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Distribution of enamel on the incisors of old world monkeys.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 71:103-113 (1986)
Distribution of Enamel on the Incisors of Old World Monkeys
R.P. SHELLIS AND KAREN M. HIIEMAE
MRC Dental Unit, Dental School, Bristol BSl2LI: England (R.P.S.);
Department of Oral Anatomy, University of Illinois, Chicago, Illinois 60680
(K.M. H.)
KEY WORDS Allometry, Dental histology, Feeding, Primates,
Cercopithecidae
ABSTRACT
Longitudinal ground sections of 29 Old World monkey central
lower incisors were studied histologically and metrically. Labiolingual incisor
width tended to scale isometrically with body weight but with important
deviations in relative incisor size, which appeared to be correlated with diet in
accord with work by Hylander. Lower incisors of the predominantly folivorous
colobine monkeys had a substantial layer of enamel on both lingual and labial
aspects and consequently had blunt incisal edges. These teeth in both cercopithecins and papionins, which are omnivorous or frugivorous, had little or no
enamel on the lingual aspect, resulting in sharp incisal edges. It is suggested
that colobine incisors are used mainly in gripping and tearing leaves, whereas
cercopithecine incisors are better adapted to cutting and scraping. Crown
height showed a positive allometric relationship with overall incisor height, so
that the tall incisors of papionins, especially Papia and Mandrillus, were more
hypsodont than the shorter incisors of colobines and cercopithecins. This appears to be related to differences in the rates of incisor wear between the
groups.
The two subfamilies (Colobinae, Cercopithecinae) of Old World monkeys have different diets (Napier and Napier, 1966). The Colobinae feed predominantly on leaves,
whereas the diet of the Cercopithecinae includes more fruit and other food items. Hylander (1975) found that cercopithecines,
particularly papionins, had relatively larger
incisors than colobines. He concluded that
this was correlated with increased use of the
incisors in preparation of foods, probably attributable to the inclusion in the diet of such
items as tough-skinned fruits.
Noble (1969) observed that the lingual aspects of the lower incisors of Papio were incompletely covered by enamel and that the
enamel. where mesent. was verv thin. He
established from' histological examination of
developing teeth that this was not due to
attrition and concluded that the reduction of
the lingual enamel was a n adaptation leading to production of a sharp chisel edge, a n
arrangement analogous to that seen in rodent incisors. From microscopic examination
of the crown surfaces of intact teeth, Delson
(1975)suggested that strong reduction or lack
of enamel on the lingual surface of the lower
01986 ALAN R. LISS, INC.
incisors and consequently sharp incisal edges
were characteristic of Papionini among the
cercopithecines. In contrast, these teeth in
Cercopithecini and Colobinae were said to
have a well developed enamel layer over the
whole crown. Gantt (19771, from measurements on sectioned teeth, concluded that
among Old World monkeys there were no
differences in incisor enamel distribution
that could be correlated with dietary variations. The purpose of the present study was
to investigate in more detail the distribution
of enamel in a range of Old World monkeys
and to relate that distribution to tooth morphology
-- and pattern of wear.
MATERIALS AND METHODS
The study was based on 29 lower central
incisors from six cercopithecine and two colobine genera (Table 1).For comparison, upper central incisors from Cercopithecus
pygerythrus, Macaca fascicularis, M. mulatta
(three), and Papio cynocephalus (two) were
examined.
Recewed July 30, 1985, accepted Aprll7, 1986
F
6,250
12,500
6,500
?
7.
72
.-
118
120
8,000
M
115
-
19,000
19,000
5,900
5,000
7,500
7,850
8,200
12,000
-
6,750
?
?
?
M
?
?
F
?
?
M
M
7,670
5,600
9,740
-
68
74
121
110
71
108
112
18
23
26
111
?
?
106
73
104
113
F
M
?
Sex
102
No.
Body
weight
3.9
3.5
3.8
3.3
4.2
7.25
7.0
6.4
5.1
4.6
4.4
4.25
5.3
5.0
7.8
4.0
3.8
3.9
5.1
3.7
Incisor
width
(mm)
11.5
13.15
11.65
15.6
26.8
27.1
17.8
26.0
22.85
17.3
18.35
15.5
18.7
15.7
16.2
19.0
13.2
Incisor
height
(mm)
IR
IR
IR
5.5
5.25
5.7
7.1
17.3
17.8
9.0
16.75
12.75
9.5
9.45
10.7
9.8
7.3
7.55
10.0
5.15
Crown
height
(mm)
Unworn or slightly worn’ incisors
‘ND, not detectable; IR,incomplete roots.
‘In “sliahtlv worn” incisors, onlv the extreme tip of the dentine was exposed.
‘Identifiled i s “Kasi“ in records bf British Museum; assigned a body wefight value for P. senex.
Colobinae
Colobus sp.
C. badius
C. guereza occidentalis
C. uerus
Presbytis aygula
P. entellus
P. senex or P. johnii3
Papio cynocephalus
Mandrillus leucophaeus
M. mulatta
Cercopithecinae
(Papionini)
Cercocebus torquatus
Macaca sp.
M. fascicularis
Cercopithecinae
(Cercopithecini)
Cercopithecus diana
C. lhoesti preussi
C. mitis
C. mona
C. sabeus
Erythrocebus patas
Taxon
-
TABLE 1. Composition of the sample and summary
of
390
480
330
400
505
630
630
530
540
580
500
530
510
480
560
480
395
410
530
410
75
125
150
65
105
50
ND
ND
40
25
ND
50
ND
ND
ND
ND
20
ND
100
ND
Enamel thickness
(pm)
Labial
Lingual
odontometric data’
117
114
122
124
125
F
?
?
?
?
M
M
M
F
-
109
107
103
101
No.
3,600
8,380
19,000
19,000
19.000
8,000
8,500
7,400
3,700
Worn incisors
Body
weight
Sex
(gm)
3.1
4.3
7.2
7.2
7.0
6.8
2.9
3.6
3.7
Incisor
width
(mm)
105
CERCOPITHECID INCISOR ENAMEL
All incisors were sectioned longitudinally
in the labiolingual plane using a n annular,
diamond-edged blade in a Microslice 2 sectioning machine (Metals Research Ltd.,
Royston, Herts, U.K.). The sections were polished to a thickness of about 80 pm with
alumina powder in water, mounted in balsam, and examined by ordinary and polarised light microscopy. Drawings of the
sections at a scale of 1 mm = 50 pm were
prepared using a projection microscope.
The following linear measurements were
made on appropriate sections (Fig. 1): T (incisor height; distance between incisal edge
and apex on the long axis of the tooth), measured for unworn or slightly worn teeth with
complete roots; H (maximum height of anatomical crown; distance between incisal edge
and cervical margin of enamel on the labial
aspect), measured on the same sections a s for
T and along the same axis; W (labiolingual
width of dentin component of crown, at right
angles to the long axis, at the widest point of
the section; just incisal to the cervical enamel
margin), measured in most sections in the
sample; E (thickness of enamel on the labial
aspect, at right angles to the enamel-dentin
junction, as close as possible to the incisal
dentin margin but cervical to the region
where the incisal enamel margin was
rounded by wear), measured in all unworn or
slightly worn teeth.
All measurements were made with calibrated eyepiece scales, using a dissecting microscope for T, H, and W (to the nearest 50
pm) and a polarising microscope for E (to the
nearest 5 pm). The ratio c = 1OOH/Tpercent
was used as a n index of the proportion of
tooth height covered by enamel. Orientation
of wear facets was estimated from the tracings of the sections. A line was drawn between the edges of the facet in the dentine
and the angle (0) between this line and the
tangent to the EDJ at the incisal margin
measured (Fig. 1).
Estimates of body weights (M) were taken
from the data reported by Bauchot and Stephan (19691, Harvey et al. (19781, and Smith
(1981). Relationships between tooth dimensions and between these and body weight
were evaluated by least-squares regression,
usually after transforming data to their natural logarithms. Because for most species
only one specimen was available, and because for species represented more than once
the specimens were not all of the same sex,
species averages were not calculated for the
purposes of regression. However, all five
Fig. 1. Diagram of a longitudinal section of a cercopithecid incisor showing the dimensions measured in this
study.
specimens of Papio cynocephalus were of unknown sex and had very similar tooth widths,
and to avoid bias from this relatively large
number for one species average values for
this species were used in regression of tooth
width and height on body weight. When the
sex of a specimen was unknown, it was assigned the average of the male and female
weights. The probability of the slope of a
given regression line being identical with a
chosen hypothetical value was evaluated by
modified t test (Snedecor and Cochran, 1980:
153-156). The value of P given after regression equations 1-5 represents the probability
that the slope of the regression line equals
zero (i.e., that regression accounts for none of
the variance of the dependent variable). The
square of the correlation coefficient r (the
coefficient of determination; Sokal and Rohlf,
1981) is given to indicate the proportion of
the variance of the dependent variable accounted for by regression.
RESULTS
Lower incisors
Size and shape: Incisor width (mm) varied
with body weight (gm) according to the
equation
106
R.P. SHELLIS AND K.M. HIIEMAE
+
logew = 0.35610geM 10g~0.185
(n = 21, P < 0.05, r2 = 0.2361).
(1) The distributions of incisor heights in relation to the regression line (not illustrated)
were very similar to those for incisor width.
The slope was not significantly different The lingual crown surface in unworn or
from 0.333. Figure 2 shows that, with one lightly worn lower incisors was rather more
exception, the incisors of Macaca mulatta and concave in colobines than in cercopithecines
M. fascicularis were larger than predicted by (Figs. 3, 4). The concavity was particularly
regression, and those of Cercocebus, pronounced in incisors from Colobus sp., C.
Mandrillus, and Papio were even larger. Of guereza, and Presbytis aygula; these teeth
the cercopithecins, Cercopithecus mona and appeared bladelike in section, and there was
two Erythrocebus specimens had incisors a distinct cingulum (Fig. 3-41. A cingulum
close to the predicted size, whereas the was not observed on cercopithecine lower inremaining specimens had smaller incisors cisors (Fig. 4A,B).
than predicted; the point for Cercopithecus Enamel distribution and thickness: Enamlhoesti was well below the line. All colobines el was present as a substantial layer on both
had smaller incisors than predicted by labial and lingual surfaces of colobine lower
regression. Incisor height (mm) had a close incisors (Figs. 3-4-D, 5). Maximum lingual
linear relationship with tooth width:
enamel thickness was 60-125 pm (17-21% of
the maximum labial enamel thickness).
(2) In contrast, the enamel layer was always
T = 3.308W + 1.91
very thin, and was often absent altogether,
(n = 17, P < 0.001, r2 = 0.871).
on the lingual surface of cercopithecine lower
conformity with this, incisor height incisors, even when unworn or only slightly
showed a similar relationship to body weight Worn (Figs. 4A,B, 6). Examination of the cut
surfaces of the remaining portions of the
a s did incisor width:
teeth showed that this was not a n artifact
log,T = 0.36810g~M+ 10g~0.427
resulting from loss of enamel during section
(n= 14, P = 0.05, r2 = 0.263). (3) preparation. Transverse sections of a lower
E
'
8t
111 A
c 71
.g
C
.-
5l
4
6
8
10
12
IL
16
18 20
body weight - kg
Fig. 2. Regression of labiolingual lower central incisor width on body weight (logarithmic
plot). Solid circles, Cercopithecini; triangles, Papionini; open circles, Colobinae. The small
numbers adjacent to the data points refer to specimen Nos. (Table 1).
107
CERCOPITHECID INCISOR ENAMEL
R
D
Fig. 3. Longitudinal sections of colobine lower central incisors. Not to uniform scale. Dashed
lines indicate dentin-cementum junction. A, Colobus guereza (slightly worn); B, Presbytis sp.
(slightly worn); C, C. badius; D, C. uerus (heavily worn). Enamel shaded black.
A
u
C
E
B
Fig. 4. Longitudinal sections of cercopithecine lower
central incisors. Not to uniform scale. Dashed lines indicate dentin-cementum junction. A , Mucucu mulatta
(partly erupted, unworn); B, Erythrocebus patas (slightly
worn; arrow indicates cervical limit of wear facet); C,
Cercopithecus lhoesti (advanced wear); D, C. mona (extreme wear); E , Pupio cynocephalus, transverse section
of crown of lateral lower incisor, showing thinning of
enamel towards mesiolingual and distolingual angles.
Enamel shaded black.
108
R.P. SHELLIS AND K.M. HIIEMAE
Erythrocebus, the maximum lingual enamel
thickness was less than 50 Fm.
Crown height (mm) varied with incisor
height (mm) as follows (Fig. 7):
logeH = 1.44410geT + 10g~0.15
( n = 17, P < 0.001, r2 = 0.919).
(4)
The slope was not significantly different from
1.5; i.e. crown height varied approximately
with tooth height to the power 3/2. The ratios
(c) of crown height to incisor height in the
various subgroups were: colobines 45.4 f 4.0
(SD); cercopithecines 54.9
8.8; cercopithecins (four specimens from only two species)
7.1. By
46.2 & 5.6; and papionins 58.8
Student's t test, the Colobinae differed significantly from both the Cercopithecinae as a
whole and from the Papionini in this feature
(P < 0.025) but not from the Cercopithecini.
The values for the Papionini and Cercopithecini were significantly different (P < 0.025).
Within the Papionini, c was 55.7 _+ 6.8 in
Macaca and Cercocebus and 64.9 & 0.7 in
Mandrillus and Papw. These values were
significantly different (P < 0.025).
Labial enamel thickness (pm) varied with incisor width (mm) as follows (Fig. 8):
+
+
logeE = 0.5210gew 1oge217
(n= 20, P < 0.001, r2 = 0.584).
Fig. 5 . Longitudinal ground section of slightly worn
lower central incisor of Colobus guereza showing substantial enamel layer on lingual surface (right).
Fig. 6. Longitudinal ground section of unworn, partly
erupted lower central incisor of Mucuca mulatta. Although a thin layer of enamel extends over the extreme
tip of the tooth, no enamel is present on the lingual
surface (right).
incisor crown of Papio (Fig. 4E) showed that
the enamel tapered off at the mesiolingual
and distolingual angles of the crown. When
enamel was present on the lingual aspect, it
was thickest in the cervical region, a t the
transition to the root surface. In one specimen of Erythrocebus, the lingual enamel
reached a thickness of 100 km in this locality,
but in all other specimens, including two of
(5)
The slope was not significantly different from
0.5; in other words, labial enamel thickness
varied approximately with the square root of
tooth width. Data points appeared to be scattered randomly around the regression lines
represented by equations 4 and 5 (Figs. 7,8).
Wear patterns: In colobines, dentin was exposed by wear only in the incisal region. On
slightly or moderately worn incisors the wear
facet was oriented at 80" to the labial surface
in Colobus (Figs. 3A,C) and at 60-85" in
Presbytis (Fig. 3B). In the only heavily worn
specimen (C. uerus, Fig. 3D),the angle was
95". Enamel was present on the lingual surface a t all stages of wear (Fig. 3A-D).
With the exception of the single slightly
worn specimen of Mandrillus, the wear facet
in early stages of wear in cercopithecines was
flat or slightly concave and formed a slightly
larger angle (45-70", mean 58") to the labial
surface than the natural lingual surface (Fig.
4B). On the Mandrillus incisor, the facet was
convex and oriented at 80" to the labial surface. With increasing wear, the facet enlarged to occupy the whole of the lingual
surface (Fig. 4). In moderately worn teeth,
109
CERCOPITHECID INCISOR ENAMEL
2ol
q
.-Q,
lo
r
'2/
118
Q102
12
16
1L
20
18
22
26
2L
28 30
incisor height - mm
Fig. 7. Regression of crown height on overall tooth height of lower central incisors Uogarithmic plot). Solid circles, Cercopithecini; triangles, Papionini; open circles, Colobinae.
600-
E
%OOI
0118
el06
in
in
Q,
C
Y
0
2 LOOY
E
z
0120
aJ
t
300
3
L
5
6
7
8
labiolingual incisor width - mm
Fig. 8. Regression of labial enamel thickness on labiolingual width of lower central incisors
(logarithmic plot). Solid circles, Cercopithecini; triangles, Papionini; open circles, Colobinae.
the facet maintained the same orientation
(52-61", mean 58") as in initial wear (Fig.
4B,C), but in heavily worn teeth the angle
with the labial surface tended to increase
(65-SO", Fig. 4D).No enamel was present on
the lingual surface in moderately or heavily
worn teeth.
Cercopithecine upper incisors
These teeth were similar in form to the
lower incisors (Fig. 9A), although in the worn
tooth from Cercopithecus a cingulum was
present (Fig. 9C). However, both lingual and
labial surfaces of the upper incisors were cov-
110
R.P. SHELLIS AND K.M. HIIEMAE
A
C
B
Fig. 9. Longitudinal sections of upper incisors of cercopithecines. A, Macaca mulatta (unworn, partly erupted); B, Papw cynocephalus (moderate wear); C, Cercopithecus pygerythrus
(advanced wear); D, P cynocephalus (extreme wear).
ered with enamel in contrast to the lowers
(Fig. 9A-C). With wear, a concave facet exposing dentine formed at 60-75" to the labial
surface in earlier stages of wear and a t about
80" in heavily worn teeth. The lingual
enamel was lost by wear only in one extremely worn incisor from Papio (Fig. 9D).
DISCUSSION
Tooth size
Previous metrical studies of incisor size
(Hylander, 1975; Gingerich et al., 1982) have
included data for Old World monkeys within
samples of wider ranges of primates. On the
basis of our sample, body weight was a poor
predictor of incisor size for this family, accounting for only 23.6% of the variance in
tooth width and 26.3% of that in tooth height.
Thus, although incisor size tends to increase
isometrically with body weight (in agreement with Hylander, 1975, and Gingerich et
al., 19821, there are wide departures from the
values predicted by regression.
Hylander (1975) suggested that the variations were correlated with the nature of the
diet. Relatively narrow incisors tended to be
found in monkeys with a predominantly folivorous diet or that consumed items such as
seeds requiring little treatment by the incisors. Relatively wide incisors were associated
with diets including items, especially fruits,
that require more extensive reduction before
processing by the molars. This hypothesis is
supported by complementary work on the size
and morphology of the molars (Kay, 1978,
1981). Although the position of the regression line with respect to the data points is
predictably different from that reported by
Hylander, because of the different composition of the sample and because both males
and females were included in the calculation,
our results support Hylander's general conclusion that colobines have relatively small
incisors, whereas cercopithecines on the
whole have relatively large incisors. With
respect to the cercopithecines, our results also
agree with Hylander's in more detail. Thus
Cercocebus, Mandrillus, and Papio had exceptionally large incisors, whereas those of
Cercopithecus mitis were relatively small and
those of C. lhoesti smaller still. The points
for Erythrocebus of known sex fell close to
CERCOPITHECID INCISOR ENAMEL
the line. The third specimen (No. 73) appeared to have smaller incisors, but, if, as
suggested by the similarity in dental dimensions with specimen No. 104, this specimen
was a female, the distance below the line
would be considerably reduced.
Among our very small sample of colobines,
Presbytis entellus appeared to have much
smaller incisors for its size than indicated by
Hylander’s data. Again, this could be due to
lack of information about sex of the specimen. Our specimen (No. 1201, belonging
either to F! johnii or to I! senex, had incisors
of about the same size as F! aygula when
assigned a body weight for F! senex, but, if
the specimen was actually the larger F!
johnii, the relative incisor size would be
smaller than for F! aygula, as reported by
Hylander. With regard to the distribution of
incisor sizes, therefore, we are in agreement
with Hylander, and we support his hypothesis that incisor size is correlated with the
composition of the diet.
Enamel distribution
Our results confirm the finding (Noble,
1969; Delson, 1975)that enamel is extremely
thin or absent on the lingual surfaces of lower
incisors of Papionini. However, we have also
found this to be the case in the lower incisors
of Cercopithecini. The cercopithecines as a
whole are thus distinguished in this character from the colobines, in which the lower
incisors have a substantial layer of enamel
on both lingual and labial aspects. Delson
suggested that the colobine type of incisor
structure is primitive for Old World monkeys. It is worth noting that in colobines the
lingual enamel is thinner relative to the labial enamel than i t is in man. In human
lower incisors, the thickness of the enamel
on the lingual surface is about two-thirds
that on the labial surface (Gillings and Buonocore, 1961; Shillingburg and Grace, 1973).
Gantt (1977) examined lower incisors from
two cercopithecine and two colobine genera.
In his Table 4.12 (Gantt, 1977:114-115) a lingual enamel thickness of 331 pm is recorded
for the lower incisors of Papio cynocephalus,
but no values are reported for Macaca mulatta or M. fascicularis and a value of only 56
pm for M. nemestrina. The smaller lower incisors of Colobus and Presbytis are reported
as having lingual enamel thickness of 95128 pm. Gantt concluded that lingual enamel
thickness was highly variable between species and that, bearing in mind the small
number of teeth examined, his results did
not support the concept of a difference in
111
enamel distribution between folivores and
frugivores. However, apart from the value
for Papio, Gantt’s results agree with ours.
Noble (1969) observed that enamel distribution was irregular on the lingual aspect of
lower incisors of l? cynocephalus, and Gantt’s
measurements in this species may relate to
isolated islands of enamel that had a thickness greater than we have encountered. Alternatively, they may be artifactual; Gantt
used rather thick sections (about 100 pm), did
not polish them, and apparently did not check
the identity of the outer layer of the tooth,
e.g., by polarising microscopy.
The blunt, flattened incisal edges resulting
from the presence of enamel on both lingual
and labial aspects of colobine lower incisors
are likely to reduce the efficiency of these
teeth in scraping, or in cutting through tough
foodstuffs, because of reduced stress concentration at the edge. However, they would provide a means of gripping leaves firmly while
they were torn by movements of the head or
by use of the hands. It has been shown that
in grass most of the strength of the leaf resides in the sclerenchyma fiber bundles,
which have to be severed either by tearing
or by nibbling (Vincent, 1982). It seems reasonable to assume that the fiber bundles are
also mechanically important in the broader
leaves of trees. In that the bundles here are
associated with the radiating vascular ribs,
it is likely that animals of the size of colobine
monkeys would be able to feed most efficiently by tearing leaves, since a relatively
large proportion of a leaf could be detached
by breaking only some of the fiber bundles.
Walker (1976) observed that a lower proportion of colobine incisors showed wear striations compared to cercopithecine incisors and
that, when present, the striations tended to
be oriented mesiodistally, i.e., parallel to the
incisal edge rather than perpendicular to it.
If the main function of colobine incisors is
to provide grip, it seems likely that attrition
rates would be low, because of the limited
movement of foodstuffs against the incisal
edge, and that the forces exerted on the tooth
during tearing of leaves would be relatively
low. The small size and short crowns of colobine incisors are consistent with modest mechanical demands. These features reflect the
greater importance in colobines of the postcanine teeth, with their tall cusps and long
shearing edges (Delson, 1975; Kay, 1978,
1981; Eaglen, 19841, in reducing the food.
The incisors of colobines normally occlude
edge-to-edge (Colyer, 19361, but a n adequately firm grip would probably be obtained
112
R.P. SHELLIS AND K.M. HIIEMAE
with less perfect occlusion. This might account for the high incidence of mandibular
incisor protrusion in Colobus and Presbytis
(Colyer, 1936; Zingeser, 1970). Among our
specimens, only that of C. badius (Fig. 3C)
showed evidence, from rounding of the wear
facet and thinning of the incisal region of the
lingual enamel, of this trait.
Because the lingual enamel layer on cercopithecine lower incisors is vestigial, the
gouge-shaped crown form present at eruption
is maintained with only slight modification
during most of the functional life span. Such
enamel as is present lingually is remote from
the incisal edge and the small variations observed in its thickness are thus unlikely to
influence crown form during wear to any significant extent. Undercutting of the dentine,
which would reduce support for the enamel
cutting edge, does not occur, perhaps because
the mechanical loads are concentrated at the
incisal enamel ma; gin. The increase in angle
between the lingual and labial surfaces in
late stages of wear suggests that secondary
dentine, which by then is exposed at the surface, wears slowly relative to primary
dentine.
In the upper incisors, a sharp incisal edge
is also maintained, apparently through attrition by the lower incisors occluding at the
incisal region of the lingual surface. The wear
surface has the form of a mesiodistally oriented groove (described by Walker, 1976,
from replicas). In longitudinal section, the
surface of this groove forms a larger angle to
the labial surface than the wear surface in
the lower incisors, presumably because of the
presence of enamel on the lingual surface.
The gougelike crowns of cercopithecine incisors seem to be mechanically well adapted
to reduction of tough materials, e.g., fruit
rinds, for which a scraping action is required.
They would also be effective in cutting materials, such as tubers, that fail by brittle
fracture, since the sharp edges would produce the high stress concentrations necessary to initiate cracks in the food substance.
Such use implies a predominantly vertical
movement of the incisors and this is supported by the finding that scratch marks on
cercopithecine incisors tend to be oriented
labiolingually (Walker, 19761, i.e., perpendicular to the incisal edges. It is of interest in
this connection that on the lingual surfaces
of the lower incisors of the marmosets Cebuella and Callithrix, which are used to
pierce tree bark during sap-feeding, enamel
is either absent or extremely thin (Rosenberger, 1978; Gantt, 1978).
The morphology of the lower incisors does
not prevent cercopithecines from feeding on
leaves. However, biting leaves with sharpedged teeth would sever fiber bundles only
over a limited area, and several bites might
be required to cut large bundles. The more
efficient strategy would be that reported by
G . Hausfater (quoted by Walker, 1976), in
which leaves are dragged between the incisors, so stripping the parenchymatous tissue
from the fibrous components.
Incisor height in general increased linearly
with tooth width, but crown height had a
positive allometric relationship with tooth
height. Thus, among Old World monkeys,
stouter incisors are not only taller but are
also more hypsodont than small incisors.
Large incisors would thus be structurally
adapted to higher rates of wear, associated
with processing tough foods (Hylander, 1975);
the taller crowns would allow the incisors to
function longer. Lateral mandibular radiographs of young and old specimens of Papio
in our collection suggest that extensive eruption of the lower incisors occurs as wear progresses; this brings into function the more
cervical portion of the enamel crown, which
is initially located subgingivally. The similarity in the ratio of crown height to tooth
height between the cercopithecins and colobines and the difference in the ratio between
these monkeys and the papionins appear to
be consequences of the distributions of crown
height, incisor size, and body weight.
The thickness of the labial enamel varied
over a rather restricted range and tended to
be smaller, relative to tooth size, in larger
incisors. All incisors of 4 mm and above belonged to cercopithecines, and in these teeth
the presence of a relatively thin layer of
enamel is probably sufficient to ensure differential wear and maintenance of the incisal
cutting edge.
CONCLUSION
It appears that Old World monkey incisors
show adaptive variations in two features,
enamel distribution and tooth size, and that
the two are partly linked. The relative uniformity of diet among the folivorous colobine
monkeys is reflected in their incisor structure: small size, relatively short crowns,
blunt incisal edges resulting from the presence of enamel on both labial and lingual
surfaces. Gouge-like lower incisors, resulting
from there being only a vestigial enamel
layer on the lingual surface, seem to be a n
important factor in the ability of cercopithecine monkeys to exploit a wider range of
CERCOPITHECID INCISOR ENAMEL
foods than the colobines. Although the form
of the lower incisor crowns is uniform within
this group, these teeth are adapted to widely
differing, diet-related mechanical demands
through striking variations in tooth size and,
concomitantly, in relative crown height.
Our results suggest that further histologic
and odontometric work on incisor structure
among a wider range of species would contribute to understanding the functional adaptation of these teeth to their role in the
dentitions of primates with different diets. In
such work, examination of cut polished surfaces by scanning electron microscopy (Martin and Boyde, 1984) would be a more
conservative technique for studying valuable
material than that used here.
113
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ACKNOWLEDGMENTS
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