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Dental microwear and diet Implications for determining the feeding behaviors of extinct primates with a comment on the dietary pattern of Sivapithecus.

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Dental Microwear and Diet: Implications for
Determining the Feeding Behaviors of
Extinct Primates, With a Comment
on the Dietary Pattern of
Department of Anthropology, Duke Uniuersity, Durham, North Carolina
27706 (H.H.C.)and Department of Anatomy, Duke University Medical Center,
Durham, North Carolina 27710 (R.l?K.)
Dental microwear, Diet, Jaw movements
Dental microwear is of special interest for two reasons. First, it
has been proposed that specific dental microwear patterns are associated with
specific diets and therefore that the diets of extinct forms may be deduced by
analysis of microwear. Second, it has been suggested that the geometry of wear
striations indicates the direction of masticatory movement. We tested these
ideas by analyzing microwear of laboratory animals fed different diets. Twelve
American opossums (Didelphis marsupialis) were fed soft cat food for 90 days.
Two control animals were fed only this base diet, five animals had plant fiber
added to their diet, four animals had chitin added to their diet, and one animal
had fine ground pumice added to its diet (for the last 30 days of the feeding
period). We examined the wear surface below the paracristid on the Ms and M, of
each animal by SEM. No microwear pattern differences were observed on the
plant fiber-fed,chitin-fed, or control animal's molars. The pumice-fed opossum had a
distinct microwear pattern with many parallel striations, resembling those foundon
the teeth of grass-eating hyraxes (Walker et al., 1978).These results suggest that 1)
exogenous grit (this study) or plant parts containing opaline phytoliths (Walker
et al., 1978)produce similar microwear patterns, and 2) the diets of extinct forms
cannot always be deduced by the analysis of dental microwear. The absence of
fine parallel striations on teeth of Siuapithecus examined by us suggests that
grass parts were not an important part of its diet and that it avoided dietary fine
grit. Furthermore, we found striations on opossum molars with deep, broad heads
and shallow, narrow tails oriented in opposite directions on the the same Phase I
wear facet. This suggests that the geometry of striations on Phase I wear facets
does not allow one to determine the direction of masticatory movement.
Recent interest has focused on the use of
scanning electron microscopy (SEM) to study
dental wear in primates and other mammals.
Evidence of microwear patterns has been
advanced to support hypotheses about dietary
pattern and/or differential tooth use, as well as
to make inferences about jaw movements
during mastication and other oral activities. If
microwear distinctions can be recognized and
firmly established for many sorts of diets and
oral activity patterns, this method of analysis
would provide an important adjunct for
0 1981 ALAN R. LISS.INC.
determining the oral behavior of extinct
primates. At present, we can make reliable
generalizations about these behaviors in fossil
primates only based on the structure of their
teeth by analogy with those of living species
with known oral behavior. Similarly, if it can
be established that the conformation of wear
is indicative of the direction of movements
during tooth-tooth contacts this would be an
Received April 18. 1980: accepted December 29. 1980.
important adjunct to in vivo studies of jaw
movements in the intercuspal range.
In one well-documented instance, enamel
microwear was shown to be altered by a
seasonal variation in diet. The enamel
microwear of two sympatric hyraxes, Procavia
johnstoni and Heterohyrax brucei, were
analyzed by Walker et al. (1978) using SEM.
The microwear of the two species was similar
during the dry season when both browsed on
bush and tree leaves. However, during the wet
season Procavia johnstoni ate grass and its
microwear was quite distinct from that of
Heteroliyrax brucei, which remained strictly
a browser. The molar enamel of grass-eating P.
johnstoni was covered with many fine parallel
striations, possibly produced by opaline phytoliths in the grasses. Work is presently underway by Walker and colleagues who hope to use
other similar “natural-experiments” to recognize dietary patterns.
Because of the potential that microwear
studies may have for dietary interpretations
of extinct species, it would be desirable to
make comparisons of microwear among many
animals with a variety of different diets. In
most instances, however, data of the sort used
by Walker et al. (1978) is unavailable. A
museum specimen is practically never accompanied by information about its diet covering
the interval of several months before its death.
An alternative strategy followed here is to
feed specially formulated diets to laboratory
animals and analyze the resulting dental wear.
An added advantage of this approach over
studies of feral animals is that there can be
more precise control over all aspects of the
dietary pattern, allowing one to consider and
eliminate possible complicating dietary factors. Laboratory studies of the sort reported
here provide data about which food constituents produce wear (and which do not). Clearly, if we cannot establish diagnostic criteria
for specialized dietary regimes in a controlled
laboratory situation, it would be overly optimistic to expect to interpret dietary patterns
of extinct species based on wear except in
special, unusual circumstances.
Ryan (1979a,b)investigated the relationship
of the direction and shape of microwear striations to the direction of tooth movement. He
found, in in vitro studies simulating
masticatory movement, that the initial point
of contact between striation-forming grit and
tooth surface was often deeper and wider than
the rest of the striation, which tapers in the
direction opposite to relative tooth movement.
Ryan concluded by examining dental
microwear that the direction of tooth movement in mastication could be determined. He
documented several cases where wear striations produced in vivo during Phase I mastication, for which jaw movement direction is
known, had the predicted configuration. The
data supplied by our studies allow us to
evaluate critically the studies of Ryan.
In this study we compare the pattern of
enamel microwear developed on the molars of
American opossums (Didelphis marsupialis)
fed specially prepared laboratory diets which
include plant fiber, chitin, and dietary grit
(pumice) - three materials which could produce enamel striations on the teeth of feral
mammals. We examine the implications of our
findings with respect to the use of microwear
for assessing the diets of extinct species and
for the determination of jaw movement direction during mastication.
A laboratory experiment has a major advantage for studying the pattern of enamel
microwear which is lacking in a study of
animals in their natural habitat. Because there
is precise control over what the animals eat,
more certainty is possible concerning the
agent causing the wear patterns. We selected
the American opossum as our research animal
because its masticatory behavior is well
understood (Crompton and Hiiemae, 1970),
and because opossums will readily eat a variety of foods.
Twelve animals were selected whose lower
left third molar was little worn. The criterion
of selection was that each of the trigonid
cusps (paraconid, metaconid, and protoconid)
have little, if any, dentin showing on the
leading edge of the shearing crests.
Each animal was fed daily 184 gm (one can)
of the base test diet, “Friskies Buffet - liver
and chicken parts” for an initial 4-day habituation period. This diet is soft and free from
abrasives or grit which could cause wear on
the opossum’s teeth. After 4 days the diets of
nine of the 12 animals were changed. To
simulate a “herbivorous” diet, 15% (by dry
weight) plant fiber (coarsely ground soybean
hulls, particle size range of 3 to 9 mm) was added to the diets of five animals. To simulate an
“insectivorous” diet, 15% (by dry weight)
chitin (particle size range of 3 to 9 mm) (supplied by United States Biomedical Corporation, Cleveland, Ohio) was added to the diets
of four animals. Three control animals were
fed only the base diet, All animals were continued on these diets for 90 days with one exception. One of the control animals was fed
90% base diet and 10% finely ground pumice
(by dry weight) for the last 30 days of the
feeding period.
The 15% plant fiber is taken as a reasonable
approximation of the amount of this constituent incorporated into the diet of herbivorous
primates, For example, the leaves eaten by the
primate herbivore, Presbytis johni, contain
27% to 69% plant fiber (by dry weight), and
this species eats leaves 46% of the time (Oates
et al, 1980),giving an overall figure of 13-34%
fiber in the total diet. The chitin content of insects ranged from 3 to 8% (by dry weight) for
five species examined by Tsao and Richards
(1952). We have found that crickets contain
approximately 10% chitin. The 15% chitin added to the opossums’ diet is a slightly higher
quantity than the chitin ingested by insectivorous primates. The 10% grit added to the
diet of the opossums may be a less realistic approximation of grit content in the natural diet
of any primate, but it may be supposed that
animals which root and dig for their food may
include close to 10% soil in their diet.
At the end of the 90-day feeding period the
opossums were sacrificed. The left M, and M,
were extracted from each mandible, cleaned
with a water-pik (Sears Aqua Jet by Water
Pik), and allowed to dry. The molars were
rinsed in acetone, mounted on metal stubs,
and sputter-coated with approximately 200 A
AuPd alloy under a vacuum in a “Film-Vac
Inc. mini-coater.”
Each tooth was given a code number different from the animal’s number so that the
researcher examining the teeth by SEM would
not be prejudiced by the knowledge of the
animal’s diet. The wear surface below the
leading shearing edge of the paracristid was
examined on each molar at various magnifications under a JEOL-T20 scanning electron
microscope. Special care was taken to examine
each tooth at a nearly identical orientation
and location, about normal to the leading
shearing edge of the paracristid, as diagramed
in Figure 1. This surface was chosen for examination because it is the primary Phase I
shearing surface on the lower molars of
Didelphis (Crompton and Hiiemae, 1970).
Micrographs were taken at this location on all
specimens at the same magnification ( X 350)
to allow for comparisons of microwear patterns. The overall appearance of the wear surface was recorded including the approximate
Fig. 1. Didelphis marsupialis. Anterolateral view of A,
the lower left molar series and B, M, (of the same molar
series), illustrating the orientation used in this study to
analyze the microwear pattern of Phase I wear facets. The
stippled area on B indicates the wear facet examined on
each molar.
number of striations and their size, shape, and
In addition to analyzing the microwear on
these molars, we obtained an Indian
Siuupithecus specimen (high-precision casts
were generously supplied by Dr. P.D.
Gingerich) and examined the microwear on a
Phase I shearing surface (facet la).
The microwear patterns of representative
animals fed various diets are shown in Figure
2A-D respectively. There are no qualitative
differences between the microwear patterns of
the plant-fiber-fed, chitin-fed, or control
groups in these micrographs or in others taken
on the remaining animals. The microwear of
most specimens (Fig. 2A-C) is relatively
smooth, with a few pits and striations oriented
in various directions. In contrast, the wear
surface below the leading edge of the pumicefed opossum (Fig. 2D) has relatively rough
wear with few pits and a great number of
parallel and relatively uniform-sized striation.
This pattern of microwear is quite distinctive
from that of the other opossums and appears
to be similar to the microwear pattern of the
grass-eating hyrax figured in Walker et al.
These results lead us to conclude that it is
not always possible to deduce the diets of
animals from their microwear patterns. Thus,
Fig. 2. Didelphis marsupidis. Micrographs ( X 350) of microwear pattern on wear surface below the paracristid on lower
left (right for C) molar of A,plant-fiber-fed animal, B. chitin-fed animal, C. control animal, and D.pumice-fed animal. E.
Siuapithecus sp. Micrograph ( X 350) of microwear pattern on Phase I wear facet ( # l a )on upper right first molar.
while specific wear patterns may be produc-
ed by a specific diet (for example, all chitin-fed
animals have similar microwear), these wear
patterns may not be diagnostic for a particular diet (the plant fiber and chitin-fed
animals have similar microwear). This sug-
gests that in many instances molar wear patterns may have little resolving power for
determination of diets of fossil animals.
An alternative interpretation of these
results is that 90 days (the length of the
feeding period in this experiment) may not be
long enough for animals to develop dietspecific wear patterns. We consider this
unlikely for two reasons. First, as noted
above, Walker et al. (1978) report that
microwear patterns change seasonally in one
hyrax species. The dry season is only 5
months long and animals sampled during the
dry season had distinctly different wear patterns than those in the wet season. This suggests that microwear patterns are changing in
less than 150 days. Second, in this study, one
animal had a very distinctive wear pattern
after having pumice added to its diet for 30
days, demonstrating that dental microwear
can change quite rapidly.
While dental microwear may not always
allow one to deduce the diets of fossil animals,
these data are potentially informative in some
cases. As reported above, the pumice-fed
animal has a wear pattern similar to that of
grass-eating P. johnstoni. An important structural component of grasses is opaline
phytoliths. Baker et al. (1959) suggested that
opaline phytoliths may be the major agent
causing dental wear in sheep. They reported
that plant opal is harder than enamel (5.5-6.5
for opal and 4.5-5.0 for enamel on Moh’s hardness scale), lending credence to the suggestion
of Walker, et al. (1978)that opaline phytoliths
are the agent causing striations on the teeth of
P. johnstoni. If wear facets on teeth lack fine
parallel striations then this animal probably
excluded exogenous grit andlor grasses or
other plants with hard particles from its diet.
Such a conclusion is interesting in light of the
wear pattern we observed on a molar of
Siuupithecus (GSI D-185). This molar had a
smooth wear pattern with few pits or striations (Fig. 2E), suggesting that this animal
was not eating food containing exogenous grit
or grasses or other plants containing phytoliths (at least during the time period immediately preceding its death).
In Didelphis the majority of wear striations
below the leading edge of the paracistid are
parallel to the direction of Phase I jaw movement during mastication. Most striations are
of nearly uniform width throughout their
length. Those which have a deep, wide head
and narrow, shallow tail like that described by
Ryan (1979a,b)do not always or even typically
have the tapered tail in the direction opposite
tooth movement. contrary to Ryan’s suszes-
tion. Figure 3A-C shows striations from the
same Phase I wear facet. These micrographs
were made a t various magnifications, all
within the range utilized by Ryan. In Figure
3A the head of a striation points toward the
leading edge of the paracristid, whereas in
Figure 3B the head points in the opposite
direction. If the paracristid occludes only in
Phase I mastication and hence, in one direction as described by Crompton and Hiiemae
(1970), Ryan’s contention that masticatory
direction can be deduced by striation geometry is not supported. Further evidence
that does not support this hypothesis is seen
in striations below the paracristid which have
their deepest enamel penetration in the middle, with shallower and narrower tails occurring at both ends of the striation (Fig. 3C).
A knowledge of the diets of extinct primates
is essential for understanding their evolution.
Currently, we can make a few generalizations
about the diet of extinct primates on the basis
of the relative development of shearing,
crushing, and grinding features on their
molars (e.g.,Kay, 1975). The study of dental
microwem also has proved useful as a means
of distinguishing browsing from grazing
dietary regimes (Walker et al., 1978). Here we
have attempted a preliminary assessment of
whether one can recognize other diets based
on the wear characteristics of the teeth. This
was done by feeding different diets to
opossums for 90 days in a controlled
laboratory environment and examining the
dental wear. We were unable to distinguish
between the microwear of the plant-fiber-fed,
chitin-fed, or control animals. Thus, we cannot
expect to distinguish between the microwear
of insectivorous or herbivorous fossil
primates. Dental microwear data are informative in some ways. The opossum with
pumice added to its diet has quite distinctive
microwear, similar to microwear of the grasseating hyrax described by Walker et al. (1978).
This suggests that the analysis of microwear
can provide information that animal ate grit
or parts of plants containing silica. The
smooth wear observed on the Siuapithecus
specimen we examined suggests that this
animal was excluding grass and exogenous
grit from its diet.
We also examined the geometry of wear
striations on the Phase I wear facet below the
Daracristid. On the basis of Rvan’s 11979a.b)
Fig. 3. Didelphis marsupialis. A. Micrograph ( X 1,000)of a striation with a deep, broad head and shallow tail pointing
away from the paracristid. B. Micrograph ( X 1,500) of a striation with a deep, broad head and shallow, narrow tail pointed
toward the paracristid (on same molar as A). C. Micrograph ( X 350) of a striation which is deepest and broadest in its middle, with shallow and narrow tails at both ends of the striation. In all instances, the paracristid is located a t the top of the
experimental work it would be predicted that
these striations would have a deep wide head
and shallow, narrow tail oriented away from
the leading edge of the paracristid, parallel to
the direction of mastication. We found striations with this geometry, with the opposite
orientation, and with a deep, wide middle section with shallow and narrow tails at both
ends. This suggests that Ryan's hypothesis
about the geometry of striations and their
relationship with the direction of mastication
is incorrect.
This work was supported by NSF grant
BNS-77-08939 to R.F. Kay. We thank Susan
Hutchinson for her help with the SEM. We
also thank Dr. K. Rose who read and commented on this manuscript.
Baker, G. Jones, LHP. and Wardrop, ID (1959) Cause of
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Crompton. AW, and Hiiemae. K (1970)Molar occlusion and
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Kay, RF (1975) The functional adaptations of primate
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Oates, JF, Waterman, PG, and Choo. GM (1960)Food selection by the South Indian leaf-monkey, Presbytis johni, in
relation t o leaf chemistry. Oecologia 4545-56.
Ryan, AS (1979a)Wear striation direction on primate teeth:
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implications, determinism, diet, feeding, extinct, behavior, patterns, dietary, primate, dental, comments, sivapithecus, microwear
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