The Horns and Teeth as Indicators of Age in Bison
W. A. Fuller,
Canadian Wildlife Service,
Reprinted from The Journal of Wildlife Management, Vol. 23, No. 3 July, 1959 pp.342-344
Five age classes of male bison and four age classes of female bison can be recognized in the field on the basis of horn growth. Tooth eruption and replacement permit -the recognition of five age classes separated by 1 year ( 0-4 years inclusive) .Tooth wear permits recognition of an additional three age classes, designated "young adult," "adult," and "aged," which coyer the remainder of the life span of the animals.
The Horns and Teeth as Indicators of Age in Bison
This paper describes changes, associated with advancing age, in the form of the horns and in the eruption and subsequent wear of the teeth in the bison ( Bison bison.) The study was carried out in Wood Buffalo National Park, northern Alberta and southern Mackenzie District, N.W.T., and was part of a larger study of the biology and management of that animal. The material examined came from two sources. Sixty-five animals were either shot or found dead, mainly in the summer months. From this sample 26 jaws, representing a series of advancing age, were cleaned and preserved for permanent reference. Approximately 1,800 jaws were examined at the annual reduction slaughters, held in mid- winter, in the years 1950 to 1955. Since a rather complete post-mortem inspection was carried out, only a short time was available for examination of the horns and teeth. For this reason, only jaws of unusual interest were preserved and the stage of tooth development or wear was entered on a specially prepared card by placing a check mark in the appropriate square. A brief description of the horns, often accompanied by a sketch, was entered on the same card.
Calves and yearlings can be recognized by body size, and therefore their approximate ages in months can be calculated. The study reported here suffers from a lack of older specimens of known age.
The horns of males of various ages have been figured by several previous writers, notably Allen (1876), Hornaday (1889), and Garretson (1938). Seton (1929) has reproduced Hornaday's figure in part. Any one of these plates shows clearly how the horns, which project nearly laterally in the calves, gradually turn upward and finally inward. In mature animals the tips become blunted and shredded. According to Mohr (1949), this is caused by the new horns piercing through the old sheath. She reported that it occurs regularly at 4 years of age in the wisent ( Bison bonasus ), but the writer has found that it occurs later in American bison, probably at 7 or 8 years of age. .
In using the horns for aging male bison in the field, it was found impractical to attempt to distinguish between 2- and 3-year-olds except under the most favorable conditions. Both were therefore lumped under the designation "spike-horn." It was also difficult to differentiate in all cases between a spike-horn and a young adult-the 3- and 4-year- olds of Seton's plate ( op. cit., p. 665) .It was relatively easy to spot the stepped horn tips of the mature and aged animals, and also to identify calves and yearlings where general body size affords an important clue. Thus, it is practical to set up five age-classes that are recognizable in the field, as follows:
- calf-less than 1 year
- yearling-1 to 2 years
- spike-horn-2-, 3- and some 4-year-olds
- young adult-4 years to ? ( 7 or 8 years )
- adult and aged-more than 7 or 8 years.
The horn of the female has not been described in comparable detail. It differs from that of the male in several respects. First, there is practically no spike-horn stage. By the third winter, the inward curve has been attained. Second, there is a pronounced posterior deflection of the tips, so that the mature horn curves inward and backward. Third, there is no continued enlargement of the basal diameter. Fourth, after the full curvature has been attained in early maturity, the basal part of the horn grows straight away from the head, thus carrying the curved portion farther laterally with each succeeding year. Fifth, there is seldom any fraying of the horn tips. Sixth, there is quite commonly a fracture of the entire horn 5 or 6 in. from the base, which results in loss of the entire curved portion and retention of a mere stub about as long as the bony core. This seems to be a result of the form of development described above.
From the practical viewpoint of using the horns as criteria of age, it is difficult to recognize any female counterpart of the spike-horn group in males. This is not too serious, because about one-third of the cows attain sexual maturity at 2 years of age, and most of the others, probably, as 3-year-olds. There is little need, therefore, to recognize a sub- adult, nonbreeding age group.
Aged cows are recognized by the long, straight basal portion of the horns, or by the presence of "stubby" horns on one or both sides.
Annual growth rings are too difficult to detect to be of much value as age indicators. I made a study of aging by both horn rings and dental characters during the November-December 1952 slaughter. Horn rings gave too low a value for yearlings and 2-year-olds, and too high a value for 4-year-olds and older. Horn rings were not discernible on many animals classed as 3-year-olds by dental characters. False annuli, being indistinguishable from true growth rings, appear to be common in older specimens and were therefore discarded in favor of dental characters for all subsequent age determinations on slaughtered animals.
The teeth of the bison are similar to those of the domestic milk cow, not only in dental formula but also in the form of the individual teeth, and the time and manner of replacement. The dental formula is: I 0/3, C 0/1, P 3/3, M 3/3 = 32.
The lower canine is incisiform and is often called ( incorrectly) the fourth or "corner" incisor. The most anterior premolar (P.) has a relatively simple architecture, whereas Pa and P. are progressively more complex and molariform. The premolars and molars are selenodont to a marked degree. On each molar, between the anterior and posterior cusps, there is a style which is found on the lingual side in the upper jaw, and on the labial side in the lower jaw ( Fig. 2) .The occasional anomalous development of the first premolar has been described by Fuller (1954).
FIG. I. Stages of wear on incisiform teeth of bison. At the upper left is a 3-year-old, showing slight wear on the incisors and no wear on the canine. At the upper right is a late 4-year-old, showing the beginning of wear on the canine. The lower left depicts the "adult" condition with moderate wear on the canine, and the lower right, the "aged" condition, showing heavy wear on all the occlusal surfaces and "necks" below the crown.
The normal sequence of eruption of the deciduous and permanent dentition, and subsequent stages of wear, has been worked out and applied as a means of aging carcasses at the annual slaughters in Wood Buffalo National Park. The description of these processes that follows is based on the lower jaw, but the replacement sequence is pretty well synchronized between upper and lower teeth.
At birth the complete deciduous dentition is present, but is covered by a thin, semitransparent membrane that is sloughed off in the first few days of life. The next major change occurs at about 3 months when the anterior cusp of the first molar (M1, ) pierces the gum. The posterior cusps do not emerge fully until sometime between the ninth and twelfth months. During the second year, the second
Fig. 2. Stages of wear on molar teeth of bison. The upper drawing depicts a 4-year-old. The style (St) on the first molar (M1) is worn to a loop; on the second molar (M.), it is worn to a circle; and it is not yet in wear on the third molar (Ma). The lower drawing represents the appearance of the molars in old age with all styles worn to loops.
molar (M2) appears and, like M1 requires several months for its complete eruption. In the third year, several important changes occur. The two central pairs of incisors (I1 and 2) are shed and replaced; deciduous premolars two and three (dP2 and dP3) are likewise replaced; and the third molar (M3) erupts. The deciduous fourth premolar (dP4 ) usually is retained longer than dP2 and dP3., but occasionally it is shed before the others. The pre- molars in general are more erratic than the incisors in their development and consequently are not reliable as indicators of age. M3 in bison, as in many other ruminants, possesses a rudimentary third cusp which does not come into wear for some months after the two main cusps. A "full mouth" or complete set of permanent teeth is acquired during the fourth year when dI1 and dC1, are shed and replaced and the third cusp of M3 comes into wear. From this point on, age must be judged by the amount of wear on the teeth and, consequently, aging becomes less precise.
By the time the full mouth is acquired, there is a thin line of wear apparent on 1, to I. (Fig. 1, upper left) and the style of M1 which is at or below the gum line when the tooth first erupts, is in wear. At first, only the free tip of the style wears away, producing a circular enamel pattern on the occlusal surface ( Fig. 2, upper, M2 ) .As the tooth is progressively worn away, the enamel pattern of the style becomes a loop (Fig. 2, upper, M1). In the fifth year, a slight amount of wear appears on the canine, and progressively more wear appears on the incisors, particularly 1, ( Fig. 1, upper right) The style on M1, always shows a loop, the style on M2 is usually a circle or occasionally a loop, while the style on M3 is not yet in position to wear ( Fig. 2, upper).
I did not attempt to recognize individual year classes for animals older than 4 years. However, three additional groups were defined with consider- able precision. "Young adults" have the style on M3 worn to a circle (never a loop), but still show only slight wear on the canine. The limit for slight wear is arbitrarily defined as 2 mm. By this time, the worn area on the incisors is approaching 4 mm., and is classed as moderate. In the "adult" group the wear on one, two, or three incisors may exceed 4 mm., whereas on the canine it is between 2 mm. and 4 mm. in width ( Fig. 1, lower left) .The styles on M1, and M2 usually show loops, whereas the style on M3 is typically still in the circle stage. When wear on C1 exceeds 4 mm. (Fig. 1, lower right) and the style on M3 wears to a loop (Fig. 2, lower), the animals are classed as "aged."
It will only be possible to assign a numerical age to these last three classes when known-aged animals become available. In the meantime it is my thought that the young adult group contains mostly 5- and 6-year-olds, and that the division between adults and aged probably comes at about 12 or 15 years of age. A good correlation has been observed between the appearance of a shoulder on the horn tips and the development of the dental characteristics associated with the adult category.
To make these criteria more useful as a practical means of age determination, the following dichotomous key has been constructed.
1. Deciduous teeth all present.
2. Second molar absent.
3. First molar not visible...........................................0-3 mo.
3. First molar erupting to fully growth........................3-12mo.
2. Second molar erupting to fully grown........................................1 yr.
1. At least central I's permanent.
4. C1 deciduous or shed, never permanent; I2-3 deciduous or permanent
5. I3 deciduous; I2 deciduous or permanent;3rd cusp of M3 not in wear; style on M1 just commencing to wear, or worn to circle (rarely a loop).............................................2 yr.
5. 13 permanent; C1 deciduous or shed; style on M1 typically a loop; occasionally circle or not worn............................ (in part)............................................ 3 yr.
4. All teeth, including C1, permanent.
6. No visible wear on C1; style of M3 not in wear.
7. Typically no wear on I3 as well as C1; style of M2. not in wear or with wear on tip only..............................(in part).......................3yr.
7. Typically slight wear on 13; style of M2 usually a circle. Occasionally slight wear on C1 if style of M2 not in wear.......................... 4 yr.
6. Visible wear on C1.
8. Wear on C1 not exceeding 2 mm.; M2 style circle or loop; M3 style not in wear or tip only worn................................................ young adult.
8. Wear on C1 exceeding 2 mm.
9. Wear on C1 2-4 mm.; M. style rarely a loop..............................adult
9. Wear on Cl more than 4 mm.; M3 style usually a loop...................aged
Five age classes of male bison and four age classes of female bison can be recognized in the field on the basis of horn growth. Tooth eruption and replacement permit -the recognition of five age classes separated by 1 year ( 0-4 years inclusive) .Tooth wear permits recognition of an additional three age classes, designated "young adult," "adult," and "aged," which cover the remainder of the life span of the animals.