R.N. Pal, S. Patnaik and T.K. Mohanty
National Research Center on Yak, Dirang-790 101, West Kameng District, Arunachal Pradesh, India
Age determination is essential in studies on the comparative anatomy, physiology, morphology and taxonomy. In case of ungulate the age is determined on the basis of replacement of the milk teeth by the permanent teeth and by studying the rate of wear of the permanent teeth. In some of the methodology the ratio of the length of the root to the crown is used in age determination. This study presents some observations on the appearance and physical structure of tooth in yak from birth to old age. Basic differences with cattle are: the yak calf is not born with incisors; after a day or two a small speak of white dentine appears on the hard red mass of the incisor gum pad; the first pair of incisors is subsequently followed by eruption of the rest of the incisors; the red hard mass gum pad separates out to make space for the erupting incisors; and by two years of age all the milk teeth come out, the length and width varying from 1.2 to 1.6 cm and 0.6 to 1.1 cm, respectively. On completion of two years the first pair of baby incisor is crowed out by permanent incisor. The process repeats each year until at five years all the four pairs of the baby incisor are replaced by permanent teeth. From five years onwards, the age is estimated by observing the wear and tear of the incisors, which are proportional to the age of the animal.
Keywords: Age, dentition, yak
Accurate age determination is an essential requirement to understand some of the major biological phenomenon in animals. Without reliable data on animal age, the rate of growth, sexual maturity, span of reproductive life or the entire life span is rather very difficult to determine. Age determination is also essential in studies on the comparative anatomy, physiology, morphology and taxonomy. Age determination thus enables one to perceive the rate of reproduction and the longevity of the animal. It also helps in the determination of the fluctuations in animal numbers that is going on in a population, thus in turn it can be used to establish the age composition of a population, relative size of the different age groups etc. Age structure is also useful in assessing population status in terms of potential risk of extinction.
Age determinations in the animal species have specific significance. The domestic animal's reproductive life span is important. For economical exploitation, herdsmen would like to keep a herd structure, which consists of greater number of animals in the most productive age group so as to optimise profit over time.
Different methods for the determination of age have been used. One of the oldest methods is to determine the age of the animal depending on the amount of wear and tear of the teeth, specifically in case of the carnivores and rodents. In the case of ungulates, the age is determined on the basis of replacement of the milk teeth by the permanent teeth and by studying the rate of wear of the permanent teeth. In some of the methodology the ratio of the length of the root to the crown is used in age determination. The relative breadth of the tooth canal has also been used in determining age in some species of animals.
In mammals, and also in other species, the external structure of the skull, diameter of the foramen (long preserved specimen), the rate of the fusion of the cranial sutures, the character of the surface and rate of ossification of the epiphyses of the limb bones are used to ascertain the age of the subject. In mammals, the weight of the lens of the eyeball is also occasionally used to determine age.
Examination of the number of ridges on the hooves and horns has been used to determine age in cattle. This method has been found to be somewhat reliable only when claws have not been subjected to extensive wear and tear (Klevazel and Kleinenberg 1969).
Possibly among the Bovidae family, age determination based on horn, hoof or dentition yield different results. This is not surprising since animals are reared under variable agro-climatic conditions where uniform nutrition is not available. This study presents some observations on the appearance and physical structure of tooth in yak from birth to old age.
Before focusing on the processes of wear and tear of the teeth in relation to age, a brief description of the morphology of the tooth is warranted. The tooth of a mammal has two distinct parts: a) the crown, and b) the root. The portion above the gum or jaw is the crown while the embedded portion is the root. The crown is covered from outside to inside by: a) enamel b) dentine, and c) pulp. The root is covered by `cement' on outside followed by dentine and pulp. The newly erupted tooth is a thin-walled dentine cap whose crown is already covered with enamel.
The dental formula in case of cattle has been established. There are two types of teeth, the deciduous (milk teeth), which later is replaced by a set of permanent teeth. The dental formula is as follows:
Deciduous: 2 (DI 0/4; DC 0/0; DP 3/3)
Permanent: 2 (I 0/4; C 0/0; PM 3/3; M 3/3)
DI = Deciduous Incisor; DC = Deciduous Canine; DP = Deciduous Premolar; I = Incisor; C = Canine; PM = Premolar; M = Molar
The dental formula holds good for cattle and buffaloes, and is considered applicable in the case of yak as well, but with modifications to accommodate some basic differences at birth and at subsequent ages. Results presented here are based on observations based on yak at various stages of growth and are compared to the pattern of dental development in cattle.
At birth: Yak calf is not born with any baby tooth. The lower jaw at birth is a continuous red hard mass. In some of the newborn calf, very small-pointed edges of dentine appear on the outer corner of both the first pair of incisors. Conspicuous changes take place within seven days of birth. The first pair of incisors appears, the red hard mass starts showing demarcations and there are distinct signs for the appearance of incisor teeth.
After seven days following birth: The beginning of the second week is marked by the complete exposure of the first pair of incisors. The second pair of incisors is still embedded in the red hard mass but the third pair shows up as a small pointed dentine speck on the hard mass. The fourth pair of incisors is still inside the red hard mass.
After fourteen days following birth: The first pair of incisors is broader and bigger. The second incisor has appeared but smaller than the third but the fourth incisor is yet to show up. The red mass in between the incisor is retracted producing gaps between the incisors.
At completion of third week: The first, second and third pairs of incisors have grown bigger and comparatively rounder compared to the second week. The fourth pair of incisors is yet not erupted but there is sign of eruption, the bulging of the red mass at the place where the fourth incisor is to come out is prominent.
At completion of fourth week: The fourth pair of incisors has just erupted and starts to grow. The rest of the incisors have also grown in length and breadth. The wide gaps between the incisors have been reduced to a great extent.
Basic differences with cattle up to one month of age:
Crowding out of baby teeth by permanent teeth: At about two years of age, the first pair of the baby incisors is crowded out by a pair of permanent teeth. At three years of age the second pair of the incisors is replaced by the permanent teeth. The third pair at fourth year and the fourth pair at fifth year of age are crowded out. These phenomena are similar to cattle. The baby teeth are flat, long and wide at the initial stages but as age advances these become smaller and smaller in size until they are completely weeded out by permanent teeth.
The diametrical distance between the two fourth incisors also varies according to the age of the animal. The average distance in yak is 8.46 ± 0.25 cm, with minimum and maximum distances of 7.00 and 10.50 cm, respectively. The distance increases with age. The milk incisors up to two years are large, chiselled out and angular at the upper end and as the time of crowding out comes closer, the upper edges become flat. From a visual examination of the milk teeth on or before two years, an expert can predict approximate age of the subject. The milk incisors' length and width vary from 1.2 to 1.6 cm and 0.6 to 1.1 cm. At five years of age when the mouth is full (all baby teeth replaced by permanent incisors), the maximum length and width of the incisors are 2.0 cm and 1.4 cm, respectively and the corresponding minimum length and width are 0.8 and 0.8 cm. The length and width proportionately decrease with the age, with a tendency to flatten out almost at a prime age, but this will depend on the type of the grazing available. In old yak above 20 years, only a remnant of the incisors is left.
The authors are most grateful to Indian Council of Agricultural Research, Krishi Bhavan, New Delhi-110001, for funding the Emeritus Scientist Project to the senior author and are also very much obliged to the Director, National Research Centre on Yak, Dirang, for providing necessary facilities during the study.
Klevazel G.A. and Kleinenberg S.E. 1969. Age determination of mammals from annual layers in teeth and bones. Program for Scientific Translations, Jerusalem, Israel. 128 pp.