Canine Role in Dental Wear Patterns: Macaca nemestrina RONALD J. BUTLER AND IRWIN S. BERNSTEIN D e p u r t m e n t of Anthropology, University of Georgia, A t h e n s , Georgia 30602 K E Y WORDS Canine . Interlocking . Wear . Tooth . Macaca. ABSTRACT A study of variables and patterns in dental wear among 3 0 individuals in a colony of Macaca nernestrina shows that consideration of age and sex are crucial for understanding differential wear degrees on molar occlusal planes. With advanced age, this species of non-human primate undergoes obliteration of initial morphological characteristics through the gradual erosion of enamel. Wear gradients are differential from P M l - M 3 in both sexes. It appears that there is a functional relationship between degrees of occlusal plane wear and the degree of wear on the canines, and that females show a greater degree of wear changes relative to males of equivalent age because of initial differences in canine length and robusticity due to sexual dimorphism. It appears that there is a direct relationship between occlusal wear plane changes and the degree of wear on the canines, with advanced differential wear showing up in individuals in whom years of maxillary canine honing, canine damage, and the normal wear of mastication has reduced dimensions of unworn permanent canines. Other considerations included in this study are the honing functions of the deciduous first mandibular molars and molar cusp height relative to canine function. One of the supposedly unique diagnostic dental characteristics of hominids is the flattening of occlusal wear planes of molar teeth due to “rotary chewing” motions of the mandible during mastication. Presumably these motions are made possible by the absence of large, interlocking canines. However, despite the implications of such a generalization, supportive data are nearly as scarce as functional, systematic approaches to tooth wear studies. For example, the role of interlocking canines, as it pertains to patterns of tooth wear which might be useful in making taxonomic assessments, remains speculative and inferential. Studies of the variables in tooth wear for living populations of Homo are rare, and almost non-existent for living non-human primates. Only very recently have the age-correlated patterns of helicoidal and differential occlusal wear plane changes in Homo sapiens been abstracted on the basis of objective data (Butler , ’72). As St. Hoyme and Koritzer have observed (’71), flattening of molar occlusal wear planes is not absolutely indicative of hominid status; they point to the flatAM. J. PHYS. ANTHROP.,40. 391-396 tening of molar crowns in Gigantopithecus as a n example. A number of workers also have noted both occlusal plane flattening and marked interstitial wear in Pan (see Wolpoff, ’71). Further, Jolly (‘70), in one of the rare applications of systematic analysis, suggested that in the case of Theropithecus, at least, the dentition may be regarded as a functional whole: what happens to wear of the cheek teeth may be directly related to functional aspects of the anterior dentition. Regrettably, Theropithecus does not make a n ideal laboratory animal for studies on living populations because of size, temperament, and problems in anesthesia. In short, those who attempt to follow the threads of primate evolution via dental studies are handicapped by the relative lack of data concerning age and sexrelated variables in tooth wear for either fossil or recent populations of primates in general. Despite the importance of dental wear This research was supported in part by a grant from the National Institute of Mental Health MH-13864, and by a grant from the National Institute of Health PHSFF-00165. 39 1 392 RONALD J. BUTLER AND IRWIN S. BERNSTEIN patterns and their functional elaboration the statements of many early workers have not been verified or have been discarded after the substitution of data for speculation. For example, in the first quarter of this century, Campbell suggested that interlocking canines in pongids may not absolutely limit lateral mandibular movement during mastication (‘25). This bit of speculation, at least, is less dogmatic than that of Jones, who, working with Macaca mulatta, comments that “there are certainly no lateral excursions of the mandible, movement being practically straight up and down” (‘47:257). The author neglects to provide the size of the sample and does not consider the effects of differential wear due to age or sex. TABLE 1 B i r t h d a t e s a n d s e x of a n i m a l s e x u m i n e d in 1972 Males Females 1957 2 1958 1.2 1958 1.2 1960 1.2 1960 1.2 20 Nov 64 2 21 Jan 65 2 24 Feb 65 2 23 Mar652 11 Feb662 7 Sep 66 2 4 Dee 66 2 11 Dec 67 2 31 Dec 68 2 19 Apr 69 2 21 Oct 69 2 22 May 70 11 Jul70 2 4 Jul71 2 15 Jul71 2 2 Sep 71 2 24 Sep 71 1956 1.2 1959 1.2 27 Jul64 2 28 Jun 65 19 Jun 66 5 Sep 66 27 Jul67 2 8 Dee 67 19 Sep 68 2 24 Sep 68 2 Jun 69 2 21 Feb 70 2 22 Aug 70 2 15 Sep 71 10 Oct 71 METHOD AND SAMPLE With this background in mind, we chose a living colony of Macaca nemestrina to determine dental wear pattern and correlate it with age and sex. Observations were made on some 37 individuals in the spring and confirmed on subsequent reexamination for 30 individuals during the summer of 1972 at the Lawrenceville Field Station facilities of the Yerkes Regional Primate Research Center (see table 1). Large adult males were anesthesized prior to examination, while younger males and females of all ages were examined while their limbs were pinioned. Color slides and black and white photographs were taken, and data on wear was recorded on tape. Observations were repeated by the second author several months after the initial examination, and the data were then oriented with recorded vital statistics. We regard the sample as adequate in regard to wear patterns, but it is too skewed in terms of age and sex to permit precise statements about such matter as tooth eruption sequences. Further, no attempt is made to present data in terms of actual differences in degrees of occlusal plane wear. The techniques for this form of approach are still being developed. All animals lived together in an open air compound which originally included the natural vegetation of northern Georgia. They were maintained on monkey chow and a variety of fruit and vegetable supplements. In addition, animals were seen Age estimated i n 1962; all others, including 1957 female, were born in captivity. Animals reported on in this paper. eating a variety of insects, small animals and earth. Animals chewed sticks, mouthed stones and bit wooden and other objects in play and exploration. These patterns, plus horizontal molar and incisor gritting and vertical canine grinding patterns were considered within the normal range of activities for wild specimens based on field observations in Malaysia by the junior author. RESULTS In this sample of Macaca nemestrina, enamel loss on the occlusal surfaces of the tooth leads, as in other genera, to gradual obliteration of initial crown morphology, exposure of dentin and pulp chambers, and to degrees of change in occlusal wear planes. An occlusal wear plane may be oriented lingually or buccally, or it may be flattened. In our sample, permanent dentition was completed by the age of 5 or 6 years. Enamel loss at this age is more marked on the anterior teeth, with relatively little dentin exposure visible on the cheek teeth. Although we are aware of the significance of time differentials in tooth eruption sequences as variables in wear, we can state only generally that M 1 seems to be func- CANINE ROLE tionally erupted between the ages of 2 and 3 years, M2 between the ages of 3 and 4, and M 3 by the seventh year at the latest and usually earlier. Permanent canines ordinarily follow functional eruption of M2. In at least one instance, permanent maxillary canine eruption followed functional eruption of M 3 in a 5-year-old male. Graduated wear in the cheek teeth is fully evident by the age of 7 years. Although this age category is represented only by three females, two of these individuals have sharply-sloped buccal occlusal wear planes on the mandibular M1, while the maxillary M1 is worn to an even greater degree in a lingually-oriented plane. In all age categories, there is considerable variation in regard to both eruption times and degrees of wear. We do not overlook the possibility that differential wear from front to back may be present at an earlier age. Obvious sex-related degrees of occlusal plane changes are apparent in all individuals 8 years and older. Although differential wear is evident in both sexes, the degree of wear from the age of 8 onward is consistently greater for females, with M1 worn to a greater degree than M2, and M2 more than M 3 for both mandible and maxilla. Suggested relationships between canine size in their initial, unworn state and wear on the cheek teeth correlated with age are seen in two 12-year-old females. One of these females has drastically worn canines, with those of the mandible worn to the incisal level of the adjacent teeth. In addition to wide areas of secondary dentin on the premolars, molars in this individual are worn to the extent that not even a rim of enamel remains around the contour of the molar occlusal surfaces. The second 12-year-old female has relatively large canines. Incisor wear is almost to the gum line, but molar wear is not so advanced as in the other female, with large areas of intact enamel and retention of much of the original area of the cusps. It is of some interest that the female with the well-worn canines shows relatively more premolar and molar wear than the alpha male, a 13-year-old individual whose canines remain robust despite a modest degree of blunting. In the alpha 393 male, differential wear gradients on molar surfaces are present, but much of the crown enamel is still intact. The oldest male available for study is a 16-year-old individual in whom the maxillary canines are greatly worn and chipped (fig. 1). The entire maxillary molar series shows occlusal wear planes which are sharply sloped lingually, while the wear planes of the mandibular molars all slope to the buccal side. Wear is greater in the maxillary dentition than in the mandibular. It should be noted that, in this old male, while upper and lower canines show considerable damage and blunting, the remaining portions of the teeth are still relatively robust. The oldest female, aged 15 years, has mandibular canines worn nearly to the level of the incisors. Her right maxillary canine is severely worn relative to the left. On this animal, all of the mandibular molars show sharply-sloped buccal occlusal wear planes, with the maxillary series showing a strong lingual orientation of the wear planes. In this case, as in the others, M l is worn to a greater extent than the rest of the series. The 15-year-old female retains lips of enamel around the occlusal contour of M2-3, but the upper and lower M1 show almost no enamel, with pulp chambers visible although not actually exposed. No enamel elevations are visible on any of the molar series in this individual. A slightly younger female, 14 years of age, shows canine wear to the extent that there is no apparent impediment to lateral mandibular excursion. Incisors are heavily worn, and all of the mandibular molars are worn sharply to the buccal side while those of the maxilla all are worn to the lingual side. By way of contrast, another 14-year-old female with less severely worn canines shows a lesser degree of occlusal plane wear on the molars. In this individual, original cusp sites are represented by areas of elevated enamel. In all individuals in whom deciduous molars were retained, the mandibular M1 is an elongated tooth relative to other deciduous molars and has a raised anterior surface which seems to function in much the same way as the permanent first mandibular molar, as a honing surface for the maxillary canine. It would 394 RONALD J. BUTLER AND IRWIN S. BERNSTEIN Fig. 1 Maxillary dentition of a 16-year-old male shows advanced wear on all teeth, including the chipped and worn canines. Molar wear is differential but nonhelicoidal. seem that deciduous upper canines are honed in the same manner as the permanent upper canines. DISCUSSION Degrees of wear on the occlusal wear planes of Macaca nemestrina seem to be more pronounced in females than in males provided that comparisons are made of animals of equivalent age. In all cases, changes of degree in occlusal wear planes are differential, with PM1 normally showing greater wear than PM2 and M1 showing the greatest amount of wear in the molar series. In no case was differential wear found associated with helicoidal patterns. That is, there were no changes in the occlusal wear planes from buccal to lingual or vice versa on any of the cheek teeth as occurs in Homo and earlier hominids (see Butler, '72). In this population, the pattern with advancing age, regardless of sex, seems to be for the entire maxillary molar row to assume a sharply lingual wear plane, with the opposite true for the mandibular molars. Data for premolars are insufficient for generalizations. The data do support statements by various workers that large projecting canines impede lateral mandibular motions, pro- vided that these statements take into account the suggested strong relationship between degrees of canine wear associated with age and resultant changes on occlusal surfaces of the molar teeth. This sample further suggests that the degree of wear on the cheek teeth increases as size and robusticity of the canines decreases through mastication, honing, or fighting. A variable which could not be checked was the amount of wear advanced by molar teeth gritting, and canine grinding which may be part of a general anxiety syndrome for the species. Finally, this sample also strongly suggests that canine length and robusticity attributable to sexual dimorphism should be taken into account when considering either degrees of wear or patterns of wear on the cheek teeth. Explanations of differential but nonhelicoidal wear in our sample are not clear-cut, but examination of skeletal material of the same species offers several possibilities. One lies in the relatively pronounced height of molar cusps and the functional relationship with unworn canine size. In young adults, before wear has proceeded to any marked degree, lateral mandibular excursion is limited by con- 395 CANINE ROLE tact of the distal aspect of the mandibular canine with the mesial surface of the maxillary canine. As this occurs, unworn mandibular canines on the opposite side come into impeding contact with the maxillary lateral incisor. Further, lateral mandibular movement is limited by contact of the distal aspect of the maxillary canine with the mesio-buccal aspect of the mandibular PM1. Functionally related to these basic considerations is the fact that differences in dental arch width at different tooth positions causes considerable overlap of the maxillary teeth so that the lingual cusps of the mandibular teeth fit, in the unworn state, between the buccal and lingual cusps of the maxillary teeth. All unworn molar cusps are relatively high. It would seem that the initial emphasis of wear is on the lingual aspect of the maxillary molars and the buccal aspect of the mandibular teeth. By the time individuals of this species have reached the age where sharply differential occlusal wear planes are evident, it appears that so much of the original tooth structure has been lost that even increased lateral movement of the mandible will not result in helicoidal patterns (see Brace and Molnar, '67; Molnar, '68) for a review of experimental work in human tooth wear. CONCLUSIONS Based on this sample of Macaca nemestrina, we conclude that initial wear patterns are strongly influenced by the devel- oping canines. The fully erupted canines, of males especially, limit lateral mandibular movements to the extent that molar wear is minimal on mandibular lingual cusps and maxillary buccal cusps. In older individuals with heavily worn canines increased lateral movement of the mandible permits wear of these cusps, but the initial wear emphasis on the buccal aspect of the mandibular molars and the lingual aspect of the maxillary molars persists. Occlusal wear planes thus may undergo wear changes in terms of degree but there is no similarity to true helicoidality, including reversals of initial occlusal wear planes. LITERATURE CITED Butler, R. J. 1972 Age-related variability in occlusal wear planes. Am. J. Phys. Anthrop., 36: 381-390. Brace, C. L., and S. Molnar 1967 Experimental studies in human tooth wear: I. Am. J. Phys. Anthrop., 27: 213-221. Campbell, T. D. 1925 Dentition and Palate of the Australian Aboriginal. Hassell Press, Ade1aide. Jolly, C. J. 1970 The seed-eaters: A new model of hominid differentiation based on a baboon analogy. Man, 5: 5-26. Jones, H. G. 1947 The primary dentition of Homo sapiens and the search for primitive features. Am. J. Phys. Anthrop., 5: 251-282. Molnar, S. 1968 Experimental studies in human tooth wear. 11. Am. J . Phys. Anthrop., 28: 361-368. 1971 St. Hoyme, L. E., and R. T. Koritzer Significance of canine wear in pongid evolution. Am. J. Phys. Anthrop., 3 5 : 145-148. Wolpoff, M. H. 1971 Interstitial wear. Am. J. Phys. Anthrop., 34: 205-228.