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Age Determination of the Mongolian Wild Ass (Equus Hemionus Pallas, 1775) by the Dentition Patterns and Annual Lines in the Tooth Cementum

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Age Determination of the Mongolian Wild Ass (Equus Hemionus Pallas, 1775) by the Dentition Patterns and Annual Lines in the Tooth Cementum Journal of Species Research 2(1):85-90, 2013 Age determination of the Mongolian wild ass (Equus hemionus Pallas, 1775) by the dentition patterns and annual lines in the tooth cementum Davaa Lkhagvasuren1,*, Hermann Ansorge2, Ravchig Samiya1, Renate Schafberg3, Anne Stubbe4 and Michael Stubbe4 1Department of Ecology, School of Biology and Biotechnology, National University of Mongolia, PO-Box 377 Ulaanbaatar 210646 2Senckenberg Museum of Natural History, Goerlitz, PF 300154 D-02806 Goerlitz, Germany 3Institut für Agrar- und Ernährungswissenschaften, Professur fuer Tierzucht, MLU, Museum für Haustierkunde, Julius Kuehn-ZNS der MLU, Domplatz 4, D-06099 Halle/Saale, Germany 4Institute of Zoology, Martin-Luther University of Halle Wittenberg, Domplatz 4, D-06099 Halle/Saale, Germany *Correspondent: [email protected] Based on 440 skulls recently collected from two areas of the wild ass population in Mongolia, the time course of tooth eruption and replacement was investigated. The dentition pattern allows identification of age up to five years. We also conclude that annual lines in the tooth cementum can be used to determine the age in years for wild asses older than five years after longitudinal tooth sections were made with a low- speed precision saw. The first upper incisor proved to be most suitable for age determination, although the starting time of cement deposition is different between the labial and lingual sides of the tooth. The accurate age of the wild ass can be determined from the number of annual lines and the time before the first forma- tion of the cementum at the respective side of the tooth. Keywords: age determination, annual lines, dentition, Equus hemionus, Mongolia, Mongolian wild ass, tooth cementum �2013 National Institute of Biological Resources DOI: 10.12651/JSR.2013.2.1.085 ence of poaching on the population size and population INTRODUCTION structure. With respect to population analysis, the deter- mination of the accurate age is important baseline data. The Mongolian wild ass or Khulan (in Mongolian) Poached animals can be used as a sample for age deter- Equus hemionus hemionus Pallas, 1775 is one of the flag- mination of the Mongolian wild ass population. ship species that occurs in extreme ecosystems of the Several methods have been developed to assess the true World (Kaczensky and Walzer, 2008). Its population is age of dead individuals of many species (Sergeant, 1967; still decreasing year by year due to direct impact of ille- Morris, 1972; Harris, 1978; Habermehl, 1985; Lüps et al., gal hunting, habitat degradation and limited water access 1987). In the present study, two most accurate methods in Mongolia (Feh et al., 2002; Stubbe et al., 2005; Clark were chosen: 1.) dentition pattern and 2.) annual lines in et al., 2006; Kaczensky et al., 2006; Wingard and Zahler, the tooth cementum. The eruption and replacement of the 2006; Stubbe et al., 2007; Kaczensky and Walzer, 2008; teeth has also been used for the age determination of sev- Moehlman et al., 2008). However, almost 80% of the eral wild-living equids: mountain zebra Equus zebra ze- total population of the Mongolian wild ass in the World bra, Hartmann zebra Equus zebra hartmannae and plains is found in Mongolia (Feh et al., 2002), hence the Mon- zebra Equus burchellii (Erz, 1964; Klingel and Klingel, golian population is critical to the preservation of the 1966; Joubert, 1972; Penzhorn, 1982), as well as two species (Reading et al., 2001). For long-term conserva- domestic species of the Equidae closely related to the tion of the Mongolian wild ass, specific knowledge on Mongolian wild ass: the domestic donkey Equus asinus its biology and ecology is essential, including the influ- (Svendsen, 1997) and the domestic horse Equus caballus 86 JOURNAL OF SPECIES RESEARCH Vol. 2, No. 1 AB C Cementum Dentine Root Crown Fig. 1. First lower incisor (A) and its longitudinal section (B) with annual lines (C). (Habermehl, 1985). Nevertheless, no usable information nated from the southern Gobi (mostly from the Bordzo- exists regarding the dentition of the wild asses (Orlov, ngijn Gobi) and 64 (20♀, 24�, 20 unknown) from the 1961; Ansorge et al., 2007). Only Orlov (1961) gives south-western Gobi (Dzungarian Gobi). some short remarks, and Heptner et al. (1966) described Skulls of asses found dead were often preserved in a the tooth replacement of the Mongolian wild ass incor- mummified condition covered by skin. This made it pos- rectly by “die Milchzaehne werden mit dem 4. Jahr durch sible to estimate the season of death by the different sum- bleibende ersetzt” (“milk dentition is replaced by the per- mer and winter coat. manent teeth as of the fourth year”). Nevertheless, com- The sequence of eruption and replacement of the teeth parison of the well known dentition of the domestic equids could be evaluated by comparing different development and the zebras with the collected skulls of the Mongolian of the teeth of 127 individuals showing unfinished denti- wild ass is used to clarify the dentition of the species. tion. The time of the consecutive appearances of the teeth The accurate age of an older animal can be determined were assessed, including further comparative investiga- by the incremental cementum lines of tooth sections tions on related equid species like zebras, domestic horse (Laws, 1962; Morris, 1972; Grue and Jensen, 1979; Kle- and donkey (Erz, 1964; Klingel and Klingel, 1966; Jou- vezal, 1996). This method has been used successfully to bert, 1972; Penzhorn, 1982; Habermehl, 1985; Svendsen, estimate the age of mountain zebra Equus zebra zebra 1997). and plains zebra Equus burchelli (Smuts, 1974; Penzhorn, Based on the periodic growth, there are annual lines in 1982). Klevezal and Kleinenberg (1967) mentioned “re- the cementum of the tooth root, which can be used for gular, probably annular, layers in the cementum and sec- determining the age in years for many mammals. In order ondary dentin” of Equus hemionus, but did not consider to assess if such lines exist in the Mongolian wild ass the features for age determination. and to check which kind of teeth provide the most promi- The objective of this study was to evaluate a method nent lines, a total of 455 permanent teeth from 314 indi- of accurate age determination for Mongolian wild ass viduals were cut using a standard technique with a dia- using teeth eruption and replacement, as well as by annu- mond wheel of a low-speed precision saw (Buehler al lines in the dental cementum. IsoMet® 1000) (Johnston and Watt, 1980; Driscoll et al., 1985; Ansorge, 1995). Tooth roots were longitudinally sectioned by a diamond wheel of a low-speed precision MATERIALS AND METHODS saw without any pre-treatment. Generally, three cuts of every tooth were mounted on a slide with Histofluid made The study is based on 440 skulls of the Mongolian wild by Paul Marienfeld GmbH - a water-clear adhesive of ass collected between 2002 and 2010 mainly from two acrylic resins dissolved in xylene. The unstained sections regions of southern Mongolia (Stubbe et al., 2005; 2007). show annual lines - if present - under reflected light by Of these, 376 skulls (177♀, 147�, 52 unknown) origi- lateral illumination (Fig. 1) (Ansorge et al., 2007; Lkha- February 2013 LKHAGVASUREN ET AL.-AGE DETERMINATION OF EQUUS HEMIONUS 87 M3 M2 Milk tooth M1 Permanent tooth P4 P3 P2 Tooth P1 C I3 I2 I1 Birth 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 Age (by year) Fig. 2. Times of eruption and replacement of the teeth in the Mongolian wild ass. gvasuren et al., 2009). Time of the first development of tooth cementum and of the first incremental line were taken from skulls of known age, which was determined by their sequence of tooth replacement. The season of death could be estimated for 226 cases by the winter or summer coat on the skull. RESULTS As a first result of examination of the extensive mate- rial of Mongolian wild ass, a basic outline of tooth devel- opment can be given for the species (Fig. 2). Fig. 3. Occurrence of deciduous and permanent first premolars in Deciduous dentition the Mongolian wild ass skull. Although our sample of skulls did not include newborn animals, the eruption of all four deciduous premolars The deciduous dental formula of the Mongolian wild and the deciduous canine can be assumed at the time of ass is the same as in the other equid species: specially, a birth because the youngest specimens of our material small canine may occur in females, which is not visible. (⁄3 weeks) already show these teeth. However, the first small premolar and the canine mostly do not appear but Id 3 Cd 1 Pd 3(4) Male: mmmmmmmmmmmmmmm=28(32) exist below the gum line (Fig. 3). In some cases, the first Id 3 Cd 1 Pd 3(4) premolar is totally missing from the tooth row. Shortly Id 3 Cd 0(1) Pd 3(4) after birth the two inner incisors appear to erupt nearly Female: mmmmmmmmmmmmmmmmm=24(32) Id 3 Cd 0(1) Pd 3(4) simultaneously. The corner incisor follows after about (Id=deciduous incisor, Cd=deciduous canine, Pd= half a year. deciduous premolar) The time between eruption of these deciduous teeth is relatively short. This makes it possible to determine the Permanent dentition age of a Mongolian wild ass in months up to an age of one year. As the initial permanent tooth, the first premolar app- 88 JOURNAL OF SPECIES RESEARCH Vol. 2, No. 1 ears a few weeks after birth (Fig. 3), but sometimes does The respective permanent dental formula is: not cut through the gum or may be missing. The first I 3 C 1 P 4 M 3 molar erupts from the following spring to summer, and Male: mmmmmmmmmmmmm=44 I 3 C 1 P 4 M 3 the second molar does the same one year later.
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