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Dental fluctuating asymmetry in the Gullah Tests of hypotheses regarding developmental stability in deciduous vs. permanent and male vs. female teeth

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 129:427–434 (2006)
Dental Fluctuating Asymmetry in the Gullah: Tests of
Hypotheses Regarding Developmental Stability in
Deciduous vs. Permanent and Male vs. Female Teeth
Debbie Guatelli-Steinberg,1* Paul W. Sciulli,1 and Heather H.J. Edgar2
1
Department of Anthropology and Department of Evolution, Ecology, and Organismal Biology, Ohio State University,
Columbus, Ohio 43210-1364
2
Maxwell Museum of Anthropology and Department of Anthropology, University of New Mexico, Albuquerque,
New Mexico 87131
KEY WORDS
Gullah; dentition; teeth; asymmetry; developmental stability
ABSTRACT
In this investigation, deciduous teeth
(canines, c; first molars, m1; second molars, m2) and their
permanent successors (canines, C; first premolars, P1;
second premolars, P2) were used to test two related
hypotheses about fluctuating asymmetry (FA). First,
based on the biology of the developing dentition, it was
predicted that deciduous teeth would be more developmentally stable and thus exhibit less dimensional FA than
their permanent successors. Second, based on sex differences in tooth development, it was predicted that female
canines would have greater developmental stability (less
FA) than male canines. Bucco-lingual measurements were
made on dental casts from a single Gullah population.
Using a repeated-measures study design (n ¼ 3 repeated
measures), we tested these hypotheses on sample sizes
ranging from 63–82 antimeric pairs. Neither hypothesis
was supported by our data. In most cases, Gullah decidu-
ous teeth did not exhibit statistically significantly less
FA than their permanent successors; indeed, statistically
significant differences were found for only 3 of 12 deciduous vs. permanent contrasts, and in two cases, the deciduous tooth had greater FA. Female mandibular canines
exhibited statistically significantly greater FA than those
of males, while there was no statistically significant
sex difference in FA for the maxillary canine. FA in these
Gullah samples is high when compared to Archaic and
late prehistoric Ohio Valley Native Americans, consistent
with historical and archaeological evidence that environmental stress was relatively higher in the Gullah population. We suggest that when environmental stress in a
population is high, the impact of differences in tooth formation time spans and developmental buffering upon FA
may be minor relative to the effect of developmental noise.
Am J Phys Anthropol 129:427–434, 2006. V 2005 Wiley-Liss, Inc.
In bilaterally symmetric organisms, fluctuating asymmetry (FA) can be defined as asymmetry ‘‘that results
from the inability of organisms to develop in precisely
determined paths’’ on both sides of the body (Van Valen,
1962). This inability reflects a compromise between two
sets of processes: developmental noise, ‘‘a suite of processes that tend to disrupt precise development’’ (Palmer,
1994, p. 337), and developmental stability, ‘‘a suite of processes that tend to resist or buffer the disruption of precise
development’’ (Palmer, 1994, p. 337). Factors that may
increase developmental noise include, but are not limited
to, extreme temperature, audiogenic stress, malnutrition
and disease, congenital defects, chromosomal defects, maternal alcohol consumption, maternal obesity, maternal
smoking, congenital defects, and possibly also inbreeding
(e.g., Bailit et al., 1970; Kieser, 1992; Kieser et al., 1997;
Siegel and Doyle, 1975a,b; Siegel et al., 1977; Sciulli et al.,
1979; Narayanan et al., 1999; Niswander and Chung,
1965; Townsend, 1983; Townsend and Garcia-Godoy,
1984). In contrast to what is known about causes of developmental noise, current knowledge regarding the physiological processes underlying developmental stability is
much more limited (Lens et al., 2002), although Quetisch
et al. (2002) identified a cellular protein (heat-shock protein 90) that stabilizes other cellular proteins under conditions of external stress.
In a sample of individuals, fluctuating asymmetry is
manifested as a pattern of ‘‘bilateral variation . . . where
the mean of R L [right minus left] is zero and variation
is normally distributed about that mean’’ (Palmer, 1994,
p. 338). It differs from directional asymmetry in which
there is normally ‘‘greater development of a character
on one side of the plane of symmetry . . . than the other’’
(Van Valen, 1962, p. 125). Van Valen (1962) cited the mammalian heart as an example of directional asymmetry.
Fluctuating asymmetry also differs from antisymmetry in
which there is ‘‘a negative correlation, usually implying a
negative interaction, between the development of a character on the two sides’’ (Van Valen, 1962, p. 126). For antisymmetry, there is a consistent mean difference between
sides, but the ‘‘side that is larger varies at random among
individuals’’ (Palmer 1994, p. 337).
Fluctuating asymmetry is of particular interest to biologists and anthropologists as an indicator of environmental
stress in populations (reviewed in Lens et al., 2002;
Kieser, 1990; Fraser, 1994). Lens et al. (2002, p. 27) found
that FA does not ‘‘consistently index stress or fitness’’
in animal populations. Fraser (1994, p. 319) similarly
C 2005
V
WILEY-LISS, INC.
C
*Correspondence to: Dr. Debbie Guatelli-Steinberg, Department of
Anthropology, Ohio State University, 124 West 17th Ave., 244 Lord
Hall, Columbus, OH 43210-1364. E-mail: guatelli-steinbe.1@osu.edu
Received 8 October 2004; accepted 16 November 2005..
DOI 10.1002/ajpa.20237
Published online 1 December 2005 in Wiley InterScience
(www.interscience.wiley.com).
428
D. GUATELLI-STEINBERG ET AL.
reviewed studies of FA in human dental dimensions, stating that these studies provide no ‘‘convincing evidence’’
that stressful environments increase FA. Lens et al.
(2002) found that the lack of consistency across studies is
in part the result of conceptual issues (e.g., our limited
understanding of the mechanisms underlying developmental stability) and in part the result of statistical issues
involved in comparing FA levels between or among
groups. FA differences between samples are assessed by
testing for differences in variances where small differences (e.g., F ¼ 1.25 at ¼ 0.05) are not likely to be
detected unless sample sizes are large. Smith et al. (1982)
found that sample sizes of several hundred are required
for this variance ratio, while Kieser (1990) argued that
sample sizes of 75 or more are adequate. Sample sizes
smaller than this may have led to type II errors in some
studies attempting to test the relationship between FA
and environmental stress.
In this paper, we examine two additional factors complicating the use of dental FA as a stress indicator in
humans: differences in permanent vs. deciduous and male
vs. female tooth formation. Differing conditions specific to
each (deciduous vs. permanent, and male vs. female) may
cause them to differ in the degree to which they are buffered against growth disturbances. First, because deciduous tooth crowns take less time to form than permanent
tooth crowns, both in their soft-tissue and mineralization
stages (e.g., Mizoguchi, 1980, 1998), and may also be buffered against developmental disturbances by the intrauterine environment in which they grow (Cook and Buikstra, 1970; Sciulli, 1978), we predicted that deciduous
teeth would exhibit less FA than their permanent successors. Second, female permanent canine teeth were expected to show less FA than those of males for various reasons: females may be better buffered against developmental disturbances (Stinson, 1985), paired X-chromosomes
may confer on females greater dimensional control during
odontogenesis (Garn et al., 1965, 1966), and the permanent canine teeth of females, because they mineralize in
relatively less time (Moss, 1978; Moss and Moss-Salentijn,
1976), may have a smaller window of opportunity for
developmental disturbances to occur (Townsend and
Farmer, 1998; Townsend, 1981). The period during which
developmental disturbances can have their greatest
impact on tooth size begins with the formation of the
enamel organ and ends during the bell stage, when mineralization begins (Harris, 2002). Thus, the duration of
the soft-tissue stage of development is especially relevant.
Previous research did not find consistent dental FA difference between deciduous and permanent dentitions or
between males and females. Research on FA in deciduous
vs. permanent teeth suggests that the former exhibits less
FA than the latter for dental morphological traits (Saunders and Mayhall, 1982), but Hershkovitz et al. (1993)
found only two cases out of 12 comparisons in which permanent and deciduous teeth differed significantly in
metric FA, and in both cases, the deciduous teeth had the
greater mean rank (Kruskal Wallis one-way ANOVA).
Garn et al. (1966) and Townsend and Brown (1980) found
males to have greater FA in their permanent dentition
than females; Harris and Nweeia (1980) found greater FA
in females; and Kieser et al. (1986) did not find a statistically significant sex difference.
The inconsistent findings of the above studies may represent real population differences but are also likely to
reflect different methods of analysis, and the fact that not
all studies achieved adequate sample sizes, tested for anti-
symmetry, or partitioned out measurement error and
directional asymmetry from the between-sides variance.
Here, we reexamine the question of permanent vs. deciduous and male vs. female differences in dental FA by taking
these complicating factors into account in our statistical
analysis. We thus test for antisymmetry and adjust our
FA estimates for directional asymmetry, size or shape variation, and measurement error.
MATERIALS AND METHODS
Sample
The data derive from dental casts of a single population
of Gullah, African Americans from the Outer Banks of
South Carolina, living on St. James Island during the
1950s (Menegaz-Bock, 1968). The dental casts (N ¼ 469)
were made as part of a larger study of Gullah biology and
ancestry (Menegaz-Bock, 1968), and are now housed in
the Renée M. Menegaz-Bock Dental Anthropology Collection of Ohio State University. Living anthropometrics as
well as genealogies are available for the collection. Individuals included in this study were selected to minimize
their interrelatedness: only one member of each group of
related individuals was included. Antimeric pairs of the
following teeth were measured: maxillary and mandibular
deciduous canines (c), first molars (m1), second molars
(m2), and their permanent successors (canines, C; first
premolars, P1; second premolars, P2). These particular
teeth were chosen because there were few dentitions with
deciduous anterior teeth. Tables 1–3 give sample sizes.
For comparative purposes, we also included data from a
previous study of dental asymmetry in Native American
Late Archaic hunter-gatherers and late prehistoric maize
agriculturalists of the Ohio Valley (Sciulli, 2002). Details
and sample sizes for these groups are mentioned in Sciulli
(2002), so they are not reproduced here. The data from
these groups provide a context for evaluating the magnitude of dental asymmetry in the Gullah.
Measurement
Bucco-lingual (BL) diameters were measured using
Mitutoyo digital calipers (instrument accuracy ¼ 0.01
mm). Buccolingual diameters are the ‘‘distances between
points of maximum curvature of the buccal and lingual
surfaces of a tooth’’ (Sciulli, 2002) and are measured perpendicularly to the mesio-distal axis (Kieser, 1990). Teeth
were excluded from the study if excessive wear or carious
lesions affected the maximum BL diameter. D.G.-S. measured each tooth three times on separate occasions, with
intervals of at least 1 week between them. P.W.S. reviewed
these measurements, noting recording errors and perceived measurement errors. D.G.-S. then remeasured the
teeth in question without knowing the previous measures
or the exact nature of the error. These corrections were
then used in the analysis.
Statistical analyses
Descriptive statistics were generated for each tooth
type: mean size, standard deviation, coefficient of variation,
and measurement error. Measurement error is (MSM)1/2,
where MSM is the error mean square from a sides individual ANOVA (Palmer, 1994).
To analyze dental asymmetry, we used a two-way,
mixed-model analysis of variance (ANOVA) with repeated
measures, where sides are fixed and individuals are ran-
429
GULLAH FLUCTUATING DENTAL ASYMMETRY
TABLE 1. Sample sizes and descriptive statistics: Gullah deciduous teeth
Left
Right
Tooth
N
Mean
S
CV
Mean
S
CV
Measurement
error (mm)1
Max C
Max m1
Max m2
Mand c
Mand m1
Mand m2
68
67
63
67
65
63
5.78
8.21
9.56
5.35
6.72
8.67
0.44
0.44
0.44
0.39
0.50
0.49
7.57
5.33
4.63
7.24
7.43
5.63
5.72
8.29
9.56
5.29
6.65
8.75
0.49
0.47
0.49
0.44
0.52
0.51
8.59
5.66
5.10
8.37
7.85
5.79
0.13
0.16
0.15
0.12
0.19
0.15
1
From side individual ANOVA; (mean square measurement)1/2, where mean square measurement ¼ error mean square.
Max ¼ maxillary, Mand ¼ mandibular.
TABLE 2. Sample sizes and descriptive statistics: Gullah female permanent teeth
Left
Right
Tooth
N
Mean
S
CV
Mean
S
CV
Measurement
error (mm)1
Max C
Max P1
Max P2
Mand C
Mand P1
Mand P2
82
79
72
73
81
73
8.09
9.75
9.75
7.24
8.10
8.63
0.57
0.57
0.59
0.59
0.56
0.61
7.09
5.81
6.01
8.09
6.97
7.02
8.12
9.74
9.74
7.23
7.98
8.53
0.55
0.58
0.60
0.57
0.55
0.64
6.83
5.97
6.17
7.84
6.83
7.54
0.10
0.09
0.12
0.12
0.13
0.13
1
From side individual ANOVA; (mean square measurement)1/2, where mean square measurement ¼ error mean square.
Max ¼ maxillary, Mand ¼ mandibular.
TABLE 3. Sample sizes and descriptive statistics: Gullah male permanent teeth
Left
Right
Tooth
N
Mean
S
CV
Mean
S
CV
Measurement
error (mm)1
Max C
Max P1
Max P2
Mand C
Mand P1
Mand P2
74
82
71
70
81
69
8.60
9.93
9.90
7.75
8.42
8.82
0.61
0.65
0.60
0.64
0.63
0.60
7.14
6.50
6.03
8.21
7.53
6.82
8.54
9.96
9.93
7.71
8.29
8.71
0.63
0.62
0.58
0.60
0.62
0.64
7.42
6.17
5.85
7.80
7.51
7.31
0.10
0.14
0.13
0.13
0.13
0.17
1
From side individual ANOVA; (mean square measurement)1/2, where mean square measurement ¼ error mean square.
Max ¼ maxillary, Mand ¼ mandibular.
dom (Palmer and Strobeck, 1986). Table 4 contains the
ANOVA model with the expected mean squares, in which
S is the number of sides, J is the number of individuals,
and M is the number of measures per side.
Three kinds of asymmetry were considered: antisymmetry, directional asymmetry, and fluctuating asymmetry
(Van Valen, 1962). In the ANOVA with one individual per
genotype, the contribution of antisymmetry to nondirectional asymmetry cannot be estimated. In extreme cases,
antisymmetry results in a bimodal distribution of the
signed difference between sides, while in more subtle
cases, the distribution will tend toward platykurtosis. We
assessed antisymmetry by testing for the normality of the
(R L) distributions, including tests for kurtosis and
skewness (D’Agostino, 1986; Zar, 1999). Fluctuating
asymmetry was evaluated by the F statistic, using the
remainder mean square (MSSJ) over the mean square
due to measurement error (MSM). If significant, the variance component due to nondirectional asymmetry is i2
¼ (MSSJ MSM)/3, where 3 is the number of repeated
measures per tooth. Nondirectional asymmetry contains
contributions from antisymmetry and fluctuating asymmetry. The presence of antisymmetry is evaluated by the
normality test, and if absent, leaves only fluctuating
asymmetry in the nondirectional component. Size and
shape variation were eliminated as sources of variation by
dividing each side by (Ri þ Li)/2. Thus the sum of squares
and mean squares for the individual source of variation is
in all cases zero. Directional asymmetry was evaluated
using the mean square for sides (MSS) over the remainder
mean square (MSSJ).
We also calculated the repeatability for each tooth in
order to investigate how much accuracy would be gained
by repeated measurements (Falconer and MacKay, 1997).
The ANOVA model for repeatability is given in Table 5.
RESULTS
Descriptive statistics
Descriptive statistics (mean, standard deviation (SD),
coefficient of variation (CV), and measurement error) are
given in Tables 1–3. Because there were too few deciduous
teeth of each sex to conduct and analysis by sex and
because deciduous teeth are compared to permanent teeth
in the asymmetry analysis, the descriptive data for deciduous teeth are presented for both sexes combined. There
were approximately equal numbers of each sex in the sam-
430
D. GUATELLI-STEINBERG ET AL.
TABLE 4. ANOVA model for evaluating asymmetry
Source of variation
Sides
Individuals
Remainder
Measures
1
Degrees of freedom1
Mean square
Expected mean square
Interpretation
(S 1)
(J 1)
(S 1)(J 1)
SJ(M 1)
MSS
MSJ
MSSJ
MSM
2
m
þ M(i2 þ (J/S 1)S2)
2
m
þ M(i2 þ Sj2)
2
m þ Mj2)
2
m
Directional asymmetry
Size/shape variation
Nondirectional asymmetry
Measurement error
S ¼ 2, J ¼ number of individuals, M ¼ 3 (measures per side).
TABLE 5. ANOVA model for evaluating repeatability
Source of variation
Degrees of freedom1
Among individuals
Within individuals
J1
J (M 1)
1
Mean square
MSB
MSW
W2
W2
þ
MB2
Repeatability
B2/(B2 þ W2)
B2 ¼ (MSB MSW)/6.
TABLE 6. Repeatability values for deciduous and permanent
(females, males) teeth1
Tooth
Deciduous
(SE)
Maxillary canine
Maxillary P1 or m1
Maxillary P2 or m2
Mandibular canine
Mandibular P1 or m1
Mandibular P1 or m2
0.87
0.84
0.85
0.86
0.83
0.86
1
Expected mean square
(0.03)
(0.03)
(0.03)
(0.03)
(0.03)
(0.03)
Females
(SE)
0.92
0.96
0.92
0.88
0.87
0.89
(0.02)
(0.01)
(0.02)
(0.02)
(0.02)
(0.02)
Males
(SE)
0.92
0.92
0.91
0.88
0.89
0.88
(0.02)
(0.02)
(0.02)
(0.02)
(0.02)
(0.02)
SE, standard error.
ple of deciduous teeth. In general, mandibular teeth, both
permanent and deciduous, show greater variation (CVs)
than their maxillary counterparts. Except for the mandibular m2, the relative variation of deciduous teeth is as
great as their permanent successors.
Measurement error ranges between 0.09–0.19 mm. Measurement error as a percentage of mean BL diameter
ranges between 0.9% (female maxillary right P1) and
2.9% (mandibular left m1). Our tests for repeatability
(Table 6) revealed that with our degree of measurement
error, we could not have increased our accuracy by taking
more than three measurements (Falconer and Mackay,
1997).
Normality
We performed 18 tests of normality for the (R L) distributions of the deciduous teeth (three for each tooth).
Thirty-six such tests were performed for the permanent
teeth (three for each tooth for each sex). For the deciduous
teeth, there was skewness in measure 2 of the mandibular
m1 (P ¼ 0.022) and measure 1 of the mandibular m2
(P ¼ 0.010). Skewness in these measures results from
some individuals exhibiting extreme values. These outliers were checked and kept in the data set. None of the
teeth showed consistent deviation for all three measures.
Importantly, none of the deciduous distributions were platykurtic, indicating that antisymmetry was not present.
For the permanent teeth, there was no skewness,
although five measures were slightly leptokurtic: female
maxillary P2, measure 1 (P ¼ 0.030) and measure 2
(P ¼ 0.0220); female mandibular P1, measure 1 (P ¼ 0.033);
male maxillary P1, measure 3 (P ¼ 0.010); and male maxillary P2, measure 1 (P ¼ 0.040). Graphical inspection of
these data revealed that outliers caused the leptokurtosis.
These outliers were checked and retained in the data set.
Again, in the permanent dentition, there was no platykurtosis in (R L) distributions, indicating that there was no
antisymmetry.
Directional and fluctuating asymmetry
T-tests of the means of (R L) distributions indicate
that the means are significantly different from zero for
five deciduous teeth and four permanent teeth. Thus these
teeth showed directional asymmetry. In the deciduous
dentition, right was greater than left in the maxillary
canine, mandibular canine, and mandibular m1, while left
was greater than right in the maxillary m1 and mandibular m2. In the permanent dentition, right was greater
than left for both male and female mandibular P1 and P2.
However, in all cases, the mean (RL) was 2–6 times
smaller than 0.798 (variance (RL))1/2, indicating that the
predisposition toward one side is less than the average
deviation about the mean (RL). (If developmental instability varies among individuals, the distribution of RL
is no longer normal, and the expected value (mean) of
the distribution differs from the expected standard deviation by (2/)1/2 ¼ 0.798.) Directional asymmetry is thus
unlikely to confound interpretations about FA variation,
as deviations about the mean directional asymmetry
are largely due to developmental instability (Palmer and
Strobeck, 2003).
Results of ANOVA tests are given in Tables 7–9. The
size-correction transformation eliminates the size-shape
(individual) source of variation (MSJ ¼ 0). Thus, only
three sources of variation remain: sides, fluctuating asymmetry (this is equal to nondirectional asymmetry because
antisymmetry is not present), and measurement error.
As seen in Tables 7–9, there is significant nondirectional as well as directional asymmetry (but see above) in
both dentitions and in both males and females. Here, we
list the teeth from those with most to least FA and group
within parentheses those that do not differ significantly
from each other, as revealed by F-tests. For the deciduous
dentition, the order from greatest to least FA (i2) is: (maxillary canine, mandibular canine, and mandibular m1)
and (maxillary m1, maxillary m2, and mandibular m2).
For the female permanent dentition, the order from greatest to last FA is: (mandibular C (mandibular P2, mandibular P1) (maxillary canine) maxillary P2) (maxillary P1).
For the male permanent dentition, FA from greatest to
least is: (mandibular P1, mandibular P2, maxillary canine
(mandibular canine) maxillary P2, and maxillary P1).
Figure 1 plots the variance component due to FA (i2 104)
for each tooth type. Enclosed in each rectangular box
431
GULLAH FLUCTUATING DENTAL ASYMMETRY
TABLE 7. Two-way mixed-model ANOVA: Gullah deciduous
teeth with 3 repeated measures per side1
Tooth
N
xc
68
xml
xm2
dc
dml
dm2
1
2
3
4
5
6
7
67
63
67
65
63
Source of
variation2
DF
MS3
F4
i2
DF5
Sides
Remainder
Measurement
Sides
Remainder
Measurement
Sides
Remainder
Measurement
Sides
Remainder
Measurement
Sides
Remainder
Measurement
Sides
Remainder
Measurement
1
67
272
1
66
268
1
62
252
1
66
268
1
64
260
1
62
260
127.25
23.30
2.30
113.20
10.78
1.53
0.44
0.50
9.37
166.91
23.10
2.25
107.37
21.22
4.50
76.50
10.77
1.04
5.466
10.11
7.00
54
10.50
7.03
3.08
51
0.05
9.60
2.80
61
7.226
10.25
6.95
53
5.066
4.71
5.57
39
7
7.10
10.30
TABLE 9. Two-way mixed-model ANOVA: Gullah female
permanent teeth with three repeated measures per side1
Tooth
N
XC
82
XP1
XP2
DC
DP1
7
DP2
3.24
48
1
x, maxillary; d, mandibular.
Individual source of variation ¼ 0 with size correction.
104.
Boldface indicates P < 0.05.
Sheffé (1959).
R > L.
L > R.
2
3
4
5
6
79
72
73
81
73
Source of
variation2
DF
MS3
F4
i2
DF5
Sides
Remainder
Measurement
Sides
Remainder
Measurement
Sides
Remainder
Measurement
Sides
Remainder
Measurement
Sides
Remainder
Measurement
Sides
Remainder
Measurement
1
81
328
1
78
316
1
71
288
1
72
292
1
80
324
1
72
292
12.47
14.06
0.56
0.57
4.85
0.36
1.97
10.21
0.47
0.84
27.69
0.97
290.95
17.80
0.80
132.90
20.16
0.99
0.89
25.14
4.50
74
0.11
13.47
1.50
66
0.19
21.62
3.24
61
0.03
28.60
8.91
67
16.356
22.13
5.67
73
6.59
20.41
6.39
65
6
X, maxillary; D, to mandibular.
Individual source of variation ¼ 0 with size correction.
104.
Boldface indicates P < 0.05.
Sheffé (1959).
R > L.
TABLE 8. Two-way mixed-model ANOVA: Gullah male permanent teeth with three repeated measures per side1
Tooth
N
XC
74
XP1
XP2
DC
DP1
DP2
1
2
3
4
5
6
82
71
70
81
69
Source of
variation2
DF
MS3
F4
i2
DF5
Sides
Remainder
Measurement
Sides
Remainder
Measurement
Sides
Remainder
Measurement
Sides
Remainder
Measurement
Sides
Remainder
Measurement
Sides
Remainder
Measurement
1
73
296
1
81
328
1
70
284
1
69
280
1
80
324
1
68
276
63.17
15.24
0.61
16.65
9.27
0.99
6.42
9.11
0.62
29.95
15.04
1.06
305.27
19.06
0.73
153.85
16.38
1.50
4.15
25.01
4.88
66
1.80
9.33
2.76
66
0.70
14.61
2.83
59
1.99
14.12
4.66
44
16.016
26.14
6.11
75
9.396
10.96
4.96
56
X, maxillary; D, mandibular.
Individual source of variation ¼ 0 with size correction.
104.
Boldface indicates P < 0.05.
Sheffé (1959).
R > L.
are values of i2 that are not statistically significantly different from each other as assessed by F-tests, given in
Table 10. There are 12 comparisons of deciduous teeth
and permanent teeth: each of the six deciduous teeth is
compared to the six male and six female permanent counterparts.
Of the 12 deciduous vs. permanent comparisons (Table 10),
the permanent successor has a greater FA component
than the deciduous tooth in seven cases, with one compar-
Fig. 1. Bilateral variance component due to FA (i2 104)
for each tooth type. Values enclosed in rectangles do not differ
significantly from each other, as assessed by F-tests shown in
Table 10. GullahD, Gullah deciduous; GullahPF, Gullah permanent female; GullahPM, Gullah permanent male; LP, late prehistoric Ohio Native Americans; LA, Late Archaic Native Americans; X, permanent maxillary; D, permanent mandibular; x,
deciduous maxillary; d, deciduous mandibular.
ison reaching statistical significance (mandibular m2female P2). Of the five cases in which the deciduous tooth
has the greater FA component, two reach statistical significance (deciduous maxillary canine female maxillary
canine and maxillary m1-female P1). Three comparisons
in Table 10 resulted in F-values near 1.5 which, given
our sample sizes, indicate that the power of the test at
¼ 0.05 is approximately 50% (Smith et al., 1982). In two
of these cases, the deciduous tooth has a greater FA component than the permanent replacement. Likewise, four
comparisons yielded F-values between 1.10–1.28 which,
given the sample sizes, should have a power of 25% or
less. In these comparisons, three permanent teeth have
432
D. GUATELLI-STEINBERG ET AL.
TABLE 10. F tests of i2 between groups1
Deciduous vs. male perm.
Deciduous vs. female perm.
Male vs. female perm.
Tooth
F value
i2 greater in
F value
i2 greater in
F value
XC or xc
XP1 or xm1
XP2 or xm2
DC or dc
DP1 or dm1
DP2 or dm2
1.43
1.12
1.01
1.49
1.10
1.53
Deciduous
Deciduous
Male perm.
Deciduous
Male perm.
Male perm.
1.56
2.05
1.16
1.28
1.02
1.97
Deciduous
Deciduous
Female perm.
Female perm.
Female perm.
Female perm.
1.08
1.84
1.14
1.91
1.08
1.28
i2 greater in
Male
Male
Female
Female
Male
Female
1
X, permanent maxillary; D, permanent mandibular; x, deciduous maxillary; d, deciduous mandibular; perm. ¼ permanent. F values
in boldface are statistically significant.
greater FA components than their deciduous predecessors.
If the power of the tests was increased so that F-values as
low as 1.10 could be confidently accepted as showing statistically significant differences, then in this sample there
would be five deciduous teeth and five permanent teeth
with greater FA components. The results of the F-tests are
not consistent with our first hypothesis, that deciduous
teeth should exhibit less FA than their permanent successors, and we reject this hypothesis for this sample.
The second hypothesis, that females would have permanent canines with significantly less FA than those of
males, is also rejected. For the mandibular canine, females
show statistically significantly greater FA than males. For
the maxillary canine, males and females do not differ significantly in FA, though FA in the male maxillary canine
is higher. The F-value for this comparison is only 1.08
(Table 10).
For comparative purposes, data from late prehistoric
(LP) and Late Archaic (LA) Ohio Valley Native Americans
are included in Figure 1. Note the generally higher i2 values of the Gullah.
DISCUSSION
This investigation was undertaken to determine if
deciduous teeth would exhibit less fluctuating asymmetry
than permanent teeth, and if female canines would
exhibit less fluctuating asymmetry than those of males.
As explained in the introduction, these hypotheses were
based on the biology of the developing dentition. Our data,
derived from a single Gullah population, reject both
hypotheses. We note, however, that the Gullah population
we sampled has much higher FA than our samples of prehistoric Ohio Native Americans. Thus, we suggest that
the impact on FA of permanent vs. deciduous and male
vs. female differences in tooth formation time spans and
developmental buffering (intrauterine environment, female development) may be minor when compared to the
effect of developmental noise when stress is high.
The fact that the Gullah dentition shows generally
greater FA than Archaic and late prehistoric Ohio Valley
Native Americans is consistent with what is known about
population stress from historical descriptions (Gullah)
and archaeological evidence (Ohio Valley Indians). There
is strong evidence indicating that the Gullah, despite
being a 20th century American population, were subjected to high levels of stress throughout their lifetimes.
Through at least the early 1960s, economic and social conditions kept the Gullah in poverty, landless, and in jobs
requiring hard manual labor (Newby, 1973a). The medical
community of South Carolina at the time was aware of the
health problems that resulted from these conditions, and
several reports are available documenting the Gullah’s
(and other African American South Carolinians’) plight.
Aycock (1964) found a high correlation between infant
deaths and poverty, illiteracy, illegitimacy, and race. In
1951–1952, four times as many blacks as whites used public child health facilities, while the black population of
South Carolina was dropping from two thirds to one third
of the total population (Aycock, 1964). More than 4% of
black children seen in these clinics were malnourished;
0.7% had rickets (Newby, 1973b). In 1960, the overall life
expectancy for South Carolina was 66.41 years, the lowest
for the country. However, life expectancy in the state was
approximately 10 years shorter for black South Carolinians than for white (US Department of Health, Education,
and Welfare, 1961). By the 1950s, diseases that had been
problematic, such as malaria, hookworm, pellagra,
typhoid, smallpox, and diphtheria, could only be found
among the poorest South Carolina blacks, including the
Gullah. However, venereal disease and tuberculosis were
still problematic. During the 1950s and 1960s, approximately 85% of all cases of venereal disease treated in public health clinics were in African Americans (Newby,
1973a). Congenital syphilis and other venereal diseases
would likely be a source of developmental stress and possibly dental asymmetry.
Skeletal and dental stress indicators in the Ohio Valley
Native American populations suggest a ‘‘low prevalence of
pathological conditions’’ (Sciulli, 2002, p. 42). Steckel et al.
(2002) compared Ohio Valley Native American Archaic
and Pearson archaeological samples with other Western
Hemisphere archaeological samples in terms of a health
index based on stature, enamel hypoplasias, skeletal evidence of anemia, dental pathology, periostitis, trauma,
and degenerative joint disease. The health indices in these
archaeological populations range from a low of 53.5 (indicating poor health) to 91.8 (indicating good health). The
Pearson population health index is 73.7, and that of the
Archaic population is 77.9, both above the median score
of 72.8. African American slave populations had lower
health indices than these, with an average health index of
67.1 (Steckel et al., 2002).
The presence of some directional asymmetry in our sample may also reflect population-level stress. Harris (1992)
suggested that directional asymmetry may be related to
developmental timing differences between antimeres, which
may in turn result from developmental stress in a population. Sharma et al. (1986) proposed that directional asymmetry might result from environmental or genetic stressors
that cause unilateral acceleration of mitotic rates in enamel
organs. However, as Townsend and Farmer (1998, p. 253)
noted, while directional asymmetry is not a statistical artifact, ‘‘its exact nature and causes remain to be solved.’’
While our data rejected the hypotheses (and we suggest
that the high level of population stress in the Gullah is
one possible reason for this), there are three other points
to consider in interpreting our results. First, although we
GULLAH FLUCTUATING DENTAL ASYMMETRY
compared deciduous and permanent teeth from a single
population, a more refined test of our hypothesis would be
to test FA in the deciduous and permanent teeth of the
same individuals. The dental casts of the Burlington
Growth Study, for example, would provide such an opportunity. Indeed, in the Burlington sample, Saunders and
Mayhall (1982) found that deciduous second molars were
less asymmetric than permanent molars for the Carabelli
trait and protostylid. Interestingly, our data show that
maxillary and mandibular m2s have less FA than the
other deciduous teeth we measured (maxillary and mandibular canines, and m1s). Liversidge and Molleson (1999)
found m2 to be the least variable and least asymmetric
tooth of the deciduous dentition, arguing that it may be
the polar tooth of the deciduous molar field.
A second point to consider in interpreting our results
surrounds assumptions embedded within our hypothesis
that male canines should show more FA than female canines. Although male permanent canines take longer to
mineralize than those of females (Moss, 1976; Moss and
Moss-Salentijn, 1974), it is not known if the former actually spend more time in the soft-tissue stage of development than the latter. In addition, some cultures may favor
children of one sex over another, leading to sex-biased
child-care practices that can impact tooth development
(e.g., Goodman et al., 1987). While sex certainly played a
role in educational and economic opportunities among 20th
century South Carolina African Americans, it is not clear
from ethnographic records of the Gullah if there were sex
biases in child care that may have had health effects
(Weiner, 1998).
Thirdly, Palmer and Strobeck (2003) noted that FA
might be affected by random phenotypic variation that
does not result from developmental instability. It is important to note, however, that this factor most affects tissues,
such as bone, that have the ability to remodel and are thus
phenotypically plastic. Once crown formation is completed,
teeth are minimally phenotypically plastic (secondary dentin is deposited, but this does not affect tooth size). In addition, any random phenotypic variation unrelated to developmental instability accruing during the period of crown
formation would be expected to involve both sides.
The fact that our data reject the hypothesis that male
permanent canines are less developmentally stable than
those of females is interesting in a comparative nonhuman primate context. Manning and Chamberlain (1993)
demonstrated an association across 21 species of Old World
primates between male, but not female, FA and canine
sexual dimorphism. They interpreted this result to mean
that there is a causal link between sexual selection on the
canines of nonhuman primate males and fluctuating
asymmetry in these teeth. It is thought that sexual selection results in diminished developmental canalization,
such that structures are more affected by external conditions affecting growth (Manning and Chamberlain, 1993).
The fact that male canines do not exhibit more FA than
female canines in our study is consistent with the small
degree of sexual dimorphism that human canines exhibit
(Kieser, 1990), indicating that human male canines are
not undergoing sexual selection for large canine size.
In terms of avenues for further investigation, we suggest that future studies aim to 1) compare deciduous teeth
and permanent teeth from the same individuals, 2) test
these hypotheses in populations which are minimally
stressed, and 3) test these hypotheses in populations with
no evidence of sex-bias in child care. Basic information
regarding potential differences in male-female soft-tissue
formation stages is also needed. Finally, we point out that
433
more studies focusing on the issue of developmental stability in the human dentition will ultimately help improve
our understanding of fluctuating dental asymmetry as an
environmental stress indicator.
CONCLUSIONS
This study tested two related hypotheses about fluctuating asymmetry (FA): 1) deciduous teeth were predicted to
be more developmentally stable than their permanent
successors; and 2) female canines were predicted to be
more developmentally stable than male canines. To test
these hypotheses, bucco-lingual measurements were
made on dental casts from a single Gullah population on
sample sizes ranging from 63–82 antimeric pairs, using a
repeated-measures study design to partition out measurement error from between-sides variance. Neither hypothesis was supported by our data. In most cases, Gullah
deciduous teeth did not exhibit statistically significantly
less FA than their permanent successors; indeed, statistically significant differences were found for only 3 of 12
deciduous vs. permanent contrasts, and in two cases, the
deciduous tooth had greater FA. Female mandibular
canines exhibited statistically significantly greater FA than
those of males, while there was no statistically significant
sex difference in FA for the maxillary canine. Compared to
Archaic and late prehistoric Ohio Valley Native Americans,
FA in these Gullah samples is high. This FA difference is
consistent with historical and archaeological evidence that
environmental stress was higher in the Gullah population
relative to these Native American groups.
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