Enumerating Megapopulations of Wild Bears with an Ingested Biomarker

The effort required to capture and handle animals limits the utility of mark-recapture population estimation to relatively small study areas, which, for large, mobile species may yield unrepresentative results. We employed baits containing tetracycline, a commonly-used biomarker of bone and teeth that fluoresces under ultraviolet light, to mark a large sample of American black bears (Ursus americanus) across 2 states (Minn. and Mich.). Baits were small enough to be completely devoured by individual bears and were distributed widely enough within each state (1 bait/23-65 km 2 ) that only a small percentage of bears consumed >1 bait. The proportion of each population marked with tetracycline was estimated by examining thin sections of teeth or ribs collected mainly from hunter-killed bears. The number of animals marked was equated to the number of baits consumed by bears (distinguished by their claw marks on trees where baits had been hung), adjusted for the extent of within-year multiple-marking observed in teeth and rib samples. The year of marking was distinguishable by the location of marks in relation to annulations in the tooth; this enabled us to mark bears over several years. Moreover, because the marks are long-lasting in bone and permanent in teeth, we could reassess the proportion marked in any given year with ribs and teeth collected during later years. The addition of samples collected >1 year after marking caused population estimates for the year of marking to increase. We believe that population estimates produced from samples collected during the year of marking were low, possibly because the bears that had been attracted to our baits and consequently marked were more prone than other bears to be killed by hunters. However, because most bears were eventually killed by hunters, the cumulative harvest of bears provided an aggregate sample of teeth that became progressively less biased with respect to marking. Low-biased estimates are common in mark-resample studies; the potential to correct this bias by accumulating samples over time is an advantage of marking with tetracycline. The primary advantage, however, is that animals can be marked over a large area at relatively little expense. This technique could be applicable to other species of bears, as well as other harvested, solitary animals.