Transcript Posters_Presentations_files/Zaleski AIA 2011

Age and Sex Estimates of Subadult Ilia from Charnel House A22, Bab-edh-Dhra':
A Model for Determining Subadult Sex
Sarah Zaleski, Dept. of Anthropology and Middle Eastern Cultures, Mississippi State University
Results and Discussion
Introduction
The determination of sex in human skeletal remains is crucial to making conclusions about demography and cultural trends of
past populations. The bony pelvis exhibits sexually distinct features and is the most accurate indicator of sex for adult specimens.
However, tests of subadult (ages 0-15) sex indicators have produced varying, often low accuracy rates. Methods involving the ilium,
the largest bone of the pelvis, have demonstrated the most consistent results across multiple tests (Vlak et al. 2008). Previous
studies show that the accuracy rates of ilial features as subadult sex indicators vary with age. This study therefore recommends an
age-adjusted model for determining subadult sex using four ilial features. The skeletal collection used to develop this model is an
Early Bronze Age (EBII-III; 3000-2300 BCE) commingled collection from Charnel House A22 in Bab edh-Dhra’, Jordan. This
collection is now housed at the University of Notre Dame.
Background
During the period in which A22 was in use, the surrounding region saw developments in
agricultural practices and urbanization. A22 is Bab edh-Dhra’s largest charnel house, and contained
prestige items including gold jewelry. A22 individuals were possibly of a higher social status with
greater access to nonlocal goods (Chesson and Schaub 2007).
The fragmented nature of this study’s sample necessitates the analysis of multiple ilial features
in order to obtain sex estimations for all ilia. This requirement stimulated the development of a
multi-feature model for subadult sex determination. This model differs from previous methods for
subadult sex determination which are based on single feature evaluations. This study’s sample is of
unknown sex; therefore, the accuracy of the proposed model is unknown. Nevertheless, this model
provides a foundation for important potential bioarchaeological and forensic applications.
The model proposed in this study is based on the assessment of four features of 61 right subadult ilia – auricular surface
elevation, greater sciatic notch (GSN) angle, GSN depth, and the arch criterion – according to the methods set forth by Weaver
(1980) and Schutkowski (1993) and expanded upon by Sutter (2003) and Vlak et al. (2008). Each feature was evaluated separately
and independently. The ilia were seriated in order of feature expression for auricular surface elevation and arch criterion to provide
for a more objective analysis. Digital pictures of the pelvic and gluteal aspects of the GSN were taken to objectively measure the GSN
angle and depth with ImageJ computer software. Graphical trends of the GSN angle and depth values established male, female, and
indeterminate ranges. A preliminary study describes these feature assessments and initial sex estimations (Zaleski et al. 2010).
Arch
Criterion
Figure 3.
Standardized landmarks
for GSN Angle (A) and GSN
Depth (notch
breadth/notch depth) (B).
Modified from Vlak et al.
(2008) Fig. 2B and 2C.
A.
B.
GSN
Notch
Depth
Auricular
Surface
To address the fact that the accuracy rates of subadult sex indicators vary
with age, this study integrates age data with the preliminary feature assessments.
Age was estimated using ilium width measurements. Rissech and Malgosa (2005)
found that because ilium width does not present significant differences between
males and females, it has great utility as a subadult age indicator for individuals
ages 0-20 years. Due to the fragmentary nature of the ilia, not all bones have the
osteological points from which to take the measurement. The sample was
therefore seriated from smallest to largest while accounting for the nature of
fragmented bones. Ilium width was measured for the bones that had the
appropriate points. Using the seriation, the ilium widths of the remaining bones
were estimated.
Sample
<2 year-old Subsample
2-5 year-old Subsample
6.5-7.5 > x > 3.5-4.5 Subsample
6-9 year-old Subsample
12.5-13.5 > x > 7.5-8.5 Subsample
Figure 4. BD 1408.30. Ilium width is the
distance between the two blue dots.
Photograph taken by author.
Age was estimated via ilium width with reference to a table of subadult Arikara ilium widths and corresponding age data (Ubelaker
1999). Percent deviations of the A22 sample’s ilium widths from the Arikara means were calculated to ensure that the A22 ilium widths
fell within the range of the Arikara data (Franciscus and Churchill 2002). A modified t-test showed that the A22 ilium widths do not
differ significantly from the Arikara means (Sokal and Rohlf 1981). Six age brackets were created based on the nature of the A22 and
Arikara data.
Feature Evaluations
Figure 2.
Bone ID: BD.1217.84
(pelvic aspect) with
features designated.
Photograph taken by
author.
Table 4. Sex Distribution
Age Estimations
Figure 1. Bab-edh-Dhra’ is highlighted.
Modified from Chesson (1999), Fig. 1.
Methodology
Ilium Width Measurements
Feature Weighting
This model gives weight to features that are the most reliable subadult sex indicators for particular ages. The reliability of the
features is based on a consensus of their accuracy rates reported in previous studies (Weaver 19080, Mittler and Sheridan 1992,
Schutkowski 1993, Sutter 2003, Cardoso and Saunders 2008, and Vlak et al. 2008).
Table1. Feature Weighting for Age Brackets
Ilial Features
Auricular
Surface
GSN Angle
GSN Depth
Arch Criterion
<2
2-5
0%
0%
33.33% 25%
33.33% 50%
33 .33% 25%
Feature Weights by Age Bracket
3.5-4.5<x<6.5-7.5
6-9
7.5-8.5<x<12.5-13.5 11-15
0%
33.33%
33.33%
33.33%
0%
33.33%
33.33%
33.33%
25%
25%
25%
25%
Each bone in the sample was classified as female, likely female, male, likely male, or indeterminate. The ilia’s four pre-existing
feature scores (Zaleski et al. 2010) were transformed. Female scores became scores of 1; male scores became scores of 0; and
indeterminate scores became scores of 0.5. Features that were missing or unscorable due to fragmentation were also given a score of 0.5,
based on the idea that there is a 50-50 chance that that feature would score as male (0) or female (1). For each ilium, each of its feature’s
scores was multiplied by the weight given to that feature for the particular age bracket into which that particular ilium fell. Then, the
products for each bone were summed. The resulting sum or total score became a percent estimation of how likely a bone was to be
female. Score ranges for sex estimations are specified in Table 2.
Notch Breadth
Table 2. Total Scores and
Corresponding Sex Estimations
The preliminary research for this study was supported by the NSF-REU (SES 0649088) Summer Research in Biological Anthropology at the
University of Notre Dame. This study was supported by Dickinson College Academic Affairs and Research and Development Committee.
Thank you to the Mississippi State Graduate School TAGG program and Mississippi State’s Dept. of Anthropology and Middle Eastern
Cultures. Many thanks as well to: Robert J. Smith (Emory University), Susan G. Sheridan, Ph.D. (University of Notre Dame), Jaime Ullinger
(Ohio State University), Brenda Baker, Ph.D. (Arizona State University), Karen J. Weinstein, Ph.D. (Dickinson College), Ann M. Hill, Ph.D.
(Dickinson College), Peter A. Zaleski, Ph.D. (Villanova University), Ryan P. Shears, and Nicholas P. Herrmann (Mississippi State University).
Sex Estimation
.75-1
.5-.75
0.5
.25-.5
0-.25
Female
Likely Female
Indeterminate
Likely Male
Male
Table 3. Example from the 11-15 Age Bracket
Ilium ID
Weight
BD1500.13 Feature Score
Auricular Surface GSN Angle GSN Depth Arch Criterion Total Score
0.2
1
0.4
0.5
0.2
0.5
Likely Males
20.00% (3/15)
Indeterminate
26.67% (4/15)
Likely Females
40.00% (6/15)
0% (0/9)
33.33% (3/9)
22.22% (2/9)
20.00% (2/10)
10.00% (1/10)
40.00% (4/10)
15.38% (2/13)
23.08% (3/13)
46.15% (6/13)
33.33% (3/9)
22.22% (2/9)
11.11% (1/9)
0% (0/4)
50.00% (2/4)
25.00% (1/4)
Females
13.33% (2/15)
72.72% (8/11)
33.33% (3/9)
83.33% (5/6)
10.00% (1/10)
55.56% (5/9)
0% (0/13)
60.00% (6/10)
0% (0/9)
14.29% (1/7)
0% (0/4)
50.00% (1/2)
The model produces a classification of 42.22% males and 57.78% females in the A22 sample. When the study sample is
broken down by age brackets, the youngest two brackets – <2 years and 2-5 years – contain much greater female percentages
(72.72% and 83.33%, respectively) than male percentages. These results suggest preferential treatment of males over females at
weaning age.
Certain aspects of the A22 sample should be considered along with the suggested preferential treatment of males during
weaning. It is important to note that the A22 collection is not representative of the entire Bab edh-Dhra’ population, but rather of
the elite with access to nonlocal goods and prestige items (Chesson and Schaub 2007). Additionally, the sample size of 61 right
ilia was limited to bones that possessed at least one assessable feature of the four ilial features assessed in this study.
One aspect that should be noted when considering the A22 sample at large is that during late EB III (2300 BCE), Charnel
House A22 and the bones inside burned with the destruction of the town (Chesson and Schaub 2007). Burning at high
temperatures or for long durations can result in bone shrinkage, which will affect metric measurements. The temperature and
burning environment specification for a bone based on its color is its burn score (Walker et al. 2008). Despite the burning, the
analysis of sexual characteristics in the A22 sample should not be affected greatly (Schmidt, 2009 personal communication). An
examination of bone color in this study’s sample suggests that about 30% of the sample may have undergone shrinkage;
however, this possibility could not be addressed in age estimations. The ilia that for which ilium width measurements were taken
did not exhibit bone colors that suggested shrinkage.
Conclusions
20%
40%
20%
20%
Sex Estimations
Total Score
11-15 year-old Subsample
Males
0% (0/15)
27.27% (3/11)
11.11% (1/9)
16.67% (1/6)
20.00% (2/10)
44.44% (4/9)
15.38% (2/13)
40.00% (4/10)
33.33% (3/9)
85.71% (6/7)
25.00% (1/4)
50.00% (1/2)
0.2
0.5
1
0.6
Sex Estimation
Likely Female
Because the A22 collection is of unknown sex, future research must begin with the assessment of this study’s model on
subadult skeletal collections of known sex. For collections with more complete individuals, age should be estimated via dental
eruption, and cleaner age brackets should be created when possible. It is likely that the features within each age bracket should
be weighted differently for different population groups. Future research should seek to determine how certain environments and
stressors impact the growth and sexual dimorphism of the ilium and its features. The proposed model’s use of multiple features
and feature weighting based on age provide a foundation for further development.
Ultimately, accurate subadult sex determination contributes to a better understanding of subadults in archaeological
contexts. This clearer picture of subadults is crucial to understanding archaeological population groups at large.
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