Atypical tooth wear found in fossil hominins also presentin a Japanese macaque population
Ian Towle
ianetowle@hotmail.co.uk cs 2022 doi 10.1002/ajpa.24500
... Accidental ingestion of sand & oral processing of marine mollusks likely creates these atypical wear patterns. Implications for similar wear that has been ass.x tool-use in fossil hominin samples were discussed.
... Discussion: The atypical tooth-wear found in all Koshima Island individuals suggest they vary in some way to other macaque populations, likely relating to differences in behavior, diet and/or environment. ... Despite 7 decades of behavioral research
at Koshima ... on behaviors related to object manipulation over the past decade, there seems to be no observational evidence suggesting deliberate placement of tools or other non-dietary items into the mouth which could explain the atypical wear patterns
observed in these individuals. These observational studies have been continuous, and range from following macaques across their natural range, to more detailed studies at specific locations, incl. studies explicitly looking at how the macaques interact
with non-masticatory items: Koshima Island macaques rarely placed stones in or near their mouths (it was recorded as scarce or absent, depending on the exact behavior described). A tool-use behavior that would cause such extensive & unusual tooth-wear
would have to be performed regularly: it is unlikely that these macaques undertook such a habitual behavior, over a large span of time, without it being recorded by an observer studying their behavior: Watanabe (2001: pp.405): “observations on social
interaction in the Koshima group have generated an enormous amount of data”, yet despite all this observational data, no non-masticatory related behaviors have been recorded that could explain the atypical tooth-wear observed in this sample. Lastly, on
site managers & researchers have recorded macaque behavior on the site neardaily over 7 decades: it is also unlikely that tool-use behaviors have occurred when no one was present. Moreover, most macaques do not use tools frequently, apart from long-
tailed macaques who are known regular tool users. Only a few cases of tool-use have been reported in Japanese macaques, all at sites where they were heavily provisioned and/or under experimentalconditions, e.g. dental flossing has been described in a
Japanese macaque from a different site, but such behavior has not been reported for Koshima Island macaques: placing non-masticatory objects (rocks, stones, hair, sticks) into the mouth is rare in Jap.macaques in general - Koshima macaques are no
exception. Koshima macaques have been provisioned regularly on a sandy beach since 1952, so they are likely to accidentally ingest sand. The non-provisioned diet of these macaques seasonally changes with locally available foodsand consists largely of
mature leaves & fruits. Around a 6th to a 10th of the calorie intake of the Koshima macaques' diet is from provisioned foods, equating to 60–90' feeding c 2/week. Accidental sand ingestion (sand particles covering ingested food items, or food being
directly eaten from the sand substrate) has been regularly observed. The behavior of washing provisioned food (e.g. sweet potatoes) has spread throughout the population, with duration varying among individuals, many individuals undertake wheat placer
mining behavior (grains are dropped into the water to remove sand, and subsequently eaten), but the macaques still end up accidentally ingesting sand alongside food. Another food processing behavior with potential to contribute to the atypical wear
described seems unique to the Koshima Island macaques: the extraction & processing of marine mollusks. This behavior has a long history on the island (more prevalent during winter). Cellanatoreuma (the limpet spp commonly consumed) is typically dislodged
from rocks utilizing the mouth, followed by removing the edible contents, using the anterior dentition. The exact actions involved & the fre-quency of the behavior likely varies from individual to individual, and through time. Teeth must regularly
contact rocks that limpets are attached to during extraction, and also the hard shell when contents are removed, using the anterior dentition. Given the hardness of these materials, this process may contribute to the fm of the macro-striations visible &
also to the atypical wear observed.
Lastly, the role of other hard dietary items cannot be ruled out: this group has also been observed eating acorns & other items containing a hard coating. OtherJap.macaque groups have been observed using a variety of feeding techniques to consume hard-
shelled food items: particular oral processing behaviors may have the potential to lead to unusual wear patterns. Further research on living individuals is needed to assess the role of these factors informing the atypical wear patterns observed, but
regardless of the role of sand ingestion, limpet extraction/processing & abrasion by other dietary items, it seems clear: tool-use was not the cause of the atypical wear observed here. Wallace (1974) suggested: NCCL's were ass.x grit mastication. An
argument against this: all teeth & other crown/root surfaces should also be affected by atypical wear. Earlier studies have suggested: labial striations on incisors likely formed due to hard object mastication, but later research suggested such items
would not leave large homogeneous scratches on dental surfaces. The present study showed: the accidental ingestion of sand-grains & marine mollusk processing can potentially lead to localized atypical wear, incl. large directional macro-striations &
NCCLs, supporting earlier researchers. This does not mean these processes necessarily caused NCCL's & labial striations on incisors in fossil hominins, but does highlight such behaviors can lead to similar localized atypical wear, without the need for
inferring tool-use behaviors: additional evidence may be required to infer non-masticatory related etiologies for these wear-patterns.The reason why certain locations (labial surface of incisors & root surfaces of molars) are more commonly affected by
atypical wear likely relates to the way items enter the oral cavity & movements of the masticatory cycle, saliva flow, swallowing & tongue movements. The types of atypical tooth-wear observed in this Jap.macaques likely share a common etiology: the
scratches potentially caused by sand scraping along the tooth surface. This could also contribute to the larger areas of tissue removal also observed in this population (e.g. root grooves, rounded/labial incisor wear & beveling). Wallace (1974): grit is
forced along root surfaces during the swallowing process, creating NCCL's & associated striations in hominins. How large directional striations form on root surfaces requires further research, but the present study supports this conclusion. The role of
periodontal bone-loss & gaps between posterior teeth is likely also crucial in explaining why these surfaces commonly show atypical wear (the exposed root dentine is weaker than enamel). Limpet removal & processing may help explain the extreme tooth wear
on anterior teeth & associated striations, although it seems likely that sand ingestion might be related to macro-striations & grooves observed on posterior teeth. Incisors are morelikely to contact the hard object & environmental grit first, esp. in
the initial stage of biting into a sand-laden food item, or when trying to process marine mollusks. This may explain why the labial surface of incisors were commonly affected, and why anterior teeth show more extensive atypical wear. Through the normal
masticatory cycle, lower incisors move in a non-linear, sub-vertical motion. In the Koshima population, performing masticatory movements whilst biting into gritty food, or directly onto the substrate itself (sand, rock, limpet, other dietary items) could
have led to a predominance of (sub)vertical scratches on the labial surface of incisors. Based on macroscopic observations, scratches in fossil hominins look remarkably similar to those in this study, with numerous large grooves observed in a predominant
vertical or subvertical orientation. The wear pattern described here also looks similar from a micro-scopic perspective, although at present, only a single tooth has been studied using these methods. The striations on the labial surface of the upper
incisor studied are within the 20–100 μm range of striation width that is often used to support a tool-use related origin for scratches in fossil hominins. There are several other microscopic features also cons.x fossil hominin examples: V-shaped
sections, micro-scratches at the bottom of grooves, raised areas adjacent to the grooves, the lengthf the striations (typically several mm). A more in-depth-microscopic study is required on a larger sample of teeth/individuals, to investigate variation
in size & angle of striations & the concentration of scratches in different regions. This will allow further conclusions on how the wear in this Jap.macaques match different hominin samples. But based on the present study, ,additional criteria are
required to rule out a masticatory-related originfor this sort of atypical tooth wear. The Koshima Jap.macaques analyzed here has a few similarities with fossil hominis in terms of the presence of NCCL's on posterior teeth & macroscopic striations on
anterior teeth. High rates of chipping, extensive interproximal wear & inter-dental spaces between molars, unusual wear on the buccal/labial surface of teeth (incl. rounded/beveled tooth wear on anterior teeth) & general extensive occlusal wear were also
observed. This study shed light on the etiology of NCCL's & macroscopic striations: tool use does not necessarily explain atypical & localized tooth-wear as observed in fossil hominins, it may suggest an alternative etiology (accidental grit consumption
or specific types of food processing). These non-tool related behaviors could be potentially linked with atypical tooth-wear in fossil hominins, incl. NCCL's, macro-striations on anterior teeth, buccal/labial incisor tooth wear and bevel-ing; extensive
tooth chipping; and extreme anterior tooth wear. Extreme tooth wear is well documented in Neanderthals, with interproximal wear & inter-dental space between teeth linked to high-masticatory force levels, abrasive diets & non-masticatory behaviors. In a
recent study on Krapina indiv.20, the posterior teeth showed chipping, directional macroscopic scratches on an occlusal surface & inter-proximal & lingual grooves on root surfaces. Other Neanderthal spms show NCCL's & striations on the labial surface of
incisors & extensive wear (incl. lingual/labial wear: beveling) on anterior teeth. This was also the case in some H.sapiens samples with similar wear across the dentition. Earlier Homo samples also commonly display similar atypical wear, with NCCL's &
macro-striations on the labial surface of anterior teeth reported: atypical tooth-wear recorded in fossil Homo spms could potentially be related to accidental grit/sand ingestion and/or specific food processing behaviors, based on the results of the
present study. High rates of chipping in Homo & the Koshima Island macaques is another similarity, it adds further support to the grit ingestion hypothesis. Some researchers have argued: striations on anterior teeth (if caused by “stuff+cut”-type
behaviors) would be indicative of the handedness of the individual, due to the orientation of striations. In the present study, there was evidence of side differences in striation & wear patterns, which may relate to handedness differences, but is more
likely caused by other factors: the masticatory cycle & occlusion of teeth is likely also crucial in generating these wear patterns. Jap.macaques from Koshima would often eat directly from the sand at first, but then shift to using 1 hand to pick up the
grains, although some individuals had no hand preference. Individuals may also prefer a certain side of the mouth for placing the food item: asymmetrical tooth-wear may be expected regardless of whether the wear is caused bytool-use, or normal food
processing behaviors. This study supports other evidence which has been overlooked in recent literature. Bax & Ungar (1999) compared labial tooth striations in different human samples with known differences in food processing practices (Aleut, Arikara,
Illinois Bluff, Puye populations), they found no evidence that these striations are linked to "stuff+cut-like" behaviors, and were skeptical of such striations being used to infer handedness in fossil spms. Re. the 'tooth-picking' hypothesis, a criticism
of this theory is: these grooves have not been documented in posterior teeth of contemporary populations, even among people who regularly use tooth-picks. Experimental studies which support these types of atypical wear also have limitations: sand can
create large directional striations, similar to the ones observed in fossil spms. Even in studies suggesting a tool-use origin for NCCL's, it has been suggested that grit adhered to tools may be required to create the observed striations/grooves. Unusual
interproximal tooth wear (grooves & striations) & large directional scratches on occlusal & buccal/labial surfaces have also been observed in various mammals (extant & fossil), incl. bears & several ungulate spp: this study adds to the growing evidence
that tool-use behaviors are not necessarily required to explain common types of atypical, localized wear, incl. types commonly described in fossil hominins.This study supports earlier claims: accidental ingestion of grit can lead to localized atypical
tooth wear (NCCL's, oriented macrostriations) as observed in fossil Homo (e.g. Puech 1981, Wallace1974, Wallace 1975). Other similarities in terms of micro-wear, chipping, overall occlusal wear & wear on non-occlusal surfaces may suggest a common
etiology of atypical tooth-wear as observed in the Koshima macaques studied here & for fossil hominins. Further research is needed to understand characteristics that may differentiate wear caused by tool-use & wear caused through regular mastication/
ingestion of hard objects. Food preparation patterns likely play a large role in generating these wear patterns. Although this does not mean hominins were not placing tools in their mouths, our study suggests the accidental ingestion of grit and/ornormal
masticatory food processing behaviors could have also been responsible for this atypical wear pattern.
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