Age determination and body growth of the Common duiker Sylvicapra grimmia(Mammalia)

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  • J. Zool., Lond. (1984) 202,283-297

    Age determination and body growth of the Common duiker Sylvicapru grimmiu (Mammalia)

    V. J . WILSON*, J. L. SCHMIDT** A N D J . HANKS Institute of Natural Resources, University of Natal, P.O. Box 375, Pietermaritzburg, 3200

    Republic of South Africa

    (Accepted I4 June 1983)

    (With 3 plates and 8 figures in the text)

    Age determination in the Common duiker Sylvicapra grimmia was investigated by analysis of tooth eruption and replacement sequence, incremental lines of tooth cementum and tooth wear in a unique collection of 48 known-age skulls, and also by analysis by post-natal body growth in known-age duiker. In both the mandible and maxilla, permanent molariform teeth were fully erupted and in wear by 26 months of age. There was little variation in the age of eruption and replacement of all molariform teeth, making this a particularly useful feature of the duiker for age determination purposes. In contrast, the variability in eruption of the incisiforms, coupled with the difficulty in distinguishing deciduous incisiforms from the permanent counterparts, placed an unexpected limitation on the use of these teeth. Although the apparent linear relationship between tooth attrition and age has potential for further investigation as an age determination technique, the cementum annuli were not correlated with chronological age. Theoretical Von Bertalanfly equations were used to analyse body growth with age. It was concluded that because the asymptote of growth was reached at such an early age, and because there is so much individual variation in growth, body growth, including horn growth, is of very limited value for age determination. Female duiker were significantly larger than males.

    Contents

    Page Introduction . . .. . . .. . . .. .. .. .. . . 281 Materials and methods . . .. . . .. . . .. .. . . 282 Results .. . . .. . . .. .. .. .. .. . . 284 Discussion . . . . .. . . .. . . .. .. .. .. 290 References . . .. .. . . .. . . .. . . . . .. 294

    Introduction The Common duiker (Sylvicupru grimrnia) is one of the more widely distributed of the

    African ungulates (Meester & Setzer, 1971). It is typically a savanna species, but also occurs in quite open country, and extends into the alpine zone in mountain areas. Although the species is widespread in Africa, nowhere is it abundant. It is rarely seen in groups larger than two, and usually occurs singly. The Common duiker is mainly nocturnal, and because

    *Present address: Chipangali Wildlife Orphanage, P.O. Box 1057, Bulawayo, Zimbabwe. **Present address: Department of Fishery and Wildlife Biology, Colorado State University, Fort Collins, Colorado

    80523, U.S.A.

    0022-5460/84/020283 + 15 $03.00/0 283

    0 1984 The Zoological Society of London

  • 284 V. J . WILSON ET AL

    of its non-gregarious and retiring habits, it is a difficult species to study in the field. Thus in spite of its wide distribution, the Common duiker has been the subject of comparatively few investigations, the major studies concentrating on feeding habits (Wilson, 1966a), aspects of reproduction (Child & Mossman, 1965; Symington & Patterson, 1970; Von Ketelhodt, 1977), behaviour (Hopkins, 1966; Dunbar & Dunbar, 1979), blood parasites (Keymer, 1969), predators (Wilson, 1966b), effects of hunting (Child & Wilson, 1964) and drug immobilization (Wilson, 1967). Only two studies have examined techniques for the age determination of the species. Riney & Child (1960) developed nine age classes based on tooth eruption and wear, using four known-age skulls from 1.5 to 21 months to assign chronological ages to the classes. Riney & Child (1964) later investigated horn height as a technique for age determination, but concluded that it was of limited value. No published studies could be found of body growth with age in the Common duiker.

    The objectives of this paper are to examine age determination in the duiker based on tooth eruption and replacement sequence, incremental lines of tooth cementum and tooth wear in a unique collection of 48 known-age skulls, to describe and analyse post-natal body growth with age of known-age specimens and to investigate the use of growth data for age determination purposes.

    Materials and methods

    This study is based on 48 duiker of known-age, ranging from a neonate to 21.5 years of age. All of the animals were raised in captivity in large enclosures in natural to semi-natural conditions, 7 being raised in eastern Zambia, the remainder near Bulawayo in Zimbabwe. Thirty-two duiker were raised naturally by their mothers in enclosure of at least 1000 m2, all of which contained indigenous grasses, shrubs and trees which provided ample food and cover. Sixteen duiker were hand-reared on plain, undiluted cows milk to which a small quantity of Terramycin powder was added for the first few days of feeding to prevent stomach disorders. Initially they were fed 4 times a day at approximately 6 hourly intervals. This was gradually reduced to 3 times, twice and once a day until they were weaned at about 4 months of age. Indigenous vegetation was always available to the hand-reared duiker, although they were confined to small cages until they were weaned.

    The 48 known-age upper and lower jaws were grouped into age classes based on the sequence of tooth eruption and replacement. X-rays were taken of the mandibles of all specimens with permanent molariform dentition to determine and examine the sequence of the erupting incisiform teeth. The height of the anterior and posterior enamel crown of each of the three fully erupted molars was measured to the nearest 0.1 mm, following the method of Grimsdell (1973) in order to investigate the possible use of tooth attrition as a method of age determination. All fully erupted first, second and third molars in known-age specimens were extracted from one side of the mandible, fixed in 10% buffered formalin and decalcified in 5% nitric acid. The specimens were dehydrated, embedded in paraffin wax, sectioned at 7-15 pm and stained with Haris haematoxylin and eosin. The stained sections were examined microscopically with transmitted light for the presence of cementum annuli. Six mandibles from duiker of unknown age collected from various localities in Zambia and Zimbabwe were used to investigate the presence and distribution of tooth cementum annuli in the roots and root pads of the first and third molars using reflected light as described by Morris (1972). Cross-sections and longitudinal sections were taken of each tooth.

    Where possible, the known-age duiker were caught each month, weighed to the nearest 0.1 kg and standard body measurements taken as described by Ansell (1965). Horn length was measured along the front surface of both horns, and the mean length calculated and used for subsequent analysis. Five males and 3 females were weighed and measured each month from hi& to death. Theoretical Von

  • DUIKER AGE DETERMINATION AND BODY GROWTH 285

    TABLE I Tooth eruption and replacement in the mandible of the Common duiker

    Incisors Canine Premolars Molars

    Age Known-age class n (months) 1 2 3 1 2 3 4 1 2 3

    1 2 3 4 5 6 I 8 9 10 1 1 12 13

    1 4 3 3 2 3 3 5 I 4 I 4 3

    0 1-3 2.5-3 4.5 I

    8'5-1 0.5 9-10 15-20 11-23 22-25 2648 34-31

    60-older

    d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d

    d/P d/P d/P d P P P P

    e e e d d d e d d d E d d d P d d d P e d d d P E d d d P P d d d P P e d d d P P E

    d/e d/e d/e P P P P P P P P P P P P P P P P P P P P P

    Explanation of symbols: d = deciduous tooth; d/e = deciduous tooth still present, with permanent tooth erupting below; d/P=erupting incisor, I, to I, may be at any stage from d/e to just P; e=permanent tooth just erupting; only cusps or tips visible above bone; E=erupting permanent tooth; all four cusps well above bone but not fully erupted; P= fully erupted permanent tooth.

    TABLE I1 Tooth eruption and replacement in the maxilla of the Common duiker

    Premolars Molars

    Age Known-age class n (months) 2 3 4 1 2 3

    1 2 3 4 5 6 I 8 9 10

    I 4 2 4 1 3 5 9 3 14

    0 1-2.5 34.5 4.5-1 9

    8.5-10.5 10-18 18-23 22.5-2 5 26-older

    e e e d d d e d d d E d d d P d d d P e d d d P E d d d P P d d d P P e

    d/e d/e d/e P P E P P P P P P

    Symbols as in Table I.

  • 286 V. J . WILSON E T A L .

    Bertalanffy equations to describe growth with age were obtained by calculating the three coefficients in the function by the program of Hanks (1972).

    Results Tooth eruption and replacement

    Duiker dentition is diphyodont and similar to that of most bovids. There are 20 deciduous teeth and 32 permanent teeth. The complete tooth eruption and replacement sequence in the mandibles of the known-age duiker is shown in Table I, and in the maxilla in Table 11. There was little variation in the age of eruption of the molariform teeth. In both the mandible and the maxilla, all the permanent molariform teeth were fully erupted and in wear by 26 months of age, and there was little overlap between successive age classes leading up to that state of eruption. In the mandible in Class 6, one duiker of 10.5 months had M, still erupting, overlapping with the three duiker of 9, 10 and 10 months in Class 7, where M, was fully erupted. In Class 9, two duiker of 17 and 18 months had a more advanced eruption of M, than the five other animals in that group, overlapping slightly with two animals of 18 and 20 months in Class 8. Class 9 also had one duiker of 23 months in which M, was not fully erupted, which overlapped with Class 10 by one month. In the maxilla, none of the successive age classes overlapped by more than 0.5 month.

    Eruption of the permanent incisiform teeth was much more variable in relation to chronological age. The greatest variability occurred in Class 11. X-rays revealed that one 26-month-old duiker was at the same state of tooth development as a 48-month-old animal (Plate I). In both animals, the permanent incisiform teeth were clearly visible beneath the deciduous teeth. The 48-month-old duiker showed a surprising lack of wear on the deciduous incisors and canines (Plate 11) and in this animal and in several others it was impossible to distinguish deciduous incisiforms from the permanent counterparts. The four animals in Class 12 showed various gradations of eruption of the incisors from the state where the deciduous tooth was still present with the permanent tooth erupting below to the state where I, and I, were fully erupted and I, was still deciduous. A further collection of material is required to confirm the sequence of replacement of the incisiforms.

    PLATE I. X-ray of the deciduous incisiforms of (a) a 26-month-old duiker and (b) a 48-month-old duiker, illustrating the erupting permanent teeth below the bone.

  • DUIKER AGE DETERMINATION AND BODY GROWTH 281

    PLATE 11. The deciduous incisifoms of a 48-month-old duiker (left) and a 26-month-old duiker (right), the same animals as in Plate I.

  • 288

    Age (months)

    FIG. 1 . The relationship between molar enamel crown height and age, M, anterior cusp -0; M, anterior cusp. . . 0; M, posterior cusp - - - A. Equations in text.

    PLATE 111. The skull and lower jaw ofa 21.5-year-old duiker, the oldest known-age Common duiker on record.

  • 289 DUIKER AGE DETERMINATION AND BODY GROWTH

    Tooth attrition

    Of the 14 duiker with three fully erupted molars, only four animals were 4 years of age and above (Fig. l), including one animal of 21.5 years (Plate 111), which is the oldest known-age duiker on record. The relationship between age and attrition was investigated, and a linear regression gave the best fit (Fig. 1).

    The relationship between molar crown height and age is given by the following equations, where x is age in months, and y is crown height (mm).

    M, anterior cusp: y=6.799-0.026x (r=0.812; P

  • 290 V. J . WILSON ET AL.

    Body growth

    Growth with age in the body parameters measured, together with the mean size at birth and an indication of the age at which the asymptotes are reached, are summarized in Table 111, with growth curves for male shoulder height, tail length, total body length and body weight illustrated in Figs 2-5 respectively. The theoretical Von Bertalanfi equations indicate that males are slightly smaller than females for all the parameters measured, and when all the measurements for each parameter that fell within 5% of the respective asymptotes were compared, the differences between the sexes were significant for ear length, hindfoot length, hoof length and tail length (P0.05). With the exception of shoulder height and body weight, there was little difference between the sexes in the ages at which the asymptotes were reached (Table 111). The theoretical Von Bertalanfi equations indicate that males reach asymptotic

    I . .

    520 - T t t 480.

    1

    E

    1 1 I I 1 I I I I 1 I I 1 I l l I I 1

    0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36

    Age (months)

    FIG. 2. Theoretical Von Bertalanffy growth curve for shoulder height in male duiker of known-age (range-vertical line; mean-crossbar; standard error-broad ponion of line). The equation is:

    h I- - 55 1.3 (I-e-0.147('+4-856)) mm

    140 -

    40 A A k A Ib ;2 Ib Ik ;8 ;O ;2 ;4 ;6 ;8 ;O 42 ;4 ;6 Age (months)

    FIG. 3. Theoretical von Bertalanffy growth curve for tail length in male duiker of known-age (range-vertical line; mean-crossbar; standard error-broad portion of line). The equation is:

    / I - - 124.5 (I+-O224('+3276)) mm

  • DUIKER AGE DETERMINATION AND BODY GROWTH 29 1

    1100- 4A-L-i ' L I ' 1000 - I

    - 900- E E

    f C 700- 3

    - 0

    400 d h d h A ;O /2 Ib Ik /8 ;O i2 ;4 ;6 ;8 ;O ;2 ;4 d6 Age (months)

    FIG. 4. Theoretical Von Bertalanffjr growth curve for total body length in male duiker of known-age (range- vertical line; mean-crossbar; standard error-broad portion of line). The equation is:

    1 t - - 1061.3 (1~-0~161(f+4~140)) mm

    20

    18

    16

    14

    0 12 c

    r - g 10 .- $ 8

    6

    4

    2

    0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36

    Age (months)

    FIG. 5. Theoretical von Bertalanffjr growth curve for body weight in male duiker of known-age (range-vertical line; mean-crossbar; standard error-broad portion of line). The equation is:

    w - 18.31 (1_e-0.136(~+5.416))3 kg 1-

    weight and shoulder height at 22 months, 6 months later than females. This apparent sexual dimorphism in growth rate is confirmed by Figs 6 and 7, which also illustrate the extent of the individual variation in growth with age. The drop in weight of 2-4 kg by all three females (Fig. 7) was unrelated to season or reproductive status. The three duiker were not reared simultaneously, and there was no record of any environmental or stress factors which could have caused the drop.

    Horn growth with age was not amenable to analysis by the Von Bertalanffy equation. Although horns were first visible externally as points of keratinization at three months of

  • 292 V. J. WILSON E T A L .

    L Q x

    0

    .- a

    Age (months)

    FIG. 6. Growth in body weight with age in five...

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