American Journal of Primatology 15:l-15 (1988)

RESEARCH ARTICLES

Age Assessment Using Cementum Annulus Counts and Tooth Wear in a Free-Ranging Population of Macaca mulaita

RICHARD F. KAY ' AND JOHN G.H. CANT? 'Department of Anatomy, Duke University Medical Center, Durham, North Carolina; 'Department ofAnatomy, School of Medicine, Unrversity of Puerto Rico, Sun Juan, Puerto Rico

Estimates were made of the tooth wear and the number of cementum annuli on lower first molars of Macaca mulatta of known age that had lived on Cay0 Santiago, h e r t o Rico. It is demonstrated that both these measure- ments are significantly correlated with age. Neither measurement by itself, however, is strongly enough correlated with age to provide a reliable guide to the true age of individuals older than about 14 years, although cementum annulus counts clearly provide a more reliable guide to age determination than does wear. A combination of tooth wear and annulus number is a somewhat better predictor of age, with a multiple regression explaining 79% of the overall variance in age. As has been reported previously in tropical ungulates, there is more than one cementum annulus per year deposited on the Mls of our sample. Comparison with rainfall data indicates that the number of dry intervals in the animal's life corresponds on a one- to-one basis with the number of annuli. It is hypothesized that such dry intervals cause nutritional stress, which in turn is reflected in periods of arrested or slowed growth in the tooth cementum. Also, more annuli are formed per year on the teeth of males than those of females. Stress engen- dered by intermale competition may play a role in this phenomenon.

Key words: age determination, cementum annuli, dental attrition

INTRODUCTION Estimation of chronological age of individuals has assumed increasing impor-

tance in primate biology for several reasons related to developments in diverse areas of research. Field studies of natural populations that seek to elucidate the contribu- tion to reproductive success of morphological, physiological, and behavioral traits require knowledge of age because the ways in which animals maximize fitness change with age.

Age estimation is particularly important for evolutionarily oriented field studies of primates because of the unusual life history strategies that characterize primates,

Received June 17,1987; revision accepted December 10,1987.

Address reprint requests to Richard F. Kay, Department of Anatomy, Duke University Medical Center, Durham, NC 27710.

0 El88 Alan R. Liss, Inc.

2 I Kay and Cant

emphasizing relatively slow maturation and long life spans relative to other mam- mals of comparable body size. Now that capture (by tranquilizing darts) and release of primates is becoming more common, it is desirable for us to develop accurate, simple methods of estimating age in the field. In addition, estimation of age of osteological specimens can be useful in laboratory studies of the onset and course of diseases such as osteoarthritis and osteoporosis, and also for estimating maximum life span of individuals collected from natural populations.

Three methods of age determination have been used for primates. First, it is possible in field studies to follow an individual over its lifetime, starting when it is a juvenile (when age can be estimated with reasonable accuracy if there have been developmental studies of captive conspecifics). Unfortunately this requires research of much longer term than is normally feasible. Second, estimation of the ages of individuals that are not yet fully adult can be accomplished by assessing the degree of dental development and eruption in captives and applying resulting criteria to wild populations. But in addition to the fact that captives may develop faster than wild individuals, this method does not help determine the ages of adults.

The third method, measurement of tooth wear (attrition of the occlusal surfaces), has received a great deal of attention in attempts to estimate chronological age of several species of mammals. We should expect variation in average rates of attrition between populations because of variation in diet. Within a population it is necessary to calibrate degree of wear with actual age.

In his study of Papw cynocephalus, Bramblett [1969] used methods of Miles [1963], which involved “projecting forward” degree of wear from individuals of known age (determined by tooth eruption) to older animals. Froehlich et al. [1981] attempted to ascertain rates of tooth wear from a few howling monkeys (Alouatta palliata) that had been captured twice. In neither case was there adequate indepen- dent confirmation of the relationships between age and wear. Any future method of using dental attrition to estimate chronological age needs to determine more pre- cisely the variation in the relationship between age and wear (i.e., the correlation coefficient).

In view of the foregoing difficulties, it is not surprising that investigators have sought some better method that is both simple and not plagued by environmental variation, as is dental wear. Such an alternative approach is to count the annuli (rings) that develop over time in cementum as it is deposited on tooth roots. (See Kay et al. [1984] for a brief description of the process of cementum deposition.) Incremental cementum annuli have been used successfuly for age estimation in many temperate mammals, where the general pattern is for one annulus to appear every year of the animal’s life starting with initiation of cementum deposition at about the time of tooth emergence. Far fewer tropical mammals have been examined for cementum annuli, and there is some evidence, though far from certain, that two annuli may be deposited per year as a consequence of bimodal rainfall and vegeta- tion patterns [Grimsdell, 1973; Sinclair, 1974; Spinage, 19761.

Very little work has been done on cementum annuli of nonhuman primates. Three studies [Stott et al., 1980; Wada et al, 1978; Yoneda, 19821 were inconclusive about the value of cementum for age determination for either or both of these reasons: the animals used were of unknown age, or they lived in captivity where environmental influences on cementum deposition are likely to be different from those in the wild. Kay et al. 119841 analyzed the patterns of annuli on the teeth of eight individual rhesus macaques (of known age) that had lived in relatively natural conditions in colonies maintained in Puerto Rico, and their results suggested the potential of this method of age determination and prompted the present, more thorough study.

The research reported here used teeth from Macaca mulatta that had lived on Cay0 Santiago, Puerto Rico. Although provisioned with monkey chow, the animals

Macaque Age Assessment by Dental Criteria I 3

were subject to considerable seasonal variation in weather and in the availability of natural plant foods that constitute part of their diet. A distinct advantage of using this material is that all individuals in the sample lived in the same basically natural tropical habitat while developing cementum annuli, and all had the opportunity to consume similar diets while using (and wearing) their teeth. The basic approach was to determine the correlations of age with wear and with cementum annuli. This information then permitted assessment of the reliability with which wear, cemen- tum annuli, and the combination of wear and annuli could be used to estimate the age of individuals. Two hypotheses were addressed regarding the formation of annuli. These hypotheses had been raised in the literature: does one annulus form per year because of a biological clock, or do annuli form in response to slowed growth (in turn caused by nutritional stress)?

MATERIALS AND METHODS Mandibular first molars of rhesus macaques (Mmucu rnututtu) were obtained

from the Caribbean Primate Research Center (CPRC) skeletal collection. The sample is composed of 65 animals, all of known age, that lived on the island of Cay0 Santiago. Table I presents information on age, sex, and analytic variables for the individuals of the sample, and Figure l a and b are histograms of age according to sex.

The lower first molar was chosen for study because of previous reports that cementum annuli are readily observed and counted on this tooth [Kay et al., 1984; Wada et al., 19781. Moreover, the chronological age of eruption and the first deposi- tion of cementum on the lower first molar roots is much less variable than that on the other cheek teeth, a desirable characteristic because it is not known precisely when cementum began to form in any individual.

The monkeys of the Cay0 Santiago colony are free-ranging. They are provisioned with commercial monkey chow but also have access to naturally occurring plant and animal foods. Although the proportion of their diet that comes from natural sources is unknown, there is marked seasonal variation in the time spent feeding on these foods [Lauer, 19761. This variation appears to be significantly influenced by patterns of rainfall, as will be demonstrated below. Records of monthly rainfall at Cay0 Santiago were used for parts of the analysis. For some time intervals when such records were unavailable, we used weather data from San Juan, PR. Comparison of rainfall data for the same periods from these locations indicates that the two are highly correlated, and the timing and duration of dry periods were similar. There- fore, we have used the San Juan data when Cay0 Santiago information was unavailable.

Molars were removed from their alveoli by cutting away the surrounding bone using a circular grinding disk. Care was taken to avoid damaging the outer surface of the cementum layer investing the roots. High-resolution epoxy casts were made of all teeth used for sectioning following the protocol described in Covert and Kay [1981]. Casts were substituted in the mandibles for the orginal teeth. In this way future mensurational studies or analysis of microwear are facilitated. (For all but six specimens used to compare antimeres, only one lower first molar was removed.)

Two sorts of information were gathered from the teeth under study: estimates of tooth wear and counts of cementum annuli. For the quantification of tooth wear, we made a scale drawing of the outline of the tooth crown viewed in the occlusal plane as well as of the outlines of all areas of exposed dentin. The drawings were made at x 12 using a camera lucida attached to a Wild M5 binocular microscope. The area of the tooth crown in the occlusal plane as well as the summed areas of the exposed dentin were measured with a polar planimeter. A wear index was calculated as 100 times the ratio of dentin area to total tooth area.

For the analysis of cementum annuli, all teeth were assigned a code number to

4 / Kay and Cant

TABLE I. Basic Data on the Individuals in the Sample

CPRC Age dry No. Wear index Coll. No. bears) periods annuli

No.

Males 645 562 509 595 490 625 526 609 576 496 438 528 599 591 439 551 635 523 512 566 437 499 535 636 633 500 603 529 524 539 594 498

Females 604 486 622 507 466 513 559 494 634 611 572 632 639 577 627 579 590 444

1.5 2.8 3.3 3.3 4.0 4.3 5.4 5.8 6.0 6.4 6.6 6.6 6.6 6.7 7.6 8.1 8.6 8.8 9.6 9.6 9.7

10.5 11.3 11.3 11.4 12.5 12.7 13.4 14.5 14.7 15.1 20.8

1.9 2.1 3.2 3.5 3.8 3.9 4.8 5.2 5.3 5.9 6.2 6.3 6.4 6.9 7.2 8.0 8.9 9.8

0 3 3 3 5 4 7 6 8

10 10 8 8 8

14 11 13 13 18 15 17 20 21 18 18 23 23 24 25 26 28 37

1 1 1 3 5 4 6 7 7 6 9 8 8

10 9

12 13 17

0.5 0.5 3.0 3.0 4.0 3.0 8.5 6.0 8.0 9.0

11.0 7.0

15.5 6.0 8.5

11.0 12.0 10.5 17.5 20.5 8.5 9.0

26.0 26.5 20.5 29.5 17.5 21.0 49.5 34.5 25.0 51.0

1.0 1.0 4.0 3.0 5.5

11.0 8.0 7.0 7.0 9.5 6.0 9.5

10.0 11.0 11.5 10.5 16.5 22.0

0.0 0.8 2.6 1.5 2.5 0.5 3.2 5.7 5.7 4.7 4.8

11.6 9.8

15.6 4.0 7.0

38.9 7.0 2.3

23.9 14.1 13.3 7.7

16.0 18.1 6.0

23.3 24.9 10.7 11.4 22.2 16.0

0.3 0.1 1.1 0.6 5.1

14.6 3.2

20.8 7.8

15.2 8.6 5.4 4.9 6.3 4.0 7.0

10.3 11.0

Macaque Age Assessment by Dental Criteria / 5

(continued from preceding page) 580 10.0 16 20.5 9.9

495 11.0 21 11.5 10.8 588 11.0 18 25.5 8.3 626 11.3 17 28.5 12.2 589 11.9 23 17.5 17.1 641 11.9 19 13.5 12.0 550 12.6 23 18.5 40.6 502 14.9 24 24.0 31.5 497 15.3 27 15.5 9.4 450 16.6 27 21.5 10.5 570 17.1 29 46.5 13.2 441 17.4 30 17.0 11.5 493 19.8 35 34.5 26.9 435 23.7 39 18.0 66.0

436 10.5 19 12.0 14.2

12 141

Age in Years

12 141

Age in Years Fig. 1. Age distributions of the sample according to sex.

6 I Kay and Cant

TABLE 11. Ages of Animals in the Sample

Sample N age deviation Range Mean Standard

All animals 65 9.1 4.9 1.5-23.7 Males 32 8.7 4.3 1.5-20.8 Females 33 9.5 5.4 1.9-23.7

TABLE 111. Statistical Analysis of Wear on Lower First Molar as a Function of Chronological Age*

Sample

I. Raw data A. Wear as dependent variable

All animals Males Females

All animals Males Females

B. Age as dependent variable

11. Log-transformed data (wear) A. Wear as dependent variable

All animals Males Females

All animals Males Females

B. Age as dependent variable

Correlation N coefficient Slope Intercept

65 0.64 1.47 (.22) -1.80

33 0.69 1.64 (.31) -2.9 32 0.56 1.17 (.32) 0.32

65 0.64 1.64 (.04) 5.86 32 0.56 0.27 (.07) 5.93 33 0.69 0.29 (.06) 5.87

64 0.67 0.17 LO21 0.46 31 0.66 0.16 (.04) 0.51 33 0.68 0.17 (.03) 0.42

64 0.67 2.73 (.38) 3.82 31 0.66 2.65 (56) 3.72 33 0.68 2.77 (.54) 3.77

*Correlation coefficients, slopes (with standard errors), and intercepts for equations predicting age from wear and wear from age for both raw and log-transformed data (using natural logs of wear).

avoid the possibility that counts of annuli might be influenced by prior expectations. The teeth were decalcified, dehydrated, and embedded in paraffin following the protocol of Bhaskar [1976]. Sections 8-12 pm thick were made with a steel knife on a rotary microtome. The sections were cut parallel to the occlusal plan at seven different levels between the root apex (level 1) and the cementum-enamel junction (level 7). Representative sections at these levels were mounted and stained with hematoxylin and eosin (H&E) or by the Mallory trichrome method [Humason, 19791. The two strains gave equivalent results, and we used primarily H&E because it involved fewer steps and was faster. An illustration of eosin and hematoxylin- stained annulus bands in Macaca is shown in Kay et al. [1984] Fig. 1).

Cementum annuli were counted at level 1, close to the cementum-enamel junc- tion, because 1) cementum forms here at the earliest stage of root formation, 2) there is relatively little cellular cementum (in which annuli are frequently disrupted), and 3) annuli are most clearly demarcated in acellular cementum. Slides were examined under phase contrast with an orange-red filter at magnifications between x 100 and

Macaque Age Assessment by Dental Criteria 1 7

5.

P i

0. a . 0

0

0

0

0

0 . 0 0

0 0

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Fig. 2. Relationship between tooth wear and age. Solid circles are males, open circles are females. Wear index = 1.47 (age) - 1.8. See Table III.

45 501 0

0 /

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5 20. a

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10.

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0 2 4 6 8 10 12 14 16 18 20 22 24 26

Age in Years

Fig. 3. Relationship between number of cementum annuli and age. Symbols as in Figure 2. Number of annuli = 1.84 (age) -2.01. See Table IV.

8 I Kay and Cant

~150. The filter brings out the greatest discrimination between light and dark bands in H&E-stained sections. Each investigator made independent counts of the number of annuli. (An annulus consists of a pair of lines, one light and one dark.)

Prior investigators have exhibited considerable disagreement as to whether all bands on a section should be counted. The literature on annulus counts abounds with references to the existence of so-called “false annuli” and “double annuli.” The authors were unable to develop objective criteria for identifying these classes of annuli and suspect that such categories may simply be post-hoc classes designed to improve the agreement between the number of annuli and the chronological age of the animal. It was decided that the most objective approach to counting annuli is to locate the part of the section where the most lines are visible and to count the maximum number of lines even when some lines appear to merge with others at some points. Using this approach, the authors reached very close agreement on the number of annuli. We disagreed roughly 20% of the time, but the total number of annuli counted rarely differed by more than one annulus in either direction or approximately 5% variation. The number of annuli listed in Table I represents the mean of the counts made independently by each author.

RESULTS The specimens used in all analyses are listed by individual animal in Table I.

The sample included 32 males and 33 females, ranging in age from 1.5 to 23.7 years. The mean ages of males and females were similar (Table ID. Not surprisingly, fewer specimens of advanced age were available than we might have desired (Fig. 1).

Tooth Wear The wear index (100 times the ratio of exposed dentin to total occlusal area) for

each specimen is given in Table I, and the values are plotted against age in Figure 2. The correlation of wear with age was low, r = .64 for both sexes combined, so prediction of age from wear was necessarily poor (Table III). Thinking that wear might not be linear with age, we performed the same analyses using log-transformed wear data, but this did not improve the correlation. There were no significant differences (at the 95% confidence level) between the sexes for any of the analyses of wear and age.

Number of Cementum Annuli Annulus number and age. Age is the first factor likely to influence the deposi-

tion of annuli. The number of annuli for each specimen was derived by calculating the mean of the independent counts of the two authors and is indicated in Table I. Figure 3 depicts the relationship of the number of annuli with age, and Table IV provides relevant statistics. There were about 1.6 annular lines per year, and the fact that annuli accumulated at a rate considerably greater than one per year was shown by regressing the number of annuli on age, which yielded a slope of 1.84, significantly greater than 1 ( P < .05). The overall correlation (r = .80) was greater than that for wear with age ( P c .029, one-tailed test of significance; Sokal & Rohlf [1981, p. 58911, but prediction was still not very good; 64% of the variance in the dependent variable was explained.

To assess the consistency of annulus deposition on different teeth of the same individual, the number of annuli on left and right lower first molars of six individ- uals was determined. All other analyses used the right molar only of these partic- ular animals. This comparison showed that antimeres could differ substantially (Table V), and the difference between low and high counts for an animal ranged from 3% to 40% (specimen 450) of the higher value.

Macaque Age Assessment by Dental Criteria I 9

TABLE IV. Statistical Analysis of Cementum Annulus Counts and Age*

Sample N coefficient Slope Intercept

Annuli as dependent variable All animals 65 0.80 1.84 (.18) -2.01 Males 32 0.90 2.67 (.24) -8.19 Females 33 0.76 1.38 (.21) 1.51

All animals 65 0.80 0.35 (.03) 4.03 Males 32 0.90 0.30 (.03) 4.16 Females 33 0.76 0.42 (.07) 3.47

Correlation

Age as dependent variable

*Correlation coefficients, slopes (with standard errors), and intercepts for equations predicting No. of annuli from age and age from No. of annuli.

Annulus number and environmental factors. A second class of factors that might affect the accumulation of annuli are environmental and behavioral varia- tions potentially capable of influencing growth processes directly or indirectly. Sufficiently detailed data on the life history or health status of the individuals under study were not available, so many kinds of analysis were precluded. There were, however, two possible influences on growth that could be assessed the sex of the individual and seasonal variation in rainfall. For the former, a number of growth stresses might affect males and females differently because of sex differ- ences in behavior and physiology associated with diverse aspects of reproduction. In the latter case, seasonal variation in rainfall could ultimately affect the nutri- tional status of the animals by causing seasonal variation in the availability of some foods, which in turn could produce seasonal variation in competition for food.

Sex and the number of cementum annuli. When annulus number was regressed against age for the sexes separately, males exhibited a significantly higher slope than did females (Table N; P < .05). This suggests that annuli accumulate more rapidly in males than females. Also, the correlation between age and annulus number was significantly tighter in males than in females (female r = .76 and male r = .90, P < .034).

Rainfall and the number of cementum annuli. There is strong circumstantial evidence that rainfall affects the availability and use of natural foods on Cay0 Santiago. Figure 4 summarizes data from Lauer [1976] on the number of bouts spent feeding on vegetable foods by a group of Cay0 Santiago macaques over a 1- year period. The rainfall for those months is also indicated. In the peak of the dry season, January through April, there was very little consumption of naturally occurring foods. Presumably, during this period there was increased reliance on, and possibly competition for, provisioned food. By contrast, in the wet season, July through November, the animals spent much more time feeding on natural vegeta- tion. In the seasonal tropics, both new leaves, favored by primates over mature leaves, and fruit are produced by plants in greater abundance at the beginning of the wet season than during the dry season [e.g., Milton, 19801. Thus, seasonal variation in rainfall produces seasonality in the nature and abundance of available foods, presumably altering the nutritional status of the animals, although exact processes and effects are unknown. It is interesting, therefore, to examine the relationship between rainfall and the number of cementum annuli.

To correlate number of annuli with variation in rainfall, a criterion was first developed for what constitutes a “dry period,” and the number of periods during

10 / Kay and Cant

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Fig. 4. Dry periods and plant consumption. The abscissa depicts months of the year; the ordinate shows the number of dry months in the interval 1950-1983 (open triangles). The same ordinate scale gives monthly plant food consumption as a percentage of annual plant food consumption (solid squares).

TABLE V. Comparison of the Numbers of Annuli on the Right and Left MIS of an Individual

Animal Age Right/ No. of No. Sex hears) left annuli

632 F 6.3 R 9.5 L 9.0

580 F 10.0 R 20.5 L 15.5

603 M 12.7 R 17.5 L 15.5

497 F 15.3 R 15.5 L 15.0

450 F 16.6 R 21.5 L 13.0

570 F 17.1 R 46.5 L 33.0

the years when cementum was being formed were counted for each animal. An arbitrary definition of “dry period” was devised prior to any analysis of annuli number and rainfall. The basic data were monthly precipitation records for Cay0 Santiago and San Juan, Puerto Rico (sources were records from Cay0 Santiago and US. Weather Bureau Reports). From 1950 to 1983, encompassing the lifespans of all animals in the sample, rainfall averaged 1501 mm per year or 125.1 mm per

Macaque Age Assessment by Dental Criteria I 11

TABLE VI. Statistical Analysis of Cementum Annulus Counts and Dry Periodst

Sample N coefficient Slope Intercept

Annuli as dependent variable All animals 65 0.79 0.93 1.09) 1.38

Females 33 0.75 0.71 (.11) 3.89

All animals 65 0.79 0.68 (.07) 4.48 Males 32 0.87 0.61 (.06) 4.80 Females 33 0.75 0.79 (.13) 3.49

tCorrelation coefficients, slopes (with standard errors), and intercepts for predicting No. of annuli from No. of dry periods and vice versa. See text for calculation of the No. of dry periods in each animal‘s life.

Correlation

Males 32 0.87 1.26 1.13) -2.46

Dry periods as dependent variable

TABLE VII. Stepwise Multiple Regression Analysis for “Prediction” of Age From Number of Annuli and Tooth Weart

Correlation Coefficient of Variable coefficient determination

Step No. entered (r or R) (r20r R2)

1 No. of annuli 0.797 0.635 2 Toothwear 0.887 0.786 ?Regression equation: age = 2.85 + 0.282 (No. annuli) + 0.182 (wear).

month. A dry month was defined as a month in which less than one-half the average monthly rainfall occurred, and a dry period consisted of 1 or more consecutive dry months.

By this criterion, the longest dry period experienced by any animal in the study was 5 months (February through May, 1974). Dry periods of 3 months duration were common. Although there was typically one dry period per year, there were none in some years (e.g., 1956) and up to four in others. Overall, from 1950 through 1983, there were 58 dry periods, a mean of 1.76 per year, averaging 2 months’ duration. Dry periods tended to cluster between January and April, but every month of the year was part of a dry period at least once during the entire study period.

For the following calculations we counted for each animal the number of dry periods to which it was exposed from 1.5 years of age to its death. The age of 1.5 years was chosen because it was assumed that cementum deposition coincides with the initiation of root formation on the lower first molar a t about this age. Figure 5 illustrates the relationship between the number of annuli and the number of dry periods, and Table VI provides statistics. About 0.9 annuli were formed for each dry period during that portion of the animal’s life when cementum was being deposited. The slope (0.93) of the regression with number of dry periods as the independent variable and number of annuli as the dependent variable was not significantly different from 1.0 at the 95% confidence level. Separating the sexes, the slope for males was greater than that of females (P < . 05). The strength of the correlation between annuli and dry periods did not differ from that between annuli and age (compare Tables IV and VI for the sexes separately and combined). The strength of the correlation of annuli with dry periods did not differ between the sexes (male r = .87, female r = .75, P < .08).

12 I Kay and Cant

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Number of Dry Periods Fig. 5. Relationship between number of annuli and dry periods. Symbols as in Figure 2. No. of annuli = 0.93 (No. of dry periods) + 1.38. See Table VI.

Multiple Regression of Age on Annuli and Wear Although the correlation between tooth wear index and number of annuli was

low (r = .35), we wondered whether using both tooth wear and annulus number would “predict” age appreciably better than either variable alone. Table VII gives the results of multiple regression with age as the dependent variable, entering the number of annuli in the first step and tooth wear in the second step. The correlation coefficient of the first step (r = .797) and coefficient of determination (3 = .635) are familiar from the analysis reported in Table IV. Addition of tooth wear to the equation produced a multiple correlation coefficient (R) of .887 and a coefficient of multiple determination (R2) equal to .786 (proportion of explained variance). The addition of wear to annulus number did, in fact, yield a signifkantly better correla- tion than did annulus number alone @‘-test of difference between r2 and R2, P < <.001).

Figure 6 depicts the distribution of residuals from the multiple regression of age on annuli and wear. The residual for an animal was the difference in years between its actual age and the age predicted by the regression equation. For more than one- half of the entire sample, the use of annuli and wear together provided an estimate of within 1 year of actual age. The three lower histograms of Figure 6 show that accuracy of estimation declines with increasing chronological age.

Fig. 6. Distribution of residuals (absolute values) from multiple regression of age on annuli and wear. The residual for an individual animal is the difference in years between actual age and age predicted by the regression equation. See Table W.

Macaque Age Assessment by Dental Criteria I 13

0 to 4.9 Years

15 Years or Older

Age Residual (years)

14 I Kay and Cant

DISCUSSION The value of tooth wear and cementum annulus counts for predicting age was

considered for a sample of Macaca mulatta from Cay0 Santiago. These animals were free-ranging and were of known sex and chronological age. The area of exposed dentin on MI corrected for tooth size was used as a measure of tooth wear. Expressed in this way, wear was shown to explain only about 40% of the variance in chronolog- ical age in our sample. This is a significant correlation but not particularly useful for predicting the age of an individual or the age structure of the population. More needs to be done with the Cay0 Santiago sample of known-age animals to determine whether predictability of age improves when more than one tooth is considered. Based on the current evidence, age estimation of wild-caught primates on the basis of tooth wear alone, e.g., studies by Bramblett [1969] and Froehlich et al. [1981] should be viewed with caution.

The number of cementum annuli on the lower first molar was also examined as a predictor of age. A higher correlation with age was found, but only 64% of the variance in age was explained by annulus counts. Thus, annulus counts were of limited value for estimating age of individuals. Moreover, it was determined that more than one annulus formed per year in the molar cementum of the Cay0 Santiago macaques. This finding agrees with earlier reports of annulus counts in tropical African ungulates in which up to two annuli per year prevail [Sinclair, 1974; Robinette & Archer, 1971; Grimsdell, 1973; Spinage, 19761. It contrasts with one annulus per year in temperate mammals (see Morris [1978] for a review) and in Japanese macaques living in temperate regions [Wada et al., 19781. These results indicate that environment plays a key role in the production of cementum annuli and suggest that considerably more knowledge about environmental stress would be needed before annulus counts could be used reliably for age estimation in tropical primates.

Taken together, tooth wear and number of annuli predict age somewhat more accurately than does either measurement individually. Nearly 79% of the variance in age in this sample was explained by a multiple regression including both factors. Examination of residuals showed that age could be confdently predicted to within 1 year for most animals less than 10 years old; however, the accuracy of prediction decreased markedly from that point onward. In the case of animals older than 15 years, the accuracy in the sample declined to an average of plus or minus about 5 years.

The finding of more than one annulus per year in our sample indicates that the formation of annuli is not rigidly dependent on a biological clock, and that other factors must be involved. With the data available two possible factors-sex and rainfall-could be investigated.

Some workers have suggested that “extra” lines (i.e., more than one per year) may be present in some animals because of nutritional stress caused by intermale competition for access to mates [Saw, 19731. If so, one should expect to find more cementum lines in males than in females, although this effect could be counteracted if the stress of pregnancy causes the formation of additional annuli in females that have had one or more pregnancies. In fact, the finding that males accumulate significantly more annuli with increased age lends support to the intermale compe- tition hypothesis.

The analysis of rainfall patterns in relation to numbers of cementum annuli was particularly revealing. Seasonal differences have been observed in feeding patterns of the macaques at Cay0 Santiago that seem to be triggered by changes in rainfall [Lauer, 19761. Given this situation, it would not be surprising if some periods of the year were more stressful with respect to nutrition. To investigate this, the number of dry periods in each animal’s life span was examined in relation to the number of annuli, and roughly a one-to-one correspondence was found between these

Macaque Age Assessment by Dental Criteria I 15

variables. In our view, this supports the hypothesis that dietary stress may be responsible for introducing incremental lines in cementum.

CONCLUSIONS 1. Both tooth wear and number of cementum annuli are sigmfkantly correlated

with chronological age in Cay0 Santiago M. muluttu, but these measures, either separately or in combination, cannot predict age reliably. This is especially true for animals over 14 years of age.

2. Regardless of whatever biological clock may exist, formation of cementum annuli in Cay0 Santiago macaques is probably affected by variation in nutritional stress. Support was found for two possible sources of stress: intermale competition and, more convincingly, dry intervals with attendant decline in nutritional plane.

ACKNOWLEDGMENTS We thank Dr. Matt Kessler, Director of the Caribbean Primate Research Center,

and Drs. Dean Falk and Jean Turnquist, curators of the CPRC skeletal collection, for material support, assistance, and information. The CPRC collection is supported by grants from the U.S. National Public Health Service to the University of Puerto Rico, Medical Sciences Campus. This research was supported by a grant from the Duke Medical Center Research Council.

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