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Dental asymmetry as a measure of environmental stress in the Ticuna Indians of Colombia.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 53:133-142 11980)
Dental Asymmetry as a Measure of Environmental Stress in
the Ticuna Indians of Colombia
EDWARD F. HARRIS AND MARTIN T. NWEEIA
Department of Orthodontics (EEH.),and School of Dental Medicine (E.F.H.,
M.T.N.), University of Connectccut,Farmington, Connecticut 06032
KEY WORDS
South America
Odontology, Asymmetry, Amerindians,
ABSTRACT
The magnitude of fluctuating dental asymmetry is reported for a
marginally Westernized, horticultural Indian group, the Ticuna of the Region
Amazonas, Colombia. Asymmetry is lower than in other Amerindian and Eskimo
groups reported to date, which accords with the adequacy and reliability of traditional food sources and complements the claim that protein intake is a t or above
minimum requirements. Partitioning the variation by sex, arcade, dimension, and
tooth discloses several statistically significant effects. Among these: 1)females are
proportionately more asymmetric than males; 2) maxillary teeth are more asymmetric than their mandibular counterparts; 3) the mesiodistal dimension is less
canalized than buccolingual width in the maxilla, but is more asymmetric in the
mandible; and 4) the pattern of asymmetry corresponds closely with the morphogenetic gradients within each tooth field, suggesting that bilaterality provides
an additional measure for disclosing underlying genetic and ontogenetic patterns
in the dentition.
Several approaches are aimed a t assessing
the biological effects of different environments
on individuals and/or populations. Demographic, epidemiologic, and physiologic parameters each measure how a group is coping
with its environment (e.g., Damon, '74). Another approach, and the one used here, is to
quantify the degree of developmental stability
exhibited by a population sample. The issue is
that paired organs (or tissue systems) are seldom exactly symmetric, even though the genetic information is presumed to be (Waddington, '42, '57; Lerner, '54;Rendel, '67). The cause
of asymmetry is generally referred to the idiopathic category "stress," since few studies on
karyotypically normal humans have access to
quantified nutritional or epidemiologic histories, and because the biologic response is
non-specific (Hinkle, '73).Attempts a t relating
the degree of asymmetry to specific agents in
humans have not been successful (e.g., DiBennardo and Bailit, '78).
"Stress" incorporates a multitude of inherent
and extrinsic agents, including genetic and
chromosomal disharmonies, nutritional deprivations, and diseases, infectious and otherwise (Cannon, '32; Selye, '76). Localized side
differences in the degree and duration to which
growth is retarded by physiological entrench-
0092-948318015301-0133$02.00 G 1980 ALAN R. LISS. INC.
ment to the stress condition will result in bilateral asymmetry. Asymmetry is a host response.
Heat, cold, and audiogenic stress, pre- and/or
postnatally, each produce increased levels of
dental asymmetry in rats (e.g., Sciulli et al.,
'79). Wild and random-bred animals are more
symmetric than are inbred strains (e.g., Sumner and Huestis, '21; Bader, '65; Boesiger, '731,
and asymmetry can be increased or decreased
by selection (e.g., Mather, '53; Waddington,
'601, though human studies (Bailit et al., '70;
Potter et al., '76) find no evidence of a genetic
component. Within an individual, some features are inherently more variable than others
(Pearson and Woo, '35; Schultz, '37; Van Valen,
'62; Jolicoeur, '63).
As part of a descriptive odontometric study of
a South American Indian group, the Ticuna of
the Region Amazonas, Colombia (Harris and
Nweeia, '801, we have assessed dental asymmetry, which we report here. In addition to
broadening the narrow baseline of comparative
data on dental asymmetry in preliterate
groups, the Ticuna are of interest as representatives of the technologic and sociocultural
stage encountered throughout the history and
much of the prehistory of northern South
Received July 27, 1979; accepted February 4, 1980.
133
E.F. HARRIS AND M.T. NWEEIA
134
America outside the areas of cultural florescence (e.g.,Steward and Faron, '59; Willey, '71).
In addition, a different statistical approach is
used for the study of asymmetry, one which
circumvents the inefficiency and the multiple
comparisonproblem involved in using variance
ratios (e.g., Garn et al., '66a, b; Doyle and
Johnston, '77) or intraclass correlations (Bailit
et al., '70). The factorial analysis of variance
design described below provides a unified approach for the examination of asymmetries by
tooth, tooth dimension, arcade, and sex.
THE TICUNA
Tribes speaking Ticuna (or Tukuna,
T i k u n a b a n unaffiliated language possibly related to Arawakan (Mason, '50)-are distributed in and around the intersection of Peru,
Brazil, and Colombia. The people live in scattered, semi-permanent, kin-based villages
similar to other tropical-forest farming groups
of north-central South America (e.g.,Steward,
'48; Steward and Faron, '59). The Ticuna are
best known ethnographically from the works of
Nimuendaju ('48, '52) and Medina ('77).
The specific village studied, Arara, about 30
km northwest of Leticia, Region Amazonas,
was formed in 1969 as a result of non-Indian
coercion from a kin-based unit of a village located in northern Amazonas, adjacent to
Brazil. The Ticuna of Arara have access to
Western foodstuffs and medical attention (e.g.,
Medina, '77). Importantly, though, all individuals examined spent their childhood-when
crown tooth formation was completed-in an
isolated, essentially traditional setting.
The Ticuna are horticulturalists, raising
primarily platano, yucca, and maize. Fishing
provides an important dietary component, but
mammals are scarce and hunting contributes
little to protein intake. Collecting wild, seasonally available fruits and roots adds substantially to the diet. Data on similar rain forest
tribes suggest both that calorie intake (Johnston e t al., '71) and protein consumntion
(Chagnon and Hames, '79) are above daily nutritional requirements.
Medical examinations of the Ticuna were not
conducted. Presumably, they are exposed to a
similar spectrum and intensity of problems
found in other lowland forest groups. Examinations of a similar group, the Xavante of Brazil,
reported by Neel et al. ('64, '67) and Weinstein
et al. ('67),emphasize stresses of repeated infections and parasitizations. Neel et al. conclude
that, "Physical examination reveals a group
which is superficially healthy. The greater evidence of disease in women (lower hemoglobin
levels, more common palpable spleens and
livers, and soft systolic murmurs) than in men
may reflect not only the heavier demands on
the women but also the fact that their mode of
life results in greater exposures" (Neel et al.
'64:128).
Insofar as their findings are remesentative of
the physical conditions i f other-Indian groups
inhabiting virtually identical cultural and
natural settings, expectations are that the
magnitude of fluctuating dental asymmetry
ought to be fairly high by contemporary Western standards, and that a sex difference ought
to exist, with females being more asymmetric,
on average, than males.
MATERIALS AND METHODS
Full-mouth dental study models of 57 Ticuna
adults (17 to 30 years of age) were measured for
maximum mesiodistal (MD) and buccolingual
(BL) crown diameters of all permanent teeth
(Moorrees, '57) not noticeably affected by attrition, caries, or the like (Harris and Nweeia,
'80). The sample has no known bias; cooperation was complete, and only essentially edentulous individuals were omitted.
The size difference between left and right
antimeres (d = L - R) is used as the measure
of asymmetry (Parsons, '64; Garn et al., '66b,
'67b). Summary statistics of d provide sample
estimates of mean asymmetry and its variability, as well as a test for directional differences
(e.g., a negative sign denotes right antimere
larger than left).
For the analysis of within-group distributions of asymmetry (but not for the inter-group
comparisons),the data are transformed to eliminate size differences among teeth. The side
difference, d, is divided by the mean size of the
left and right teeth. In addition, the absolute
value is taken to preclude positive and negative
values canceling one another,
d"
=
IL-R(
L+R
2
Our rationale is that a given amount of
asymmetry in a small tooth pair is of more
biologic importance than the same amount in a
large pair, when the asymmetries are compared one to another. Likewise, when using
d = L - R, other sources of variation in crown
size, such as individual, sex, and population
differences, may enter into the variance statistic.
RESULTS AND DISCUSSION
Results are divided into two sections, first a
consideration of the degree of asymmetry presented by the Ticuna relative to published re-
135
TICUNA DENTAL ASYMMETRY
the buccolingual dimension, Ohio Whites <
Eskimo < Ticuna < Puebloans. Consequently,
the Ticuna exhibit less asymmetry, on average,
than either the Puebloan agriculturalists or
the specialized Eskimo hunters. All three of
these groups exhibit degrees of asymmetry far
in excess of contemporary American Whites.
ports of other groups, and second, an analysis of
the within-group distributions of asymmetry.
Table 1presents summary statistics of d for
the Ticuna. In none of the 64 cases is the mean
of d significantly different from zero. Moreover,
nod is even as large as the standard deviation,
so there is no indication of preferential development of any tooth dimension on either side of
the midline (i.e., no directional asymmetry).
Similarly, the overall distribution of positive
(31/64)and negative (33/64)signs of the means
is effectively that expected, assuming equal
odds. The major source of variation is, then,
attributable t o random side differences, i.e.,
fluctuating asymmetry (Van Valen, '62).
2. Differences within the Ticuna
Three considerations have prompted us to
examine the within-sample distribution of
asymmetries in more than usual detail: 1)laboratory experiments by Siege1 and Doyle ('75)
suggest that maxillary and mandibular teeth
may respond differently to a given stress; 2) sex
and population differences in the length-width
ratio of teeth (e.g., Garn et al., '67b; Rosenzweig, '70) and the modest association between
length and width (r' = .26, Garn et al., '68)
suggest that the two dimensions may respond
differently to stress; and 3) cultural and medical features of the Ticuna and similar tropical
forest farming groups (Steward, '48) suggest
that sex differences may be present in the degree of asymmetry.
A three-way analysis of variance (Model I),
with the three factors being sex (male, female),
arcade (maxilla, mandible), and tooth (11,12,C,
P1, P2, M1, M2, M3), is used to circumvent
multiple comparison problems and to increase
the degrees of freedom (Sokal and Rohlf, '69;
Steel and Torrie, '60). Again, data are the
transformed antimeric differences d" defined
above.
1 . Inter-group comparisons
Table 2 presents the statistics for d, with
sexes pooled, for the Ticuna and three comparative groups: contemporary American Whites
(Garn et al.,'66b, '67a), prehistoric Hopi Indians from Arizona, and prehistoric Alaskan Eskimo (Doyle and Johnston, '77). Differences in
the magnitude of fluctuating asymmetry between the Ticuna and each of the other three
groups are determined from the ratio of variances. Most of these F-ratios (43/68, 63%) are
statistically significant.
The four groups can be sequenced relative to
their degree of fluctuating asymmetry by ranking the variances of the groups for each tooth
and then calculating the mean ranks. The sequence for the mesiodistal dimension is: Ohio
Whites < Ticuna < (Puebloans, Eskimo). For
TABLE 1 . Summary statistics of asymmetry, d', of mesiodistal and buccolingual
diameters (mmi of Dermanent teeth o f the Ticuna.
Mesiodistal
Tooth
Buccolingual
MalesZ
Females
Males
n
X
X
n
X
S
n
21
24
26
24
26
24
25
10
,026
-006
-.058
-019
-.092
-.073
-222
-.375
24
24
27
26
24
19
15
-.038
.027
,046
.048
,102
S
n
S
Females
X
S
Maxilla
I1
I2
C
P1
P2
M1
M2
M3
.219
,292
,320
,365
.382
,378
,430
,590
14
15
25
23
26
19
22
4
-.046
.545
,163
,411
-.084
,237
,211 .445
.123
,449
-.132
,440
,018 .823
-.213 1.260
23
23
26
25
26
23
25
15
.026
,013
,033
-.OOO
p.173
-.072
-.064
-.347
,280
,487
,341
,416
,621
.313
,723
.448
16
16
23
24
26
20
22
,215
,191
,221
,299
.332
,066 ,285
-.033
.335
,337 1.115
23
22
25
24
23
11
13
4
,000
.030
,034
,117
,089
-.091
,031
-.050
,174
.207
,207
.316
,374
,418
,467
,385
23
22
26
26
24
19
15
13
-.lo4
-.052
p.056
-.006
-.031
,050
-.133
,788
.298
-226
-386
.383
-202
252
,402
,992
8
-.037
-.230
-.063
-.la1
p.238
-.155
p.294
.376
514
,474
,358
,404
,375
,655
531
21
19
24
25
23
11
13
6
-.052
,013
,044
,138
,180
-.091
,108
.267
,286
209
,237
,300
.375
.297
,396
,529
p.088
Mandible
I1
I2
C
P1
P2
M1
M2
M3
8
'd = left-right. See text for additional explanation.
%nis number of antimeric tooth pairs (= individuals); Z
IS arithmetic
mean of d; s is standard deviation of d
E.F. HARRIS AND M.T. NWEEIA
136
situation: The arcade-dimension effect is statistically significant (Table 3), and, as shown in
Figure 1,there is a difference in the direction of
response. While the magnitude of fluctuating
asymmetry exhibited in tooth width is subequal in the two arcades, tooth length is more
asymmetric than width in the maxilla, and less
in the mandible. This finding, again, represents the combined effects of all tooth types.
The following results are based on the
three-way anovas of asymmetry of the mesiodistal (Table 4) and buccolingual (Table 5 )
crown diameters.
Tooth dimension (MD, BL) was omitted from
the analysis of variance simply to keep the
number of interactions within interpretable
limits. Examined by a one-way analysis of
variance, pooling tooth type, sex, and arcade,
there is no difference in the magnitude of fluctuating asymmetry between mesiodistal and
buccolingual dimensions (F = 0.02, with 1and
1,288 degrees of freedom). One interactive effect is tested: An arch-dependent difference in
the stress-response of crown length and
breadth has been reported in laboratory animals (Siege1 and Doyle, '75). This finding is
that upper molar breadth (BL) and lower molar
length (MD) exhibit more asymmetry than the
alternate dimensions. A test for interaction in
the Ticuna Indian sample discloses a similar
Main effects. All three variables-sex, arcade, and tooth-exhibit statistically significant differences in the MD dimension. Only
TABLE 2. Sample size (n) and variance (sz) ofantimeric differences,d , for the Ticuna
and three comparative groups (sexes pooled) and F-ratio tests for differences in the
magnitudes o f s 2 between the Ticuna and each comparative group.
~
~
Tieuna
Tooth
n
Ohio Whites'
SZ
n
F
S2
Eskimo2
Puebloansi
F
n
52
F
n
52
42
40
48
.33
.14
.33
.55
.20
2.W
1.3
2.W
1.3
3.4*
26
30
42
42
28
.38
.28
.28
.16
.16
2.3*
1.6
1.7
42
40
48
44
28
.29
.10
27
.66
3.1*
1.3
4.F
1.7
1.3
44
42
48
46
30
.08
.49
22
.40
.31
1.2
3.9
1.9
2.5*
2.8*
32
28
40
38
24
.30
.66
.26
22
.34
2.W
2.4*
2.F
2 2
1.5
26
30
40
40
28
.ll
.06
.05
.04
.16
1.4*
4.F
2.5*
12.W
1.4
42
40
48
44
26
.22
.19
.25
.18
.16
1.9
2.1*
3.4*
1.1
4.8*
44
.04
42
48
46
28
.ll
3.01
1.2
1.8
1.2
5.W
Mesiodistal
Maxilla
I1
I2
C
P1
P2
M1
M2
M3
35
39
51
47
52
43
47
14
.15
.18
.08
.17
.17
.17
.42
.70
310
289
278
293
272
309
217
47
46
52
50
47
30
28
12
.04
.04
.05
.09
.13
.12
.16
.88
300
312
304
297
261
300
171
.06
.08
.05
.03
.05
.07
.14
2.4*
l.e
1.7*
5.W
3.8*
2.5*
2.F
44
34
2.e
4.4*
Mandible
I1
I2
C
P1
P2
M1
M2
M3
.03
.02
.04
.03
.04
.08
.ll
1.5
1.2
1.1
3.W
2.5*
1.6
1.4
50
Buccolingual
Maxilla
I1
I2
C
P1
P2
M1
M2
M3
39
39
49
49
52
43
47
23
.10
25
.17
.15
27
.12
.48
23
113
104
70
105
99
114
51
.05
.I1
.06
.04
.07
.04
.16
2.1*
2.3*
44
41
50
51
47
30
28
19
.09
.05
.10
.12
.09
.07
.16
.76
111
112
98
107
92
111
31
.04
.06
.09
.05
.07
.06
.10
2.1*
1.3
1.2
2.9
4.F
4.W
2.7*
3.W
Mandible
I1
I2
C
P1
P2
M1
M2
M3
*P C 0.05, two-tailed test. Values computed before rounding
G a r n et al. '66b, '67a.
ZDoyleand Johnston, 1971.
2.e
1.4
1.2
1.6
.13
.19
.15
137
TICUNA DENTAL ASYMMETRY
tooth type differences are significant for BL
breadths. The analyses of interactive effects
also yield different results for the two dimensions. This disparity is in keeping with the low
correlation between length and breadth and
the lack of correlation between tooth size and
asymmetry (Garnet al., '67a). This presumably
reflects differences in the evolutionary forces
affecting the two crown diameters, though the
mechanism remains unclear (e.g., Sofaer et al.,
'71; LeBlanc and Black, '74; Frayer, '76).
Females are significantly more asymmetric
in the Ticuna, but just for the MD diameters.
Garnet al. ('66b) and Bailit et al. ('70) found no
consistent MD sex differences. In contrast,
Garn et al. ('67a) found males to be more
TABLE 3. Results of the two-way analysis of variance (Model I ) testing for differences in
the magnitude of asymmetry (d*) between dimensions (MD, BL) and arcades (maxilla,
mandible).
Source of
variation
Treatments
Arcade
Dimension
Arcade-dimension
Residual
Total
D.F.
sum of
squares
Mean
square
1
1
1
1,288
1.291
.00832
.00002
,00593
1.67407
1.68835
.00832
.00002
.00593
.00130
F-ratio
6.41*
0.02
4.5e
*P < 0.05
TABLE 4. Results of a three-way factorial analysis of uariance (Model I) for sex, arcade,
and tooth differences in bilateral asymmetry (d*) i n the mesiodistal dimensions.
Source of
variation
Main effects
Sex
Arcade
Tooth
First-order
interactions
Sex-arcade
Sex-tooth
Arcade-tooth
Second-order
interaction
Sex-arcade-tooth
Residual
Total
D.F.
sum of
squares
Mean
square
F-ratio
1
1
15
,00874
,01414
,06910
,00874
.01414
,00461
7.F
12.3*
4.P
1
15
15
.00533
,00891
,00429
.00533
,00059
,00029
4.F
0.5
0.2
15
.01591
,66468
.79109
.00106
.00115
0.9
586
639
*P < 0.05.
TABLE 5. Analysis of variance table for buccolingual dimensions+.
Source of
variation
Main effects
Sex
Arcade
Tooth
First-onler
interactions
Sex-arcade
Sex-tooth
Arcade-tooth
Second-order
interaction
Sex-arcade-tooth
Residual
Total
_ _ _ _ _ _ _ ~
*P < 005
+Particulars are as in Table 4
Sum of
squares
Mean
square
F-ratio
1
1
15
.00016
.00011
,05370
.00016
.00011
,00358
0.1
0.1
2.7*
1
15
15
,00217
.01101
,03648
,00217
,00073
.00243
1.6
0.5
1.8*
15
587
650
,01251
.78108
,89723
,00083
.00133
0.6
D.F
E.F. HARRIS AND M.T. NWEEIA
138
asymmetric for BL diameters. The present findings suggest that the genetic hypothesis that
females are better canalized by virtue of their
double X chromosome complement may not
hold in all populations. Replication of the test
using intraclass correlations yields the same
results of females being significantly more
asymmetric (Nweeia and Harris, '79).
Both dimensions exhibit significant arcade
differences,which confirms earlier studies that
maxillary teeth are more asymmetric (Garn et
al., '66b, '67a; Siege1 and Doyle, '75) and complements findings (e.g., Garn et al., '68; Potter
et al., '76) that tooth dimensions among the two
arcades are largely independent.
Mean differences among the eight tooth
types are significant in both dimensions. Since
Mesiodistal
__---Buccolingual
,051
*
.04-
V
c
0
0)
5
.03-
Q2-
the asymmetry measure d* controls for dimensional differences in overall size, the interpretation is that certain teeth are proportionately more asymmetric. The sex-tooth and
arcade-tooth interactions (below)clearly illustrate morphogenetic field gradients for asymmetry, with the pole tooth being developmentally more stable, less asymmetric, in each
field.
Sex and arcade. These interactions, plotted
in Figure 2, are different for the two dimensions. Mesiodistally, the interaction is a difference in magnitude, so, while females are more
asymmetric in each arcade, they are disproportionately so in the maxilla. Buccolingually,the
interaction constitutes a difference in the direction of response, but is not significant in this
series,
Sex and tooth. Neither the MD nor the BL
interactive effect is significant, as shown by the
approximately parallel responses of the two
sexes across the eight tooth types (Fig. 3). The
association between tooth asymmetry and
morphogenetic patterning is evident, both here
and in the arcade-tooth interactions (below).
The stable tooth in each field is less asymmetric
than the others which are also characterized by
increased metric and morphologic variability
(e.g., Dahlberg, '45;Henderson, '75). The increase in asymmetry from M1 through M3 is
particularly apparent.
There is no good indication in these resulk
(Fig. 4) that later developing teeth are more
sexually dimorphic than those, notably I1 and
MI, whose crowns are completed during infancy. So, while medical data on technologically and environmentally similar groups
Males
----- Females
,051
i
4
.04
.04
U
c
0
0)
5
.03-
A. Mesiodistal
.02J
.02
Maxilla
Mandible
Maxilla
Mandible
Fig. 2. Plots of interactions between dental arcade and sex for the mesiodistal (A) and buccolingual (B)
dimensions. Data (d*) are pooled across tooth types.
139
TICUNA DENTAL ASYMMETRY
.09
0.
Buccalinpud
oe
.02J
.02
11
I2
C
PI
P2
MI
M2
Ii
M3
1'2
c
PI
Pz
MI
~5
Mn
Fig. 3. Plots of interactions between sex and tooth type for the mesiodistal (A)and buccolingual (B)tooth dimensions.Sex
differencesare primarily of magnitude. The one clear-cut difference in direction, from C to P1for tooth width, probably reflects
the lesser variability of lingual cusp size on LP1 among males.
IC
I
,
II
I
I
11
12
C
PI
Pi!
MI
Mh
Mb
Fig. 4. Plots of interactions between arcade and tooth for the mesiodistal (A) and buccolingual (B) tooth dimensions.
(Nee1 et al., '64; Weinstein et al., '67) suggest
that females would be more asymmetric because of a more debilitating milieu, differences
found here are being initiated before sociocultural differences in sex roles are likely to be
manifest.
Arcade and tooth. The MD lengths of maxillary teeth are consistently more asymmetric
than their mandibular counterparts (Fig. 4A),
and the slopes are fairly parallel between arcades. Relationships are more variable for
tooth widths (Fig. 4B).Neither effect is signifi-
140
E.F. HARRIS AND M.T.NWEEIA
cant; the MD difference is, thus, an additive
difference in mean asymmetry, while the BL
dimension is not sexually dimorphic.
Studies that have tested for associations between tooth size and crown morphology (e.g.,
Dahlberg, '61; Garn et al., '66a; Lombardi, '75)
find significant, positive relationships. By extension, the differences among tooth types
ought to be interpretable in terms of crown trait
variability, and, indeed, this seems to be the
case: The BL dimension of upper I2 is particularly asymmetric, and inspection shows.this to
be primarily due to variable development of the
cingulum. The assertion that I2 is the stable
tooth in the mandible (e.g., Dahlberg, '51) is
also borne out in these data, where LI1 is more
asymmetric than L12 in both dimensions. The
canine, well known t o be stable (e.g., Gregory,
'22; Garn et al., '62; Grahnen, '62; Hanihara,
'76), expresses a moderate degree of fluctuating
asymmetry in this sample, with LC being
somewhat less asymmetric than UC, perhaps
because of the variable occurrence of tubercuZum dentale on the maxillary tooth (Scott, '77).
Tooth length exhibits the typical premolar relationship, P1 <P2, in both arcades. Tooth
breadth shows the same P1 <P2 relation in the
maxilla, and it is not clear why mandibular P1
breadth is more asymmetric than lower P2.
Lingual cusp number, for example, is more variable on LP2 (e.g., Kraus and Furr, '53; Turner, '67; Scott, '73). Asymmetry in the molar
field is straightforward: M1 <M2 <M3. The
maxillary molars are more asymmetric, probably because variability of one cusp, the
hypocone, has a major influence on both upper
molar length and breadth (e.g., Biggerstaff, '69;
Corruccini, '75).
It is evident that fluctuating asymmetry is
not expressed uniformly across sexes, arcades,
or teeth. Sex differences conform to expectation: Females are significantly more asymmetric than males when, as here, asymmetry is
expressed as a proportion of tooth size. This
difference holds throughout the dentition, from
teeth such as I1 and M1, which begin mineralization near birth, through those such as the
distal molars, which form in later childhood.
This uniformity argues against a simple cultural-environmental explanation that females
are stressed more by their roles, though behavioral differences favoring greater female stress
are reported for analogous lowland peoples
(e.g., Maybury-Lewis, '65; Chagnon, '68; Murphy and Murphy, '74). For example, as adolescents and adults, males spend less time in the
village; hunting, fishing, and gardening tend to
reduce the risks of parasitism and infectious
disease, which are harbored by the village setting.
CONCLUSIONS
The Ticuna, a lowland forest tribe, exhibit
only moderate dental asymmetry. As with
other preliterate groups studied to date, their
magnitude of fluctuating asymmetry is considerably greater than that of contemporary,
technologically advanced groups, but it is significantly lower than in two prehistoric groups
used for comparison, Puebloans and Eskimo
(Doyle and Johnston, '77).
Analyses of the distributions of asymmetry
between sexes, arcades, and tooth types affirms
the utility of examining the data with a unified
statistical approach such as analysis of variance. Females are more asymmetric than
males. Maxillary teeth, although morphologically somewhat simpler (at least in the premolar, molar fields), are more asymmetric, especially in the MD dimension. Asymmetry also
varies in accordance with tooth position; the
stable or pole tooth in each morphogenetic field
is least asymmetric.
ACKNOWLEDGMENTS
We thank R.J. Smith for his helpful discussions and gratefully acknowledge the Explorers Club of New York City for its support.
Analysis was conducted under Connecticut Research Foundation grant 35-044 and NIDR
grant 1-T32-DE-O70Z7.
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