J. Zool., Lond. (1983) 201,247-257

Age determination and growth in the hyrax Procuviu cupensis (Mammalia: Procaviidae)

D A N ~ L E STEYN A N D J . HANKS Institute of Natural Resources, University ofNata1, PO Box 375, Pietermaritzburg, 3200

Republic of South Afvica

(Accepted 12 April 1983)

(With 5 figures in the text)

The use of eye lens weight, tooth eruption and tooth attrition has been investigated as a method for age determination in the hyrax. Illustrations are presented on the stages of eruption to reduce subjectivity of eruption criteria and to aid age determination. All teeth are fully erupted and in wear by five years of age, from which point age determination can be based on attrition of M3. Growth with age is described by means of the von Bertalanffy equation. Asymptotic weight is reached by 60 months, asymptotic body length and body girth by 40 months, and hindfoot length by 35 months. The asymptotes and the coefficient of catabolism (K) are compared with values obtained in other studies.

Contents

Introduction . . . . Material and methods Results . . . . . . Discussion . . . . References . . . .

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Page . . . . . . . . . . 241 . . . . . . . . . . 248 . . . . . . . . . . 250 . . . . . . . . . . 254 . . . . . . . . . . 256

Introduction

An essential prerequisite of any animal population study is that the ages of individual animals are known in order to ascertain life-span, age at puberty and at reproductive senes- cence, age structure of the population, age-specific reproductive potential and a schedule of mortality with age. Furthermore, a study of growth in body parameters with age will also require an accurate method of age determination. Numerous techniques are available, and the majority have been reviewed by KlevezaI & Klinenberger (1967), Friend (1968), Morris (1972, 1978) and Spinage (1973, 1976). In the hyrax (Procavia capensis), Millar (1972), Roche (1 978) and Fairall ( 1 980) have presented preliminary results on age determination techniques, and in this study three of these namely, eye lens weight, tooth eruption, and tooth attrition have been investigated further.

Growth of an animal is measured by the change in weight and linear measurements with time. Growth studies on wild animals are scarce because of the difficulty involved in collecting data. Information on growth is necessary for the estimation of population biomass and possible rates of exploitation, and for taxonomic studies. There are two quantitative aspects of growth studies. The first is concerned with changes in weight and length of various

0022-5460/83/020241+ 1 1 $03.00/0 0 1983 The Zoological Society ofLondon 241

248 D. STEYN A N D J. HANKS

body parameters with age, while the second is concerned with the relationship between two dimensions of a single animal (Hanks, 1972); both have been used in this study. Previous descriptions of growth in the hyrax have been presented by Millar (1 972) and Fairall (1 980), but these data were limited to animals up to and including three years of age.

Material and methods From December 1978 to November 1979, 101 hyrax (57 males and 44 females) were collected from

the Muden Valley, Natal, Republic of South Africa (28"58'S, 30"30'E). A minimum of 3 males and 3 females were collected each month as part of a study of reproduction in the hyrax in relation to environmental factors (Steyn, 1980).

Eye lenses were removed from each animal, fixed in 10% formal saline for a minimum of 48 h, cleaned of any extraneous tissue, dried to a constant weight in an oven at 9 6 T , and then weighed to 4 decimal places. Each eye lens was treated in exactly the same way and care was taken to keep them dry even while weighing. Both left and right lenses were collected from 26 hyrax and fixed for equal times to see if the 2 lenses differed in weight. To test the effect of different fixation times on the dried eye lens weight, left and right lenses were collected from 16 hyrax. The left lens from each of these animals was fixed for 7 days. Seven of the right lenses were fixed for 2 days, and the other 9 fixed for 14 days.

The 101 skulls were divided into age classes on the basis of tooth eruption and attrition. Eight eruption criteria were used (Fig. l), and attrition was quantified by measuring the crown height of mandibular M3 (from the bone to the top of the cusp on the lingual side of the right mandibular M3). Chronological ages were subsequently assigned to the age classes. The hyrax in the Muden Valley is a very pronounced seasonal breeder (Steyn, 1980), and if the date of collection of a skull is known, the hyrax concerned can be easily identified as being in either its first, second or third year of life on the basis of body size and stage of tooth eruption, and relatively precise ages can be assigned accordingly. In addition, 6 known-age hyrax skulls (2.5 weeks and 12, 17, 30, 45 and 114 months old) were obtained from various sources in South Africa, and these specimens were also used to assign chronological ages to the designated age classes.

Body growth data were collected from 99 hyrax (56 males and 43 females). Body length, girth and hindfoot measurements followed the method of Ansell (1 965). Theoretical von Bertalanffy growth curves (von Bertalanffy, 1938) were calculated for each growth parameter. For growth in weight the cubic growth equation of Beverton & Holt (1 957) was used:

Where: w,= weight at age t ; w - W , (1 -e - K ( f - l ) 3 I - 0 ) ks.

W - =asymptotic weight; K = a coefficient of catabolism; t=age in months; t,=theoretical age at which the hyrax would have zero weight.

FIG. 1. Tooth eruption criteria used for assigning hyrax to age classes. (a) not yet visihle--only black hole of crypt present. (b) Visible in the crypt-from a lingual view the tooth would not protrude above the bone line. (c) At start of eruption-the tooth protrudes slightly above the hone line. The adjacent tooth is completely erupted. (d) Mid-eruption-the lingual view shows that the tooth has reached half-way up the length of the adjacent tooth. (e) Nearly level-the tooth is almost level with the adjacent tooth hut is not yet in wear. (f) Partly in wear-the tooth is fully erupted but is not completely in wear. (g) Completely in wear, but still sloping down to hack ofjaw- crypt edge still closely adhered. This criterion applies only to maxillary M,. (h) Completely in wear-tooth free of crypt edge. This criterion applies only to maxillary M,. For (a) to (h), A=anterior jaw.

(ad)-Dorsal view of right lower jaw. (e) Lingual view of right lower jaw. (f) Lingual view of left lower jaw. (g and h)-Buccal view of right upper jaw.

A

i

I - R

Anterior

250 D. STEYN AND J . HANKS

The three coefficients (W- , K and to) were calculated from observed data by an iterative method with a computer (IBM 1 130, University of Natal) using the program of Hanks (1972). Growth in body dimensions (l,) was analysed by the same method using the non-cubic equation:

I , = L , (1 -e -K([-fJ) cm.

Results

Tooth eruption and attrition The 101 skulls were divided into 13 age classes, and the chronological ages assigned to

these classes are presented in Table I. Eruption of all the teeth was complete at 60 months, and the remaining four age classes (X-XIII) were distinguished on the basis of an assumed

TABLE I Hyrax age classes (based on tooth eruption and attrition), and assigned chronological a ~ e s

Criteria

Age class Maxilla Mandible

Chronological age (months)

I1

111

IV

V

VI

VII

VlII

IX

X

XI

XI1

XI11

pm4 at start oferuption to erupted completely MI in process of eruption

MI complete. M2 not yet visible. Permanent incisors could be erupting M2 visible to erupting but not in wear. Permanent incisors could be erupting M2 in wear. M3 not yet visible. Incisors replaced M3 visible in crypt

M3 in process of eruption but not in wear M3 partly in wear

M3 completely in wear but still sloping down to back of jaw. Crypt edge still visible Eruption complete and all teeth fully in wear Eruption complete and all teeth fully in wear Eruption complete and all teeth fully in wear Eruption complete and all teeth fully in wear

pm4 at mid-eruption to erupted completely MI in process of eruption to erupted completely M1 complete. M2 not yet visible. Permanent incisors could be erupting M2 visible to erupting to partly in wear. Permanent incisors could be erupting M2 in wear. M3 not yet visible. Incisors replaced M3 visible to erupting but not in wear M3 partly in wear

M3 nearly level to erupted completely M3 complete. M3 crown height greater than 6.0 mm

M3 crown height 5.1-6.0 mm

M3 crown height 4.1-5.0 mm

M3 crown height 3.1-4.0 mm

M3 crown height 2- 1-3-0 mm

0-4

5-1

8-10

11-16

17-19

20-22

23-27

28-43

44-59

60-75

76-9 1

92-107

108-124

HYRAX AGE DETERMINATION A N D G R O W T H 25 1

Age (months)

0.01

0 20 40 60 00 100 120

FIG. 2. Growth in dried eye lens weight with age. 0 Males; 0 females. Solid line is fitted curve, and the equation is:

y=0.01 x0'40 (r=0.95; P<O.OOI).

linear rate of attrition of M3 with an incremental interval of 16 months. The oldest known- age skull (1 14 months) had an M3 crown height of 3.0 mm, which would place it in class XIII.

Eye lens weight and growth

The weights of the left and right eye lens differed by less than I%, and accordingly one lens, usually from the left eye, was used on its own for subsequent analysis. There was no effect of varying fixation time (from two days to 14 days) on the eventual dried lens weight. The relationship between eye lens weight and age is shown in Fig. 2.

Growth in body weight with age

Both males and females attained their asymptotic weight at about 60 months. The asymp- totic weight of males was approximately 5% lower than that of females. The von Bertalanffy equations for growth in weight with age are as follows.

Males: w t - - 3267 (1 - e -0.07 ( t + 9 . 2 9 ) ) 3 g (Fig. 3). Females: w,= 3450 (1 - e -0.06 @+ 13,25))3 g.

Growth in body dimensions with age

Both males and females attained their asymptotes of body length and body girth at about

252 D. STEYN AND J. HANKS

' ' Ib 2(0 $0 4b & 60 70 80 90 100 0 I I I I I I

Age (months)

FIG. 3. Theoretical von Bertalanffy growth in body weight curve for male hyrax. The equation is:

w -3267 (1 -e -0.07 ([+9'29))3 g. t -

40 months, and hindfoot length about five months earlier. The relevant von Bertalanffy equations are as follows.

Body length

Males: 1,=535 (1 -e -0.09(t+5.89)mm (Fig. 4). Females: It = 542 (1 - e -0.06 (I+ 14.34)) mm.

Hindfoot length

Males: 1,=68.37 (1 -e - O . l l ( t+7 .82) ) mm. Females: It= 68.34 (1 - e -0.07 @+ 15.88)) mm.

Body girth

Males: g,=257 (1 -e -0.06(t+12.70)) mm. Females: gt=259 (1 -e -0.05 (t+16.80)) mm.

Relationships between growth parameters

The relationships of body length, hindfoot length and body girth with weight were allometric. Body length showed the highest correlation with weight, and the equation is:

where y=body weight (g) and x=body length (mm). y=0.000107 x2.74 (r=0.97; P<O.OOl) (Fig. 5 ) ;

HYRAX AGE DETERMINATION A N D G R O W T H 253

As linear relationships are more useful for predictive purposes, a number of isometric relationships were investigated. The most useful for predictive purposes was that between body weight and girth2 x body length. The equation is:

y=480+0*78 x (r=0.90; P<O*OO1) where y= body weight (8) and x=girth2 x body length (mm).

a v

m

200

600

-

'- I I I I I I I I I I

J . o

300 400 500 600 Body length (rnrnl

FIG. 5. The relationship between body length and weight. 0 Males; 0 females. Solid line is fitted curve, and the equation is:

y=O.OOO 107 x2'74 (r=0.97; P< 0.00 1 ).

254 D. STEYN AND J. HANKS

Discussion Tooth eruption and attrition

Millar (1972) presented tables on tooth eruption in the maxilla and mandible ofthe hyrax up to two years of age. He used four different descriptive criteria: “no tooth present”, “tooth visible in crypt”, “tooth half emerged” and “tooth fully emerged”. The age at which teeth fulfill a certain description varies by up to four months between his study and this study. For example, M2 (maxilla) according to Millar ( 1 972) is first visible at nine months and is fully emerged at 15 months. In this study M2 (maxilla) is first visible at I 1 months and is fully emerged at 17 months. Also M3 (mandible) (Millar, 1972) is first visible at 17 months and fully erupted at 24 months, whereas this study shows M3 (mandible) first visible at 20 months and completely erupted by approximately 28 months. Rather than these discrepan- cies reflecting different eruption rates in the two different populations, it is suggested that the apparent discrepancies arise from subjective assessment of “visible” and “erupted completely” when different observers describe any particular tooth.

The data presented by Roche (1978) for known-age skulls and the data presented in this study correspond precisely, the most important factor being that in both it is recognized that it takes about five years for the permanent teeth to be completely erupted and in wear. The presence of the crypt edge behind M3 (maxilla) is of importance in this regard. This is the area of biggest disagreement between the study of Fairall (1980) and Roche (1978) and the present one. Fairall (1980) reported M3 (maxilla) to be fully erupted at 36 months; Roche (1978) at 68 months. This is clearly a case of “fully erupted” criteria being different. The illustrations presented in this study should eliminate this source of confusion. Verbal descriptions will not suffice because of subjectivity of judgement.

Fairall’s (1980) data deviate from Roche’s (1978) and Millar’s (1972) further in that pm4 (maxilla) was not erupting at birth in Fairall’s study whereas in the other two, pm4 was present at birth. This study confirms that pm4 is present even in a full-term foetus.

The time of replacement of deciduous incisors with permanent teeth is variable and this study shows that they erupt any time from eight months to 17 months. This is in agreement with Millar (1972), Roche (1978) and Fairall (1980). Taking into account the highest recorded age for a hyrax (148 months (Mendelssohn, 1965)) and Fourie’s (1978) suggestion that 108 to 120 months is the true life span ofa hyrax, the chronological age assigned to age class XI11 in this study (108 to 124 months) is probably very close to normal longevity for the species.

Care must be taken in referring to either maxillary or mandibular teeth since it is evident in the age determination schedule presented that the upper and lower jaws show slight differences. The investigation of dental cementum annuli was not undertaken in this study although Fairall (1980) reports that annuli are visible and that the technique is valid for the hyrax. Usually this technique is only of value in animals from temperate regions where there is a marked drop in condition and growth rate during the winter months. The hyrax from Muden showed no significant decrease in condition in winter (Steyn, 1980) and thus it is unlikely that the cementum annuli technique would have been of value.

The major drawback of determining age by tooth attrition studies is that a schedule drawn up for a population from one area cannot be used for another area because different food consumed could result in different attrition rates. This limitation might be relevant for the hyrax since they are opportunistic feeders (Lensing, 1978) and hence eat what is available in their habitat.

HYRAX AGE DETERMINATION A N D G R O W T H 255

The use of tooth emergence as a criterion for age determination is of differing applicability in animals. In the hedgehog (Erinaceus europaeus), for example, it is of limited use because tooth replacement is complete within three or four months (less than 5% of the animal’s total life span) and in certain seals tooth replacement is complete before birth (Morris, 1978). The discovery that the hyrax take up to five years (50% of the life span) to exhibit completion of permanent dentition, makes them good candidates for age determination by tooth replacement schedules.

Eye lens weight and growth

The weights of the left and right eye lenses differed by less than 1%, concording with reports by Lord ( I 959) for cottontail rabbits (Syfvifagusflori$anus), and Kolenosky & Miller (1962) for pronghorn antelope (Antilocapra americana). Morris (1 972) advised the use of both eye lenses although Edwards (1962) and Phillips (1970) obtained satisfactory results using only one. Experiments by Friend (19674 showed that considerable inaccuracies can be introduced by varying the fixation time before drying and weighing the lenses. The effect of varying fixation time in this study was negligible over the periods used for fixation (2-14 days).

After all sources of error are minimized there is still variability in the eye lens weight from animals aged according to dentition. This could be due to a number of things. First the soundness of the age determination technique based on teeth is questionable. Secondly, in any natural population there is individual variability (Berry & Truslove, 1968). Thirdly, there is the possibility of variation due to factors in the life of the animal. Stress may affect the lendage relationship (Myers & Gilbert, 1968) but a special controlled study of the effects of dietary variation on lens weight of laboratory rats (Mus musculus) (Friend, 1967b) showed that, despite dramatic changes in body weight in response to different diets, lens weight retained its normal relationship with age. Fairall (1980) discarded the eye lens technique for the hyrax as unreliable over 24 months of age. However, the unreliability of his data could be the result of lack of precise consistency especially in weighing in a non-desiccated environment rather than an unreliability of this method of age indication. In addition Fairall’s (1980) raw data for dried eye lens weights at various ages do not agree with data from this study and it is impossible to ascertain the extent to which this is due to variation in the animals themselves or to variation in the technique of eye lens weight determination.

Growth with age Comparison of von Bertalanfy growth curves

The only previous in-depth study on growth in the hyrax in which von Bertalanffy equations were used was by Fairall (1 980) based on captive artificially fed animals up to three years of age. Petrusewicz & MacFadyen (1 970) have indicated the dangers of comparing captive and wild animals, although Millar (1971) showed that captive hyrax (in the same colony as those in Fairall’s study) had the same growth curve as their wild counterparts. Only two of Fairall’s equations (body weight and body growth) can be compared with equations in this study, as all the other parameters were measured in different ways. Fairall (1980) presented the following for growth in body weight.

256 D. STEYN AND J. HANKS

Males: ~ ~ 1 3 3 7 7 . 1 5 (1 -e-0.057 g. Females: w,=2664-02 (1 - e -0.060 (1+0.597)) g.

The equations for body girth were as follows.

Males: g,=327-73 (1 -e -0.058 ( 1 - 7 . 8 8 3 ) ) mm. Females: g,= 305.42 (1 -e -0,075 (1-6.452)) mm.

The asymptote. Reimers (1972) cited several studies which demonstrated that the skeletal tissues of mammals achieve maximum rate of growth (and therefore the asymptote) sooner than muscle or adipose tissue. Roseberry & Klimstra (1975) working on the wild deer Odocoileus virgin ianus found that near-maximum size was achieved earliest in hindfoot length, followed by dentary length, chest girth and finally body weight. Findings from this study agree with the above, asymptotic measurements being reached first in hindfoot length, followed by body length and body girth and finally body weight.

With the exception of female body weight, all of the asymptotic values of Fairall (1980) presented above are higher than for the Muden colony. This is probably the consequence of a sampling artefact which has influenced the calculation of the theoretical growth curve. If the sample contains only individuals which have not yet reached the asymptote, the method of iteration employed in the calculation of the von Bertalanffy equation must inevitably result in an over-estimation of this coefficient. All of Fairall’s (1980) sample had not reached the asymptote of growth, and the theoretical values he has presented are clearly an over-estimation.

Coefficient of catabolism. The coefficient of catabolism ( K ) , also known as the growth rate constant, varied between parameters from 0-05 to 0-1 1, with a mean of 0-07. This is similiar to a mean of 0.06 obtained by Fairall (1980) for his hyrax specimens, and a mean coefficient of catabolism of 0.08 in elephants (Hanks, 1972). Fairall (1980) suggested that this close proximation of values for the coefficient could possibly be related to the proposed common phylogenetic origin of the two species. This is most unlikely, as the von Bertalanffy equation serves as a purely empirical representation of weight, height or length-at-age data and there is little or nor biological or physiological significance in the parameters it contains (Hanks, 1972; Roff, 1980). The coefficient of catabolism values for other ungulates has no phylogen- etic or biological significance, varying from a value of 0.05 (similar to the hyrax and elephants) in the eland (Taurotragus oryx) (Jeffery & Hanks, 1981) to a high value of 1.76 in the impala (Aepyceros melumpus) (Howells & Hanks, 1975), with intermediate values of 0-397 in the buffalo (Syncerus cafer) (Grimsdell, I969), 1.12 in the zebra (Equus burchelli) (Smuts, 1974), 1.25 in the nyala (Tragelaphus angasi) (Anderson, 1978) and an identical value of 1.25 in the Blue wildebeest (Connochaetes taurinus) (Attwell, 1977).

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