Tooth Wear Rate in Northern Bison

Abstract

Wood Buffalo National Park is a boreal preserve that spans the border between Alberta and the Northwest Territories. In its 44,668-km2 area there are about 6,000 free-roaming bison (Bison bison) (Anions, 1982, pers comm.; Anions and Lukiwski, 1980). From 1977 until 1981, the Canadian Wildlife Service and Parks Canada performed field studies of the interactions between wolf (Canis lupus) and bison in the Park, allowing thorough documentation of a number of bison deaths due to predation (Oosenbrug et al., 1980a, 1980b). Carcasses were examined after wolves completed feeding and teeth were collected for the purpose of determining age. I collected additional lower jaws from skeletons that I encountered during ground surveys for bison bones (Haynes, 1983). Attributes of tooth eruption and wear as they reflect age in bison have been discussed by Fuller (1959), Armstrong (1965), Frison and Reher (1970), and Reher and Frison (1980). The latter two studies are the most detailed, but involve archeological samples of B. bison from prehistoric sites on the northern Great Plains. Although the subspecific status of bison in Wood Buffalo National Park is still unsettled (Geist and Karsten, 1977; Peden and Kraay, 1979), the animals clearly are conspecific with the northern Plains population. Additionally, bison in the two populations possess nearly identical dentition, and the sequence and timing of tooth eruption are the same for both (Frison and Reher, 1970; Haynes, unpubl. data). Therefore, it seems valid to compare wear rates of teeth in the northern and southern bison. The first mandibular molar was selected for age determination because it is a large tooth that usually persists even in weathered jaws and because it is fully erupted and in function at about 12 months of age (Frison and Reher, 1970). The thin layers of cementum that are laid down annually on the roots of the first molar will thus record all years of life (Armstrong, 1965). The enamel height on the metaconid was measured for each first molar from 20 dentaries. This measurement was taken with a sliding caliper, and records the height of the crown from occlusal surface to the boundary of the enamel with the cementum-covered root, measured parallel to the long axis of the cusp (Table 1). Next, the first molars from these 20 dentaries were examined microscopically to determine age, in some cases by use of both optical and scanning electron microscopes (SEM). Before examination, the molars were removed from dentaries by sawing out the alveolar bone around roots, using a 0.25 horsepower hand-held rotary saw, which polished as it cut. The root segment was sagittally sectioned, to expose the thickest part of the layered cementum at the interradicular zone between the two roots (Fig. 1). Hand-held magnifying lenses or stereo dissecting microscopes were used to make three separate and blind counts of the visible annular cementum layers on the sections. These microscopic paired light colored and dark bands are assumed to represent a full year of cementum deposition (Armstrong, 1965). Layers that appear very light-colored (nearly white) are in fact opaque and are richer in cementocyte cells than the darker layers, which are translucent when seen in thin section. Each 1 cm thick specimen section was soaked 15-30 min in a 5% solution of formic acid and dried for 12 h. The acid bath dissolves proportionately more of the lighter colored layers, and hence etches the cementum by causing alternating layers to collapse through partial dissolution. The resulting appearance of the cementum is of valleys (the collapsed layers that were originally white) between raised ridges (the less dissolved layers that were originally dark colored) (Armstrong, 1965). After three blind counts were made, 11 tooth segments were selected as a sample of age categories best represented by the dentaries. These specimens were sputter-coated with gold: palladium (40:60) and then micrographed in a Cambridge Mark IIA Stereoscan scanning electron microscope (SEM) operated at 20 kV. Using the micrographic prints, counts were made of paired ridges and valleys in the cementum layers (Fig. 1B). Cementum band counts from the optical and SEM examinations are presented in Table 1. The counts are thought to reflect to within ?2 years the actual ages of the sample animals. The plus/minus factor results from the fact that the first one or two paired light colored and dark layers of cementum are usually August 1984 487