Age determination and growth in the hyrax Procavia capensis (Mammalia: Procaviidae)

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  • 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.


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

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    . .

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


    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 - R


  • 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.


    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


    Age class Maxilla Mandible

    Chronological age (months)













    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











    76-9 1




    Age (months)


    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


    ' ' 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


    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


    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.

    Fairalls (1980) data deviate from Roches (1978) and Millars (1972) further in that pm4 (maxilla) was not erupting at birth in Fairalls 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 Fouries (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.


    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 animals 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)...


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