Supernumerary Teeth in a Captive Population of American Bison Bison bison

Kevin R. CROOKS* and Dirk VAN VUREN


A link between dental abnormalities and loss of genetic variation has been reported for unconfined populations of American bison Bison bison (Linnaeus, 1758) but not for captive populations. From a zoo herd with a small founder population and likely history of inbreeding, we report the first recorded occurrence of dental abnormalities in captive bison and the first case of supernumerary second premolars in bison.

Department of Wildlife and Fisheries Biology, University of California, Davis, California 95616, USA

Key words: Bison bison, supernumerary teeth, inbreeding

Reprinted from Acta Theriologica 39 (3):325-328, 1994

Supernumerary teeth in a captive population of American bison

Bison bison

European bison Bison bonasus (Linnaeus, 1758), and American bison B. bison (Linnaeus 1758) both have suffered severe population bottlenecks or founder because of human activities, thus there has been considerable interest in the effects of inbreeding on genetic (McClenaghan et al.1990), demographic (Slatis 1960), and morphological (Van Vuren 1984, Kobrynczuk 1985) traits of extant bison. Dental abnormalities, particularly supernumerary teeth, are morphological traits that have been linked to inbreeding in American bison (Wilson 1974, Frison et al 1976, McDonald 1981, Van Vuren 1984), but only in unconfined populations. We know of no reports of abnormal dentition from captive bison, yet it is captive ungulates that are particularly vulnerable to inbreeding (Ralls et al. 1979). Herein we reported the first instance of abnormal dentition in captive bison and the first supernumerary second premolar in bison.

Four adult male and two adult female American bison from a herd maintained by the San Francisco Zoo were sacrificed after testing positive for bovine tuberculosis. Subsequent examination revealed that the two females had supernumerary teeth; teeth of the four males were normal. One female (Female 1) had a supernumerary " lower premolar located in the tooth row on the left dentary between the P2 and P3 (Fig. 1). The other female (Female 2) had bilateral supernumerary premolars, one on each dentary. The supernumerary premolar on the left dentary was outside the tooth row on the labial side, between the P2 and the P3 (Fig 2). The supernumerary premolar on the right dentary was located in the tooth row between the P2 and the P3 (Fig 3). The skull of Female 2 was deposited in the Museum of Wildlife and Fisheries Biology at the University of California, Davis (specimen #WFB 2637). All permanent teeth in the two females were fully erupted and in regular wear, including the third cusp of the M3, so these bison were mature individuals at least five years of age (Frison and Reher 1970).

Fig. 1. Lower left dentary (from the left: P2, supernumerary premolar, P3, P4) of Female 1, a mature female American bison. Supernumerary premolar is in the tooth row between the P2 and P3.

Fig. 2. Lower left dentary (P2, supernumerary premolar, P3, P4) of Female 2, a mature female; American bison.

Supernumerary premolar is outside the tooth tow on the labial side, between the. P2 and P3.

Fig. 3. Lower right dentary {P4, P3, supernumerary premolar, P2) of Female 2, a mature female American bison. Supernumerary premolar is in the tooth row between the P2 and P3.

Although ancestral forms of bison contained four premolars, the cases re- ported here likely do not represent recovery to this ancestral state. Vestigial first premolars do occasionally appear in bison, but they occur in the middle of the diastema, not adjacent to the other cheek teeth (Fuller 1954). Further, the supernumerary premolars were very similar in size and appearance to the normal P2 but very dissimilar to the normal P3. As is characteristic of normal P2 teeth in bison (Wegrzyn et at. 1990), each supernumerary tooth had a short and wide crown, a sharp peak of enamel in the cranial (anterior) lobe, and small depressions in the caudal (posterior) lobe. Thus, we conclude that the supernumerary pre- molars are P2 teeth, resulting from a splitting of the P2 tooth germ (Wolsan 1984).

Increased frequency of supernumerary teeth in the San Francisco Zoo bison may have resulted from a small founder population and subsequent inbreeding. The herd directly descends from the first herd of bison acquired by the zoo ca. one hundred years ago. In 1946 the bison were quarantined when some individuals were diagnosed with bovine tuberculosis. Information from available records indicate this herd has remained as a closed population until the present; the herd currently numbers only eight individuals (F. Dunker, pers. comm.). Substantial inbreeding is therefore likely to have occurred following the initial founder events. Interestingly, in addition to the supernumerary teeth, both female bison showed grossly deformed occipital condyles.

Recent studies suggest that inbreeding in small captive populations may have harmful effects due to loss of genetic variation (Senner 1980, Ralls and Ballou 1983, Ralls et at. 1988). The occurrence of dental and cranial abnormalities in the small, closed herd of bison at the San Francisco Zoo may serve to further emphasize the importance of genetic management in captive populations.

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