Accuracy of Moose Age Determinations from Incisor Cementum Layers

Allen Press

Accuracy of Moose Age Determinations from Incisor Cementum LayersAuthor(s): William C. Gasaway, David B. Harkness and Robert A. RauschSource: The Journal of Wildlife Management, Vol. 42, No. 3 (Jul., 1978), pp. 558-563Published by: Wiley on behalf of the Wildlife SocietyStable URL: http://www.jstor.org/stable/3800818 .

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ACCURACY OF MOOSE AGE DETERMINATIONS FROM INCISOR CEMENTUM LAYERS WILLIAM C. GASAWAY, Alaska Department of Fish and Game, 1300 College Road, Fairbanks 99701 DAVID B. HARKNESS, Alaska Department of Fish and Game, 333 Raspberry Road, Anchorage 99502 ROBERT A. RAUSCH, Alaska Department of Fish and Game, Subport Building, Juneau 99801

Abstract: Incisors from 68 known-age moose (Alces alces) were sectioned and the ages of the animals estimated from cementum layers. Mean age determined in this manner overestimated true age by 0.5 yr with errors ranging between minus 1 and plus 3 yr. Cross sections were preferred to longitudinal sections. The major error resulted from difficulties in distinguishing discrete cementum layers. Other sources of error are discussed and the annual sequence of cementum deposition is described.

J. WILDL. MANAGE. 42(3):558-563

Age determination of ungulates from cementum layers in teeth has become an accepted technique during the past 2 de- cades. There are inherent errors with the technique, however, and when relating age to growth, reproduction, or population dynamics, the age estimate should be relatively accurate and the frequency and magnitude of errors known. The errors appear small for some species. Com- plete agreement of age estimates with known ages was found for 20 black-tailed deer (Odocoileus hemionus columbi- anus) (Low and Cowan 1963), 46 white- tailed deer (0. virginianus) (Lockard 1972), 16 mule deer (0. hemionus) (Erickson and Seliger 1969), and 18 elk (Cervis canadensis) (Keiss 1969). Never- theless, Sauer (1973) reported that 16 percent of the age estimates from cementum layers of 69 known-age teeth of white-tailed deer were in error. Similar- ly, Ranson (1966) did not find total agreement between observable cementum layers and known age in white-tailed deer.

Previously published reports on moose age determination using cementum layers (Sergeant and Pimlott 1959, Markgren 1969, Wolfe 1969) suggested a range of accuracy of +1 or 2 yr. None of these works were based on large numbers of known-age teeth. This report describes

the accuracy and precision of age estimates from the cementum layers of incisors from 68 known-age moose, compares results from longitudinal and cross sections of cementum, and describes the annual sequence of cementum deposition.

We are grateful to E. Kootuk and the members of the Alaska Department of Fish and Game who were responsible for securing and sectioning the known-age teeth and to W. Heimer and D. McKnight for valuable criticism of an early draft of the manuscript. This work was supported by Federal Aid in Wildlife Restoration Project W-17-9.

MATERIALS AND METHODS

Incisors (1,) were collected from 68 moose tagged as calves in the Matanuska and Tanana Valleys, Alaska and later re- covered from hunter kills, live recap- tures, train and auto collisions, and nat- ural mortality.

If both I, were available, then longitudinal and cross sections of the teeth were prepared. Longitudinal sections were prepared by grinding lateral sur- faces of the tooth to produce a thin, me- dial, dorsal-ventral section of the root. Alternate sides were ground until the section became translucent and cemen-

558 J. Wildl. Manage. 42(3):1978



ACCURACY OF CEMENTUM MOOSE AGES. Gasaway et al. 559

tum layers could be distinguished readily using a binocular microscope at 10 to 30x magnification. The dorsal cementum has the most distinctive layers and was the portion used for estimating age.

A water-bath grinder with a fine abra- sive wheel was preferred since it re- duced odor, prevented burning of the tooth, and was self-cleaning. Tooth sections were stored in a solution of 70 percent ethanol, 29 percent water, and 1 percent glycerine.

Serial cross sections of the 11 root were cut with a lapidary saw fitted with a fine tooth blade (modified from Gilling and Buonocore 1959). Oil was dripped over the tooth to prevent burning. Five 0.3 mm sections were prepared from the proximal half of the root and stored in the above solution. The time required for an experienced individual to cut cross sections is approximately the same as grinding longitudinal sections; however, the initial cost of equipment to cut cross sections is greater.

Cementum layers were counted and age was interpreted generally as described by Sergeant and Pimlott (1959). In this study age was reported in whole years for convenience of analysis, although the actual age of each moose ex- ceeded its reported age by the number of months it lived past its last birthday. An- nual cementum deposition consists of translucent (light) and opaque (dark) layers when viewed with transmitted light (Fig. 1). Opaque layers are usually deposited during mid to late winter and appear to be mostly complete by April or 2 months prior to the animal's birthday. On a moose's birthday the number of opaque layers theoretically equals its age in years. An age estimate to the nearest month can be made when the month the animal died is known. Most births occur during late May in Alaska; therefore, if

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Fig. 1. Cross section of an I, from a moose 4 yr 6 months old. Four opaque layers were identified. Readability ratings by 2 observers were 2 and 3.

an animal with 4 opaque cementum layers died in late April, it would be 3 yr 11 months old, or if it died in late Septem- ber, its age would be 4 yr and 4 months.

The age of each moose was estimated independently by 2 of the authors, each of whom was experienced with the technique. Both age estimates for each moose are included in the data in order to illus- trate the variability of age estimates between observers.

RESULTS AND DISCUSSION

Accuracy of Age Estimates

Using both longitudinal and cross sections the mean ages of moose were overestimated by an average of 0.5 yr (Table 1). Twenty-three estimates for yearling animals resulted in only 2 errors, but less than 56 percent of the sections provided correct age estimates of moose 2 through 11 yr of age (Table 1). Overestimation of age was the most frequent error; about 30 percent of moose over 1 yr old were estimated 1 yr class too great while the frequency of an error by 2 and 3 yr was con- siderably lower. About 10 percent of all

J. Wildl. Manage. 42(3):1978



560 AccURACY OF CEMENTUM MOOSE AGES* Gasaway et al.

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Fig. 2. Cross section of an I1 from a moose of unknown age. Opaque layers deposited in the first few years tend to be wider and less distinct than those deposited in later years.

age determinations underestimated the moose's age. Sauer (1973) reported that the most common errors for 69 known- age white-tailed deer were overestimates of age ranging up to 3 yr while underestimates were consistently 1 yr. Although mean accuracy of age estimates for moose was the same using longitudinal and cross sections, longitudinal sections provided a slightly greater frequency of underestimates and cross sections provided more frequent overestimates. This discrepancy probably resulted from bet- ter resolution of cementum layers in cross sections which increased the like- lihood of layer identification.

Fig. 3. Longitudinal section of an I, from a 4-yr 11-month- old moose which was given a readability rating of 0 (un- readable). Layers which are more distinctive cannot be produced by varying the thickness of the section from a tooth such as this.

The pattern of errors for age estimates was generally consistent for age classes older than yearlings. Age estimates of yearlings were relatively uniform since there was no chance of underestimating the age, and overestimates could only result from the deposition of additional opaque layers during the 1st yr of life.

Sources of Error

Determining the number of cementum layers in certain teeth is somewhat subjective. Ccnentum layers deposited between year 1 and 3 are often broader, have less contrast, and are less concentric than layers deposited in later years (Fig. 2). In some teeth all layers may be diffi- cult to identify (Fig. 3). The difficulty of defining cementum layers precisely is

Table 1. Differences between the known age and the age estimated from cementum lines in moose teeth.

Error in years from estimated age Known (N [% of total]) Mean

Type of age error Sample section (yr) -1 0 +1 +2 +3 (yr) sizea

Cross 1 0 6(100) 0 0 0 0.0 6 2-11 7 (10) 40 (56) 20 (28) 4 (6) 1 (1) +0.5 72

Longitudinal 1 0 15 (88) 2 (12) 0 0 +0.1 17 2-11 11(13) 42 (51) 24 (29) 2 (2) 1 (1) +0.5 80

a The results include 2 estimates of each tooth, made by the 2 observers.




ACCURACY OF CEMENTUM MOOSE AGES - Gasaway et al. 561

4

Fig. 4. Longitudinal section of an I, from a 4-yr 6-month- old moose which was given readability ratings of 2 and 3.

demonstrated by the frequency with which the 2 observers differed in the number of layers counted. Observers agreed on 57 and 64 percent of the longitudinal and cross sections, respective- ly. The range of differences was greater in longitudinal sections than in cross sections; 4 percent of estimates for longitudinal sections differed by 2 yr while the greatest difference was 1 yr for cross sections.

Variations in the number of layers counted in moose teeth depended partly on the clarity, contrast, and distinctive- ness of opaque and translucent layers. A subjective readability rating was given to each tooth section by observers. The rating ranged from 0 (not readable, Fig. 3) to 3 (easily read, Figs. 1, 4 and 5). Cross sections received a significantly greater

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Fig. 5. Cross section of an I, from a 4-yr 6-month-old moose which had 6 definable opaque cementum layers and a readability rating of 3.

(P < 0.01) readability index than longitudinal sections when compared by the Mann-Whitney U-test, but relatively few tooth sections were rated as easily read (Table 2). Most sections required some subjective interpretation by the observers (Fig. 6). The difficulty in identifying layers is a major source of error along with the frequent presence of readily definable layers numbering greater or less than the known age.

More than 1 opaque cementum layer per year is occasionally deposited, or less frequently, cementum with no detectable opaque layer is deposited. Figures 5 and

Table 2. A comparison of the readability of cross and longitudinal sections of moose teeth.

Readability indexa (N [% of total])

Type of Known Sample section age 0 1 2 3 x (SD) size

Cross 1 0 0 4 2 2.3 (0.5)*b 6 2-11 0 15 (21) 36 (50) 21(29) 2.1 (0.7)* 72

Longitudinal 1 1 (6) 12 (67) 5 (28) 0 1.2 (0.5) 18 2-11 2 (2) 31(37) 40 (49) 9 (10) 1.7 (0.7) 82

a Readability scores were: 0-not readable; 1-barely readable, low contrast; 2-readable, moderate contrast, but some difficulty in counting; 3--high contrast, lines distinct and easily counted.

b * Significant difference between cross and longitudinal sections (P < 0.01).




562 AccURACY OF CEMENTUM MOOSE AGES * Gasaway et al.

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Fig. 6. Longitudinal section of an I1 from a 4-yr 6-month- old moose which has 5 opaque cementum layers and a readability rating of 1 and 2.

6 show tooth sections of moose which have opaque layers numbering in excess of their age in years. Sauer (1973) reported that white-tailed deer occasionally produce multiple annuli during some years.

Potential errors in age estimates must be considered when biologists are cor- relating age with parameters such as growth, reproduction, age classes select- ed by predators, animal behavior, etc. Data presented indicate that an overestimation of age is the most frequent error. Consequently, the effect on moose growth curves, for example, would be to underestimate the rate of growth. In studies of population dynamics errors in age determination may result in the loss of cohort identity.

Description of Annual Cementum Deposition

Observations of cross sections with high resolution indicate that the first layer deposited was very thin and translucent (Fig. 5). It was frequently followed by a very fine, often intermittent, opaque layer and then by another translucent lay-

er (Fig. 5). These layers may be formed prior to I1 eruption which occurs at 7 to 8 months of age in Alaskan moose. The first wide, opaque layer, similar to those formed by adult moose, was probably deposited during mid to late winter when the moose was 7 to 10 months of age. Translucent cementum appeared to be deposited during summer, fall, and early winter in adult moose. Opaque cementum in adults and in calves was appar- ently formed during winter, but the pre- cise period was not identified because sufficient specimens were not collected from December through April. Occasion- ally dark speckling in the outermost cementum was detected in teeth collected during November and December indicating the onset of opaque cementum deposition. Completion of the opaque layer occurred by April based on observations that teeth collected from several moose of unknown age and a 10-month-old calf all had a thin translucent cementum layer between the outermost opaque layer and the edge of the cementum. The timing of cementum layer deposition by moose ap- parently followed the pattern described by Sauer (1973) for white-tailed deer. Sauer reported that opaque cementum deposition occurred during winter and was completed between February and June by all deer, indicating a high degree of variability in completion dates of opaque cementum layers. Therefore, for deer and possibly moose, accurate age determination of animals dying in late winter and spring will be the most diffi- cult since the formation of the opaque layer is not synchronous in all individu- als nor does it appear to occur during a short time interval. Fortunately, most hunter harvest occurs in the autumn, when relatively good accuracy in age determination can be achieved. Observa- tions of Sergeant and Pimlott (1959) on




ACCURACY OF CEMENTUM MOOSE AGES. Gasaway et al. 563

the timing of cementum deposition differ from those of the present study.

LITERATURE CITED

ERICKSON, J. A. AND W. G. SELIGER. 1969. Effi- cient sectioning of incisors for estimating ages of mule deer. J. Wildl. Manage. 33(2):384-388.

GILLING, B. AND M. BUONOCORE. 1959. An appa- ratus for the preparation of thin serial sections of undecalcified tissues. J. Dent. Res. 38(6):1156-1165.

KEIss, R. E. 1969. Comparison of eruption-wear patterns and cementum annuli as age criteria in elk. J. Wildl. Manage. 33(1):175-180.

LOCKARD, G. R. 1972. Further studies of dental annuli for aging while-tailed deer. J. Wildl. Manage. 36(1):46-55.

Low, W. A. AND I. McT. COWAN. 1963. Age de-

termination of deer by annular structure of dental cementum. J. Wildl. Manage. 27(3):466-471.

MARKGREN, G. 1969. Reproduction of moose in Sweden. Viltrevy 6(3):127-299.

RANSON, A. B. 1966. Determining age of white- tailed deer from layers in cementum of molars. J. Wildl. Manage. 30(1):197-199.

SAUER, P. 1973. Seasonal variation in physiology of white-tailed deer in relation to cementum annulus formation. Ph.D. Thesis. State Univer- sity of New York at Albany. 85pp.

SERGEANT, D. E. AND D. H. PIMLOTT. 1959. Age determination in moose from sectioned incisor teeth. J. Wildl. Manage. 23(3):315-321.

WOLFE, M. L. 1969. Age determination in moose from cemental layers of molar teeth. J. Wildl. Manage. 33(2):428-431.

Received 8 February 1977. Accepted 20 February 1978.




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Accuracy of Moose Age Determinations from Incisor Cementum Layers