sex Estimation
The means and standard deviation for each measurement for males and
females are shown in Table 3.2.
Var
MXL
LAL
LAW
PCF
N
148
148
146
150
Table 3.2 - Simple statistics
Males
Mean
87.55
54.87
44.85
113.8
SD
4.83
3.17
2.66
7.3
N
109
108
111
112
Females
Mean
80.21
49.68
40.42
102.0
SD
4.31
2.90
2.33
6.3
Sectioning points developed for the current study were found to be close to
those of the study done by DiMichele and Spradley (2012), who also looked at modern
American individuals. Both the sectioning points from this study and those created by
DiMichele and Spradley (2012) provided high accuracy rates for sex estimation (Table
3.3).
Table 3.3 – Sectioning points and accuracy rates
MXL
LAL
LAW
PCF
Pooled accuracy rates
78.2
79.3
80.5
80.2
Pooled accuracy rates
78.2
80.9
79.8
76.7
Sectioning points from present study
Sectioning points from DiMichele & Spradley
(2012)
83.9
83.8
52.3
51.6
42.6
42.0
10.8
11.0
Sex Estimation Using the Calcaneus – Wharton 16
The Pearson correlation coefficients showed statistically significant
correlations between sex and each measurement. Additionally, cadaver stature also
showed high correlations with each of the measurements, indicating that calcaneal
measurements may be more closely associated with stature than with sex.
Table 3.4 – Pearson correlation coefficients
Sex
MXL
Sex
1.00
LAL
LAW
MXL
LAL
1.00
0.84
0.62
LAW
PCF
CS
0.64
0.66
0.65
0.72
1.00
0.74
0.72
0.70
PCF
CS
0.74
1.00
0.68
0.74
1.00
0.77
0.71
0.67
1.00
Each of the univariate equations provided high classifications for sex
estimation, ranging from 77.43% to 80.86%. All four equations classified males
correctly more often than females, meaning females were misclassified more often.
This could be due to the sample size and the ratio of males to females ( approximately
4:3). With sectioning points set at 0.5, it is assumed that there is an equal likelihood
of an individual being a male or a female. However, this is not the case with the
skeletal sample used in this study.
The multivariate equation was also able to classify sex with high accuracy, and
with similar results for males and females.
Sex Estimation Using the Calcaneus – Wharton 17
Table 3.5 – Univariate and multivariate equations and accuracy rates
l
MXL
0.0523
LAL
LAW
PCF
Constant
Sectioning point
Males classified correctly
(%)
Females classified correctly
(%)
Total classified correctly
(%)
2
-0.0798
3
-0.0976
4
-0.0356
5
-0.0067
-0.0263
-0.0356
-0.0144
-3.8405
0.5
4.6278
0.5
4.6230
0.5
4.2980
0.5
5.4653
0.5
71.56
75.00
76.58
72.32
88.68
81.76
77.43
85.14
80.86
83.56
80.54
86.67
80.53
90.28
89.60
DISCUSSION
Despite low error rates, some clarification on the LAW and LAL measurements
may help inexperienced observers. Because it can be difficult to hold a single
calcaneus in exact anatomical position when in isolation, the exact points from which
to measure are not always clear.
The present project had very similar results to the study done by DiMichele
and Spradley (2012), with high accuracy rates and similar sectioning points.
However, as shown in other studies (Lee et. al, 2012; Bethard & Seet, 2013), metric
methods of sex estimation tend to be population specific. While these sectioning
points are accurate for majority-white American skeletal populations, they may not be
accurate for individuals of other ancestral groups or from different geographic
regions. In the contexts of American forensic anthropology, future studies should look
toward developing standards for other demographics commonly found in the United
States.
Sex Estimation Using the Calcaneus – Wharton 18
The univariate equations also provided high overall accuracy rates. Since
those and the sectioning points only require one simple measurement, these are the
preferred methods compared to using a multivariate equation. While the calcaneus
may often be present in archaeological settings (Waldron, 1987), it may also be
damaged or broken, making it more difficult to get all four measurements needed for a
multivariate equation.
CONCLUSION
This study validates the findings of DiMichele and Spradley (2012), and
demonstrates that the calcaneus is sexually dimorphic and produces high
classifications for sex estimation in a modern American skeletal population.
Like many authors have done in the past, future studies could focus on more
measurements of the calcaneus to determine which metric(s) are the most useful in
estimating sex regardless of ancestry or for different ancestral groups/populations.
Additionally, it may be useful to look into what causes the misclassifications of sex.
Differences in calcaneal metrics could be caused by a variety of factors including body
weight, lifestyle, whether an individual overpronates or oversupinates, or has had a
foot injury. Further, more advanced techniques may be necessary to determine what
can affect sex estimation with the calcaneus.
Sex Estimation Using the Calcaneus – Wharton 19
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and ulna. Journal of Forensic Sciences, 58(6), 1413-1419.
Barrett, C., Cavallari, W., & Sciulli, P. (2001). Estimation of sex from the talus in
prehistoric Native Americans. Collegium Antropologicum, 25(1), 13-19.
Bethard, J. D., & Seet, B. L. (2013). Sex determination from the second cervical
vertebra: A test of Wescott’s method on a modern American sample. Journal of
Forensic Sciences, 58(1), 101-103.
Bidmos M. A., & Asala S. A. (2003). Discriminant function sexing of the
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Bidmos M. A., & Asala S. A. (2004). Sexual dimorphism of the calcaneus of South
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Bidmos, M. A., & Dayal, M. R. (2004). Further evidence to show population specificity
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