J . Zool., Lond. (1991) 225, 293-307

Mountain reedbuck Redunca fuhorufula growth and age determination using dentition

P. M. NORTON A N D N. FAIRALL

Chief‘ Directorate Nature & Eni~ironmentul Consertwtion, P.O. Box 456, Kimberley, 8300 R S A

(Accepted 9 October 1990)

(With 1 plate and 6 figures in the text)

Lower jaws and measurements were obtained from 473 mountain reedbuck Redunca/uh~orufulu culled on two reserves in the north-eastern karoo of South Africa. All mandibles were allocated to subjective age classes that were based on tooth-eruption and tooth-wear. In a sample of 69 jaws. the molar and premolar cementum annuli were counted by examining bisected whole teeth under reflected light. One light line appeared to have been deposited per year, and a usable relationship between tooth-wear category and cementum line count was obtained. although indistinct and paired lines often confused the picture. Good correlations were also obtained between the number of cementum lines and the crown height of the first molar (r? =0.68) and the second molar (r’z0.77). Actual ages of immatures were estimated by comparing peaks in the distribution in the different age classes with the age since expected mean birth date in different culls. The age of transition to adult dentition (f 30 months) was determined from two females that were earmarked when young and shot two years later. Von Bertalanffy ( 1 957) growth curves were fitted to measurements of body mass, jaw length and horn length in males to aid field identification of immatures. Asymptotic body mass was reached at 30 months in females and 40 months in males, and asymptotic jaw length was reached at 30 months in both sexes. Horns in males only reached full length at about five years.

Contents

Introduction . . . . . . . . . . . . . . . . Methods . . . . . . . . . . . . . . . . . .

Age classes from tooth-wear and tooth-eruption Age determination using cementum lines . . . . Age determination of immatures.. . . . . . . Age determination from known-age material , . Growth curves . . . . . . . . . . . . . .

Results . . . . . . . . . . . . . . . . . . Age determination of immatures . . . . . . . . Age determination using cementum lines . . . . Growth curves . . . . . . . . . . . . . .

Discussion . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

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Page 293 294 294 297 291 298 298 298 298 298 302 304 306

Introduction

Most studies of population dynamics of large mammals are based on a classification of various sex and age classes, either based on visual observation in the field or examination of dead

0952-8369/91~010293+ 15 $03.00 293

1991 The Zoological Society of London

'94 P. M . NORTON A N D N . FAIRALL

specimens. In a population study of mountain reedbuck Reduticu f idzmgfidu in three nature reserves in the karoo biome of South Africa (Norton, 1989). both field data and cull samples were available. In order to analyse and model the dynamics of the population, a system of classification into age cohorts was required.

The only age classification available for mountain reedbuck was the estimation in the field of the age of immature animals, based on growth curves. and the arbitrary tooth-wear and tooth- eruption classes used for carcasses by Irby (1976, 1977). His growth curves were derived from measurements of two tame immature females, and very small cull samples. Management culls carried out during the present study provided the opportunity to construct more reliable growth curves based on larger samples, and to relate these to tooth-wear and tooth-eruption classes.

Classification of immature mountain reedbuck according to Irby's age classes, which are based on tooth-eruption, appears to give a fairly reliable estimate of age when related to growth data. However, the age determination of adults using tooth-wear gives only relative age, and no actual ages can be assigned to the animals. This severely limits the interpretation and modelling of population processes. I t was therefore necessary to develop a more accurate classification scheme based on actual ages.

One of the most widely-used methods for determining the absolute age of mammals is the evaluation of annual lines in the dentine or cementum of teeth (Morris, 1972; Spinage, 1973). These are layers of different densities that show up as light and dark hands when stained or examined under reflected or polarized light (Morris. 1972).

In most African ungulates that have been studied so far, the best results have been obtained with lines in the cementum, particularly in the pad under the first molar (Spinage, 1973, 1976). In temperate regions it has always been found that one layer is deposited in the cementum each year, but in the tropics of Africa there are often two layers per year (Simpson & Elder, 1969; Grimsdell, 1973; Spinage. 1976). I t is not clear how this relates to differences in seasonal availability of resources and there is still much debate on exactly what causes the formation of the layers.

In a field study. i t is usually only possible to estimate the age of the live animal at a distance, and this is achieved on the basis of allometric growth characteristics. Different body components exhibit different growth rates, which results in physical conformations in the younger age classes which differ from those of mature animals. The point at which the mature size is achieved is indicated when the asymptote in growth is reached; this is conveniently illustrated by fitting an asymptotic growth function.

Methods The study was carried o u t on the Rolfontein and Doornkloof provincial nature reserves in the Cape

Province, which are situated on the shores of the P.K. le Roux Dam in the Upper Orange River Basin (c. 30's 25 E). Rainfall is variable (mean 350 mm p.a.). and falls mostly in late summer or autumn, with a lesser peak in spring.

The study material was obtaincd from mountain reedbuck culled for managcment purposes on Rolfontein N .R . in 1981 (39 rams, 83 ewes) and 1984 (58 rams. 103 ewes), and on Doornk1oofN.R. in 1985 (58 rams, 132 ewes). as well as 81 unsexed animals from Doornkloof in 1982.

The approach followed for the detailed age classification of individuals was to use a rapid method in the ficld that was based on tooth-eruption and tooth-wear for the total culled samples (473 individuals). A sub- sample of these was then selected for more detailed analysis of annual cementum lines.

Age classes ,fiotti tooth-rcecir atid tooth-rruptiotl Lower jaws from the mountain reedbuck obtained during the culls. arid from carcasses found in the veld.

M O U N T A I N REEDBUCK D E N T I T l O N 29 5

TABLE I

R = replacing, M = molar, PA4 =premolar) ARe classes o/ nlounruin reedbuck h a d 011 Iooih-eruptiotl and rooih-wear ( N = neo~iaIe, J = jurrriile, Y = j ,rarlir~g,

Class

NM -t NMf

YMo Y YM+ 1 YMu

R

IA IB

I IA IIB

111

IVA IVB

V

VI VII

VIII

Criteria ~~~ ~

M I just visible above jawbone M I half erupted, but not up in tooth-row

M I fully erupted, M2 not visible above jawbone M , fully erupted, M? just visible above jawbone M I fully erupted, M2 half erupted

M2 fully erupted, M3 not visible above jawbone M? fully erupted, M3 half erupted M2 fully erupted, M3 fully erupted and up in tooth-row. PM4 still deciduous

M, fully erupted. deciduous PM4 in process of being replaced

Permanent PM4 fully erupted, but very lightly worn. not down to anterior inter-conid ridge PM4 worn down to anterior inter-conid ridge. PM4 posterior infundibulum present

PM4 worn flat and posterior infundibulum worn away, M I anterior infundibulum wide and deep M I anterior infundibulum worn, but still forms long narrow slit

M I anterior infundibulum worn away or pinpoint. posterior infundibulum clear cup form

M I posterior infundibulum long narrow slit M I posterior infundibulum worn away or pinpoint

Mz anterior infundibulum narrow slit or absent, posterior infundibulum clear cup form

M2 posterior infundibulum narrow slit M2 posterior infundibulum worn away or pinpoint. M3 anterior infundibulum clear cup form

M I & M2 infundibula all absent, M j anterior infundibulum absent

were assigned to age classes based on tooth-wear and tooth-eruption. In the culled samples the lower jaws were removed from the head and cleaned, first by scraping, and then by boiling in a weak detergent solution. During the whole process each mandible was marked with either a plastic cattle tag or else with an aluminium ‘Dymotape’ tag attached with wire.

Originally, the jaws were grouped into the tooth-wear and tooth-eruption classes used by Irby (1976. 1977). However, the numbers of animals in each class differed markedly, suggesting that some classes were too broad. It was considered likely that they would later be found to include animals differing in age by more than one year. Irby’s classes were therefore further subdivided (Table I) so that animals in the different classes could later be grouped according to the results of the cementum line analysis. In addition, the immature age classes were further subdivided so that the age of young individuals could be more accurately determined.

Following Irby’s (1976) method, assignment ofjaws to the various adult age classes was based on the wear of the infundibula of the molars (Plate I). The average wear on the teeth of the 2 jawbones was estimated, since there was often some difference between the 2 sides of a lower jaw. Where individual teeth were abnormally worn. so that there were major depressions in the wear surface, the wear was estimated as if the teeth had worn evenly.

Age classes of immature animals were grouped according to the eruption stage of the permanent molars.

296 P. M . N O R T O N A N D N . F A I R A L L

PLATE I . Photographs of the right mandibular tooth-row of mountain reedbuck in thc age classes described in Table I (lateral view for subadult age classes and dorsal view for adult age classes: light line denotes gap between PM, and MI) .

MOUNTAIN REEDBUCK DENTITION 297

The transition from immature to adult was marked by the replacement of the characteristic 3-cusped deciduous fourth premolar, which differs markedly in shape from the permanent premolar.

Age determination using cenzentuni lines

The numbers of cementum lines in tooth-sections from 69 mountain reedbuck in the study areas wcre counted to see whether they would indicate the absolute ages of the different tooth-wear classes. The method used was to examine bisected whole teeth under reflected light, following the technique described by Mitchell (1967) for red deer in Scotland. This method has the advantage that it is far less time-consuming than demineralization and line histological sectioning of individual teeth (Sinclair & Grinisdell, 1978).

Mandibles were cut across in front of the premolars and behind the molars. and ground down to the central plane of the molars, using a carborundum masonry-cutting disc on an electric drill or angle-grinder. The grinding was aligned so as to obtain the best possible section through the cementum pad of the first molar. The segments were boiled in a weak solution of washing soda (sodium carbonate) to remove all traces of fat. and then polished on fine carborundum water-paper. After drying they were examined under a dissecting microscope using reflected light.

The cementum pad of the first molar proved to be the best site for examining cementum lines, but the cementum on the roots of the first molar, as well as the cementum pads and roots of the other molars and the premolar, were useful if the lines in the cementum pad of the first molar were indistinct.

Cementum lines were counted as the number of evenly-spaced major light-bands in the cementum working inwards from the bone matrix to the dentine. Lines often merged or split, and closely paired lines occurred in some teeth. To aid estimation of the true number of lines it was assumed that lines were of even thickness, at least in the same part of the cementum pad, as was found by Mitchell (1967).

Because of the variability in interpretation of cementum lines, the lines in the 2 jawbones from each individual mountain reedbuck were counted independently, and a mean number of lines was calculated. The sections were selected randomly from the sample, and the lines counted without looking at the reference number, to avoid inadvertent bias due to mental cross-referencing with the results from the other section from that individual.

The lines were counted by the senior author (PMN) on 2 separate occasions 2 weeks apart. In addition, they were counted independently by a colleague who had experience of age determination using annual lines in thc dentine of seals, but no knowledge of the tooth-wear classes used for mountain reedbuck.

Because the clarity of cementum lines varied from individual to individual, reliability factors on a scale of 1-4 showing decreasing clarity were recorded for each count, to aid later interpretation of questionable results.

Cementum line counts were done on a sample of 69 jaws representing all age classes. For each of the older age classes (HA-VI), samples of 4-1 2 mandibles were counted. Initially. only small samples of 2-3 mandibles were analysed for the youngest age classes (Y-IB). However, the variation for classes IA and IB was greater than expected, and 7 additional jaws were therefore added later. The sample included material from the 1981 and 1984 culls a t Rolfontein, as well as the 1985 cull at Doornkloof.

The relationship between wear on the teeth and cementum lines was described by measuring the crown height of the first and second molars. The crown height was taken as the distance from the base of the cementum pad to a point in the midline between the anterior and posterior infundibula of the tooth, Measurements were made using vernier callipers, and were taken in a vertical plane. For the regression of crown height on number ofcementum lines the mean crown height of the 2 molars on each side of the jaw was used. This crown-height measurement does not strictly represent only wear, since cementum is also added to the cementum pad as the animal ages, but it was considered more practical for use in the field.

Age determinution of immutures

In the immature age classes the determination of age using cementum lines was generally too coarse for the

298 P. M . NORTON A N D N . FAIRALL

accuracy needed in the population study (Norton, 1989). Ages were therefore estimated by comparing peaks in the age class distribution in the different cull samples with the expected mean age. Mean age was estimated as the time between the dates on which those samples were taken and the expected mean birth dates. It was possible to obtain estimates for a range of age classes, since the culls were in different months.

Age determination f iom known-age material

No reliable known-age material could be located in South African museums or zoological gardens. To obtain known-age material, 12 juveniles from the previous birth season ( f 7 months old) were captured on Rolfontein N.R. in 1984 (3) and 1985 (12). The lengths of their lowerjaws were measured by feeling through the facial skin. and they were marked with conspicuous plastic ear tags.

Two of these marked individuals (both females) were found and shot from a helicopter 25 months later. Their jaws were preserved whole as reference material. and not sectioned for cementum line counts.

Gro)vrli curi’es

Growth curves were drawn using body mass. jaw length and horn length measurements from the culled mountain reedbuck. Body mass was measured by hanging the whole unprocessed carcass on a spring balance before evisceration.

Horn length of all males was measured in a straight line from the base of the soft but horny black skin at the base of the longest horn. to the tip. Lower jaw length was measured with vernier callipers as the maximum distance from the mandibular symphysis to the angular process of the right mandible.

The asymptote in growth was estimated using the von Bertalanffy growth equation (von Bertalanffy, 1957).

Results

Age determinaiion of inimatures

The most reliable information on ageing of young mountain reedbuck came from the two known-age females. When marked in June 1985 they were clearly juveniles from the early summer birth peak. and thus between six and eight months old. Their jaws were measured at 145 m m and 150 mm, and their body mass 18 kg and 20 kg. respectively. When shot in July 1987 the former was replacing its fourth premolar (ageclass R), and the latter had the full adult dentition, but with very little wear on the fourth premolar (i.e. age class IA). This identifies the important transition age when full adult dentition is reached, and shows that mountain reedbuck in age class R must be approximately two and a half years old.

The numbers of individuals in each immature age class showed clear peaks in each of the cull samples (Table 11). Juveniles of the year were unmistakeable in the culled sample, so their age in months could be calculated by assuming a birth peak in mid-November (see Norton, 1989). The culls each fell in a different month between March and August, and the peaks in the juvenile and yearling age classes show a clear progression through these months (Table 11). Thus. assuming that the month in which the birth peak occurs remains the same each year for each study area. the younger age classes could be assigned a n age in months.

Age determination using cementum lines

The determination of absolute ages of the sub-samples of mountain reedbuck using tooth

MOUNTAIN REEDBUCK DENTITION

TABLE I1 Numbers of mountain reedbuck in each immature age class from cull samples at Rolfontein and Doornklorg Nature Reserves, showing month of cull, months since estimated birth peak (months), estimatedposition qf the peuks in each sample ( A 1 , and the estihated age of the age class (in months) derived,from this information

Months since

299

birth Cull peak N M + NMf JMo JMf JM+ YMo YMf YMu R

Rolfontein 3 5 9 1 2 9 4 14 March 1981 A A A

June 1985 A A

July I982 A A

August 1984 A A

Doornkloof 7; 4 27 1 2 7 2 2 2 3

Doornkloof 8 1 2 3' 1 3 8 1 6

1 9 1 0 2 2 6 2 2 Rolfontein 8f

Estimated age (months) 3 4 7 9 12 15 20 24 28

cementum lines proved difficult, because the lines were seldom very distinct. Merging and splitting were common, and lines appeared to be paired in some teeth, although usually with a clear major line and a less distinct minor line.

The fact that only one major line is usually deposited per year is supported by the presence of only one or two lines in the teeth of individuals in the yearling age class, which have lived through two summer wet periods.

In spite of the difficulties, the counts showed a clear sequential increase in number of lines counted in progressively older tooth-wear categories (Fig. 1). The mean numbers of cementum lines in the three independent analyses showed close agreement, although there was considerable variance around the means. It should be noted that the results from additional sections of age classes I A and IB were not included in Fig. 1 (see Methods).

The largest standard deviations from the mean number of lines counted were obtained by the independent observer (Fig. 1 b), who was not familiar with mountain reedbuck teeth, and was not clearly briefed on the possible presence of paired lines.

Some of the variance may have been due to the indistinctness of some of the sections. To test the effect of this on the results, the counts noted as having a low reliability (reliability class 4) were excluded and the means recalculated. However, the means for each age class remained almost exactly the same, and the standard deviation decreased only marginally. In eight age classes the change in standard deviation with unreliable records excluded was less than 3%, in age classes IIB and IVB the standard deviation decreased by 1 1 % and 18%, respectively, and in age class VI the standard deviation increased by 7%. It was therefore decided to include all the counts made in the analysis.

The differences between counts in the two halves of the jaw from each individual mountain reedbuck were generally low. In 7 1 YO of the individuals the number of lines counted in the left and right jaws differed by no more than one line, and in 95% the difference was no more than two lines. The closest results were obtained during the second count by the senior author (PMN), and the greatest differences during the count done by the independent observer.

A further indicator of the relevance of the cementum lines for determining age was the

300 P. M . N O R T O N A N D N . F A I R A L L

A C

7 A - 1 s& * (sample size) 2 3 3 3 11 4 10 12 8 5 3

Y R IA IB IIA llB 111 IVA IVB V VI

Age class

0 1 I I 1 I 1 I I I I I

(b) 3’0 I

0.0 ‘ ’ I I 1 I 1 I 1 I L 1

Y R IA I6 IIA 118 111 IVA IVB V VI

Age class

F I G I, (a ) Mean and (b) standard deiiation ofnumbers ofcementum lines found in mountain reedbuck teeth ofdifferent age classes during three separate assessments. two by the senior author (PMN I (~ - + ~ -) and PMN2 (&))and one by an independent observer (. . * ’ .). Overall standard deviation (- ) .

regression of tooth wear, as measured by crown height. on the mean number of cementum lines (Fig. 2 ) . This showed that 68% of the variation in height of the first molar (r’=0.68) and 77% of the variation in height of the second molar (r’ = 0.77) could be explained by the relationship between crown height and the number of cementum lines. When the regression was done with the results of PMN’s more reliable second count the correlation was even better (r’ =0.74 and 0.83 for M I and M,, respectively).

For the assignment of mean ages to the different tooth-wear classes some interpretation of all the relevant information is required. Based on the assumption that most of the mountain reedbuck were born in early summer. the ages when culled would have been full years plus six months. The

MOUNTAIN REEDBUCK DENTITION 30 1

25 r I

20 h

E E - 15 E

5 10

E

m a, r

0

.-

5

I I I I I

0 2 4 6 8 10

Mean number of cementum lines

FIG. 2. Regression of mountain reedbuck tooth crown height of the first (MI, 0) and second (M2, +) molars on the mean number of cementum lines counted (mean of all three assessments).

three mandibles of age class R showed a mean of close to two cementum lines (Fig. 1) and, since it is known that individuals in this class are approximately 2.5 years old, the ages of the different age classes are taken as the mean number of cementum lines counted plus a half.

The mean number of cementum lines in age class IA in Fig. 1 suggests that the age should be closer to 3.5 years than 2.5. However, for two reasons the mean age of this age class was taken as 2.5 years. First, the known-age class IA female was clearly closer to 2.5 than to 3.5 years old. Secondly, Table I1 showed that mountain reedbuck with replacing premolars were common in March, but less frequent in the later culls, which suggests that replacement is mostly complete before June. If this is so, the mountain reedbuck with recently replaced premolars will be 2.5 years old. This was supported by three additional mandibles in this age class that were examined after the main counts. Two of these showed only two cementum lines, while one showed three.

Class IB jaws were clearly older than IA in both the initial and additional samples, and were taken as 3.5 years old. As the age increased the standard deviation increased, which meant that sampling variation became more important. The mean number of cementum lines for age class IIA was 4.6, from a larger sample (n = 1 1). This gives an estimate of 5.1 years if the half-year is added, which is just over half-way between 4.5 and 5.5 years. However, this class was clearly younger than IIB for which the estimate is also closest to 5.5 years. Histograms of population age distribution (Fig. 3) with IIA and IIB combined would have given an improbably high peak for age 5 - 5 years. The ages were therefore taken as 4.5 years for class IIA, 5.5 years for IIB, and 6.5 years for class 111.

It is clear from Fig. 1 that the cementum line counts gave no justification for separating age classes IVA and IVB by a whole year, even though their standard deviations were high and the population age distribution gave an improbably high peak for this age class (Fig. 3). Both classes were taken to be 7.5 years old.

Few mountain reedbuck from the oldest age groups (V-VIII) were obtained in the culls, but the number of cementum lines suggest that their ages were approximately 8.5 and 9.5 years for classes V and VIjVII, respectively. The small number of jaws in class VIII were assigned an age of 10.5 years.

302 P. M . N O R T O N A N D N. FAIRALL

(a) I

NM+NM/JMoJM+JMNMoYMNMu R IA I6 IIA llB Ill IVA IVB V VI VII VIII

Age class

0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5

Age (years)

F I G . 3. Population age distribution of all culled samples of mountain reedbuck pooled (n=473): (a) individual age classes (see Table I); (b) estimated age based on cementum lines.

Growth ciirres

The ages assigned to the different age classes outlined above make i t possible to fit growth curves to body measurements of culled animals (Figs 4, 5, 6).

The body mass of mountain reedbuck increases rapidly through the juvenile age classes and appears to reach the asymptotic adult body mass at about 30 months in females and 40 months in males (Fig. 4). The asymptotic mean adult mass is 31.4 kg for males (n = 93) and 29.5 kg for females (n = 229), with a maximum recorded mass of just over 38 kg for both sexes.

MOUNTAIN REEDBUCK DENTITION 303

0 ' I I I I I I

0 20 40 60 80 100 120 Age (months)

t I 1-

I I 1 I I I

0 20 40 60 80 100 120

Age (months)

FIG. 4. Growth in body mass of (a) male and (b) female mountain reedbuck from Rolfontein and Doornkloof Nature Reserves (n=473), showing means (D), standard deviations (+) and the fitted von Bertalanffy growth curves (-).

The horns of the male mountain reedbuck are no more than small protrusions at the age of nine months, and only start to become noticeable after about 15 months (Fig. 5). They then grow straight upwards until two years of age when they start to curve forwards. They develop the basal ridges and adult shape at the age of three years, but only attain the asymptotic length after five years.

The lower jaws grow gradually from birth as successive teeth are added to the tooth-row, and in both sexes reach the elongated adult form at approximately the same time that full body mass is attained and the last molar is fully erupted (Fig. 6). After this the mean jaw length is similar in all adult age groups.

304 P . M. N O R T O N A N D N. F A I R A L L

150

E 0 I

50

0 4 : :! I I 1 I I I

0 20 40 60 80 100 120

Age (months)

FIG. 5. Growth of horns in male mountain reedbuck from Rolfontein and Doornkloof Nature Reserves (n= 115), showing means (H). standard deviations (+ ) and the fitted von Bertalanffy growth curve (-).

I I I I I I

0 20 40 60 80 100 120

Age (months)

FIG. 6. Growth in lower jaw length (mandibular symphysis to the angular process of right mandible) of mountain reedbuck from Rolfontein and Doornkloof Nature Reserves (n=413), showing means (H), standard deviations (+) and the fitted von Bertalanffy growth curve (-).

Discussion

The age classification method based on tooth eruption and peaks in the cull samples was effective for assessing the age of immature mountain reedbuck. The comparison of peaks in the numbers in each age class with the time elapsed since the expected birth peak showed clear sequential spacing. The accuracy of these ages is based on the assumptions that the calculation of

M O U N T A I N R E E D B U C K D E N T I T I O N 305

the birth peak in November is accurate, and that this does not differ significantly from year to year (Norton, 1989). Allowance must, however, be made for some variability in the timing of eruption of the molars.

The known-age animals provided the important information that full adult dentition is achieved at the age of two and a half years, which agrees with the expected pattern from peaks in age distribution of the cull samples.

In contrast to these relatively accurate ages for immature mountain reedbuck, the tooth-wear categories for adults used by Irby (1976), and modified for the present study, are subjectively assigned classes that may be subject to substantial errors. In particular, the spacing of one year between subsequent age classes in the cull samples is based on the assumption that breeding peaks were consistent for at least 10 years. However, the cementum lines provide some justification for this classification.

The rate of wear in ungulate teeth is also likely to vary between different individuals, and between different areas and seasons, according to the nature of the diet (Spinage, 1973). For this reason, the ages assigned are only valid for the karoo study areas and similar areas nearby, and cannot necessarily be extrapolated to other areas where mountain reedbuck occur. Nevertheless, the major advantage of the tooth-wear classification is that large numbers ofjaws can be handled in a short time, and by staff who have had minimal training.

Counts of the number of cementum lines in the molar teeth involved a degree of subjectivity, since the clarity often obtained in ungulates from temperate regions was not found. However, usable relationships between tooth-wear categories, tooth-crown height and number of cementum lines were obtained. It should be stressed that the ages assigned to the tooth-wear categories merely identified the expected mean age of the age class as found during the cull and for use in the model; it was not possible to account for individual variation in tooth-wear.

The importance of practice and familiarity with counting cementum lines in this species is illustrated by the apparent increased accuracy in PMN’s second count. Both the standard deviation in cementum line counts in the different age classes and the difference in number of cementum lines between the two halves of the jaw from the same individual were substantially lower in most age classes than during the other counts. The counts by the independent observer were more variable, but the fact that similar mean values were obtained by both observers substantiates the viability of the method.

The counts of cementum lines showed a high degree of variability, both in the number of lines present and the accuracy with which they could be read. However, assignment of actual ages is essential for population analysis even if the ages are not very accurate. Cohorts with high juvenile survival in good years can be followed as they move up through the population, or can be identified in standing population distributions to represent past good years.

Various growth curves can be applied to growth data, but the von Bertalanffy curve has been most generally used for African ungulates (Hanks, 1972; Smuts, 1974; Howells & Hanks, 1975; Brooks, 1978; Penzhorn, 1978). Although there are various problems with von Bertalanffy’s equation (Hanks, 1972; Fairall, 1980), it was used in this study because it gave an acceptable fit and, particularly, the results could be compared to other studies of medium-sized African ungulates.

The growth curves presented provide justification for the subjective criteria used in the field (Norton, 1989). Relative size is the most consistently used field criterion of age and is generally used to distinguish between immature or juvenile and mature classes. Mass is an indication of size, and the growth in mass curve shows a consistent increase up to an age of two years, indicating that

306 P. M. N O R T O N A N D N . F A I R A L L

yearlings of both sexes should be distinguishable from mature mountain reedbuck. However, after 12 months the variability in mass makes accurate field classification doubtful, even though considerable growth still takes place.

The only other growth data in the literature for mountain reedbuck are the body mass growth curves for two tame females obtained by Irby (1976), and a few measurements of a tame male recorded by Rowe-Rowe (1973). One of Irby’s animals was larger than the present series at seven months of age, but generally the tame animals fall within the range of measurements of our study.

Comparing the present data with results for impala Aepyceros melampus (Howells & Hanks, 1975: Brooks, 1978) and springbok Antidorcas marsupialis (Penzhorn, 1978) shows that growth in mass follows a similar pattern, with males tending to mature more slowly in all the studies, while females mature shortly after first mating. Growth is affected by the sex hormones and it is likely that the observed pattern is caused in this way. While not directly comparable, growth rates are similar in all these studies. as would be expected in antelope of similar size.

Irby (1976,1977) used facial morphology, especially the length of the snout, as one of the criteria for assigning immatures to different age classes, and the same approach was employed in the field study of mountain reedbuck. Although the head matures earlier than many other body components, the growth pattern exhibited by jaw length, and the effect this would have on relative snout length, substantiates the use of this subjective criterion in the field.

Horn length increased up to an age of five years in the present series and, although this character is very variable, it is a valuable field criterion when taken together with the change in morphology that is not adequately reflected in the length measurement alone. Rowe-Rowe (1 973) provides a few measurements of horn growth. His values of 30 mm at 15 months and 80 mm at 19 months are slightly lower than the means, but well within the ranges, obtained in the present series, again indicating that the present ages based on time since the birth peak are close to the true ages.

The study was carried out under the auspices of the Chief Director of Nature & Environmental Conservation, Cape Provincial Administration, and this paper formed part of a PhD thesis submitted by P. M. Norton to the Zoology Department. University of Stellenbosch. Special thanks are due to Professor J. Nel for criticism and encouragement during the preparation of the thesis. C. Fabricius commented on the manuscript, and H. Oosthuizen assisted with the interpretation of tooth-sections. K . Coetzee, Q. Hahndiek and T. Farr provided hospitality and enthusiastic help with the collection of material from culled mountain reedbuck.

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